Minnesota Department of Agriculture
Summary of Groundwater Nitrate-Nitrogen Data
March 2012
MAU-12-100
Minnesota Department of Agriculture
625 Robert Street North
Saint Paul, MN 55155-4194
http://www.mda.state.mn.us/
651-201-6000 or 800-967-2474
TDD: 1-800-627-3529
Minnesota Department of Agriculture
Pesticide and Fertilizer Management Division
Monitoring and Assessment Unit
Report Review and Collaboration
PFMD Staff
In accordance with the American with Disabilities Act, an alternative form of communication is available upon
request. TDD: 1-800-627-3529. The MDA is an Equal Opportunity Employer and Provider.
TABLE OF CONTENTS
TABLE OF CONTENTS ………………………………………………………………..…...
i
LIST OF FIGURES ……………………………………………………………………..……
ii
LIST OF TABLES …………………………………………………………………..……….
iii
LIST OF APPENDICES …………………………………………………………..…………
iv
SECTION 1: INTRODUCTION …………………………………………………………..
1
1.1. Summary of the MDA Groundwater Monitoring Program …………………….…...
1
1.2. Sample Collection Efforts by Other Entities in Minnesota for Nitrate Analysis ...…. 7
SECTION 2: SUMMARY OF NITRATE DATA FROM REGIONAL
MONITORING ……………………………………………………………………………...
8
2.1. Central Sands (PMR 4) …………………………………………………..…………
10
2.2. Southeast Karst (PMR 9) …………………………………………………………....
14
2.3. Other Agricultural Regions (PMRs 1, 5, 6, 7, & 8) ………………………………...
19
2.3.1. Northwest Red River (PMR 1) …………..………………….………………..
19
2.3.2. East Central (PMR 5) ………….……………………….…………………….. 22
2.3.3. West Central (PMR 6) ……………...…………………………………...……
26
2.3.4. Southwest (PMR 7) ……………………………………...……………………
30
2.3.5. South Central (PMR 8) ……..………….……………………………………..
34
2.4. Urban Groundwater Monitoring …………………………………..………………..
38
SECTION 3: ANALYSIS OF AVAILABLE NITRATE DATA FOR PMR 4 ….………
42
3.1. Temporal Analysis ………………………………………………………………….
42
3.2. Spatial Analysis …………………………………………………………………….. 45
3.3. Depth Analysis ………………………………………………………………...……
53
SECTION 4: CONCLUSIONS ………………………………………………...…………..
56
SECTION 5: REFERENCES ……………………………………………………………..
57
i
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
FIGURES
Figure 1. MDA’s Pesticide Monitoring Regions (PMRs) ………………………………..…..…....
4
Location of groundwater sampling sites in 2010 ………………………………...….......
6
Figure 3. MPCA’s groundwater contamination susceptibility map with MDA’s PMRs ……...…..
8
Figure 4.
PMR 4 median nitrate results per site …………………………………………………...
13
Figure 5.
PMR 9 nitrate concentration comparison by source for the current network....................
17
Figure 6.
PMR 9 median nitrate results per site ……………….…………………………………..
18
Figure 7.
PMR 1 median nitrate results per site ……………………………..………………....….
21
Figure 8.
PMR 5 median nitrate results per site ……………………………………………..….....
25
Figure 9.
PMR 6 median nitrate results per site ……………………………………………....…...
29
Figure 10. PMR 7 median nitrate results per site ……………………………..………………….....
33
Figure 11. PMR 8 median nitrate results per site ………………………………………………..….
37
Figure 12. Urban groundwater monitoring median nitrate results per site …………..……………..
41
Figure 13. Nitrate concentration time series from PMR 4 groundwater monitoring …………….…
43
Figure 14. MDNR ECS subsections with the MDA PMR 4 monitoring wells …………………..…
46
Figure 15. Anoka Sand Plain median nitrate data in comparison to PMR 4 median nitrate data …..
48
Figure 16. Hardwood Hills median nitrate data in comparison to PMR 4 median nitrate data……..
48
Figure 17. Minnesota River Prairie median nitrate data in comparison to PMR 4 median nitrate
data ……………………...………………………………………………………………..……
49
Figure 18. Pine Moraines & Outwash Plains median nitrate data in comparison to PMR 4 median
nitrate data …………………………………………...………………...………………………
49
Figure 19. PMR 4 median nitrate results per site for the current network ………………………….
50
Figure 20. PMR 4 median nitrate results per site for the current network, along with the nitrate
concentration gradient for the Quaternary outwash geology layer and ECS subsections for
the region ………………………………………………………………………………..…..…
52
Figure 21. Location of deeper wells within PMR 4 ……………………………………………...…
53
Figure 2.
ii
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
TABLES
Table 1. MDA’s Pesticide Monitoring Region (PMR) descriptions and
characteristics………………………………...…………………………………………….……
5
Table 2.
Summary of nitrate results from the former network in PMR 4 ……………..………......
11
Table 3.
Summary of nitrate results from the current network in PMR 4……..…..……………….
12
Table 4.
Summary of nitrate results from the former network in PMR 9 ………………….....…...
15
Table 5.
Summary of nitrate results from the current network in PMR 9 ………………..…...…...
16
Table 6.
Summary of nitrate results from the former network in PMR 1 ………………..….…….
20
Table 7.
Summary of nitrate results from the current network in PMR 1………………...……….
20
Table 8.
Summary of nitrate results from the former network in PMR 5……………..…..……….
23
Table 9.
Summary of nitrate results from the current network in PMR 5……………..…………..
24
Table 10. Summary of nitrate results from the former network in PMR 6 …………..……………..
27
Table 11. Summary of nitrate results from the current network in PMR 6………………...………..
28
Table 12. Summary of nitrate results from the former network in PMR 7……………….…………
31
Table 13. Summary of nitrate results from the current network in PMR 7 ………………..……….
32
Table 14. Summary of nitrate results from the former network in PMR 8 …………..……………..
35
Table 15. Summary of nitrate results from the current network in PMR 8………………..………..
36
Table 16. Summary of nitrate results from the former network in urban areas …………………….
39
Table 17. Summary of nitrate results from the current network in urban areas …………………….
40
Table 18. Summary of trend test results for PMR 4 nitrate data ………………………...………….
45
Table 19. Summary of nitrate results in paired shallow and deep monitoring wells in PMR 4 …….
55
iii
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
APPENDICES
Appendix A. Mann-Whitney comparison test results.
Appendix B. Mann-Kendall trend test results.
Appendix C. Kruskal-Wallis comparison test results.
Appendix D. SUBSEC Mann-Kendall trend test results.
Appendix E. Geostatistical analysis background.
*NOTE*: All appendices have been placed in a document separate from this report, which is
available upon request. Please contact the Monitoring and Assessment Unit with the Minnesota
Department of Agriculture, via the information listed on the title page of this report, to obtain this
appendix document.
iv
Section 1: INTRODUCTION
The Minnesota Department of Agriculture (MDA) Monitoring and Assessment Unit
(MAU) provides information on impacts to the state’s water resources from the routine
application of agricultural chemicals. Although MDA’s groundwater monitoring
program was designed for pesticides, MDA collects and analyzes samples for nitratenitrogen (hereafter referred to as nitrate) analysis to add to the body of information that
relates to the potential environmental impact to groundwater associated with agricultural
activities in the state. This document provides a summary of MDA’s groundwater
monitoring activities and the results from these historical nitrate analyses.
1.1. Summary of the MDA Groundwater Monitoring Program
The MDA began monitoring groundwater through a series of surveys beginning in the
fall of 1985. In 1987 Minnesota enacted the Minnesota Pesticide Control Law
(Minnesota Statutes Chapter 18B), which included a requirement for the evaluation of
pesticide impacts on the environment; the Minnesota Comprehensive Groundwater
Protection Act (Minnesota Statutes Chapter 103H) in 1989 further expanded the
groundwater protection responsibilities of the MDA. In response to these laws, MDA
initiated a groundwater monitoring program in 1987 that consisted of a network of preexisting monitoring wells around the state that were selected from the earlier monitoring
surveys that were conducted. The wells in the original network (1987-1996) consisted of
monitoring wells, observation wells, and private drinking water wells that, depending on
the region, were placed in either the Quaternary aquifer, till, or karst bedrock. After
1996, MDA completed a formal evaluation of its groundwater monitoring network and
determined that this network had provided all the information it was capable of
providing. It was further determined that many of the wells were, or soon would be, past
their useful life span.
Central Sands
In January of 2000, following three years of development, the MDA implemented a new
legislatively approved network of monitoring wells specifically installed for examining
agricultural chemical impacts to groundwater in a focused area of the state. This new
network, termed the Central Sands groundwater monitoring network, was conceived by
the MDA and designed in consultation with the Minnesota Department of Health (MDH),
Minnesota Pollution Control Agency (MPCA) and Minnesota Department of Natural
Resources (MDNR), along with the counties of the region (see Table 1 for a listing of
these counties), the University of Minnesota, St. Cloud State University, the Natural
Resource Conservation Service, and the United States Geological Survey (USGS). The
design was ultimately based on a randomly oriented geospatial grid-based network. The
MPCA, the MDNR, and local water well contractors assisted with monitoring well design
and installation. The Central Sands network, as originally intended, consisted entirely of
water quality monitoring wells designed to sample the very top portion of the shallowest
aquifer in the state’s major sand plain region. The Central Sands area is considered one
of the most vulnerable regions in the state with respect to potential agricultural chemical
movement to groundwater due to the rapid movement of water through the sandy, coarse
1
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
textured soils. Accordingly, the monitoring wells in the network were designed to detect
changes in groundwater quality in the shortest possible time.
Southeast Karst
The karstic terrain of southeast Minnesota is highly susceptible to contamination from
activities at the ground surface. This is because the surface soil layers in the region are
relatively thin and lie directly on top of the highly fractured limestone bedrock. Naturally
occurring springs and domestic drinking water wells provide the two primary means of
access to groundwater for sampling in this region. Groundwater springs have been
sampled for pesticides in southeast Minnesota since 2000; nitrate samples were first
collected beginning in 2002. Springs at MDNR fish hatcheries in the region were first
utilized in the program. Springs from other locations in the region were added to the
groundwater monitoring program in 2006 after it was determined that they met site
selection criteria, which included connection to a shallow geologic formation without an
overlying confining unit and that the source area consisted primarily of row crop
agriculture. A select assemblage of 14 private drinking water wells were also added to
the groundwater monitoring program in 2009 after meeting similar site selection criteria
as the springs.
Urban
Sampling of urban monitoring wells prior to 2000 occurred exclusively in the Twin Cities
Metropolitan Area. Beginning in 2004, a cooperative effort between the MPCA and
MDA included the collection of water samples from monitoring wells located in urban
areas around the state. All the urban monitoring wells are part of an MPCA urban
groundwater monitoring program and samples are collected by MPCA staff. A
maximum of 20 urban monitoring wells are selected and sampled per year.
Pesticide Monitoring Regions
In 2004 the MDA groundwater monitoring program, with assistance from the University
of Minnesota, designed and implemented a regional groundwater monitoring network
approach that divided the state into ten regions. These regions were developed to
facilitate water quality monitoring efforts, pesticide management, and best management
practice development, promotion, and evaluation. As such, these regions were termed
Pesticide Monitoring Regions (PMRs) by the program. Figure 1 depicts MDA’s PMRs.
The PMR delineations were based on crops, soils, climate, geology, and existing
ecoregion boundaries. Table 1 provides a general summary of the characteristics of the
different PMRs. Ultimately, the boundaries were delineated along county lines to
facilitate the implementation of management practices and reporting of results.
MDA groundwater monitoring locations that were situated in PMR 4 (Central Sands),
PMR 9 (Southeast Karst), and urban areas, as previously discussed, were incorporated
into the new regional groundwater monitoring network. Monitoring wells elsewhere in
the state were assigned to PMRs 1, 6, 7, and 8 for inclusion in the regional groundwater
monitoring network. PMR 5 (East Central) was given a lower priority ranking for
network establishment due to the area having limited agricultural production. Monitoring
wells in PMR 5 were included in the regional groundwater monitoring network in 2007
2
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
after monitoring was established and implemented in the other higher priority areas.
PMR 2 (North Central) and PMR 3 (Northeast) were not selected for monitoring due to
bedrock limitations for well installation and little ground application of pesticides in these
heavily forested areas.
Initially, monitoring wells were chosen from pre-existing monitoring or observation wells
maintained by the state and completed in the Quaternary aquifer. Well construction
reports were evaluated to ensure that the screened interval of the selected monitoring
wells intersected an unconfined aquifer water table condition and would therefore
represent water recharging from the local land surface. Other monitoring wells were
installed by the MDA in these PMRs as the need arose and resources were available.
Figure 2 displays the status of the MDA groundwater monitoring network in 2010.
For further information regarding MDA’s groundwater monitoring design and PMR
characteristics, please refer to the Groundwater Monitoring Design Document at
(www.mda.state.mn.us/monitoring).
3
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Figure 1. MDA’s Pesticide Monitoring Regions (PMRs).
4
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 1. MDA’s Pesticide Monitoring Region (PMR) descriptions and characteristics.
PMR
Region
Counties Included
Physical Characteristics
1
Northwest Red
River
Kittson, Roseau, Marshall, Pennington,
Red Lake, Polk, Norman, Mahnomen,
Clay, Wilkin, Traverse, Grant
2
North Central
Lake of the Woods, Koochiching,
Beltrami, Clearwater, Itasca
Glacial lake bed w/ high clay content soils 150 to 250 ft thick;
gravel aquifers buried under clay; beach ridge deposits of
sand and gravel; high value agriculture of sugar beets and
small grains
Mostly forested and bog; little agriculture in discontinuous
areas; groundwater resources quite variable
3
Northeast
St. Louis, Lake, Cook, Carlton
Forested with shallow bedrock; agriculture nearly nonexistent
4
Central Sands
Large glacial outwash sand plains that are highly sensitive to
surface activities; high value potatoes and other crops;
irrigated fields are common
5
East Central
Becker, Hubbard, Cass, Crow Wing,
Morrison, Wadena, Otter Tail, Todd,
Douglas, Pope, Stearns, Benton,
Sherburne, Kandiyohi
Aitkin, Pine, Mille Lacs, Kanabec,
Chisago, Isanti
6
West Central
Stevens, Big Stone, Swift, Chippewa, Lac
Qui Parle, Yellow Medicine
7
Southwest
8
South Central
9
Southeast
Karst
10
Metro
Lincoln, Lyon, Pipestone, Murray, Rock,
Nobles
Wright, Meeker, Renville, McLeod,
Sibley, Nicollet, Le Sueur, Rice, Steele,
Waseca, Blue Earth, Brown, Redwood,
Cottonwood, Watonwan, Jackson, Martin,
Faribault, Freeborn
Goodhue, Wabasha, Winona, Olmsted,
Dodge, Mower, Fillmore, Houston
Anoka, Ramsey, Washington, Dakota,
Scott, Carver, Hennepin
5
Glacial outwash and lacustrine sands; low pH soils; generally
poor cropping conditions; some irrigation; some potato
production
Some areas of glacial outwash sand; thin and narrow alluvial
aquifers; many buried sand aquifers; mix of corn and
soybeans; thick glacial tills in some areas
Aquifers consist of highly sensitive alluvial river valley
deposits; fractured quartzite formations and well protected
deep cretaceous sediments; sufficient water supply is hard to
come by; rural water systems are large and growing
A mix of glacial outwash sands; deep glacial tills, glacial
lacustrine deposits; windblown silts, river valley deposits; and
deep bedrock aquifers; sensitivity varies accordingly; corn
and soybeans; intensive ag production; most productive land
in the state
Karst geology that is highly sensitive to surface activities;
shallow windblown silt and glacial till soils; springs,
sinkholes and disappearing streams; high value trout streams;
extremely shallow to very deep bedrock aquifers; some river
valley alluvial deposits
Urban, suburban and transitional areas; some irrigated
farming; hobby farms; much farming conducted on leased
land by relatively large farm operations; outwash sand and
gravel to deep bedrock aquifers
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Figure 2. Location of groundwater sampling sites in 2010.
6
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
1.2. Sample Collection Efforts by Other Entities in Minnesota for Nitrate Analysis
In addition to the MDA, there are several other state, local, and federal entities that
collect groundwater samples for nitrate analysis throughout Minnesota. Some of these
entities are listed below, along with their related web link (it should be noted that this is
not intended to be an exclusive list):




Minnesota Department of Health (MDH) – monitoring of public drinking water
supplies in the state
https://apps.health.state.mn.us/mndata/nitrate-messaging
Minnesota Pollution Control Agency (MPCA) – ambient monitoring of urban
areas around the state
http://www.pca.state.mn.us/index.php/water/water-types-andprograms/groundwater/groundwater-monitoring-and-assessment/groundwatermonitoring-and-assessment.html
United States Geological Survey (USGS) – special project areas or regional-scale
assessments for the nation’s groundwater resources
http://water.usgs.gov/ogw/programs.html
County agencies, such as Soil and Water Conservation Districts (SWCDs) –
management of local-scale natural resources
http://www.maswcd.org/SWCDs_On_The_Web/swcds_on_the_web.htm
MDA nitrate data has not been compared with data collected by these entities. The MDH
Minnesota Nitrate Work Group (1998) noted that various entities have unique reasons for
collecting nitrate data, and the comparison of data sets could lead to inappropriate
statistical conclusions complicated by differences in program design and underlying
statistical methods. Further, there are various external confounding factors that can
complicate data set comparisons, such as:









Land use (i.e., row-crop agriculture versus grassland)
Well depth (i.e., shallow versus deep)
Well type (i.e., monitoring well versus domestic well)
Geology (i.e., sand and gravel versus till)
Aquifer conditions (i.e., reducing versus non-reducing environment)
Geologic setting (i.e., confined versus unconfined aquifer)
Sampling protocols (i.e., immediate sample collection versus sample collection
after purging and parameter stabilization)
Sample collection timing (i.e., winter versus spring sample collection)
Program length (i.e., short-term versus long-term monitoring)
Because the MDA would be unable to control for these confounding factors in a
statistical analysis of data from all entities, MDA analysis in this report is limited to
MDA data. MDA program design limits confounding factors, leading to more
statistically valid analysis of its own data.
7
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Section 2: SUMMARY OF NITRATE DATA FROM REGIONAL MONITORING
This section of the report provides data summary comparisons between nitrate
concentration results from the former, pre-1996 groundwater monitoring network and the
current, post-1999 monitoring network. The former network database was vetted so that
only samples from wells that matched the monitoring type, depth and geologic material
of wells from the current network were selected. That way, data comparisons between
the former and current networks represent changes over time and not changes in
monitoring methods.
The nitrate data has been compiled and analyzed on an annual basis by network (former
versus current) for each PMR. Summaries for PMRs have been ordered based on
sensitivity to, and impact from, nitrate contamination. The Central Sands area (PMR 4)
is discussed first, followed by a discussion of the Southeast Karst area (PMR 9). The
other areas (PMRs 1, 5, 6, 7, and 8) have been placed into the same subpart for discussion
of nitrate results since they are generally not as vulnerable to nitrate contamination, as
presented in Figure 3 (MPCA, 1989). Nitrate results from urban groundwater well
monitoring efforts are discussed last.
Figure 3. MPCA’s groundwater contamination susceptibility map with MDA’s PMRs.
8
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
It should be noted that dissolved oxygen, oxidation-reduction potential, and iron
concentration have not been historically collected as part of MDA’s monitoring program,
although deemed important when characterizing the sensitivity of groundwater to nitrate
contamination (MPCA, 1999). Further, assessments of nutrient loading or weather
patterns were not conducted and tracked over time. An evaluation of MDA’s nitrate data
was still warranted, however, because this evaluation will identify areas of surficial
aquifers that have been impacted by nitrate, and the level of that impact, irrespective of
the biological processes occurring under the ground and the physical processes occurring
above the ground. These factors will greatly affect results locally and on a small time
scale, but will dampen out in a large scale, long term study such as performed in this
analysis.
Nitrate occurrence (percent detection) was calculated as a simple ratio of total number of
samples with nitrate present to the total number of samples. Descriptive statistics
(minimum, median, 75th percentile, 90th percentile and maximum values) are also
reported. This information provides a general summary of the data and is an easy way to
compare data sets with uneven numbers of samples. These statistics are commonly used
for describing the location and spread of non-normally distributed data (Helsel and
Hirsch, 2002), which is the nature of the MDA’s groundwater nitrate data. The only
difference is that the 90th percentile is reported instead of the 25th percentile. The 90th
percentile is added to give a more detailed look at samples with higher nitrate
concentrations. Average annual minimum and maximum values were also computed to
provide further comparisons between PMR former and current time frames. Lastly, the
nitrate concentration data has been compared against two benchmark values, a
background level and the MDH Health Risk Limit (HRL), and reported as a percentage
of values meeting these benchmarks. The background level, the level of nitrate typically
found in aquifers under undisturbed landscapes, is 3.0 mg/L; the MDH HRL is
10.0 mg/L. These benchmarks are intended to provide a sense with respect to the level of
nitrate impact on the shallow groundwater in each region (and shallow vs. deep
groundwater in PMR 4), based on all available sampling results. All this information has
been compiled in summary tables for each region and time frame.
Further analysis was performed on the nitrate data after the statistical summaries were
completed. Comparison tests were performed on the nitrate data to evaluate differences
in the central tendencies of the regional data sets from the former and current networks.
These were generated using the Mann-Whitney statistical test, a nonparametric method
used for comparing two independent, non-normally distributed data sets (Conover, 1971).
Tabulated results of comparison testing are presented in Appendix A.
Summary figures were generated for each PMR. These figures display the median nitrate
results by particular selected sites in each PMR for each network, based upon certain
criteria:

Only sites with a minimum of four nitrate values within each network were selected for
display in the figure. This criterion was applied to ensure that the median statistic for
each site was supported by an acceptable number of sample values.
9
Summary of MDA’s Groundwater Nitrate-Nitrogen Data

For former network sites, only those that had locational information (latitude and
longitude) provided for at least two quarters were used in calculating site median
statistics.
Median nitrate results for sites were calculated and condensed into four groups for
mapping and assessment. These four groups were:
1. Sites with no measurable nitrate levels, which are considered as sites not having nitrate
impacts;
2. Sites with median nitrate results between the Method Reporting Limit (MRL) of
0.40 mg/L (basically >0) and 3.0 mg/L, which are considered as background sites;
3. Sites with median nitrate results greater than 3.0 to 10.0 mg/L, which are considered as
sites impacted by nitrate; and,
4. Sites with median nitrate results greater than 10.0 mg/L, which exceed the state’s HRL.
These groupings were applied to both the former and current network sites.
The summary figures are intended to provide a visual sense related to the magnitude of
nitrate impacts on the shallow groundwater in each PMR on a per site basis and,
ultimately, show that there are great differences locally compared to regional scales.
2.1. Central Sands (PMR 4)
Nitrate concentration results from PMR 4 have been summarized in Tables 2 and 3 for
the former and current networks, respectively. Only data from PMR 4 monitoring wells
were used in this analysis (results from private drinking water wells were removed from
the data set). This region has the largest data set, with combined totals of over 1,000
groundwater samples collected between 1985 and 1996, and between 2000 and 2010.
Sample collection frequency changed in 2008, with a shift from quarterly to bi-annual
sampling. This was found to have no impact on the overall results due to the lack of
seasonality in the data (2009, MDA Groundwater Design Document).
Nitrate was detected in 73 percent of the groundwater samples collected from the former
PMR 4 network (Table 2). The detection frequency increased to 97 percent in the current
network (Table 3), a difference of 24 percent. In the former network, a similar
percentage of samples had concentrations at the background level (between the MRL and
3.0 mg/L) as had concentrations above the HRL of 10.0 mg/L (37 percent versus 38
percent, respectively). This similarity disappeared in the new network. The biggest
difference was in 2003 when only 8 percent of groundwater samples were at background
nitrate levels, while 69 percent of samples were above the nitrate HRL. The central
tendency of the nitrate data shifted over time, as well. The median nitrate value of the
former network was 6.50 mg/L. This value increased to 14.40 mg/L for the current
network. The 75th and 90th percentiles, as well as the average annual maximum value, for
the current network were also higher than the former network values for nitrate. This
shift in nitrate concentrations between the two networks was found to be significant at the
95 percent confidence level (Appendix A).
10
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 2. Summary of nitrate results from the former network in PMR 4.
Maximum
(mg/L)
% at or
below
background
(3 mg/L)
% at or
above
Health Risk
Limit
(10 mg/L)
16.40
18.60
29%
14%
22.44
30.70
40%
40%
9.75
18.14
27.50
52%
25%
6.72
12.80
20.70
28.50
37%
30%
0.00
5.50
14.30
21.90
33.60
40%
35%
72%
0.00
6.85
15.30
22.80
47.40
33%
41%
156/200
78%
0.00
8.20
17.95
29.95
86.80
30%
42%
1992
112/158
71%
0.00
6.90
19.00
29.80
79.80
36%
42%
1993
141/194
73%
0.00
6.85
17.30
24.90
51.00
36%
39%
1994
125/170
73%
0.00
4.40
16.60
30.40
45.00
42%
33%
1995
119/159
75%
0.00
5.80
19.57
31.76
50.20
41%
37%
1996
86/111
77%
0.00
9.84
20.90
31.27
41.80
30%
49%
37%
38%
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
5.80
7.27
5.75
15.10
0.00
2.45
67%
0.00
75/109
69%
1990
131/182
1991
Detections/
Number of
Samples
Percent (%)
detections
Minimum
(mg/L)
Median
(mg/L)
1985
5/7
71%
0.00
1986
99/134
74%
0.00
1987
34/56
61%
1988
67/100
1989
Year
Avg. Annual Min.
Overall
1,150/1,580
73%
0.00
Avg. Annual Max.
6.50
16.72
11
25.65
45.08
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 3. Summary of nitrate results from the current network in PMR 4.
Maximum
(mg/L)
% at or
below
background
(3 mg/L)
% at or
above
Health Risk
Limit
(10 mg/L)
28.00
55.70
13%
57%
32.06
72.36
15%
58%
21.42
31.94
84.20
13%
65%
15.60
24.97
33.16
77.20
8%
69%
0.00
16.30
23.75
39.77
77.90
11%
67%
99%
0.00
16.80
24.75
36.40
115.00
12%
65%
112/113
99%
0.00
15.80
26.72
30.90
86.20
11%
62%
2007
109/111
98%
0.00
14.40
22.67
30.12
104.00
10%
67%
2008
80/84
95%
0.00
14.05
22.55
32.54
66.80
18%
62%
2009
80/84
95%
0.00
15.20
20.90
33.24
51.60
24%
61%
2010
79/84
94%
0.00
11.75
24.85
36.76
61.90
24%
52%
14%
62%
Year
Detections/
Number of
Samples
Percent
(%)
detections
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Minimum
(mg/L)
2000
166/170
98%
2001
180/189
95%
0.00
12.80
21.50
0.00
12.60
21.52
2002
185/189
98%
0.00
14.00
2003
131/133
99%
0.00
2004
105/108
97%
2005
112/113
2006
Avg. Annual Min.
Overall
1,339/1,378
97%
0.00
Avg. Annual Max.
14.40
23.10
12
33.10
77.53
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Mapping of the individual sites (Figure 4) illustrates that most of the sites in PMR 4 (31
of 100 in the former network and 54 of 86 in the current network) were in the group
where median nitrate results are above the HRL of 10.0 mg/L. The number of sites with
no nitrate impacts decreased from 24 sites in the former network to only one site in the
current network. This mapping assessment of individual sites helps visualize the results
of the comparisons made in Tables 2 and 3.
Figure 4. PMR 4 median nitrate results per site.
13
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
2.2. Southeast Karst (PMR 9)
Nitrate results from PMR 9 have been summarized in Tables 4 and 5 for the former and
current networks, respectively. This region is one of the most sensitive areas to surface
contamination, but is also one of the most complex areas geologically. Samples have
been collected from various sources over time (monitoring wells, private drinking water
wells, and groundwater springs) in an attempt to collect representative groundwater
samples for analysis. For further information on these current sampling efforts, refer to
the MDA Groundwater work plans found at www.mda.state.mn.us/monitoring. Overall
sampling numbers between the former and current networks (337 samples overall for the
former and 393 samples for the current) are similar. Percent nitrate detections were high
for both the former and current networks (at 83 percent and 99 percent, respectively).
The PMR 9 network summaries suggest that nitrate contamination has decreased over
time, with higher overall concentrations and more samples above the HRL in the former
network. It is important to consider how the sampling regime changed over time when
drawing this conclusion. More groundwater samples in the former network were
collected from monitoring and domestic wells that likely represent a worst-case scenario
due to well construction, land use around the wells, and nearby likely contributing source
areas. Worst case conditions typically represent more localized conditions rather than a
region-wide condition.
A comprehensive regional groundwater monitoring effort occurred in 1991, with 126
groundwater samples collected in that year. As such, 1991 is the most representative year
of the former network’s data set for the region as a whole (as indicated by the shaded
rows in Table 4). Further, during the early years of the current network, sampling
focused on collecting samples from three MDNR fish hatchery springs (Lanesboro,
Crystal, and Peterson). These springs were useful because they provided an easy access
site for groundwater sampling and a cooperator to collect the samples. The land use and
geologic conditions surrounding these springs do not truly represent a region-wide
condition that is of interest for the program. Springs associated with other parts of PMR
9, which were more focused on the agricultural land use conditions for the region and
shallower groundwater, were not added into MDA’s groundwater monitoring program
until 2006. Residential well monitoring was not added back in to the PMR 9
groundwater monitoring regime until 2009.
14
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 4. Summary of nitrate results from the former network in PMR 9.
Maximum
(mg/L)
% at or
below
background
(3 mg/L)
% at or
above
Health Risk
Limit
(10 mg/L)
18.82
25.80
12%
30%
21.90
31.50
16%
28%
17.80
24.24
27.80
0%
48%
10.50
14.10
32.80
32.80
5%
53%
0.00
4.80
8.90
22.80
25.80
44%
24%
67%
0.00
3.60
7.50
10.49
30.60
47%
14%
25/31
81%
0.00
9.50
16.12
26.38
30.00
23%
45%
1993
16/16
100%
6.17
14.25
21.45
26.32
28.60
0%
75%
1994
16/16
100%
10.80
16.95
21.25
30.98
32.10
0%
100%
1995
13/13
100%
0.70
13.40
16.25
24.00
24.00
8%
92%
26%
35%
Year
Detections/
Number of
Samples
Percent
(%)
detections
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Minimum
(mg/L)
1986
41/43
95%
1987
22/25
88%
0.00
7.50
10.72
0.00
8.30
12.55
1988
23/23
100%
3.30
9.30
1989
19/19
100%
2.30
1990
20/25
80%
1991
85/126
1992
Avg. Annual Min.
Overall
280/337
83%
2.33
Avg. Annual Max.
7.40
11.95
____: Indicates year selected for statistical analysis
15
22.54
28.90
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 5. Summary of nitrate results from the current network in PMR 9.
Maximum
(mg/L)
% at or
below
background
(3 mg/L)
% at or
above
Health Risk
Limit
(10 mg/L)
-
-
-
-
-
-
-
-
-
3.30
3.69
3.93
4.02
50%
0%
2.58
2.97
3.80
3.86
3.95
50%
0%
100%
2.68
3.17
3.79
4.09
4.17
50%
0%
96%
0.00
3.72
5.53
6.25
16.00
24%
8%
56/56
100%
0.72
5.48
13.80
15.25
16.10
21%
34%
2007
55/55
100%
0.66
5.96
10.03
13.30
15.20
18%
25%
2008
46/46
100%
3.52
6.69
11.50
14.36
15.60
0%
41%
2009
62/62
100%
2.35
6.61
10.30
11.91
13.70
2%
27%
2010
62/62
100%
2.11
6.97
9.61
12.79
26.10
3%
24%
17%
22%
9%
30%
Year
Detections/
Number of
Samples
Percent
(%)
detections
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Minimum
(mg/L)
2000
-
-
-
-
-
2001
-
-
-
-
2002
14/14
100%
2.82
2003
24/24
100%
2004
24/24
2005
48/50
2006
Avg. Annual Min.
Overall
20062010
391/393
99%
1.94
Avg. Annual Max.
4.83
8.81
13.20
Avg. Annual Min.
281/281
100%
1.87
12.76
Avg. Annual Max.
6.49
11.00
____: Indicates year selected for statistical analysis
16
13.74
17.34
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Figure 5 is a box plot showing differences in nitrate concentration by source for the
current PMR 9 network, as displayed by the different shapes of the boxes. Box plots are
interpreted with the following guidance. The open circles represent outliers and asterisks
represent extremes. The top whisker is the largest non-outlier value, which is
representing the reasonable upper spread in the data. The bottom whisker is the
minimum value. The top of the box is the 75th percentile, followed by the median in the
center of the notch and, lastly, the bottom of the box is the 25th percentile. The length of
the notch within the box represents the 95 percent confidence interval of the median.
The shape of the boxes for MDNR springs group and other springs group are different
and the notches in the middle of the boxes do not overlap, while the boxes for the other
springs group and residential wells group are similar and the notches overlap. This box
plot illustrates the point made earlier that PMR 9 groundwater data for the current
network is best represented by data collected after 2005 (which is indicated in the shaded
rows of Table 5) because the data for that time frame includes results from springs in
other areas of PMR 9 and residential wells. Focusing on data collected from the former
network in 1991 and the current network after 2005, the nitrate concentrations were
significantly different at the 95 percent confidence level (Appendix A). Visual inspection
of the data in these two networks indicates that percent detections have increased, and
nitrate is now detected 100 percent of the time in PMR 9. Concentrations have also
increased at all percentile values (although maximum concentrations have varied little),
and a greater percentage of detections are occurring above the HRL of 10.0 mg/L, with
fewer detections found at the background level.
Figure 5. PMR 9 nitrate concentration comparison by source for the current network.
17
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Mapping individual sites (Figure 6) illustrates that for both the former and current
networks, a majority of sites in PMR 9 (5 of 10 in the former network and 8 of 15 in the
current network) could be grouped as having nitrate impacts. This mapping assessment
of individual sites helps visualize the results of the network comparisons made in
Tables 4 and 5.
Figure 6. PMR 9 median nitrate results per site.
18
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
2.3. Other Agricultural Regions (PMRs 1, 5, 6, 7, & 8)
Nitrate results for the other regions of Minnesota are placed together in this subpart and
are discussed in numerical order by their respective PMR. Only results from monitoring
wells were used in the analysis for these regions. Additionally, the screened intervals for
many of the wells in the former network covered by these regions were located below the
water table or were installed in till rather than the Quaternary aquifer. Many of the wells
used in the former network were MDNR observation wells, which are used primarily to
monitor water level fluctuation. A longer water column length is needed so that these
wells do not go dry during summer months. In contrast, most of the screened intervals of
the monitoring wells in the current network span just the water table.
2.3.1. Northwest Red River (PMR 1)
PMR 1 former and current network nitrate results are summarized in Tables 6 and 7,
respectively. This was one of the least monitored regions in the former network, with
only 31 total samples being collected in 1986 and 1987. PMR 1 also had the lowest
percentage of detections, at 6 percent. More samples were collected as part of the current
network, but nitrate detections are the lowest of all the other regions in the current
network (50 percent). While the central tendency of the PMR 1 nitrate data is nearly the
same between the two networks (0.00 mg/L for the former network versus 0.20 mg/L for
the current network), there has been a noticeable change in the upper percentiles of the
data. The 75th and 90th percentiles for the former network were non-detect
(0.00 mg/L), while these percentiles increased to 1.89 mg/L and 4.89 mg/L, respectively,
for the current network. The average annual maximum value also experienced a large
increase, from 0.90 mg/L to 16.14 mg/L. Nitrate concentrations were significantly
different at the 95 percent confidence level (Appendix A). The majority of samples in
PMR 1 are at, or below, the background detection level and this characteristic has not
changed over time.
19
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 6. Summary of nitrate results from the former network in PMR 1.
Year
Detections/
Number of
Samples
Percent
(%)
detections
Minimum
(mg/L)
1986
2/17
12%
1987
0/14
0%
2/31
6%
% at or
above
Health
Risk Limit
(10 mg/L)
1.80
100%
0%
0.00
100%
0%
100%
0%
% at or
below
background
(3 mg/L)
% at or
above
Health
Risk Limit
(10 mg/L)
Median
(mg/L)
90th
Percentile
(mg/L)
Maximum
(mg/L)
0.00
0.00
0.00
0.80
0.00
0.00
0.00
0.00
Avg. Annual Min.
Overall
% at or
below
background
(3 mg/L)
75th
Percentile
(mg/L)
0.00
Avg. Annual Max.
0.00
0.00
0.00
0.90
Table 7. Summary of nitrate results from the current network in PMR 1.
Year
Detections/
Number of
Samples
Percent
(%)
detections
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Minimum
(mg/L)
Maximum
(mg/L)
2004
4/9
44%
0.00
0.00
3.69
4.67
4.76
67%
0%
2005
10/17
59%
0.00
0.46
2.27
4.22
5.10
88%
0%
2006
3/9
33%
0.00
0.00
0.81
2.13
2.30
100%
0%
2007
3/8
38%
0.00
0.00
2.52
16.36
21.90
75%
12%
2008
4/8
50%
0.00
0.30
1.09
17.23
24.00
87%
12%
2009
2/7
29%
0.00
0.00
0.37
23.06
28.70
86%
14%
2010
10/14
71%
0.00
0.84
1.09
16.39
26.20
79%
21%
83%
8%
Avg. Annual Min.
Overall
36/72
50%
0.00
Avg. Annual Max.
0.20
1.89
20
4.86
16.14
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Mapping individual sites (Figure 7) illustrates that the majority of PMR 1 sites (3 of 3 in
the former network and 9 of 10 in the current network) were in groups with lower
concentration ranges (no nitrate impacts or background nitrate levels). This mapping
assessment of individual sites helps visualize the results of network comparisons made in
Tables 6 and 7.
Figure 7. PMR 1 median nitrate results per site.
21
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
2.3.2. East Central (PMR 5)
The nitrate results summaries from the former and current networks for PMR 5 are
presented in Tables 8 and 9, respectively. The percentage of samples at background
levels, as well as above the nitrate HRL of 10.0 mg/L, has basically held steady over time
(based upon the overall percentages for the two networks). The median nitrate
concentration increased in value, from 8.20 mg/L in the former network to 9.57 mg/L in
the current network, reflecting a difference of 1.37 mg/L. The average annual maximum
nitrate concentrations also increased in value, from 24.56 mg/L in the former network to
26.28 mg/L in the current network. However, the Mann-Whitney statistical test indicated
that the increases were not significantly different at the 95 percent confidence level (pvalue=0.8832) (Appendix A).
22
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 8. Summary of nitrate results from the former network in PMR 5.
Maximum
(mg/L)
% at or
below
background
(3 mg/L)
% at or
above
Health
Risk Limit
(10 mg/L)
13.60
13.60
33%
67%
16.50
16.50
33%
33%
11.52
17.00
17.00
40%
20%
3.95
7.90
7.90
7.90
50%
0%
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
19/22
86%
0.00
5.30
18.40
47.07
56.00
27%
45%
1992
6/9
67%
0.00
6.90
12.75
21.12
23.60
33%
33%
1993
7/10
70%
0.00
9.85
24.70
32.05
38.80
30%
50%
1994
6/7
86%
0.00
22.60
24.17
36.02
38.90
14%
57%
1995
3/4
75%
0.00
15.65
26.51
33.33
33.33
25%
30%
1996
0/1
0%
0.00
-
-
-
0.00
0%
0%
30%
44%
Year
Detections/
Number of
Samples
Percent
(%)
detections
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Minimum
(mg/L)
1985
2/3
67%
1986
2/3
67%
0.00
11.40
13.05
0.00
9.00
14.62
1987
3/5
60%
0.00
9.00
1988
1/2
50%
0.00
1989
-
-
1990
-
1991
Avg. Annual Min.
Overall
49/66
74%
0.00
Avg. Annual Max.
8.20
18.40
23
33.12
24.56
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 9. Summary of nitrate results from the current network in PMR 5.
Maximum
(mg/L)
% at or
below
background
(3 mg/L)
% at or
above
Health
Risk Limit
(10 mg/L)
-
-
-
-
-
-
-
-
-
-
-
-
-
12.20
17.82
24.68
28.40
33%
56%
0.45
13.50
16.77
19.76
21.20
33%
56%
0.00
8.49
15.97
21.66
21.90
33%
33%
0.00
10.20
14.17
22.86
33.60
35%
53%
34%
50%
Year
Detections/
Number of
Samples
Percent
(%)
detections
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Minimum
(mg/L)
2004
-
-
2005
-
-
-
-
-
-
-
-
2006
-
-
-
-
2007
9/9
100%
0.55
2008
9/9
100%
2009
8/9
89%
2010
15/17
88%
Avg. Annual Min.
Overall
41/44
93%
0.25
Avg. Annual Max.
9.57
15.90
24
21.98
26.28
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Mapping the individual sites (Figure 8) indicates that most of the sites in PMR 5 (2 of 4
in the former network and 4 of 9 in the current network) were in the group where median
nitrate results are above the HRL of 10.0 mg/L.
Figure 8. PMR 5 median nitrate results per site.
25
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
2.3.3. West Central (PMR 6)
Nitrate results for PMR 6 are summarized in Tables 10 and 11 for the former and current
networks, respectively. This is the best data set for making statistical comparisons
because the sample sizes for both networks are the same (63 samples for each network).
The frequency of nitrate detection has increased from 25 percent in the former network to
56 percent in the current network (a change of 31 percent). The median nitrate
concentration for the current network was 0.61 mg/L (compared to 0.00 mg/L for the
former network), but nitrate concentrations increased from the former network at all
percentiles. The increase is most noticeable at the higher percentile ranges. The 75th
percentile value increased from 1.72 mg/L to 4.58 mg/L, and the 90th percentile value
increased from 7.38 mg/L to 24.10 mg/L. A large difference can also be observed for the
average annual maximum nitrate value, going from 8.80 mg/L for the former network to
27.85 mg/L for the current network. The Mann-Whitney statistical test for comparing
nitrate concentrations between the former and current networks indicated a significant
difference at the 95 percent confidence level, as shown in Appendix A. Most samples in
PMR 6 had nitrate concentrations at background levels and this has not changed over
time.
26
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 10. Summary of nitrate results from the former network in PMR 6.
Maximum
(mg/L)
% at or
below
background
(3 mg/L)
% at or
above
Health Risk
Limit
(10 mg/L)
5.08
6.80
75%
0%
0.00
5.50
6.10
87%
0%
0.00
2.56
3.20
86%
0%
0.00
0.00
0.00
0.00
100%
0%
0.00
1.15
18.10
22.10
27.00
58%
42%
0.00
0.00
4.90
8.05
9.70
70%
0%
76%
8%
Year
Detections/
Number of
Samples
Percent
(%)
detections
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Minimum
(mg/L)
1986
4/16
25%
0.00
0.00
1.55
1987
2/15
1988
1/7
13%
0.00
0.00
14%
0.00
0.00
1989
0/3
0%
0.00
1990
6/12
50%
1991
3/10
30%
Avg. Annual Min.
Overall
16/63
25%
0.00
Avg. Annual Max.
0.00
1.72
27
7.38
8.80
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 11. Summary of nitrate results from the current network in PMR 6.
Year
Detections/
Number of
Samples
Percent
(%)
detections
Minimum
(mg/L)
2004
0/3
0%
2005
2/6
2006
6/9
2007
Maximum
(mg/L)
% at or
below
background
(3 mg/L)
% at or
above
Health Risk
Limit
(10 mg/L)
0.00
100%
0%
0.73
0.74
100%
0%
29.28
34.00
44%
33%
31.44
37.40
56%
33%
7.11
28.69
42.10
67%
11%
0.86
7.62
31.20
40.40
67%
22%
1.03
2.77
23.81
40.30
78%
11%
70%
17%
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
0.00
0.00
0.00
0.00
33%
0.00
0.00
0.61
67%
0.00
3.81
16.05
6/9
67%
0.00
1.69
13.80
2008
5/9
56%
0.00
2.07
2009
6/9
67%
0.00
2010
10/18
56%
0.00
Avg. Annual Min.
Overall
35/63
56%
0.00
Avg. Annual Max.
0.61
4.58
28
24.10
27.85
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Mapping individual sites (Figure 9) illustrates that the majority of sites in PMR 6 (9 of 9
in the former network and 5 of 9 in the current network) were in groups where median
nitrate results indicated either no nitrate impacts or background levels of nitrate.
However, the number of sites having no nitrate impacts decreased from 8 sites in the
former network to 3 sites in the current network.
Figure 9. PMR 6 median nitrate results per site.
29
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
2.3.4. Southwest (PMR 7)
PMR 7 nitrate results for the former and current networks have been summarized in
Tables 12 and 13, respectively. The nitrate detection frequency increased from
34 percent in the former network to 56 percent in the current network. This is a change
of 22 percent. Nitrate concentrations increased at all percentile levels. The largest
increase occurred in the average annual maximum nitrate value (7.76 mg/L in the former
network to 22.64 mg/L in the current network). Further, the median nitrate value for the
current network is 4.45 mg/L, up from 0.00 mg/L for the former network. The nitrate
concentration differences between the two networks for PMR 7 were significant at the 95
percent confidence level, as presented in Appendix A. A greater percentage of samples
were found to be at background levels versus the percentage found to be above the HRL
of 10.0 mg/L, but this varied over time. Some years showed the percentage of samples at
background levels and the percentages above the HRL were equal.
30
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 12. Summary of nitrate results from the former network in PMR 7.
% at or
below
background
(3 mg/L)
% at or
above
Health
Risk Limit
(10 mg/L)
6.40
75%
0%
0.00
100%
0%
3.30
3.30
75%
0%
16.50
16.50
16.50
50%
50%
0.00
1.30
5.20
5.20
80%
0%
0.00
6.15
9.57
9.80
9.80
20%
0%
0.00
4.58
9.63
13.10
13.10
50%
25%
69%
6%
Year
Detections/
Number of
Samples
Percent
(%)
detections
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Minimum
(mg/L)
Maximum
(mg/L)
1986
2/8
25%
1987
0/7
0%
0.00
0.00
2.45
5.95
0.00
0.00
0.00
0.00
1988
1/4
25%
0.00
0.00
1.65
1989
1/2
50%
0.00
8.25
1990
1/5
20%
0.00
1991
4/5
80%
1992
3/4
75%
Avg. Annual Min.
Overall
12/35
34%
0.00
Avg. Annual Max.
0.00
4.87
31
9.50
7.76
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 13. Summary of nitrate results from the current network in PMR 7.
Maximum
(mg/L)
6.27
6.27
50%
0%
22.30
22.30
50%
25%
34.70
38.50
43%
43%
15.00
15.64
15.70
43%
43%
3.11
15.19
26.14
28.40
43%
29%
0.00
5.22
10.36
18.00
19.60
43%
29%
0.00
4.36
8.02
17.89
27.70
43%
21%
44%
29%
Year
Percent
(%)
detections
Minimum
(mg/L)
Median
(mg/L)
2004
1/2
50%
0.00
3.13
6.27
2005
2/4
50%
0.00
4.32
15.47
2006
4/7
57%
0.00
7.22
17.75
2007
4/7
57%
0.00
3.94
2008
4/7
57%
0.00
2009
4/7
57%
2010
8/14
57%
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Avg. Annual Min.
Overall
27/48
56%
% at or
above
Health
Risk Limit
(10 mg/L)
% at or
below
background
(3 mg/L)
Detections/
Number of
Samples
0.00
Avg. Annual Max.
4.45
13.15
32
19.57
22.64
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Mapping individual sites (Figure 10) illustrates that most of the sites in PMR 7 (3 of 4
sites in the former network and 3 of 7 sites in the current network) were in the group
where median nitrate results indicate no impacts.
Figure 10. PMR 7 median nitrate results per site.
33
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
2.3.5. South Central (PMR 8)
The nitrate data summaries for PMR 8 former and current networks are presented in
Tables 14 and 15, respectively. Nitrate detection frequency increased in the current
network compared to the former network by 44 percent (62 percent versus 18 percent,
respectively). Concentrations have also increased at all percentile levels. The largest
difference was between the average annual maximum values of 27.30 mg/L in the current
network versus 9.76 mg/L in the former network. The difference in nitrate values
between the two networks was significant at the 95 percent confidence level, as presented
in Appendix A. The majority of samples in the two networks, however, are at
background levels, with a median nitrate value of 1.21 mg/L for the current network.
34
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 14. Summary of nitrate results from the former network in PMR 8.
Maximum
(mg/L)
% at or
below
background
(3 mg/L)
% at or
above
Health Risk
Limit
(10 mg/L)
-
0.00
100%
0%
3.00
25.84
33.50
77%
23%
0.77
20.14
31.50
78%
11%
0.00
2.37
9.50
9.50
80%
0%
0.00
0.00
0.00
0.00
0.00
100%
0%
10%
0.00
0.00
0.00
1.00
2.00
100%
0%
2/26
8%
0.00
0.00
0.00
0.00
8.70
92%
0%
1992
1/6
17%
0.00
0.00
0.00
2.34
2.60
100%
0%
1993
0/2
0%
0.00
0.00
0.00
0.00
0.00
100%
0%
88%
7%
Year
Detections/
Number of
Samples
Percent
(%)
detections
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Minimum
(mg/L)
1985
0/1
0%
0.00
-
-
1986
8/22
1987
2/9
36%
0.00
0.00
22%
0.00
0.00
1988
1/5
20%
0.00
1989
0/3
0%
1990
1/10
1991
Avg. Annual Min.
Overall
15/84
18%
0.00
Avg. Annual Max.
0.00
0.00
35
6.99
9.76
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 15. Summary of nitrate results from the current network in PMR 8.
Maximum
(mg/L)
% at or
below
background
(3 mg/L)
% at or
above
Health Risk
Limit
(10 mg/L)
0.00
100%
0%
-
0.00
100%
0%
32.22
45.10
67%
22%
20.95
28.80
60%
20%
5.94
21.68
41.70
75%
17%
0.86
7.90
25.08
47.90
67%
17%
1.85
9.26
14.84
27.60
56%
22%
65%
19%
Year
Detections/
Number of
Samples
Percent
(%)
detections
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Minimum
(mg/L)
2004
0/2
0%
0.00
0.00
0.00
0.00
2005
0/1
0%
0.00
-
-
2006
7/9
78%
0.00
1.21
5.61
2007
6/10
60%
0.00
0.69
7.59
2008
6/12
50%
0.00
0.67
2009
8/12
67%
0.00
2010
16/23
70%
0.00
Avg. Annual Min.
Overall
43/69
62%
0.00
Avg. Annual Max.
1.21
7.61
36
14.82
27.30
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Mapping the individual sites (Figure 11) illustrates that the majority of sites in PMR 8 (7
of 9 sites in the former network and 7 of 11 sites in the current network) were in groups
where median results indicate no nitrate impacts or background levels of nitrate. The
number of sites that had no nitrate impacts decreased from seven sites in the former
network to three sites in the current network.
Figure 11. PMR 8 median nitrate results per site.
37
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
2.4. Urban Groundwater Monitoring
Nitrate data from urban groundwater monitoring in the former and current networks has
been summarized in Tables 16 and 17, respectively. It should first be noted that only
results from former urban network monitoring wells were used in this analysis (there
were some results from private drinking water wells and irrigation wells that were
removed from the data set). Some of the former network monitoring wells were placed in
till instead of the Quaternary aquifer. Further, as mentioned previously, urban
groundwater monitoring prior to 2000 was performed solely in PMR 10, while urban
groundwater monitoring since 2004 has been performed in urban areas around the state.
These factors should be kept in mind when considering the network comparisons.
The number of samples collected from urban monitoring wells has doubled over time,
from 60 samples collected as part of the former network to 120 samples collected as part
of the current network. The percent nitrate detection increased, by 33 percent, although
the median nitrate result remained constant over time (3.10 mg/L for the former network
and 3.20 mg/L for the current network). Upper percentile values slightly decreased, but
the average annual maximum value increased to 20.57 mg/L. The percentage of samples
that were at the background level was equal between the two networks, at 48 percent,
with fewer samples being above the HRL. The difference between the two networks was
not significant at the 95 percent confidence level (p=0.1008) (Appendix A), meaning
nitrate concentrations have not increased over time for urban areas that have been
monitored as part of this program.
38
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 16. Summary of nitrate results from the former network in urban areas.
% at or
below
background
(3 mg/L)
% at or
above
Health
Risk Limit
(10 mg/L)
14.00
23%
31%
15.20
15.20
40%
20%
-
-
-
-
-
-
-
-
-
5.20
7.60
12.28
12.80
17%
17%
0.00
0.00
5.17
12.78
17.00
62%
19%
0%
0.00
-
-
-
0.00
100%
0%
2/6
33%
0.00
0.00
13.50
47.25
51.00
67%
33%
1994
2/4
50%
0.00
2.00
9.15
14.30
14.30
50%
25%
1995
1/3
33%
0.00
0.00
7.95
10.60
10.60
67%
33%
1996
0/1
0%
0.00
-
-
-
0.00
100%
0%
48%
23%
Year
Detections/
Number of
Samples
Percent
(%)
detections
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Minimum
(mg/L)
Maximum
(mg/L)
1986
11/13
85%
0.00
3.80
10.70
13.20
1987
3/5
1988
-
60%
0.00
4.10
7.02
-
-
-
-
1989
-
-
-
-
1990
6/6
100%
1.67
1991
8/21
38%
1992
0/1
1993
Avg. Annual Min.
Overall
33/60
55%
0.19
Avg. Annual Max.
3.10
7.70
39
13.25
14.99
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 17. Summary of nitrate results from the current network in urban areas.
Year
Detections/
Number of
Samples
Percent
(%)
detections
Minimum
(mg/L)
2004
10/10
100%
2005
18/23
2006
16/19
2007
% at or
below
background
(3 mg/L)
% at or
above
Health
Risk Limit
(10 mg/L)
12.00
30%
10%
8.40
30.00
61%
9%
11.60
16.00
53%
16%
9.36
12.00
59%
6%
8.00
13.00
37.00
44%
19%
4.10
7.17
14.00
22.00
33%
20%
3.60
6.21
8.45
15.00
45%
10%
48%
12%
Median
(mg/L)
75th
Percentile
(mg/L)
90th
Percentile
(mg/L)
Maximum
(mg/L)
0.11
3.60
7.70
10.10
78%
0.00
2.80
6.70
84%
0.00
3.00
7.55
16/17
94%
0.00
2.40
5.60
2008
14/16
88%
0.00
3.70
2009
15/15
100%
0.94
2010
17/20
85%
0.00
Avg. Annual Min.
Overall
106/120
88%
0.15
Avg. Annual Max.
3.20
6.80
40
11.00
20.57
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Mapping of the individual sites (Figure 12) illustrates that the majority of sites in the
former urban network (2 of 3 sites) were in the group indicating no nitrate impacts,
whereas most sites in the current network (4 of 9 sites) were in the group indicating
nitrate impacts.
Figure 12. Urban groundwater monitoring median nitrate results per site.
41
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Section 3: ANALYSIS OF AVAILABLE NITRATE DATA FOR PMR 4
Additional analyses were performed on the MDA nitrate data for PMR 4. These included
temporal analysis, spatial analysis, and depth analysis. PMR 4 was selected for these
additional analyses because it has the most comprehensive data set, which should lead to
statistically valid results. PMR 4 is the only regional monitoring network that was
established through the utilization of a statistical design.
3.1. Temporal Analysis
Time-trend Graphical Analysis
The nitrate data for the former and current networks were summarized by computing the
median, 75th percentile, and 90th percentile values for each quarter (winter, spring,
summer, and fall) in each year, similar to as what was discussed in Section 2 but at a
smaller time scale. These results were graphed over time based on quarterly sampling.
The nitrate HRL of 10.0 mg/L was also included as a reference.
The first quarter sampled was the fall of 1985 and the last quarter sampled was the fall of
2010, as presented in Figure 13. Trend lines were fitted through the use of a routine
called Locally Weighted Scatterplot Smoothing (LOWESS). Linear lines were also
added in to aid in visual assessment of the data only. It must be noted that linear
regression is an inappropriate method to use with data that is non-linear in nature or with
non-normally distributed data due to violating the assumptions inherent in that type of
analysis (Helsel and Hirsch, 2002). The LOWESS method is a robust and particularly
useful method as it generates a line to track the central tendency of the data over time
based upon the data itself rather than being constrained by assumptions associated with
an equation that is attempted to be “fit” to the data. This is particularly useful for nonnormal data, such as the MDA’s groundwater nitrate data. The drawback, however, is
that the accuracy of the line estimate degrades at the tails of the line. This is due to the
fact that the data near the midpoint of the line receives greater weight than the data
located at the ends of the line during LOWESS generation. As such, caution should be
excercised when evaluating the shape of the line at the tails and interpreting trends during
more recent time periods. Similar to regression methods, extrapolating the line beyond
the extent of the data, or to use trend lines to predict across large gaps in the data record,
is inappropriate.
Figure 13 indicates that monitoring well nitrate concentrations have generally gone up
over time since groundwater monitoring began in PMR 4, even though there is some up
and down nonseasonal fluctuation in the data. This fluctuation has occurred at all levels
(median, 75th percentile, and 90th percentile). Further, median nitrate concentrations in
the current network were consistently higher than the HRL of 10.0 mg/L, whereas
median concentrations in the former network were, in their majority, below the HRL. It
appears that the trend of nitrate concentrations in the current network timeframe are
increasing at a lower rate compared to nitrate concentrations in the former network
timeframe, based upon the linear lines fit to the percentile data.
42
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Figure 13. Nitrate concentration time series from PMR 4 groundwater monitoring.
To further explore this result, the nonparametric trend test methodology, the proper
method to use for this type of water quality data (Helsel and Hirsch, 2002), has been used
to investigate the most appropriate trend in the MDA’s nitrate data and the discussion of
these results follows.
Time-trend Statistical Analysis
Before trend testing within the former and current networks could be performed on the
PMR 4 nitrate data, the data was reviewed for its appropriateness. To perform this type
of analysis, the time interval between measurements needed to be consistent and
continuous. Second, the population size for each subcategory being tested needed to be
similar. Since groundwater monitoring in the former network ended in 1996 and
groundwater monitoring in the current network began in 2000, the two networks were
separated and trends were computed for each network. Further, because sampling
schedule varied within and between each network, there were two ways to assemble each
network’s data that would allow for comparable statistical analyses: both networks
having comparable spans of spring and fall quarter results, and both networks having
lengthy periods of consecutive quarter results. The data was evaluated within each
network using Analysis A (comparable spans of spring and fall quarter results) and
Analysis B (comparable spans of consecutive quarter results). Results from both
analyses are included to assess and illustrate the ways in which each approach might lead
to different conclusions.
43
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Analysis A (Spring and Fall Quarters Results): Due to quarterly sampling ending in
2008, when bi-annual sampling began, Analysis A used results from spring and fall
quarters for each year. For the former network, 18 quarters were used (Spring 1988 –
Fall 1996), where the number of nitrate observations in each of these quarters for
generating percentile values ranged from 17 - 69, with an average 36; for the current
network, 22 quarters were used (Spring 2000 – Fall 2010), where the number of nitrate
observations in each of these quarters for generating percentile values ranged from
27 - 49, with an average of 36.
Analysis B (Consecutive Quarters Results): For Analysis B, the amount of data used for
consecutive quarter trend testing had to be constricted to certain time ranges due to nonconsecutive sampling quarters within each network. For the former network, 34 quarters
were used (Winter 1988 – Spring 1996), where the number of nitrate observations in each
of these quarters for generating percentile values ranged from 20 - 69, with an average of
36; for the current network, 34 quarters were used (Winter 2000 – Spring 2008), where
the number of nitrate observations in each of these quarters for generating percentile
values ranged from 25 - 50, with an average of 35.
Analyses A and B were conducted for both the former and current networks, and trends
were computed and tested for statistical significance at the 95 percent confidence level
using the nonparametric Mann-Kendall test for trend (Gibbons and Coleman, 2001;
Helsel and Hirsch, 2002). Time-trend analysis on median values is the most widely
accepted method for non-parametric data. Trend analysis was also computed for the 75th
and 90th percentile concentrations, as they may also be useful for evaluating trend
dynamics at higher concentrations. Results of all trend analyses on nitrate
concentrations, including the p-value, Kendall’s tau (a nonparametric correlation
estimate), and Thiel-Sen slope estimates, have been included in Appendix B. A summary
table of the trend test results has been included as Table 18.
Analysis A indicated that the only trends that were statistically significant were at the 75th
and 90th percentiles for the former network data. No trends for the current network data
were statistically significant in Analysis A. Analysis A did indicate positive Thiel-Sen
slope estimates, which is suggestive of a weak increasing trend.
Analysis B indicated that trends for the 75th and 90th percentiles in the former network
data were statistically significant (the trend for median data was not). The trend for
median data in the current network data was also statistically significant, while the trends
for the 75th and 90th percentiles were not. Even though not all trends were significant in
Analysis B, almost all were found to give positive Thiel-Sen slope estimates, which is
suggestive of a weak increasing trend. The exception to this was for the median data
from the former network, which was suggestive of a slightly decreasing trend.
44
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 18. Summary of trend test results for PMR 4 nitrate data.
Analysis A
(spring & fall
quarters)
Former network
Current network
spring 1988 - fall 1996 (18 quarters)
spring 2000 - fall 2010 (22 quarters)
Mann-Kendall test, 95% confidence level
Mann-Kendall test, 95% confidence level
percentile
Thiel-Sen slope
estimate
Kendall’s tau
statistic
p-value/
trend significance
Thiel-Sen slope
estimate
Kendall’s tau
statistic
p-value/
trend significance
50%
+0.0225
+0.0719
0.3524/
not significant
+0.0453
+0.1515
0.1688/
not significant
75%
+0.1583
+0.3791
0.0154/
significant
+0.0319
+0.0996
0.2675/
not significant
90%
+0.1825
+0.4118
0.0094/
significant
+0.0525
+0.1558
0.1615/
not significant
Analysis B
(consecutive
quarters)
winter 1988 - spring 1996 (34 quarters)
winter 2000 - spring 2008 (34 quarters)
Mann-Kendall test, 95% confidence level
Mann-Kendall test, 95% confidence level
percentile
Thiel-Sen slope
estimate
Kendall’s tau
statistic
p-value/
trend significance
Thiel-Sen slope
estimate
Kendall’s tau
statistic
p-value/
trend significance
50%
-0.0286
-0.0838
0.7524/
not significant
+0.0944
+0.2495
0.0196/
significant
75%
+0.1678
+0.3512
0.0018/
significant
+0.0875
+0.1818
0.0671/
not significant
90%
+0.3370
+0.5401
0.0000/
significant
+0.1121
+0.1604
0.0935/
not significant
3.2. Spatial Analysis
Spatial analysis of nitrate data in PMR 4 was accomplished by dividing the region’s
monitoring wells into different subsections based on location and then performing
statistical tests of comparison on the nitrate data from each subsection. The MDNR
Division of Forestry’s Ecological Land Type Associations of Minnesota layer (MDNR,
1999) was used to divide PMR 4 into meaningful subsections for spatial analysis.
Polygon boundaries for this layer were delineated by the MDNR through manual
interpretation and visual assessment of a variety of features, such as: topography, wetland
distribution, soil characteristics, hydrography, presettlement vegetation, bedrock type,
Landsat satellite imagery, and geomorphology, as well as local knowledge of the
landscape. The determination to use this layer in this study was based upon the fact that
it integrated many sources of landscape-feature data for boundary delineation. Coverage
information down to the third level of classification from the Ecological Classification
System (ECS) was used, which is called ECS subsection. Select sites in PMR 4, as
described in Section 2, were overlain and mapped in GIS against the ECS subsection
layer (Figure 14). Through mapping, it was determined that only four subsections
(Anoka Sand Plains, Hardwood Hills, Minnesota River Prairie, and Pine Moraines &
Outwash Plains) had sufficient numbers of monitoring wells from the former and current
networks to provide statistically significant results.
45
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Figure 14. MDNR ECS subsections with the MDA PMR 4 monitoring wells.
Comparison tests were performed on the nitrate data from the monitoring wells in each
subsection, for each time period, to see if they were significantly different from each
other based upon the central tendency of the data. Tabulated results of this comparison
testing are presented in Appendix C. These comparison tests were generated using the
nonparametric Kruskal-Wallis test for comparison of multiple independent, non-normally
distributed groups, along with the Conover-Inman test for multiple pairwise comparisons
(Conover, 1971) (the Dwass-Steel-Chritchlow-Fligner test for pairwise comparisons
(Hollander and Wolfe, 1999) was also computed and included in Appendix C as a
reference, but was not used in analysis of the data). Box plots and descriptive statistics
46
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
(minimum, 25th percentile, 50th percentile, 75th percentile, 90th percentile, and maximum)
were also generated and are included in Appendix C.
For the former network, it was found that the Anoka Sand Plain nitrate data was
statistically different than the other three subsections at the 95 percent confidence level,
with a median nitrate concentration of 9.50 mg/L, significantly higher than the other three
subsections. The Hardwood Hills nitrate data was also statistically different than the
other three subsections at the 95 percent confidence level, with a median nitrate
concentration of 1.10 mg/L. The nitrate concentration data for the Minnesota River
Prairie (median of 6.50 mg/L) and Pine Moraines & Outwash Plains (median of
5.75 mg/L) subsections were not found to be statistically different than each other at the
95 percent confidence level.
For the current network, it was found that only the nitrate data for the Minnesota River
Prairie subsection was statistically different than the other three subsections at the
95 percent confidence level, with a median nitrate concentration of 18.80 mg/L,
significantly higher than the other three subsections. The median nitrate concentrations
for the other three subsections were also higher than the HRL of 10.0 mg/L. The
difference in median nitrate concentrations between the former and current networks was
largest for the Minnesota River Prairie subsection (a difference of 12.3 mg/L), while it
was the smallest for the Anoka Sand Plain subsection (a difference of 4.25 mg/L).
Time series plots, with LOWESS fit lines, were also produced for the median nitrate data
for each subsection based on sampling quarter and compared against the median nitrate
time series for all of the PMR 4 nitrate data. These are presented as Figures 15, 16, 17,
and 18 for the Anoka Sand Plain, Hardwood Hills, Minnesota River Prairie, and Pine
Moraines & Outwash Plains subsections, respectively. These plots provide a visual
corroboration of the Kruskal-Wallis test results: the Anoka Sand Plain subsection median
nitrate concentration was consistently higher than the regional median in the former
network, and the Minnesota River Prairie subsection median nitrate concentration was
consistently higher than the regional median in the current network. Further, these
graphs suggest that nitrate concentrations were increasing in the Anoka Sand Plain and
Minnesota River Prairie subsections while they appeared to be decreasing in the
Hardwood Hills and Pine Moraines & Outwash Plains subsections.
Trend testing was also performed on the median nitrate data per subsection, but only the
current network median results were used. Results of this trend testing have been
included as Appendix D. Only the trend for the Minnesota River Prairie subsection was
positive and statistically significant.
47
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Anoka Sand
Plain median
regional network
median
30
20
2010
2008
2006
2004
2002
2000
1998
1996
1994
1992
1986
0
1990
10
1988
Concentration (mg/L)
40
Year
Figure 15. Anoka Sand Plain median nitrate data in comparison to PMR 4 median nitrate
data.
Hardwood Hills
median
regional network
median
30
20
2010
2008
2006
2004
2002
2000
1998
1996
1994
1992
1990
0
1988
10
1986
Concentration (mg/L)
40
Year
Figure 16. Hardwood Hills median nitrate data in comparison to PMR 4 median nitrate
data.
48
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Minnesota River
Prairie median
regional network
median
30
20
2010
2008
2006
2004
2002
2000
1998
1996
1994
1992
1990
0
1988
10
1986
Concentration (mg/L)
40
Year
Figure 17. Minnesota River Prairie median nitrate data in comparison to PMR 4 median
nitrate data.
Pine Moraines &
Outwash Plains median
regional network
median
30
20
2010
2008
2006
2004
2002
2000
1998
1996
1994
1992
1990
0
1988
10
1986
Concentration (mg/L)
40
Year
Figure 18. Pine Moraines & Outwash Plains median nitrate data in comparison to PMR 4
median nitrate data.
49
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
As part of the spatial analysis for PMR 4, geostatistical analysis was also conducted
(median number of data points collected per site equals 17) using GIS. This analysis was
insightful because visual inspection of the point nitrate data only provided part of the
picture as to how nitrate concentrations vary spatially across PMR 4 (as shown in
Figure 19).
Figure 19. PMR 4 median nitrate results per site for the current network.
50
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Geostatistical analysis is a tool used to interpolate and predict values in between
measured data points. There are many different interpolation methods within GIS that
can be used. The method that minimizes the mean prediction error and gives a root mean
square standardized prediction error value close to one is most appropriate. It was
determined that the universal kriging method provided the best prediction surface, based
upon review of the statistical output. A summary of the kriging method options used in
this analysis, and the associated statistical summary output, has been included as
Appendix E of this report.
This prediction surface was clipped to the glacial outwash extent for PMR 4, from the
Quaternary geology layer of Minnesota (as produced by the Minnesota Geological
Survey, 1982), along with the ECS subsections for PMR 4. This overlay was provided to
emphasize that the nitrate results presented are only representative of the shallow,
Quaternary outwash conditions and are not indicative of conditions beyond this extent
spatially. The resulting map is presented as Figure 20. This map further corroborates the
findings of the comparison testing performed with the Kruskal-Wallis test, indicating that
the higher nitrate concentrations are found in the Minnesota River Prairie subsection in
the current network.
51
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Figure 20. PMR 4 median nitrate results per site for the current network, along with the
nitrate concentration gradient for the Quaternary outwash geology layer and ECS
subsections for the region.
52
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
3.3. Depth Analysis
MDA’s PMR 4 groundwater monitoring data best represents the shallowest, most
vulnerable portion of the aquifer and provides less detail about nitrate concentrations and
occurrence at lower aquifer depths. To learn further about nitrate and pesticide
concentrations at slightly greater depths, in 2010 the MDA groundwater monitoring
program installed eight monitoring wells in PMR 4. They were established at depths
between 10 and 15 feet deeper than previously installed, water table monitoring wells
(Figure 21). The installation of the new, deeper monitoring wells was completed in May
and June 2010. The first samples were collected in November 2010. Samples from the
paired shallow and deeper wells at each site were collected at the same time.
Figure 21. Location of deeper wells within PMR 4.
53
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 19 provides a summary of the nitrate results from the eight shallow and eight deep
monitoring wells from the 2010 fall sampling event in PMR 4. Nitrate was found in all
monitoring wells. The shallow monitoring wells had higher median nitrate
concentrations (23.1 mg/L) than the deeper monitoring wells (11.3 mg/L). Lower
dissolved oxygen concentrations (i.e. which may be linked to reducing conditions)
typically increase with aquifer depth, and these results are similar to those found by
MPCA in other studies conducted throughout Minnesota (MPCA, 1998, 2001; MPCA
and Met Council, 2002). Reducing conditions facilitate the decomposition of nitrate by
anaerobic bacteria. Shallow monitoring wells that straddle the water table are typically
connected to “fresher” water that has recently infiltrated. These wells in turn have higher
levels of dissolved oxygen and higher nitrate concentrations. These results will be
investigated in the future through further nitrate sampling of the shallow and deep
monitoring wells coupled with an assessment of dissolved oxygen concentrations.
54
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Table 19. Summary of nitrate results in paired shallow and deep monitoring wells in PMR 4.
Maximum
(mg/L)
% at or
above
Health Risk
Limit (10
mg/L)
Depth
Type
Detections/N
umber of
Samples
Percent (%)
detections
Minimum
(mg/L)
Median
(mg/L)
Shallow
8/8
100%
14.30
23.10
28.60
39.76
44.50
0%
100%
Deep
8/8
100%
0.68
11.30
18.90
26.59
28.00
25%
62%
55
90th
Percentile
(mg/L)
% at or
below
background
(3 mg/L)
75th
Percentile
(mg/L)
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Section 4: CONCLUSIONS
The MDA monitoring well program is designed to evaluate pesticides in groundwater in
shallow, vulnerable aquifers immediately adjacent to operating farms. This program
reflects an extremely sensitive hydrogeologic setting. Although the MDA’s pesticide
groundwater monitoring program was not designed to determine nitrate detection or
concentration status and trends, the nitrate data collected does provide some useful
results. Nitrate data reviewed for this report indicates that nitrate concentration in the
very shallow, highly sensitive groundwater monitoring wells sampled in this program
exceed health risk levels at many locations. However, this is not the situation with every
well or all of the PMRs monitored. There were many wells that have shown no
detections or very low levels of nitrate.
Nitrate data collected around the state showed that, when comparing the former and
current networks, there was a significant step increase in nitrate concentration in a
majority of the PMRs. The reasons for this step change are not known and are likely to
be varied. The nitrate concentration data from the PMRs also showed significant
fluctuation over both shorter-term and longer-term time frames. Further, the rate of
increase in nitrate concentration within PMR 4 decreased when going from the former to
the current network. The rate of increase was statistically significant in four out of the
six trend tests performed on the PMR 4 former network data compared to only one out of
the six trend tests performed on the current network data.
In addition to the trends over time, there are significant differences over space showing
that concentrations and trends may be different, both between and within various PMRs.
When viewing the PMR 4 data using the ecological classes, visual analysis suggests that
nitrate concentrations were increasing in the Anoka Sand Plain and Minnesota River
Prairie subsections while they appeared to be decreasing in the Hardwood Hills and Pine
Moraines & Outwash Plains subsections. However, only the trend for the Minnesota
River Prairie subsection was positive and statistically significant. When viewing maps of
concentration data from the various PMRs across the state, it is also clear that the level of
nitrate present in the groundwater can be quite site specific, varying greatly among
relatively nearby sites.
Identification of the causes and factors involved in the changing trends in nitrate
concentrations would require a monitoring program dedicated to understanding nitrates in
groundwater and was not addressed in this report.
56
Summary of MDA’s Groundwater Nitrate-Nitrogen Data
Section 5: REFERENCES
Conover, W.J., 1971. Practical Nonparametric Statistics, Second Edition. 462 pp.
Gibbons, R.D. and Coleman, D.E., 2001. Statistical Methods for Detection and
Quantification of Environmental Contamination. 400 pp.
Helsel, D.R. and Hirsch, R.M., 2002. Statistical Methods in Water Resources. TWRI
04-A3. 523 pp.
Hollander, M. and Wolfe, D.A., 1999. Nonparametric Statistical Methods, Second
Edition. 816 pp.
MDA, 2009, Groundwater Monitoring Network Design. 42 pp.
MDH, 1998, Guidance for Mapping Nitrate in Minnesota Groundwater. 43 pp.
MDNR, 1999, Ecological Land Type Associations of Minnesota. Digital map.
MGS, 1982, Geologic Map of Minnesota: Quaternary Geology. Digital map.
MPCA, 1988, Water Quality in the Upper Fifteen Feet of a Shallow Sand Aquifer. 38 pp.
MPCA, 1989, Groundwater Contamination Susceptibility in Minnesota. Digital map.
MPCA, 1999, Estimating Ground Water Sensitivity to Nitrate Contamination. 2 pp.
MPCA, 2001, Effects of Land Use on Ground Water Quality, St. Cloud Area, Minnesota
– Summary of Results from 1997 through 2000. 73 pp.
MPCA and Metropolitan Council, 2002, Ground Water Quality Beneath Twin Cities
Metropolitan Communities Served by Individual Sewage Treatment Systems. 60 pp.
57