Total Chromium and Hexavalent
Chromium Occurrence Analysis
Web Report #4414
Subject Area: Water Quality
Total Chromium and Hexavalent
Chromium Occurrence Analysis
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
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Total Chromium and Hexavalent
Chromium Occurrence Analysis
Prepared by:
Chad J. Seidel and Christopher J. Corwin
Jacobs Engineering, Denver, CO 80202
Rajiv Khera
U.S. Environmental Protection Agency
Office of Ground Water and Drinking Water, Washington, DC 20460
Sponsored by:
Water Research Foundation
6666 West Quincy Avenue
Denver, CO 80235
Published by the
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
DISCLAIMER
This study was funded by the Water Research Foundation (Foundation). The Foundation
assumes no responsibility for the content of the research study reported in this publication or
for the opinions or statements of fact expressed in the report. The mention of trade names
for commercial products does not represent or imply the approval or endorsement of the
Foundation. This report is presented solely for informational purposes.
Copyright © 2012
by Water Research Foundation
ALL RIGHTS RESERVED.
No part of this publication may be copied, reproduced
or otherwise utilized without permission.
Printed in the U.S.A.
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
CONTENTS
1. Introduction ............................................................................................................................. 1 2. Description of Data ................................................................................................................. 1 2.1. National Chromium and Boron Occurrence Survey (NCBOS) ....................................... 2 2.2. EPA’s Second Six Year Review ICR Contaminants Monitoring Data (6YR2) .............. 2 2.3. CDPH Data ....................................................................................................................... 6 3. Analysis of Total and Hexavalent Chromium Occurrence ..................................................... 8 3.1. Comparison of Chromium Occurrence between Datasets ............................................... 8 3.2. Analysis of Chromium Occurrence by Source Type, Region, and System Size ........... 10 3.3. Analysis of Paired Total and Hexavalent Chromium Samples ...................................... 14 4. Spatial Distribution of Chromium Occurrence ..................................................................... 16 5. Co-occurrence with Chromium ............................................................................................ 19 6. Estimated Extent of Potential Future Regulation ................................................................. 20 7. 8. Conclusions ........................................................................................................................... 23 References ............................................................................................................................. 24 v
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
LIST OF TABLES
1.
Summary of occurrence data sources ..................................................................................1
2.
Summary of non-detect results in the 6YR2 dataset ............................................................2
3.
Summary of non-detect results in the CDPH dataset...........................................................7
4.
Percent of entry points with 75th percentile total chromium concentration above the
threshold values (6YR2 data, non-detects set to one-quarter of the MRL) ...........18
5.
Correlation of co-occurring contaminants with total chromium in the
6YR2 dataset ..........................................................................................................20
6.
Estimation of the national CWS entry point inventory......................................................21
7.
Number of CWS entry points above the threshold concentration at various
threshold concentrations ........................................................................................22
vi
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
LIST OF FIGURES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
Analysis of non-detect MRLs of the 6YR2 dataset (all non-detect samples). .....................3
Cumulative probability distribution of median total chromium concentration by
entry point of the 6YR2 dataset with non-detects set to zero, one quarter of the
MRL, half the MRL, and at the MRL ......................................................................4
Cumulative probability distribution of median total chromium concentration by
entry point of the 6YR2 dataset with non-detects set to zero, and one quarter
of the MRL, compared to the NCBOS dataset and the Bayesian estimate ..............5
Cumulative probability distribution of total chromium concentration by entry
point of the 6YR2 dataset (non-detects set to one-quarter of the MRL) .................6
Cumulative probability distribution of chromium in CDPH for raw versus
treated by sample location (75th percentile concentration per each sample
location) ...................................................................................................................8
Cumulative probability distribution of total chromium data from the three datasets
(6YR2 and CDPH are the 75th percentile concentration per sample location) ........9
Cumulative probability distribution of hexavalent chromium NCBOS & CDPH
raw data (CDPH shows the 75th percentile concentration per sample location,
non-detects set to zero) ..........................................................................................10
Cumulative probability distributions of total chromium in the 6YR2 dataset by
source water type (75th percentile concentration per sample location,
non-detects set to one-quarter of the MRL) ...........................................................11
Cumulative probability distributions of CDPH hexavalent chromium data in ground
and surface waters divided into raw and treated samples (75th percentile
concentration per sample location, non-detects set to zero) .................................12
Map of EPA Regions .........................................................................................................13
Cumulative probability distribution of 6YR2 total chromium data by EPA region
(75th percentile concentration per sample location, non-detects set to
one-quarter of the MRL) ........................................................................................13
Cumulative probability distributions of 6YR2 total chromium data by system
size category (75th percentile concentration per sample location, non-detects
set to one-quarter of the MRL) ..............................................................................14
CDPH paired sample analysis of ground waters and surface waters .................................15
CDPH paired sample analysis of ground waters grouped by raw and treated waters .......15
CDPH paired sample analysis of surface waters grouped by raw and treated waters .......16
National pin map of 75th percentile total chromium concentration from the 6YR2
dataset by system (for systems with more than two samples) ...............................17
California pin map of 75th percentile hexavalent chromium concentration from
the CDPH database by system ...............................................................................19
Estimation of total number of CWS entry points with total chromium above
threshold concentrations ........................................................................................23
vii
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
1. Introduction
The purpose of this study is to update the chromium occurrence analysis originally presented
in the Water Research Foundation (WaterRF) Report #2759, “Occurrence Survey of Hexavalent
Chromium and Boron,” (Frey et al., 2004) with new data from the U.S. Environmental
Protection Agency (EPA) and the State of California. New data were obtained from two primary
sources: the California Department of Public Health (CDPH) Water Quality Analysis Database,
and the EPA’s second six year review database. While these occurrence data are already
available, the objective of this assessment was to identify general occurrence trends of chromium
and capture regional occurrence patterns and assess the geographical coverage and extent of
impact of a potential future revision of total chromium or a new hexavalent chromium regulation
on the nation’s water utilities.
2. Description of Data
Chromium occurrence information was gathered from available sources and the previous
WaterRF survey to guide the development of this updated national occurrence survey. Three
primary datasets are used in this analysis: WaterRF Report #2759, EPA’s second six year review,
and CDPH Water Quality Analysis Database (WQAD). A summary of the datasets is given in
Table 1. Other data sources considered but found not to be useful were: USGS National Water
Information System, National Water-Quality Assessment, and Water Quality Monitoring
Networks databases, US EPA STORET and voluntary water utility monitoring data.
Table 1. Summary of occurrence data sources.
Data Source
Database
Geographic
Coverage
Period of Record
Source Water
Sampled
WaterRF #2759
NCBOS
USEPA
6YR2
CDPH
WQAD
41 States
46 States and 2 Tribes
California
2003
1998 - 2005
Raw Drinking Water
Treated Drinking Water
Total Chromium
Data
MRL=0.6 µg/L
n=342, 39%<MRL
MRL varies from
0.02 -100 µg/L
n=162,823, 84%<MRL
Hexavalent
Chromium Data
MRL=0.2 µg/L
n=341, 57%<MRL
None
1995 - 2011
Raw and Treated Drinking
Water
MRL varies from
1 -50 µg/L
n=108,782, 49%<MRL
MRL=1 µg/L
or less
n=27,507, 13%<MRL
Strengths
Low and consistent
MRL
Number and geographic
distribution of samples
Number of samples for
total and hex chromium
Number and
geographic distribution
of samples
Some high MRL data, no
hexavalent chromium
data
Some high MRL data,
limited geographical
distribution
Weaknesses
MRL = minimum reporting level
1
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
2.1. National Chromium and Boron Occurrence Survey (NCBOS)
The National Chromium and Boron Occurrence Survey (NCBOS) was compiled, analyzed,
and reported in WaterRF Report #2759 (Frey et al., 2004). The survey utilities were recruited on
a voluntary basis, rather than a designed survey to adequately represent regional, system size,
and source water occurrence. Field samples were collected in 2003 and transported to a central
laboratory where the analysis of total and hexavalent chromium was performed. Reporting limits
were 0.6 µg/L for total chromium and 0.2 µg/L for hexavalent chromium. A total of 342 total
chromium and 341 hexavalent chromium raw water samples were reported. The samples were
taken at the same relative location for each of 342 different sources, from 189 different systems
in 41 states. Samples from the treatment system profiles included with the NCBOS were not
included in this analysis.
2.2. EPA’s Second Six Year Review ICR Contaminants Monitoring Data (6YR2)
The 6YR2 dataset consists of nationwide entry point samples for total chromium. The EPA
collected the 6YR2 data by a voluntary call-in of compliance data from state and primacy
agencies for samples collected between 1998 and 2005 (EPA, 2010). The samples were
analyzed by many different labs and methods resulting in reporting levels from 0.02 – 100 µg/L.
A total of 162,823 samples were included in the dataset after applying a data quality auditing
procedure. The EPA’s data auditing involved standardizing units of concentration for the
contaminants and ensuring that parameters such as the system type were consistent with data in
federal safe drinking water information system (SDWIS). Systems classified as a groundwater
include only ground water sources. However, systems classified as surface waters may include
groundwater and groundwater under the direct influence (GWUDI) of surface water among its
sources. Some of the data submissions included source water samples. Where possible the
source water data points were removed by the EPA. Also, each sampling point was inferred to
be an entry point to the distribution system.
Table 2 summarizes the non-detections in the 6YR2 dataset. The results were calculated
considering all the samples in the dataset, then grouping the data by entry point. Thus, a nondetect result at an entry point means that chromium was never detected in that source.
Table 2. Summary of non-detect results in the 6YR2 dataset.
Total count
Non-detect count Percent non-detects
162,823
136,826
84%
All Samples - Cr
74,672
58,010
78%
Entry Point - Cr
Of the 162,823 samples in the 6YR2 database, 136,826 were reported as non-detects with an
associated minimum reporting level (MRL). During the EPA’s data auditing procedure, nondetects that did not include a MRL were assigned the mode MRL value for that state where other
data in the state were available (EPA, 2010). If no other MRL value was available for that state,
the national mode MRL value was substituted. Figure 1 shows the distribution of the non-detect
results from the 6YR2 dataset. Figure 1 indicates 47% of non-detect results have a MRL of 10
µg/L, which many of these presumably resulting from samples that did not include an MRL.
Another 15% of the non-detects have a MRL of 5 µg/L.
2
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A)
60,000
80%
50,000
60%
40,000
30,000
40%
20,000
20%
10,000
0
0%
0
Figure 1.
B)
Cumulative Probability
Number of Non‐Detects in 6YR2
100%
70,000
10
20
Total Chromium MRL (µg/L)
0
10
Total Chromium MRL (µg/L)
20
Analysis of non-detect MRLs of the 6YR2 dataset (all non-detect samples).
Panel A shows the histogram of the non-detect MRLs. Panel B shows the
cumulative probability distribution of non-detect MRL concentrations.
Non-detect samples can have a profound effect on the analysis of the data. Because the
concentration of a non-detect sample can be any value from zero to the MRL, cumulative
probability distributions of the dataset were computed under four conditions to characterize the
effect of the non-detect results:
1. zero was substituted for a non-detect result,
2. one-quarter of the MRL was substituted for the non-detect result,
3. half of the MRL was substituted for the non-detect result, and
4. the MRL was substituted for the non-detect result.
Substituting zero for the non-detects results in bias towards lower concentrations because some
of the non-detects would have had detectable chromium if it were measured by a method with a
lower MRL. Similarly, substituting the MRL for a non-detect results in bias towards higher
concentrations because the actual sample concentrations were likely lower than the MRL in most
cases. Substituting one-quarter and one-half of the MRL for the non-detect attempts to balance
these biases, but the bias is unknown. See Raucher et al. (1995) for more details of non-detect
handling.
To create the cumulative probability distributions, the samples were grouped by entry point
to analyze the results on a drinking water source basis (all water source types for both
community water systems (CWS) and non-transient, non-community water systems
(NTNCWS)). Next, the median, 75th percentile, 95th percentile, and maximum value
concentrations were calculated for each of the 74,672 entry points. Figure 2 presents the
cumulative probability curves of median total chromium concentration for the 6YR2 data by the
four non-detect substitution conditions. The median value varies from zero to 10 µg/L as
3
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
influenced by the non-detects with an MRL of 10 µg/L, again many of which would have been
assigned a value of 10 µg/L because no MRL was reported with the data. The vertical portions
in each curve represent a large number of non-detect results at a particular MRL. For instance,
the vertical lines from about the 40th to 80th percentile are indicative of the non-detects with an
MRL of 10 µg/L.
Cumulative Probability
100%
80%
n.d.=0
60%
n.d.=0.25∙MRL
40%
n.d.=0.5∙MRL
n.d.=MRL
20%
0%
0
10
20
30
Total Chromium (µg/L)
40
Figure 2. Cumulative probability distribution of median total chromium concentration by
entry point of the 6YR2 dataset with non-detects set to zero, one quarter of the
MRL, half the MRL, and at the MRL.
Another method to analyze the impact of the non-detect results is a Bayesian estimate of the
posterior cumulative probability. A Bayesian analysis of the non-detects from total chromium
concentrations between 0.5 and 15 µg/L was prepared using the WinBUGS software at the mean
concentration of each entry point location. The Bayesian analysis uses the detect data and a
defined distribution to estimate the cumulative probability curve.
Figure 3 shows the cumulative probability of the 6YR2 dataset with non-detects substituted
with zero and one-quarter of the MRL with the cumulative distributions of the NCBOS dataset
and the Bayesian estimate. The NCBOS and Bayesian curves show similar results. This result is
somewhat unexpected as the NCBOS dataset was based upon voluntary participation of water
utilities that were willing to sample their source waters and therefore considered to be biased
towards utilities where chromium was suspected or known to occur. NCBOS was generated on a
sample-by-sample basis because only one sample was taken per sample location, and were raw
water samples. But notably, the NCBOS data shows the smooth, nearly log-normal curve to the
4
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
origin resulting from the low reporting limits used in the study. Raucher et al. (1995) observed
that occurrence data often display a log-normal behavior. Substitution of one-quarter of the
MRL in the 6YR2 data is close to the NCBOS and EPA Bayesian estimates while still being
conservative, particularly above 5 µg/L. Therefore, substitution of one-quarter of the MRL for
non-detects was used for all further analyses of the 6YR2 data, except where noted. Continuing
analyses with the Bayesian approach are not practical because the non-detect values are not reassigned, thus each data sub-set would require its own Bayesian analysis.
Cumulative Probability
100%
75%
6YR2 (n.d.=0)
50%
NCBOS
EPA Bayesian Estimate
25%
6YR2 (n.d.=0.25∙MRL)
0%
0
5
10
15
Total Chromium (µg/L)
20
Figure 3. Cumulative probability distribution of median total chromium concentration by
entry point of the 6YR2 dataset with non-detects set to zero, and one quarter of
the MRL, compared to the NCBOS dataset and the Bayesian estimate.
Figure 4 presents the cumulative probabilities of total chromium from the 6YR2 data by the
median (50th percentile), 75th percentile, 95th percentile, and maximum concentration at each
entry point. Using percentiles to represent the data lessens the impact of non-detect handling
over averaging; therefore, the average at each entry point was not included in this analysis. The
75th percentile, 95th percentile, and maximum value yield very similar curves, with the 95th
percentile and the max value curves being indistinguishable. The median value was eliminated
from consideration because it may not be inclusive enough of systems with variable chromium
concentrations. For instance, a utility that has source water with a variable chromium
concentration, may have a median concentration below a threshold; however, if a regulation
were in place at that threshold the utility would be in violation at times when the chromium
concentration was above the median concentration. The similarity of the 75th, 95th, and
5
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maximum curves occurs because the sample size of most entry points is low (162,823 samples
divided by 74,672 entry points equals an average of 2.2 samples per entry point). The 75th
percentile has been chosen to be consistent with the USGS analysis of arsenic national
occurrence data (USGS, 2012), and its similarity to the 95th percentile results.
Cumulative Probability
100%
75%
50%
median
75th percentile
25%
95th percentile & max
0%
0
5
10
15
Total Chromium (µg/L)
20
Figure 4. Cumulative probability distribution of total chromium concentration by entry
point of the 6YR2 dataset (non-detects set to one-quarter of the MRL).
2.3. CDPH Data
The California Department of Public Health (CDPH) maintains a water quality analysis
database, which includes total and hexavalent chromium in the state of California collected
generally between 1995 and 2011. Data are representative of raw and treated waters from
throughout the state. The database included 136,286 results, 108,779 for total chromium and
27,507 for hexavalent chromium as of November 18, 2011. Because CDPH provided data for
the USEPA 6YR2 database, there is an overlap of up to 23,397 total chromium samples between
the CDPH data and 6YR2 datasets for California (22% of the CDPH database, 14% of the 6YR2
database). CDPH data came classified as raw, treated, and unknown for treatment type.
Samples also came classified as groundwater, surface water, or non-source water.
Table 3 summarizes the CDPH data and shows a similar percentage of non-detect results on a
sample basis, but a lower percentage of non-detects on a source location basis. The lower
percentage of non-detects on a source location basis can be attributed to two factors. First, the
CDPH data is weighted on more recent data with lower detection limits. Second, from a spatial
6
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distribution perspective, California has some of the highest levels of chromium (demonstrated in
Section 3).
Table 3. Summary of non-detect results in the CDPH dataset.
Total count
Non-detect
Percent noncount
detects
108,779
90,560
83%
All Samples – Total Cr
27,507
11,387
41%
All Samples - Cr(VI)
17,846
11,104
62%
Sampling Location – Total Cr
7,880
3,096
39%
Sampling Location - Cr(VI)
The CDPH total chromium dataset includes 35,968 non-detect samples results at an MRL of
10 µg/L (29% of all non-detects), and another 40,255 had no reported MRLs and were therefore
set to the mode value of the CDPH dataset of 10 µg/L (bringing the total to 73% of all nondetects). Thus, the CDPH total chromium dataset is similarly influenced by the non-detect
results as the 6YR2 dataset. For comparable analysis with the 6YR2 data, the CDPH chromium
data have been analyzed with non-detects substituted with one-quarter of the MRL and missing
MRL values set to the mode value of 10 µg/L. The MRL for the CDPH hexavalent chromium is
generally 1 µg/L or less. The lower MRL adds very little bias to the analysis of hexavalent
chromium in the range of 1 to 50 µg/L of interest in this study. Therefore, non-detect values
were set to zero for the CDPH dataset for hexavalent chromium.
Before further analysis can be made, it must be determined whether distribution of the data
differs between raw and treated waters. Figure 5 plots the cumulative probability for raw and
treated waters for both total and hexavalent chromium. The analysis was performed with the
75th percentile concentration on a sample location basis.
The data shown in Figure 5 indicate more occurrence of total chromium than hexavalent
chromium. Higher total chromium is expected because hexavalent chromium is only a portion of
the total chromium measurement. Treated water data in Figure 5 show a lower occurrence of
both total and hexavalent chromium compared to raw waters. This could be due to treatment
removing some chromium, but a more likely explanation is that sources with high concentrations
of chromium are not used for drinking water except where they cannot be avoided. Also, the
disparity in the number of samples (treated waters only account for about 5% of the data) could
also result in some selection bias. Because the distributions of raw and treated water differ, they
will be analyzed separately throughout the remainder of this report.
7
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Cumulative Probability
100%
75%
Total Cr Raw (n=14,735)
50%
Total Cr Treated (n=1,003)
25%
Hex. Cr Raw (n=6,547)
Hex. Cr Treated (n=419)
0%
0
5
10
15
Concentration (µg/L)
20
Figure 5. Cumulative probability distribution of chromium in CDPH for raw versus
treated by sample location (75th percentile concentration per each sample
location).
3. Analysis of Total and Hexavalent Chromium Occurrence
3.1. Comparison of Chromium Occurrence between Datasets
The NCBOS, 6YR2, and CDPH datasets all include total chromium. However, the datasets
differ in the number of samples and sample type (raw versus treated). The 6YR2 and CDPH data
were analyzed with various MRLs. CDPH data include raw and treated waters, where the
NCBOS contain only raw, and 6YR2 contain only data from treated drinking waters. The
datasets are compared by plotting the cumulative probability function of each in Figure 6. The
cumulative probabilities were calculated from the 75th percentile concentration at each sample
location.
The data in Figure 6 indicates the CDPH raw waters exhibited the most occurrence and the
NCBOS exhibited the least occurrence. The 6YR2 and CDPH treated waters showed roughly
equivalent occurrence. The difference observed between these two curves is due to the 6YR2
dataset having a proportionately higher number of non-detects with an MRL of 20 µg/L (the
vertical step in the black line at 5 µg/L).
8
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Cumulative Probability
100%
75%
NCBOS (n=342)
50%
CDPH Treated (n=1,003)
25%
6YR2 (n=74,672)
CDPH Raw (n=14,735)
0%
0
5
10
15
Total Chromium (µg/L)
20
Figure 6. Cumulative probability distribution of total chromium data from the three
datasets (6YR2 and CDPH are the 75th percentile concentration per sample
location).
Figure 7 compares the cumulative probability distributions of hexavalent chromium from the
CDPH raw waters and the NCBOS dataset. The analysis was performed with the 75th percentile
concentration of hexavalent chromium on a sample location basis. The California data suggested
higher occurrence than nationally, as expected.
9
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Cumulative Probability
100%
75%
50%
25%
NCBOS (n=341)
CDPH Raw (n=6,547)
0%
0
5
10
15
Hexavalent Chromium (µg/L)
20
Figure 7. Cumulative probability distribution of hexavalent chromium NCBOS & CDPH
raw data (CDPH shows the 75th percentile concentration per sample location,
non-detects set to zero).
3.2. Analysis of Chromium Occurrence by Source Type, Region, and System Size
Figure 8 displays the cumulative probability distributions of the 6YR2 total chromium
groundwater data versus surface waters. The concentrations were calculated from the 75th
percentile at each sample location and non-detects were substituted with one-quarter of the MRL.
Results of the analysis indicate that distributions are similar, with a maximum of about 2%
difference between them. As noted previously, some groundwater sources may be labeled as
surface waters if they have any surface water, or GWUDI sources in the same system. Frey et al.
(2004) also found no significant difference in distribution of total chromium for ground waters
versus surface waters. Thus, the analysis of total chromium data will not be separated by source
water type. A similar analysis was performed for water system type. No observable difference
was found between CWS and NTNCWS. Therefore, all data has been included in the analysis
(rather than limit the data to CWS data only).
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Cumulative Probability
100%
75%
50%
Ground Water (n=67,269)
Surface Water (n=7,403)
25%
0%
0
5
10
15
Total Chromium (µg/L)
20
Figure 8. Cumulative probability distributions of total chromium in the 6YR2 dataset by
source water type (75th percentile concentration per sample location, nondetects set to one-quarter of the MRL).
Figure 9 shows the cumulative probability distributions for hexavalent chromium by ground
and surface water from the CDPH database. The occurrence in ground waters is higher than for
surface waters. Frey et al. (2004) found similar results. Figure 9 also separates the raw waters
from the treated waters. Not much difference in distribution is observed in the surface water data
as occurrence was only observed at low levels. The groundwater data; however, indicates higher
occurrences in the treated waters below 6 µg/L and higher occurrences in raw waters above 6
µg/L. An increase in hexavalent chromium could result from oxidation of trivalent chromium
during disinfection. A decrease in hexavalent chromium could be due to treatment provided at
some sites. However, the explanation for the crossover in Figure 9 cannot be determined with
any certainty, and it could also be due to the disparity in the number of samples (1,242 treated
samples vs. 14,448 raw samples).
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Cumulative Probability
100%
75%
GW Raw (n=14,448)
50%
GW Treated (n=1,242)
25%
SW Raw (n=628)
SW Treated (n=370)
0%
0
5
10
15
Hexavalent Chromium (µg/L)
20
Figure 9. Cumulative probability distributions of CDPH hexavalent chromium data in
ground and surface waters divided into raw and treated samples (75th percentile
concentration per sample location, non-detects set to zero).
The regional distribution of total chromium was investigated using EPA regions (shown in
Figure 10). Figure 11 presents the cumulative probability distributions for total chromium
grouped by EPA region. Some variability based on region was observed, but the uncertainty
surrounding the non-detect data complicates the analysis. Non-detect handling complicates
analysis of Figure 11; however, it can be seen that Region 9 (California, Nevada, and Arizona)
show the highest concentration occurrences. The vertical steps in the curves represent the reoccurring non-detect values. The Region 9 curve shows the highest concentrations towards the
end of the curves where the influence of non-detects no longer influences the analysis.
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Figure 10.
Map of EPA Regions.
Cumulative Probability
100%
75%
Region 1 (n=6,677)
Region 2 (n=4,808)
Region 3 (n=4,601)
Region 4 (n=10,143)
Region 5 (n=14,680)
Region 6 (n=8,916)
Region 7 (n=5,703)
Region 8 (n=5,367)
Region 9 (n=10,996)
Region 10 (n=2,781)
50%
25%
0%
0
Figure 11.
5
10
Total Chromium (µg/L)
15
20
Cumulative probability distribution of 6YR2 total chromium data by EPA
region (75th percentile concentration per sample location, non-detects set to
one-quarter of the MRL).
13
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
System size was investigated in the same manner as EPA region. Figure 12 presents the
cumulative distribution curves for the 6YR2 data by system size category for all systems
included in the 6YR2 dataset (CWS and NTNCWS). Non-detect handling makes interpretation
of the 6YR2 data based on system size capricious. For instance, the higher concentration
distribution curve for systems serving a population of 1,000,000 results from a significant
number of samples analyzed at an MRL of 50 µg/L.
Cumulative Probability
100%
75%
Population Range (count)
25‐100 (n=18,566)
101‐500 (n=21,909)
501‐1,000 (n=7,423)
1,001‐3,300 (n=9,397)
3,301‐10,000 (n=6,906)
10,001‐50,000 (n=6,497)
50,001‐100,000 (n=1,631)
100,001‐1,000,000 (n=2,053)
>1,000,000 (n=290)
50%
25%
0%
0
Figure 12.
5
10
Total Chromium (µg/L)
15
20
Cumulative probability distributions of 6YR2 total chromium data by system
size category (75th percentile concentration per sample location, non-detects
set to one-quarter of the MRL).
3.3. Analysis of Paired Total and Hexavalent Chromium Samples
The CDPH data were also used to study the speciation of chromium. A total of 5,976
samples with both total and hexavalent chromium results were paired by matching sample
location and date. Figures 12-14 display the results by plotting hexavalent chromium versus
total chromium by ranges from 0-100 µg/L and 0-10 µg/L by water type and raw versus treated.
In Figures 12-14 there are a number of samples that indicate a higher hexavalent chromium
concentration than total chromium, which is not truly possible. These points result from
analytical issues that are well described and explained by Parks et al. (2004).
14
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
100
90
80
70
60
50
40
30
20
10
0
GW (n=4,809)
SW (n=1,167)
Hexavalent Chromium (µg/L)
Hexavalent Chromium (µg/L)
Analysis of the ground versus surface water paired samples shown in Figure 13 indicates
surface waters have generally more trivalent chromium than hexavalent chromium. The ground
waters tend to have much of their total chromium as hexavalent chromium, consistent with Frey
et al. (2004). Despite some scatter in the data the hexavalent chromium concentration is highly
correlated with the total chromium concentration (r=0.98).
10
9
8
7
6
5
4
3
2
1
0
GW (n=3,434)
SW (n=1,147)
0
0 10 20 30 40 50 60 70 80 90 100
Total Chromium (µg/L)
1
2
3 4 5 6 7 8
Total Chromium (µg/L)
9 10
Figure 13. CDPH paired sample analysis of ground waters and surface waters.
100
90
80
70
60
50
40
30
20
10
0
GW Raw (n=3,943)
GW Treated (n=646)
0 10 20 30 40 50 60 70 80 90 100
Total Chromium (µg/L)
Figure 14.
Hexavalent Chromium (µg/L)
Hexavalent Chromium (µg/L)
Figure 14 displays groundwater paired samples similarly divided into raw and treated
categories. The data in Figure 13 also indicate treated waters have lower concentrations of
chromium. This may be due to treatment, but could also be due to sampling bias created by high
chromium sources not being used for drinking water. The disparity in sample sizes between raw
and treated waters may also contribute to bias.
The treated samples visually appear to be more heavily speciated towards hexavalent
chromium, but this is not supported by statistical analysis. Statistically, the central tendency of
the raw waters is more heavily speciated in hexavalent chromium. However, there is not a large
difference between the raw and treated waters.
10
9
8
7
6
5
4
3
2
1
0
GW Raw (n=2,798)
GW Treated (n=473)
0
1
2
3 4 5 6 7 8
Total Chromium (µg/L)
9 10
CDPH paired sample analysis of ground waters grouped by raw and treated
waters.
15
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
100
90
80
70
60
50
40
30
20
10
0
SW Raw (n=676)
SW Treated (n=473)
0 10 20 30 40 50 60 70 80 90 100
Total Chromium (µg/L)
Figure 15.
Hexavalent Chromium (µg/L)
Hexavalent Chromium (µg/L)
Figure 15 indicates surface waters tended to have little hexavalent chromium in raw or
treated waters.
10
9
8
7
6
5
4
3
2
1
0
SW Raw (n=661)
SW Treated (n=468)
0
1
2 3 4 5 6 7 8
Total Chromium (µg/L)
9 10
CDPH paired sample analysis of surface waters grouped by raw and treated
waters.
4. Spatial Distribution of Chromium Occurrence
The 6YR2 dataset was used to calculate the 75th percentile concentration of total chromium
at each entry point. The highest 75th percentile entry point concentration per system was mapped
by latitude and longitude identifiers assigned to each system from SDWIS data. No data were
available in the states of Washington, Pennsylvania, Louisiana, or Mississippi. Figure 16 shows
a national pin map of total chromium concentrations between 5 and 10 µg/L, 10 and 20 µg/L,
and above 20 µg/L. The figure also shows how total chromium is distributed across the country.
The greatest number of high chromium concentrations appears to be in the southwest US from
California to Texas. The region surrounding the Great Lakes also appears to have widespread
occurrence.
16
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Figure 16.
National pin ma
ap of 75th peercentile tootal chromiu
um concenttration from
m the
6YR2
2 dataset by system (forr systems wiith more than two sam
mples).
Tablee 4 providess a tabular an
nalysis of th
he spatial disstribution off total chrom
mium occurrrence.
The 6YR
R2 (with no
on-detects seet to one-qu
uarter of thee MRL) datta was usedd to calculatte the
percent of
o entry poin
nts that excceed total ch
hromium treeatment goalls of 1, 2, 55, 10, 20, annd 50
µg/L. The
T results indicate
i
thatt the percen
nt of system
ms impacted rises steeplyy below 5 µ
µg/L.
Much off this is du
ue to non-deetect handlin
ng. For innstance Geoorgia samplees only hadd two
detection
ns out of 2,358 samples, but becausee the MRLs of the Georggia data werre relatively high,
Table 4 indicates
i
essentially all entry
e
points exceed 5 µgg/L.
17
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Table 4.
Percent of entry points with 75th percentile total chromium concentration
above the threshold values (6YR2 data, non-detects set to one-quarter of the MRL).
Threshold
All EPs
Alaska
Alabama
Arkansas
Arizona
California
Colorado
Connecticut
Delaware
Florida
Georgia
Hawaii
Iowa
Idaho
Illinois
Indiana
Kansas
Kentucky
Massachusetts
Maryland
Maine
Michigan
Minnesota
Missouri
Montana
North Carolina
North Dakota
Nebraska
New Hampshire
New Jersey
New Mexico
Nevada
New York
Ohio
Oklahoma
Oregon
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Virginia
Vermont
Wisconsin
West Virginia
Wyoming
1 µg/L
84.3%
24.0%
100.0%
100.0%
95.1%
94.6%
66.9%
82.4%
53.7%
90.6%
100.0%
34.8%
97.8%
21.0%
99.8%
97.3%
89.5%
26.5%
85.6%
96.5%
29.4%
100.0%
100.0%
12.9%
84.6%
99.2%
66.9%
62.1%
100.0%
78.6%
71.2%
95.2%
94.4%
87.0%
90.7%
11.0%
100.0%
100.0%
97.8%
91.0%
97.8%
99.1%
99.0%
90.6%
37.4%
98.7%
99.6%
2 µg/L
69.0%
17.9%
99.8%
99.4%
85.6%
92.4%
56.8%
74.5%
34.3%
84.6%
99.9%
34.2%
66.3%
20.0%
10.9%
42.3%
62.8%
19.3%
46.4%
94.1%
11.4%
93.3%
100.0%
7.0%
83.4%
93.7%
52.0%
46.5%
99.9%
62.9%
60.9%
63.2%
14.6%
52.7%
89.8%
5.3%
100.0%
100.0%
90.7%
86.4%
96.6%
25.7%
96.8%
77.0%
19.0%
75.3%
98.0%
5 µg/L
21.6%
7.7%
2.0%
0.2%
55.9%
15.2%
41.1%
16.7%
7.8%
12.1%
99.9%
8.9%
8.7%
10.9%
7.6%
28.9%
16.4%
3.0%
10.0%
72.9%
1.3%
9.2%
0.1%
0.4%
5.2%
87.3%
30.6%
18.4%
1.3%
9.9%
28.6%
34.7%
8.4%
8.5%
13.0%
1.7%
17.6%
1.8%
18.3%
2.7%
22.3%
17.4%
2.4%
3.9%
8.7%
3.8%
73.8%
10 µg/L
7.6%
1.3%
1.0%
0.2%
29.1%
10.6%
9.5%
12.4%
1.0%
6.6%
0.1%
2.3%
1.2%
4.6%
2.9%
16.8%
2.1%
2.7%
6.0%
66.1%
0.1%
7.6%
0.1%
0.1%
1.4%
1.4%
12.9%
4.6%
0.5%
3.2%
9.7%
16.7%
6.1%
6.9%
12.7%
0.6%
5.3%
1.7%
6.5%
2.2%
6.9%
6.5%
1.6%
2.6%
4.9%
1.5%
72.3%
20 µg/L
1.7%
0.4%
0.0%
0.0%
12.3%
3.2%
2.5%
1.9%
0.2%
1.8%
0.1%
0.0%
0.8%
0.5%
0.7%
7.0%
0.5%
0.4%
0.1%
2.8%
0.1%
1.1%
0.1%
0.0%
0.4%
0.6%
2.4%
0.0%
0.3%
0.8%
1.2%
4.4%
0.2%
1.1%
8.6%
0.2%
0.0%
0.9%
0.6%
0.3%
1.9%
1.8%
0.2%
2.1%
2.3%
0.4%
0.4%
18
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
50 µg/L
0.2%
0.2%
0.0%
0.0%
1.2%
0.2%
0.2%
1.1%
0.0%
0.1%
0.0%
0.0%
0.2%
0.1%
0.1%
0.2%
0.1%
0.0%
0.0%
0.3%
0.0%
0.1%
0.0%
0.0%
0.1%
0.1%
0.4%
0.0%
0.1%
0.2%
0.1%
0.6%
0.0%
0.0%
2.8%
0.0%
0.0%
0.6%
0.0%
0.3%
0.0%
0.2%
0.0%
0.6%
0.3%
0.0%
0.2%
Figure 17
7 shows the CDPH occu
urrence of heexavalent chrromium in C
California. B
Because of thhe
lower dettection limitt for hexavallent chromiu
um concentraation ranges of 1 - 5 µg/L
L, 5 - 10 µg//L,
10 - 20 µg/L
µ
and abo
ove 20 µg/L are shown. Hexavalent chromium aappears to occcur at locatiions
across the state.
Figure 17.
Califo
ornia pin map
m of 75th
h percentilee hexavalen
nt chromium
m concentration
from the CDPH database
d
by
y system.
5. Co-o
occurrence with Chromium
The 6YR2 datab
base was qu
ueried for th
he maximum
m concentraation of eachh of 69 anaalytes
included in the 6YR2
2 dataset for each system
m. As a preliiminary anallysis, the corrrelation bettween
total chro
omium and the
t other 68
8 contaminan
nts was calcuulated. Tabble 5 shows the contamiinants
with the highest corrrelations. The
T maximum
m correlationn is selenium
m with a cooefficient of 0.05,
5 of the variation
v
con
ntaminant beeing comparred can be explained by the
meaning that only 5%
variation
n in total chro
omium conccentration. The
T most higghly correlatted compounnds on the list are
naturally
y occurring, and may bee suspected to
t be solublle under sim
milar geochem
mical condittions;
however,, the relation
nships are very weak. For
F compariison purposees, the correelation coeffi
ficient
between tetrachloroeethylene and
d trichloroetthylene from
m this samee analysis w
was 0.29. T
These
nt with the finding of Seidel (20006) which foound no co--occurrence with
results are consisten
chromium
m, except beetween total chromium and
a iron in suurface waterrs. Iron is noot included iin the
6YR2 daataset and co
ould not be investigated
i
further. Inn our opinionn, these resuults do not juustify
19
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
further investigation into co-occurrence using higher statistical techniques such as the Chi
Squared approach.
Table 5.
Correlation of co-occurring contaminants with total chromium in the 6YR2
dataset.
Contaminant
r
Selenium
0.05
Arsenic
0.04
Fluoride
0.04
Beryllium
0.03
Antimony
0.03
Cadmium
0.03
Thallium
0.02
Beta Particles (Gross beta)
0.02
Tetrachloroethylene (PCE)
0.02
Nitrate
0.02
Barium
0.02
Uranium
0.02
Trichloroethylene (TCE)
0.02
Chlordane
0.02
1,1,2-Trichloroethane
0.02
cis-1,2,-Dichloroethylene
0.02
Alpha Particles
0.01
trans-1,2-Dichloroethylene
0.01
1,1-Dichloroethylene
0.01
Ethylene Dibromide (EDB)
0.01
Combined Radium- 226 & 228
0.01
6. Estimated Extent of Potential Future Regulation
One of the primary objectives of this occurrence survey is to determine the extent a potential
future regulation may affect drinking water systems. Thus, the number of entry points greater
than a given threshold concentration were estimated for CWSs. Because the 6YR2 database
does not represent a census of all nationwide entry points, the total number of entry points was
estimated. The number of nationwide CWS entry points was estimated by comparing the 6YR2
data to the SDWIS database of CWSs (retrieved in December of 2011), broken down by system
size. Table 6 shows the results of the estimation. Columns 2 and 3 show the number of systems
in the SDWIS and 6YR2 databases, respectively. Table 6 indicates that the 6YR2 database
includes between about 60-90% of the SDWIS totals. To project this to entry points, the number
of entry points in the 6YR2 database is divided by the fraction of systems represented, which is
presented in column 6 of Table 6. Last, the average number of entry points per system is
calculated. The numbers generally increase with system size from about 2 to 14.5 for the largest
systems.
20
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Table 6. Estimation of the national CWS entry point inventory.
1
2
3
4
5
6
7
SDWIS
6YR2
Projected
Average
System
CWS
6YR2
Entry
Number
Number of
Population
PWSID
PWSID
6YR2 /
Point
of Entry Entry Points
Served
Census
Inventory SDWIS Inventory
Points
per PWSID
All
50,936
32,810
64%
74,672
115,925
2.3
0-100
11,460
8,217
72%
20,116
28,055
2.4
101-500
16,333
10,509
64%
20,977
32,602
2.0
500-1,000
5,707
3,432
60%
7,085
11,781
2.1
1,001-3,300
8,259
4,719
57%
9,189
16,082
1.9
3,30110,000
4,946
3,092
63%
6,868
10,986
2.2
10,00150,000
3,254
2,122
65%
6,509
9,981
3.1
50,001100,000
555
386
70%
1,585
2,279
4.1
100,0011,000,000
400
313
78%
2,053
2,624
6.6
>1,000,000
22
20
91%
290
319
14.5
The number of entry points greater than the threshold values was estimated by multiplying
the total number of entry points by the percent of entry points exceeding the potential regulation.
Table 7 shows these results for total chromium concentrations of 1, 2, 5, 10, 20, and 50 µg/L.
The number of entry points above the threshold concentration have been estimated based on
three conditions:
1. Using the 6YR2 database with non-detects set to one-quarter of the MRL
2. Using the 6YR2 database with non-detects set to zero
3. Using the NCBOS database with non-detects set to zero
21
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Table 7. Number of CWS entry points above the threshold concentration at various
threshold concentrations.
System
Population
Served
All Entry
Points
0-100
101-500
500-1,000
1,0013,300
3,30110,000
10,00150,000
50,001100,000
100,0011,000,000
>1,000,000
Threshold
Concentration
1 µg/L
2 µg/L
5 µg/L
6YR2 n.d.=0.25·MRL
6YR2 n.d.=0
NCBOS
6YR2 n.d.=0.25·MRL
6YR2 n.d.=0
NCBOS
6YR2 n.d.=0.25·MRL
6YR2 n.d.=0
NCBOS
6YR2 n.d.=0.25·MRL
6YR2 n.d.=0
NCBOS
6YR2 n.d.=0.25·MRL
6YR2 n.d.=0
NCBOS
6YR2 n.d.=0.25·MRL
6YR2 n.d.=0
NCBOS
6YR2 n.d.=0.25·MRL
6YR2 n.d.=0
NCBOS
6YR2 n.d.=0.25·MRL
6YR2 n.d.=0
NCBOS
6YR2 n.d.=0.25·MRL
6YR2 n.d.=0
NCBOS
6YR2 n.d.=0.25·MRL
6YR2 n.d.=0
NCBOS
97,693
23,447
50,543
23,022
5,245
12,232
27,254
6,655
14,215
9,969
2,325
5,137
13,553
3,262
7,012
9,424
2,319
4,790
8,769
2,089
4,352
2,055
489
994
2,276
705
1,144
304
76
139
79,953
18,581
29,793
19,135
4,048
7,210
22,268
5,242
8,379
7,817
1,827
3,028
10,811
2,552
4,133
7,733
1,839
2,823
7,136
1,742
2,565
1,787
422
586
2,001
602
674
289
68
82
25,044
10,417
10,897
5,957
2,332
2,637
7,248
2,827
3,065
2,554
1,032
1,107
3,101
1,289
1,512
2,144
966
1,033
2,231
1,072
938
476
303
214
749
394
247
149
37
30
10 µg/L 20 µg/L 50 µg/L
8,829
6,013
1,971
2,178
1,408
477
2,539
1,680
554
825
544
200
881
589
273
724
550
187
800
605
170
245
210
39
299
272
45
132
24
5
1,976
1,826
1,043
432
400
252
516
461
293
197
181
106
181
164
145
208
192
99
221
218
90
88
87
21
77
69
24
13
13
3
217
217
0
42
42
0
71
71
0
27
27
0
15
15
0
17
17
0
15
15
0
14
14
0
6
6
0
2
2
0
The results of Table 7 indicate the number of entry points impacted increases steeply below
treatment goals of 10 µg/L. Again this is largely due to the uncertainty associated with nondetect handling. Figure 18 shows this trend graphically aggregating all system sizes. For
instance, the estimated number of entry points with concentrations above 10 µg/L range from
1,971 (NCBOS) to 8,829 (6YR2 with non-detects set to one-quarter of the MRL). Thus, even at
a concentration above the influence of the non-detect results, over a four-fold difference is
estimated depending on which data set is selected.
22
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
Number of Entry Points Above Threshold
100,000
6YR2 n.d.=0.25∙MRL
80,000
NCBOS
60,000
6YR2 n.d.=0
40,000
20,000
0
0
10
20
30
40
50
Total Chromium Threshold Concentration (µg/L)
Figure 18. Estimation of national number of CWS entry points with total chromium above
threshold concentrations.
7. Conclusions
The purpose of this study was to update the NCBOS chromium occurrence analysis
originally presented in the WaterRF Report #2759 with new data from the EPA 6YR2 and
CDPH WQAD datasets. The comparisons also validate key findings already presented in
WaterRF Report #2759, including:
1. no significant difference in distribution of total chromium for ground waters versus
surface waters;
2. ground waters tend to have much of their total chromium as hexavalent chromium;
3. total chromium occurrence is distributed across the country; and
4. the highest chromium concentration occurrences were observed in EPA Region 9
(California, Nevada, and Arizona).
The total chromium data is limited by MRLs, making predictions of systems impacted
subject to a high degree of uncertainty below 10 µg/L. A suitable dataset is not available for
predicting hexavalent chromium occurrence on a national basis. Preliminary analysis of systems
impacted at different chromium concentrations indicates the potential for a very large number of
systems to be impacted by regulation.
23
©2012 Water Research Foundation. ALL RIGHTS RESERVED.
8. References
EPA. (2010) “The Analysis of Regulated Contaminant Occurrence Data from Public Water
Systems in the Support of the Second Six-Year review of national Primary Drinking Water
Regulations,” US Environmental Protection Agency report EPA-815-B-09-006.
Frey, M.M., Seidel, C.J., Edwards, M., Parks, J.L., McNeill, L. (2004) Occurrence Survey of
Boron and Hexavalent Chromium, Water Research Foundation, Water Research Foundation
Report #2759 Denver, CO.
Parks, J. L., McNeill, L., Frey, M., Eaton, A. D., Haghani, A., Ramirez, L., Edwards, M. (2004)
“Determination of total chromium in environmental water samples,” Water Research, 38(12):
2827-2838.
Raucher, R.S., Castillo, E.T., Dixon, A., Breffle, W., Waldman, D., and Drago, J.A. (1995)
Estimating the Cost of Compliance with Drinking Water Standards: A User’s Guide. Water
Research Foundation, Denver, CO.
Seidel, C.J. (2006) “Investigation of Hexavalent Chromium as a Future Regulated Drinking
Water Contaminant,” Doctoral Thesis, University of Colorado at Boulder.
USGS.
(2012)
Arsenic
in
Groundwater
of
the
United
States.
http://water.usgs.gov/nawqa/trace/arsenic/ accessed March 1, 2012.
24
©2012 Water Research Foundation. ALL RIGHTS RESERVED.