Technical Memorandum
To:
Commissioners, Black Dog Watershed Management Organization
From:
Barr Engineering Co.
Subject: 2011 Orchard Lake Water Quality Monitoring
Date:
March 13, 2012
Project: 23/19-0375
Orchard Lake is located in the northwest corner of Lakeville, in the southwest portion of the Black Dog
Watershed Management Organization (BDMWO). The lake is 243 acres in size, 75% of which is less
than 15 feet deep. The maximum depth is 33 feet. Average depth is 10 feet. Its tributary watershed is
2,277 acres, 100% of which is within the border of the city of Lakeville. Orchard Lake is primarily used
for fishing, but swimming, boating, and aesthetic and wildlife viewing are also popular recreational uses
of the lake.
Water Quality Goals
The BDWMO has classified Orchard Lake as a Category I “strategic” water body (i.e., used for
swimming and other direct contact recreational activities). A water quality action level of 1.8 meters
(5.9 feet) for summer-average Secchi disc transparency (SDT) was established by the BDWMO for
Orchard Lake in 2011. When a statistical trend analysis indicates that water transparency has degraded
beyond this level (i.e., summer-average SDT less than 1.8 meters), then a diagnostic study of potential
causes is recommended according to the BDWMO’s 2002 Watershed Management Plan.
Orchard Lake 2011 Water Quality Monitoring Activities
According to the BDWMO Watershed Management Plan, Orchard Lake should undergo management
level water quality monitoring once every three years. This monitoring program involves collecting more
detailed water quality data than Citizen-Assisted Monitoring Program (CAMP) data, such as collection of
water quality parameters at regular depth intervals. Management level water quality monitoring was
conducted by Barr Engineering (Barr) in 2011. Data collected in 2011 included collecting field probe
measurements of dissolved oxygen concentrations, water temperatures, specific conductivities, pH levels,
chlorophyll a concentrations, and oxidation/reduction potentials (ORP) at 1-meter depth intervals.
To:
From:
Subject:
Date:
Page:
Project:
Commissioners, Black Dog Watershed Management Organization
Barr Engineering Co.
2011 Orchard Lake Water Quality Monitoring
March 13, 2012
2
23/19-0375
Composite water samples were collected from the surface of Orchard Lake (0-2 meters depth) and sent to
a laboratory for analyses of total phosphorus, dissolved phosphorus, and chlorophyll a concentrations.
Water samples were collected at 1-meter intervals from 3 meters to 9 meters depth as well, and analyzed
for concentrations of total phosphorus and dissolved phosphorus. Tabulated water quality data collected
by Barr and CAMP in 2011 are included as attachments. Summer averages of SDT, total phosphorus, and
chlorophyll a are attached as Figures 1, 2, and 3, and incorporate data collected by Barr and CAMP.
Water quality in Orchard Lake has been variable over the past three decades. Orchard Lake summeraverage SDT varied from 1.1 to 2.4 meters (3.7 to 7.8 feet) over the period of 1989 to 2007 (see Figure 1).
The summer-average SDT in 2007 was 1.4 meters (4.6 feet), the third worst on record. However, the
following year, 2008, the summer-average SDT improved dramatically to 3.3 meters (10.9 feet). The
summer-average SDT of 2009 was the best on record: 3.7 meters (12.1 feet). Although not as good as
2009, the summer-averages of SDT in 2010 and 2011 were still good, at 3.0 meters (9.8 feet) and
2.6 meters (8.6 feet), respectively. The 2011 summer-average SDT of 2.6 meters was better than the
BDWMO action level of 1.8 meters established for Orchard Lake in 2011.
For the period of 2008-2011, summer averages of total phosphorus have been better than the Minnesota
Pollution Control Agency’s total phosphorus standard (40 µg/L) for lakes in the Central Hardwood Forest
Ecoregion. The summer-average for total phosphorus in Orchard Lake has exceeded 40 µg/L just once in
the most recent 10-year period – in 2007, when the summer average was 45.6 µg/L. Summer averages of
total phosphorus in Orchard Lake are included on Figure 2. Summer averages of chlorophyll a
concentrations are included on Figure 3.
As stated previously, concentrations of phosphorus were measured in water samples collected at regular
depth intervals in the lake as a way to gage the degree to which internal loading of phosphorus from lake
sediment might be occurring. Internal loading of phosphorus is a natural process that occurs in Minnesota
lakes, but the rate of internal loading can increase substantially in lakes that have excessive amounts of
phosphorus in the sediment. Depletion of levels of oxygen in the deeper water of the lake can also
increase the rate of internal loading of phosphorus in a lake. Data collected in 2011 demonstrated low
oxygen concentrations in the deeper waters of Orchard Lake. The low concentrations of oxygen were
most pronounced on August 8, when oxygen depletion was measured at depths of 4 meters (13 feet) and
deeper. However, phosphorus data collected at 1-meter depth intervals indicated internal loading of
P:\Mpls\23 MN\19\2319375\WorkFiles\2011_Orchard_Assessment\Orchard_Lake_2011_wq_memo.docx
To:
From:
Subject:
Date:
Page:
Project:
Commissioners, Black Dog Watershed Management Organization
Barr Engineering Co.
2011 Orchard Lake Water Quality Monitoring
March 13, 2012
3
23/19-0375
phosphorus from sediment in Orchard Lake was at a rate that would not be considered problematic for
water quality.
Conclusions and Recommendations for Orchard Lake
Improvements in lake water quality in recent years may be in part due to the City of Lakeville’s efforts to
manage Curlyleaf pondweed. Curlyleaf pondweed is a non-native aquatic plant that is known to increase
phosphorus concentrations in lakes during the summer months. Curlyleaf pondweed grows and dies
earlier in the season than native aquatic plants. The die-off of Curlyleaf pondweed in mid-summer
releases phosphorus into the lake, contributing to mid-summer algae growth and a reduction in lake
clarity. Additionally, aeration devices were installed in Orchard Pond, a wetland that contributes flow to
Orchard Lake. The aeration system is designed to reduce phosphorus export from Orchard Pond, and has
operated for the past two seasons.
Orchard Lake has experienced four seasons (2008-2011) in a row of good water quality. However, 2007
had the third worst summer average SDT on record for Orchard Lake. Barr recommends continued
monitoring in accordance with the BDWMO Watershed Management Plan: CAMP level monitoring
every year, with more intensive monitoring once every three years. Curlyleaf pondweed should also be
monitored in Orchard Lake. The City of Lakeville currently conducts monitoring of aquatic plants.
P:\Mpls\23 MN\19\2319375\WorkFiles\2011_Orchard_Assessment\Orchard_Lake_2011_wq_memo.docx
Figure 1
Lavon
OrchardLac
Lake
(Lakeville)
Summer Average Water Clarity
& Trend Analysis
0
0.0
MPCA Water Quality
Standard (1.4 m)
3.3
1
2
6.6
3
9.9
10‐Year Trend
4
13.2
5
16.5
Secchi
cchi Disc Transparency (ft)
cchi Disc Transparency (m)
Secchi
Action Level = 1.8 m (5.9 ft)
Maximum Lake Depth
Not Shown (10 m, 33 ft)
6
1975
1980
1985
1990
1995
2000
2005
P:\Mpls\23 MN\19\2319375\WorkFiles\2011_Orchard_Assessment\Orchard_trend_figs.xlsx
2010
19.8
2015
3/13/2012
Figure 2
Orchard Lake (Lakeville)
Summer Average Surface Total Phosphorus Concentrations
& Trend Analysis
BDWMO Category I & MPCA Deep Lake
80
70
60
Total Phosphoru
phorus Concentration (g/L)
MPCA Water Quality
Standard (40 µg/L)
50
40
30
20
10-Year Trend
10
0
1975
1980
1985
1990
1995
2000
2005
P:\Mpls\23 MN\19\2319375\WorkFiles\2011_Orchard_Assessment\Orchard_trend_figs.xlsx
2010
2015
3/13/2012
Figure 3
Orchard Lake (Lakeville)
Summer Average Surface Chlorophyll a Concentrations &
Trend Analysis
BDWMO Category I & MPCA Deep Lake
60
Chlorophyll
hyll a Concentration (g/L)
50
40
30
MPCA Water Quality
St d d (14 µg/L)
/L)
Standard
20
10-Year Trend
10
0
1975
1980
1985
1990
1995
2000
2005
P:\Mpls\23 MN\19\2319375\WorkFiles\2011_Orchard_Assessment\Orchard_trend_figs.xlsx
2010
2015
3/13/2012
Attachments
Table 1: Orchard Lake 2011 Water Quality Measurements Collected by Barr Engineering
Date
Max
Sample
Secchi
Depth (m) Depth (m) Depth (m)
Chl. a
(ug/L)
Turbidity
(NTU's)
D. O.
(mg/L)
Temp
(°C)
Sp. Cond.
(µmho/cm
@ 25°C)
Total P
(mg/L)
Total
Dissolved
Phosphorus
(mg/L)
pH
(S.U.)
ORP
(mv)
4/27/11
9.8
0-2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.3
1.9
4.0
2.9
-11.3
11.3
11.3
11.3
11.2
11.2
11.2
11.0
11.0
10.9
-8.2
8.2
8.2
8.2
8.2
8.2
8.2
8.0
8.0
7.9
-813
814
814
814
816
815
815
817
817
817
0.022
---0.025
0.026
0.023
0.022
0.049
0.020
0.024
0.022
---0.018
0.012
0.021
0.021
0.020
0.018
0.016
-8.3
8.3
8.3
8.4
8.4
8.4
8.4
8.4
8.4
8.4
-271
269
268
268
267
267
267
268
272
275
5/10/11
9.1
0-2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
8.6
2.0
6.7
1.8
-10.7
10.6
10.6
10.6
10.6
10.6
10.5
10.4
8.7
6.50
-13.3
13.3
13.2
13.0
12.9
12.8
12.8
12.5
9.6
9.4
-826
826
826
827
828
828
827
819
829
831
0.020
---0.021
0.019
0.020
0.018
0.018
0.021
0.020
<0.010
---0.013
0.013
0.012
0.011
0.011
<0.010
0.011
-8.2
8.3
8.4
8.4
8.4
8.5
8.5
8.4
8.2
8.0
-233
226
220
217
215
214
213
215
217
35
P:\Mpls\23 MN\19\2319375\WorkFiles\2011_Orchard_Assessment\WQ_Data\Orchard Lake WQ11 Data.xlsx
Page 1 of 6
Table 1: Orchard Lake 2011 Water Quality Measurements Collected by Barr Engineering
Date
Max
Sample
Secchi
Depth (m) Depth (m) Depth (m)
Chl. a
(ug/L)
Turbidity
(NTU's)
D. O.
(mg/L)
Temp
(°C)
Sp. Cond.
(µmho/cm
@ 25°C)
Total P
(mg/L)
Total
Dissolved
Phosphorus
(mg/L)
pH
(S.U.)
ORP
(mv)
5/24/11
9.1
0-2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
8.6
2.6
4.0
1.4
-9.4
9.3
9.2
9.2
9.2
9.1
6.8
4.3
3.1
1.7
-17.7
17.7
17.7
17.6
17.6
17.6
15.0
12.4
11.7
11.6
-819
819
819
819
819
819
830
829
832
832
0.024
---0.026
0.030
0.021
0.022
0.019
0.024
0.023
0.014
---0.017
0.021
0.012
0.015
0.013
0.013
0.015
-8.1
8.2
8.2
8.3
8.3
8.3
8.0
7.9
7.7
7.7
-124
121
119
117
117
117
123
126
127
117
6/6/11
8.6
0-2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
3.8
<0.50
1.2
-9.8
10.0
10.0
10.0
9.0
8.0
7.5
4.6
1.60
-23.3
22.5
22.2
20.8
19.9
19.2
18.7
17.3
14.8
-801
803
803
803
804
804
805
806
820
0.019
---0.022
0.020
0.019
0.019
0.019
0.021
0.013
---0.013
0.012
0.014
0.011
0.014
0.023
-8.5
8.5
8.5
8.5
8.4
8.4
8.3
8.1
7.9
-251
246
243
240
240
240
239
243
226
P:\Mpls\23 MN\19\2319375\WorkFiles\2011_Orchard_Assessment\WQ_Data\Orchard Lake WQ11 Data.xlsx
Page 2 of 6
Table 1: Orchard Lake 2011 Water Quality Measurements Collected by Barr Engineering
Date
Max
Sample
Secchi
Depth (m) Depth (m) Depth (m)
Chl. a
(ug/L)
Turbidity
(NTU's)
D. O.
(mg/L)
Temp
(°C)
Sp. Cond.
(µmho/cm
@ 25°C)
Total P
(mg/L)
Total
Dissolved
Phosphorus
(mg/L)
pH
(S.U.)
ORP
(mv)
6/20/11
9.0
0-2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
8.5
2.3
5.3
2.1
-9.0
9.1
9.1
9.1
9.1
8.0
5.5
1.1
0.24
0.19
-21.7
21.7
21.7
21.6
21.5
20.1
19.3
17.4
14.8
14.5
-785
785
785
785
785
789
789
812
827
829
0.023
---<0.010
0.019
0.019
0.018
<0.010
0.041
0.062
<0.010
---<0.010
0.012
<0.010
<0.010
<0.010
<0.010
<0.010
-8.5
8.5
8.5
8.5
8.5
8.4
8.1
7.7
7.6
7.6
-214
209
206
203
198
199
202
208
-16
-93
7/5/11
8.5
0-2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
2.8
5.3
1.3
-9.8
9.8
9.8
10.0
10.5
6.7
2.40
0.60
0.23
-26.7
26.7
26.7
26.5
22.4
20.8
19.7
18.4
15.5
-784
784
783
784
794
798
803
811
842
0.026
---0.023
0.025
0.025
0.024
0.026
0.110
0.017
---<0.010
0.017
0.017
0.015
0.017
0.019
-8.6
8.7
8.6
8.5
8.6
8.1
7.7
7.6
7.6
-206
201
193
194
197
207
216
218
-52
P:\Mpls\23 MN\19\2319375\WorkFiles\2011_Orchard_Assessment\WQ_Data\Orchard Lake WQ11 Data.xlsx
Page 3 of 6
Table 1: Orchard Lake 2011 Water Quality Measurements Collected by Barr Engineering
Date
Max
Sample
Secchi
Depth (m) Depth (m) Depth (m)
Chl. a
(ug/L)
Turbidity
(NTU's)
D. O.
(mg/L)
Temp
(°C)
Sp. Cond.
(µmho/cm
@ 25°C)
Total P
(mg/L)
Total
Dissolved
Phosphorus
(mg/L)
pH
(S.U.)
ORP
(mv)
7/18/11
9.0
0-2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
8.5
2.9
6.7
2.9
-8.4
8.4
8.4
7.6
5.4
2.1
0.29
1.10
0.24
0.18
-27.2
27.2
26.6
24.8
24.1
22.9
20.7
18.5
15.8
14.9
-744
742
728
744
741
712
794
811
841
851
0.019
---0.021
0.023
0.024
0.026
0.045
0.094
0.170
0.017
---0.014
0.016
<0.010
0.016
0.018
0.023
0.020
-8.3
8.3
8.3
8.2
7.9
7.5
7.4
7.4
7.4
7.6
-227
225
224
225
227
223
232
228
-115
-206
8/8/11
9.4
0-2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
1.8
9.3
2.8
-8.6
8.6
8.5
8.1
1.6
0.24
0.21
0.20
0.20
0.19
-27.5
27.5
27.4
27.3
27.0
24.1
20.4
17.9
15.9
15.0
-731
731
730
730
745
734
788
819
847
858
0.026
---0.027
0.028
0.032
0.031
0.043
0.110
0.460
<0.020
---0.020
<0.020
<0.020
<0.020
0.022
0.028
0.037
-8.5
8.5
8.4
8.4
7.6
7.5
7.5
7.5
7.6
7.6
-183
179
178
176
188
51
-3
-86
-168
-188
P:\Mpls\23 MN\19\2319375\WorkFiles\2011_Orchard_Assessment\WQ_Data\Orchard Lake WQ11 Data.xlsx
Page 4 of 6
Table 1: Orchard Lake 2011 Water Quality Measurements Collected by Barr Engineering
Date
Max
Sample
Secchi
Depth (m) Depth (m) Depth (m)
Chl. a
(ug/L)
Turbidity
(NTU's)
D. O.
(mg/L)
Temp
(°C)
Sp. Cond.
(µmho/cm
@ 25°C)
Total P
(mg/L)
Total
Dissolved
Phosphorus
(mg/L)
pH
(S.U.)
ORP
(mv)
8/22/11
9.4
0-2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
2.1
6.7
2.1
-8.1
8.1
7.8
7.6
7.7
7.1
0.37
0.26
0.22
0.21
-21.2
25.1
24.8
24.7
24.5
24.2
21.1
18.0
16.0
14.3
-743
744
745
744
743
743
793
834
864
887
0.044
---0.032
0.037
0.026
0.029
0.066
0.16
0.39
0.022
---0.018
<0.010
0.022
<0.010
0.031
0.031
0.031
-8.3
8.3
8.3
8.3
8.3
8.2
7.4
7.3
7.3
7.3
-211
208
206
203
200
200
209
-76
-156
-167
9/6/11
9.4
0-2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
2.6
11.0
2.9
-7.8
7.8
7.7
7.7
7.6
7.7
5.9
0.34
0.24
0.24
-22.0
22.1
22.1
22.0
22.0
21.7
21.6
18.3
16.6
15.5
-745
746
745
746
745
744
749
835
864
883
0.018
---0.025
0.022
<0.010
0.023
0.097
0.20
0.64
0.015
---0.017
0.016
<0.010
<0.010
0.029
0.026
0.021
-8.4
8.4
8.4
8.5
8.4
8.5
8.2
7.5
7.5
7.5
-203
200
198
196
195
193
194
-4
-134
-159
P:\Mpls\23 MN\19\2319375\WorkFiles\2011_Orchard_Assessment\WQ_Data\Orchard Lake WQ11 Data.xlsx
Page 5 of 6
Table 1: Orchard Lake 2011 Water Quality Measurements Collected by Barr Engineering
Date
9/19/11
Max
Sample
Secchi
Depth (m) Depth (m) Depth (m)
9.4
0-2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
2.0
Chl. a
(ug/L)
Turbidity
(NTU's)
D. O.
(mg/L)
Temp
(°C)
Sp. Cond.
(µmho/cm
@ 25°C)
6.7
3.0
-7.4
7.2
7.2
7.2
7.1
7.1
7.0
6.8
4.8
0.4
-17.4
17.4
17.4
17.4
17.4
17.4
17.4
17.3
17.2
15.2
-753
753
754
754
754
754
754
754
758
898
P:\Mpls\23 MN\19\2319375\WorkFiles\2011_Orchard_Assessment\WQ_Data\Orchard Lake WQ11 Data.xlsx
Total P
(mg/L)
Total
Dissolved
Phosphorus
(mg/L)
pH
(S.U.)
ORP
(mv)
0.023
---0.021
0.024
0.025
0.025
0.023
0.024
0.80
<0.010
---0.011
0.012
<0.010
0.012
0.014
0.011
0.018
-7.8
8.0
8.0
8.1
8.1
8.1
8.1
8.0
7.9
7.3
-193
193
192
192
192
192
192
192
196
-170
Page 6 of 6
Table 2: Orchard Lake 2011 CAMP Water Quality Data
Date
Secchi
Disc
(m)
Sample
Depth
(m)
Chl. A
(ug/L)
Temp.
(Celsius)
Total P
(mg/L)
4/24/2011
5/7/2011
5/27/2011
6/20/2011
6/29/2011
7/13/2011
7/29/2011
8/11/2011
8/22/2011
9/7/2011
9/23/2011
10/6/2011
10/19/2011
1.9
2.2
2.5
2.7
4.4
3.9
3.0
1.6
2.6
2.3
2.2
3.4
3.2
0
0
0
0
0
0
0
0
0
0
0
0
0
4.9
2.7
3.8
3.1
3.6
2.2
3.3
8.6
6.9
8.7
8.5
3.6
3.4
8.6
15.0
17.7
22.3
23.7
27.4
30.4
26.8
25.3
25.3
17.4
17.8
12.9
0.028
0.022
0.026
0.017
0.026
0.013
0.017
0.022
0.020
0.019
0.115
0.072
0.061
P:\Mpls\23 MN\19\2319375\WorkFiles\2011_Orchard_Assessment\WQ_Data\Orchard-11 wq DATA-CAMP.xlsx