Assessment Summary
The Class BLW use remains assessed as "not supported" due to high levels of total suspended solids that leads to an impairment of the narrative criteria protecting against undesirable or nuisance aquatic life. High levels of algae at this wetland also suggest impairment due to aesthetically objectionable conditions. All other designated uses are “not assessed”.
Assessment Explanation
Sources of
Data:
|
Data Source
|
Data Source ID
|
Data Type
|
Data Age
|
Site ID
|
Site Name
|
Site Description
|
|
Iowa DNR
|
6
|
WQ
|
CY 2015-2018
|
22740004
|
Virgin Lake
|
|
Class BLW - TSI's
|
Site ID
|
Data Source ID
|
Parameter Name
|
# Samples / # Years
|
Minimum Value
|
Maximum Value
|
Median Value
|
Median TSI Violation
|
Assessment Type
|
Support Level
|
|
22740004
|
6
|
Chlorophyll a TSI
|
18 / 4
|
24
|
102
|
56
|
NO
|
Monitored
|
Full
|
|
22740004
|
6
|
Total Phosphorus TSI
|
18 / 4
|
74
|
127
|
86
|
NA
|
NA
|
NA
|
|
22740004
|
6
|
Inorganic suspended solids
|
18 / 4
|
0
|
4235
|
4
|
NA
|
NA
|
NA
|
- The TSI value
for Secchi depth is not used to evaluate the attainment of aquatic life goals
in shallow lakes. Due to the depth of these shallow lakes, TSI values for
Secchi depth can be misleading. In some instances, the Secchi disk remains
visible at the bottom of the lake and the depth of the lake is recorded as the
Secchi depth. In these instances, water clarity may be sufficient to support
the Class B(LW) uses, but the index value is limited by the depth of the lake.
Thus the combination of total suspended solids and chlorophyll a will be used
to determine whether or not the Class B(LW) uses are impaired in these shallow
systems.
- Although the index value for Chlorophyll a is below the impairment trigger of 65 for this
assessment cycle, Virgin Lake was listed as partially supporting its Class A1
uses due to aesthetically objectionable conditions. Based on DNR's
methodology, the median TSI value for Chlorophyll a must be 63 or less for two
consecutive assessment/listing cycles before a lake can be removed from the
state's Section 303(d) list (IR Category 5). Therefore, Virgin Lakewill
remain listed as "not supported" for the 2020 assessment/listing
cycle.
Class BLW -
Conventional Parameters:
|
Site ID
|
Data Source ID
|
Parameter Name
|
# Samples / # Years
|
Maximum Value
|
Mean Value
|
Median Value
|
# Acute / Chronic Violations
|
% Acute/ Chronic Violations
|
Significantly >10% Violations
|
Assessment Type
|
Support Level
|
|
22740004
|
6
|
Ammonia
|
13 / 3
|
0.6827
|
0.15
|
0.07
|
0 / 1
|
0% / 8%
|
NO
|
Monitored
|
Full
|
|
22740004
|
6
|
Dissolved oxygen
|
13 / 3
|
10.43
|
6.88
|
8.26
|
NA / 3
|
NA / 23%
|
NO
|
Monitored
|
Full
|
|
22740004
|
6
|
pH
|
13 / 3
|
9.5
|
8
|
7.81
|
2 / NA
|
15% / NA
|
NO
|
Monitored
|
Full
|
|
22740004
|
6
|
Temperature
|
13 / 3
|
27.9
|
21.1
|
21.8
|
0 / 0
|
0% / 0%
|
NO
|
Monitored
|
Full
|
Class BLW -
Protection of Submerged Aquatic Vegetation:
|
Site ID
|
Data Source ID
|
Parameter Name
|
# Samples / # Years
|
Minimum Value
|
Maximum Value
|
Median Value
|
Median Value >= SAV
|
Assessment Type
|
Support Level
|
|
22740004
|
6
|
Total suspended solids
|
18 / 4
|
0
|
6190
|
10
|
NO
|
Monitored
|
Full
|
- Guidelines for
wetland assessment from the Upper Mississippi River Conservation Committee’s
Water Quality Technical Section (2003) state that an average total suspended
solids concentration during the growing season of less than 30 mg/L is
necessary to provide sufficient water clarity to support the growth of
submersed aquatic vegetation in wetlands and shallow lakes. High levels of
total suspended solids impede the growth of submersed aquatic vegetation (SAV).
Submersed aquatic vegetation is critical to the health of wetland and shallow
lake ecosystems. The elimination of SAV can degrade habitat quality such that
undesirable aquatic species such as cyanobacteria, common carp and fathead
minnows dominate. As such, the suppression of SAV constitutes a violation of
Iowa’s narrative water quality criteria protecting against undesirable or
nuisance aquatic life.
- According to
DNR's methodology, a lake must meet water quality standards for two
consecutive cycles to suggest de-listing.Therefore, Virgin Lake
will remain listed for the 2020 cycle as "not supported" its Class
BLW uses due to Total suspended solids.
Class HH - No Data
Additional Notes:
- Monitoring of fish and aquatic vegetation populations at Virgin Lake by DNR staff also suggests that the high levels of total suspended solids and algae have adverse effects on fish populations and limit the production of rooted aquatic vegetation in this wetland. These results further indicate that the aquatic life uses of this wetland should be assessed as "not supported."
