Assessment Summary
The Class BLW use was assessed as "fully supported." All other designated uses were “not assessed.”
Assessment Explanation
Data Sources:
|
Data Source
|
Data Source ID
|
Data Type
|
Data Age
|
Site ID
|
Site
Name
|
Site
Description
|
|
Iowa DNR
|
6
|
WQ
|
CY 2016-2020
|
22740004
|
Virgin
Lake
|
|
Class A1 - Indicator Bacteria: No Data
Class A1 - pH
|
Site ID
|
Data Source ID
|
# Samples / # Years
|
Minimum Value
|
Maximum Value
|
Mean Value
|
# Violations
|
Violations needed for impairment
|
Violates Significantly >10% Rule?
|
Assessment Type
|
Support Level
|
|
22740004
|
6
|
23 / 5
|
7.35
|
9.38
|
8.24
|
2
|
5
|
NO
|
Monitored
|
Full
|
Class A1 - Lake 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
|
23 / 5
|
24
|
102
|
56
|
NO
|
Monitored
|
Full
|
|
22740004
|
6
|
Total Phosphorus TSI
|
23 / 5
|
71
|
127
|
85
|
NA
|
NA
|
NA
|
|
22740004
|
6
|
Inorganic suspended solids
|
13 / 3
|
0
|
4235
|
3
|
NA
|
NA
|
NA
|
- The TSI value for Secchi depth is not used to evaluate the
attainment of aquatic life use 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.
- DNR's assessment methodology indicates that at least two assessment/listing
cycles with a TSI value less than or equal to 63 are necessary to suggest
de-listing of an impairment. The chlorophyll a TSI value for the
2010 assessment listing cycle was 83, which based on DNR's assessment/listing
methodology indicated an impairment based on the narrative criteria protecting
against aesthetically objectionable conditions. Because the TSI value for the
2020 assessment/listing cycle was less than the trigger of 65 (2020 listing for
chlorophyll a was 56), and because the TSI value for the current listing cycle
is also 56, based on DNR's methodology, these values suggest "full
support" of the Class A1 uses protecting against aesthetically
objectionable conditions and the impairment (5a) for chlorophyll a is suggested
for delisting for the 2022 assessment/listing cycle.
Class BLW - Conventional Parameters:
|
Site ID
|
Data Source ID
|
Parameter Name
|
# Samples / # Years
|
Minimum Value
|
Maximum Value
|
Mean Value
|
# Violations
|
# Acute / Chronic Violations
|
Violations needed for impairment
|
Violates Significantly >10% Rule?
|
Assessment Type
|
Support Level
|
|
22740004
|
6
|
Ammonia
|
18 / 4
|
0.0036
|
0.6827
|
0.12
|
NA
|
0 / 1
|
4
|
NO
|
Monitored
|
Full
|
|
22740004
|
6
|
Dissolved Oxygen
|
23 / 5
|
0.81
|
10.82
|
7.7
|
2
|
NA / NA
|
5
|
NO
|
Monitored
|
Full
|
|
22740004
|
6
|
pH
|
23 / 5
|
7.35
|
9.38
|
8.24
|
2
|
NA / NA
|
5
|
NO
|
Monitored
|
Full
|
|
22740004
|
6
|
Temperature
|
23 / 5
|
9.2
|
27.9
|
21.6
|
0
|
NA / NA
|
5
|
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
|
23 / 5
|
0
|
6190
|
8
|
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.
- DNR's assessment methodology indicates that at least two
assessment/listing cycles with a median growing season total suspended solids concentration of 30 mg/L to suggest de-listing of the SAV
impairment. The total suspended solids concentration for the 2010
assessment listing cycle was 54, which based on DNR's assessment/listing
methodology indicated an impairment. Because the total suspended solids concentration for the 2020 assessment/listing
cycle was less than the trigger of 30 (2020 listing for SAV was 10), and
because the total suspended solids
concentration for the current listing cycle is also 8, based on DNR's
methodology, these values suggest "full support" of the Class A1 uses
protecting against aesthetically objectionable conditions and the impairment
(5a) for SAV is suggested for delisting for the 2022 assessment/listing cycle.
Class HH: No Data
