Assessment Summary
The Class A1 use was assessed (monitored) as “not supported” due to aesthetically objectionable conditions caused by poor water transparency due primarily to high levels of non-algal turbidity. The Class BLW use was assessed (evaluated) as “WINOFI” due to high levels of (inorganic) turbidity related primarily to sediment re-suspension at this shallow lake, due to an invasive species introduction (Zebra Mussels), and due to cyanobacteria blooms. The Class BLW use was also assessed (evaluated) as “WINOFI” based on the results of a fish kill investigation. All other designated uses were “not assessed.”
Note: A TMDL for turbidity at Lower Gar Lake was prepared by DNR and approved by EPA in 2002. Because all Section 303(d) impairments identified for the current assessment/listing cycle [turbidity (algal and inorganic)] are addressed by the TMDL, this waterbody is placed in IR Category 4a.
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
|
22300012
|
Lower
Gar 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
|
|
22300012
|
6
|
12 / 4
|
8.1
|
8.7
|
8.4
|
0
|
4
|
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
|
|
22300012
|
6
|
Chlorophyll a TSI
|
15 / 5
|
36
|
72
|
53
|
NO
|
Monitored
|
Full
|
|
22300012
|
6
|
Secchi TSI
|
15 / 5
|
50
|
74
|
63
|
NO
|
Monitored
|
Full
|
|
22300012
|
6
|
Total Phosphorus TSI
|
15 / 5
|
52
|
76
|
64
|
NA
|
NA
|
NA
|
|
22300012
|
6
|
Inorganic suspended solids
|
15 / 5
|
0.3
|
22.4
|
4
|
NA
|
NA
|
NA
|
- 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 2004 assessment listing cycle was 66, 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 60), and because the TSI value for the
current listing cycle is 53, based on DNR's methodology, these values
suggest "full support" of the Class A1 use protecting against aesthetically
objectionable conditions and the impairment (5a) for chlorophyll a is suggested
for delisting for the 2022 assessment/listing cycle.
- Although the index value for Secchi is below the impairment
trigger of 65 for this assessment cycle, the Class A1 use of Lower Gar Lake remains "not supported" due to aesthetically objectionable conditions.
Based on DNR's methodology, the median TSI value for Secchi 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, Lower Gar
Lake will remain listed as "not supported" 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
|
|
22300012
|
6
|
Ammonia
|
12 / 4
|
0.0003
|
0.107
|
0.02
|
NA
|
0 / 0
|
4
|
NO
|
Monitored
|
Full
|
|
22300012
|
6
|
Dissolved Oxygen
|
13 / 5
|
6.9
|
12.5
|
9.2
|
0
|
NA / NA
|
4
|
NO
|
Monitored
|
Full
|
|
22300012
|
6
|
pH
|
12 / 4
|
8.1
|
8.7
|
8.4
|
0
|
NA / NA
|
4
|
NO
|
Monitored
|
Full
|
|
22300012
|
6
|
Temperature
|
13 / 5
|
16
|
28.1
|
22.7
|
0
|
NA / NA
|
4
|
NO
|
Monitored
|
Full
|
Class BLW - Lake Planktonic Cyanobacteria:
|
Site ID
|
Data Source ID
|
# Samples
|
Maximum Cyanobacteria Wet Mass
|
Maximum Cyanobacteria % Composition
|
Median Cyanobacteria Wet Mass
|
Median Cyanobacteria % Composition
|
Median Cyanobacteria Concentration = 75th Percentile
|
Assessment Type
|
Support Level
|
|
22300012
|
6
|
6
|
204.08
|
99.8%
|
67.42
|
94.9%
|
YES
|
Evaluated
|
3b
|
Class BLW - Fish Kill Assessment:
The Class BLW aquatic life use assessment based on the 2020 fish kill placed this segment in Category 3b. This assessment was “evaluated” because: (1) the cause of the fish kill was not identified or (2) the fish kill was attributed to a non-pollutant cause (e.g., winterkill). Segments assessed using “evaluated” assessments are placed in Category 3b and are added to the state list of Waters In Need Of Further Investigation (WINOFI). If no additional fish kills are reported in this segment for a 10-year period (2030), the Category will be changed from 3b to 3a (not assessed).
Details about the 2020 fish kill can be found here:https://programs.iowadnr.gov/fishkill/Events/995
Class BLW additional information:
This lake was infested with Zebra mussels in 2012 and the
population exploded in 2015. This invasive species will likely cause water
quality to degrade in the future. This potential impairment is not appropriate
for Iowa’s Section 303(d) list of impaired waters (Integrated Report Category
5) but is appropriate for IR Category 3b of Iowa’s Integrated Report and Iowa’s
list of waters in need of further investigation.
Class HH: No Data
