Little Wall Lake IA 03-SSK-941
Hamilton County S10T86NR24W 1 mi. S of Jewell
- Cycle
- 2018
- Release Status
- Final
- Overall IR
- 4 - Water is impaired or threatened and a TMDL has been completed or is not needed.
- Trophic
- Hypereutrophic
- Trend
- Declining
- Created
- 4/9/2019 10:49:58 AM
- Updated
- 5/28/2019 3:14:04 PM
The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to aesthetically objectionable conditions caused by poor water transparency and nuisance algae blooms and by levels of pH that violate Iowa’s water quality criterion. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported” although nutrient loading, algal blooms, and re-suspension of sediment remain concerns at this lake. Fish consumption uses are assessed (monitored) as “fully supported.” Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2012 through 2016 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) results of U.S. EPA/IDNR fish contaminant monitoring in 2007.
For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Little Wall Lake are assessed (monitored) as "not supported" due to poor water transparency and aesthetically objectionable conditions caused by algae blooms based on information from the ISU lake survey. Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 83, 72, and 77 respectively for Little Wall Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Little Wall Lake in the Hypereutrophic category. These values suggest very high levels of chlorophyll a and suspended algae in the water, extremely poor water transparency, and extremely high levels of phosphorus in the water column. The data show 2 violations of the Class A1 criterion for pH in 15 samples (13%). Note:A TMDL for algae and turbidity at Little Wall Lake was prepared by DNR and approved by EPA in 2006. Because all Section 303(d) impairments identified for the 2016 assessment/listing cycle (algal growth and turbidity) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved). The level of inorganic suspended solids was extremely high at Little Wall Lake, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Little Wall Lake (20 mg/L) was ranked 134th among the 138 lakes by the ISU lake survey. Data from the 2012-2016 ISU lake survey suggest a large population of cyanobacteria exists at Little Wall Lake, which suggests the potential for an impairment due to nuisance aquatic life These data show that cyanobacteria comprised 93% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (123.4 mg/L) was ranked 133rd of the 138 lakes sampled. This median is in the worst 25% of the 138 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa's narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values from 2012-2016. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. Assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence. The Class B(LW) (aquatic life) uses are assessed (monitored) as "fully supported." Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), one violation of the criterion for dissolved oxygen in 15 samples(7%), and 2 violations of the criterion for pH in 15 samples(13%). Based on DNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of Little Wall Lake.