Little Wall Lake IA 03-SSK-941

For the 2016 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 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 77, 69, and 70 respectively for Little Wall Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Little Wall Lake in between the Eutrophic and the Hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, extremely poor water transparency, and very high levels of phosphorus in the water column. The data show 2 violations of the Class A1 criterion for pH in 15 samples.

Note:  A TMDL for algae and turbidity at Little Wall Lake was prepared by IDNR 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 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 (10 mg/L) was ranked 117th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 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 92% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (98 mg/L) was ranked 130th 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 2010-2014. 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 2010-2014 show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and 2 violations of the criterion for pH in 15 samples(13%). Based on IDNR'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.

Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Little Wall Lake in 1998 and 2007.   The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes.   The fish contaminant data generated from the 1998 RAFT sampling conducted at Little Wall Lake show levels of contaminants are sufficiently high for concern and justify follow-up monitoring.   The composite samples of fillets from channel catfish and largemouth bass had generally low levels of contaminants.   The level of mercury in the sample of largemouth bass fillets (0.21 ppm), however, slightly exceeds the IDNR/IDPH interim trigger level of 0.20 ppm for a one meal per week consumption advisory.   Note:  the IDNR/IDPH trigger level for mercury for a one-meal/week consumption advisory was changed in 2007 to 0.3 ppm to be consistent with U.S.  EPA's recommendation for a tissue-based water quality criterion for mercury.  

According to IDNR’s assessment methodology, the single occurrence of contaminant above an advisory trigger level neither warrants issuance of an advisory nor indicates impairment of the fish consumption uses:  two consecutive samplings that show contaminant levels are above the trigger level in fillet samples are needed to justify issuance of an advisory.   But, this elevated level did indicate a concern and the need to conduct additional monitoring to better define contaminant levels in fish from this river segment.   Thus, follow-up monitoring was conducted in 2007 to better determine (1) levels of mercury in the edible portions (fillets) of fish in this lake and (2) whether a one-meal-per-week consumption advisory needs to be issued.   The level of mercury in the composite sample of fillets from largemouth bass collected as part of RAFT monitoring in August 2007 contained 0.167 ppm of mercury.   This level is well below the IDNR/IDPH trigger level of 0.3 ppm for a one meal/week consumption advisory.