Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish contaminant monitoring in 1998 and 2007.
Basis for Assessment
SUMMARY: 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 “partially supported” due to violations of the Class B(LW) criterion for pH and secondarily due to nutrient loading, algal blooms, and re-suspension of sediment. 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 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish contaminant monitoring in 1998 and 2007.
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 2008 assessment/listing cycle (algal growth, pH, and turbidity) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).
EXPLANATION: For the 2008 assessment/listing cycle, results from the ISU and UHL lake surveys indicate that the Class A1 (primary contact recreation) uses of Little Wall Lake are assessed (monitored) as “not supported” due aesthetically objectionable conditions caused by poor water transparency and nuisance algae blooms and secondarily due to a high frequency of samples that exceed the Class A criterion for pH. Data from the ISU and UHL lake surveys, show poor water quality at Little Wall Lake. Using the median values from these surveys from 2002 through 2006 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 75, 70, and 69 respectively for Little Wall Lake. According to Carlson (1977) the chlorophyll a, and total phosphorus scores place Little Wall Lake in between the eutrophic and hypereutrophic categories, while the Secchi depth value places Little Wall Lake in the hypereutrophic category. These values suggest very high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and high levels of phosphorus in the water column.
Based on data from the ISU and UHL lake surveys, the level of inorganic suspended solids was very high at this lake and suggests impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Little Wall Lake was 11.0 mg/L, which was the 21st highest of the 132 monitored lakes.
Data from the 2002-2006 ISU and UHL surveys suggest a large population of cyanobacteria exists at Little Wall Lake, which also contributes to impairment at this lake. These data show that cyanobacteria comprised 97% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (75.3 mg/L) was also the 14th highest of the 132 lakes sampled. This median is in the worst 25% of the 132 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 2002-2006. 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. The assessment category for 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 “partially supported” due to a high frequency of violations of the Class B(LW) criterion for pH. Results of the ISU and UHL lake surveys from 2002 through 2006 show no violations of the Class B(LW) criterion for ammonia in 18 samples, or for dissolved oxygen in 25 samples. The pH data, however, show 5 violations of the Class A1,B(LW) criterion for pH in 25 samples (20%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment (partial support/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.
