Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
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. Violations of the Class A1 criterion for pH also contribute to impairment at this lake. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to violations of the Class B(LW) criterion for pH. Sedimentation also remains a concern at this lake. Fish consumption uses remain “not assessed” due to a lack of information on which to base an assessment. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
Note: A TMDL for algae and turbidity at Mariposa Lake was prepared by IDNR and approved by EPA in 2004; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because all Section 303(d) impairments identified for the 2014 assessment/listing cycle (algal growth, turbidity, and pH) are addressed by the TMDL, this waterbody remains in IR Category 4a.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Mariposa Lake are “not supported.” Using the median values from these surveys from 2008-2012(approximately 14 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 77, 72, and 78 respectively for Mariposa Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values place Mariposa 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 level of inorganic suspended solids is very high at Mariposa Lake and suggests that non-algal turbidity contributes to the impairment at this lake. The median inorganic suspended solids concentration at Mariposa Lake was 19.5 mg/L, and ranked 130th of the 134 monitored lakes.
Data from the 2008-2012 ISU and UHL surveys suggest a large population of cyanobacteria exists at Mariposa Lake, which also contributes to impairment at this lake. These data show that cyanobacteria comprised 96% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (89.6 mg/L) and ranked 129th of the 134 lakes sampled. This median is in the worst 25% of the 134 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 2008-2012. 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 as “partially supported” based on information from the IDNR Fisheries Bureau and results from the ISU and UHL lake surveys. Information from the IDNR Fisheries Bureau suggests that sedimentation and algae blooms are problems at Mariposa Lake. The ISU and UHL lake survey results show poor chemical water quality at Mariposa Lake. During 2008-2012 there was one violation of the Class B(LW) criterion for dissolved oxygen in 14 samples (7%) and no violations of ammonia in 14 samples. There were also no violations of the Class A1,B(LW) criterion for pH in 14 samples. Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not constitute an impairment. However, Mariposa Lake was listed as "partially supported" for the 2012 assessment/listing cycle due to violation to the pH criterion. Based on IDNR's assessment methodology, 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the criterion are necesary to propose delisting based on pH violations. Therefore, Mariposa Lake will remain listed as "partially supported" for the 2014 assessment/listing cycle. The pH violations at Mariposa Lake likely reflect the excessive primary productivity at the lake and do not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
