Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 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 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 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 2012 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 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 73, 69, and 71 respectively for Mariposa Lake. According to Carlson (1977) the Secchi depth and total phosphorus values place Mariposa Lake in the hypereutrophic category, while the chlorophyll a value places Mariposa Lake in between the eutrophic and hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and very 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 12.0 mg/L, which was the 11th highest of the 134 monitored lakes.
Data from the 2006-2010 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 82% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (113.0 mg/L) was also the 3rd highest 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 2006-2010. 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 2006-2010 there were no violations of the Class B(LW) criterion for dissolved oxygen in 22 samples. There were 3 violations of the Class B(LW) criterion for ammonia in 22 samples (14%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(LW) uses. There were, however, 7 violations of the Class A1,B(LW) criterion for pH in 22 samples (32%). Based on IDNR’s assessment methodology these violations were significantly greater than 10% of the samples and therefore constitute an impairment (partial support/monitored) of the Class A1 and B(LW) uses of Mariposa Lake. 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.
