Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) results of the IDNR-county voluntary beach monitoring program in 2006, 2007, and 2008, and (4) 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 levels of indicator bacteria that exceed the state water quality criterion. Poor water transparency also causes an impairment at this lake. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Fish consumption uses are “not assessed” due to a lack of recent fish contaminant monitoring. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) results of the IDNR-county voluntary beach monitoring program in 2006, 2007, and 2008, and (4) information from the IDNR Fisheries Bureau.
EXPLANATION: Results of IDNR county beach monitoring from 2006 through 2008 suggest that the Class A1 uses are assessed (monitored) as "not supported." Levels of indicator bacteria at Browns Lake beach were monitored approximately once per week during the primary contact recreation seasons (May through August) of 2006 (17 samples), 2007 (14 samples), and 2008 (14 samples) as part of the IDNR county beach monitoring program. According to IDNR’s assessment methodology, all thirty-day geometric means for the three-year assessment period must be less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses. If a 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
At Browns Lake beach, the geometric means of 3 thirty-day period during the summer recreation seasons of 2006 and 2008 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. One geometric mean exceeded the standard in 2006 and two geometric means exceeded the standard in 2008. There were no violations of the geometric mean standard in 2007. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 6% in 2006, 7% in 2007, and 29% in 2008. According to IDNR’s assessment methodology and U.S. EPA guidelines, the exceedences of the geometric mean criterion suggest impairment (nonsupport/monitored) of the Class A1 (primary contact recreation) uses.
For the 2010 reporting cycle, the Class A1 (primary contact recreation) uses for Browns Lake are assessed (monitored) as “partially supported” based on results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program. Using the median values from these surveys from 2004 through 2008 (approximately 20 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 67, 61, and 67 respectively for Browns Lake. According to Carlson (1977) the Secchi depth, chlorophyll a and total phosphorus values place Browns Lake in between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, poor water transparency, and high levels of phosphorus in the water column.
The levels of inorganic suspended solids at this lake were very high and suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Browns Lake (9.0 mg/L) was the 24th highest median of the 132 lakes sampled by the ISU and UHL surveys.
Data from the 2004-2008 ISU and UHL surveys also suggest that a small population of cyanobacteria exists at Browns Lake that do not likely cause an impairment due to nuisance aquatic life. These data show that cyanobacteria comprised 52% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (7.6 mg/L) was also the 40th lowest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses for Browns Lake are assessed (monitored) as “fully supported” based on information from the IDNR Fisheries Bureau and results from the ISU and UHL lake surveys. The ISU and UHL lake surveys data from 2004-2008 show no violations of the Class B(LW) criterion for ammonia in 20 samples, or pH in 20 samples. The data show one violation of the Class B(LW) criterion for dissolved oxygen in 20 samples (5%). Based on IDNR’s assessment methodology, this violation is not significantly greater than 10% of the samples and therefore does not constitute an impairment of the Class B(LW) uses of Browns Lake.
Fish consumption uses were “not assessed” due to a lack of recent fish contaminant monitoring at this lake. The most recent fish tissue monitoring was conducted in 1995. While these results suggest that levels of contaminants were low at Browns Lake, they are now too old (greater than 10 years) to be used for an assessment.
