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) results of the IDNR-county voluntary beach monitoring program in 2006, and (4) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to poor water transparency. Levels of indicator bacteria that exceed the state water quality criterion are also a concern 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 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) results of the IDNR-county voluntary beach monitoring program in 2006, and (4) information from the IDNR Fisheries Bureau.
EXPLANATION: Results of IDNR county beach monitoring from 2006 suggest that the Class A1 uses are assessed (evaluated) 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) as part of the IDNR county beach monitoring program. Because only one year of data was collected as part of this program these data are considered not sufficient to accurately characterize current water quality conditions, therefore the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). According to IDNR’s assessment methodology, two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses: (1) all thirty-day geometric means for the three-year assessment period are less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10% of the samples during any one recreation season exceeds the state’s single-sample maximum value of 235 E. coli orgs/100 ml. 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.” Also, if significantly more than 10% of the samples in any one of the three recreation seasons exceed Iowa’s single-sample maximum value of 235 E. coli orgs/100 ml, the Class A1 uses should be assessed as “partially 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 mean of 1 thirty-day period during the summer recreation season of 2006 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml), however, was less than 10% in 2006 (6%). According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest impairment (nonsupport/evaluated) of the Class A (primary contact recreation) uses.
For the 2008 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 2002 through 2006 (approximately 21 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 65, 56, and 63 respectively for Browns Lake. According to Carlson (1977) the Secchi depth and total phosphorus values place Browns Lake in between the eutrophic and hypereutrophic categories, while the index value for chlorophyll a places Browns Lake in the eutrophic cateogory. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, poor water transparency, and moderately 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 (9.6 mg/L) was the 22nd highest median of the 132 lakes sampled by the ISU and UHL surveys.
Data from the 2002-2006 ISU and UHL surveys also suggest that a small population of cyanobacteria exists at Browns Lake that do not likely cause an impairment to the water quality. These data show that cyanobacteria comprised 67% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (10.0 mg/L) was also the 33rd 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 2002-2006 show no violations of the Class B(LW) criterion for ammonia in 15 samples, or pH in 21 samples. The data show one violation of the Class B(LW) criterion for dissolved oxygen in 21 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.
