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) information from the IDNR Fisheries Bureau, and (4) results from the IDNR-county voluntary beach monitoring program from 2006, 2007, and 2008.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to violations of the Class A1 criterion for indicator bacteria and violations of the Class A1 criterion for pH. The occurrence of aesthetically objectionable conditions caused by algal turbidity also contributes to the 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. Fish consumption uses remain “not assessed.” 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) information from the IDNR Fisheries Bureau, and (4) results from the IDNR-county voluntary beach monitoring program from 2006, 2007, and 2008.
EXPLANATION: Results of IDNR county beach monitoring from 2006, 2007, and 2008 suggest that the Class A1 uses are assessed (monitored) as "not supported." Levels of indicator bacteria at Central Park Lake beach were monitored approximately once per week during the primary contact recreation seasons (May through August) of 2006 (15 samples), 2007 (15 samples) and 2008 (13 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 Central Park Lake beach, the geometric means of 10 thirty-day period during the summer recreation seasons of 2007 and 2008 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. Five geometric means exceeded the standard in 2007 and five exceeded the standard in 2008. No geometric means exceeded the standard in 2006. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 13% in 2006, 27% in 2007 and 62% 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.
Results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program suggest that the Class A1 uses of Central Park Lake are “partially supported” due to aesthetically objectionable conditions caused by algal turbidity. Using the median values from these surveys from 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 66, 69, and 69 respectively for Central Park Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus index values all place Central Park Lake in between the eutrophic and hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, poor water transparency, and high levels of phosphorus in the water column.
The level of inorganic suspended solids at this lake was relatively low and does not suggest that water quality problems are caused by non-algal turbidity. The median level of inorganic suspended solids in Central Park Lake (2.7 mg/L) was the 43rd lowest median of the 132 lakes sampled by the ISU and UHL surveys.
Data from the 2004-2008 ISU and UHL surveys suggest that a moderately large population of cyanobacteria exists at Central Park Lake that contributes to the aesthetically objectionable conditions at this lake. These data show that cyanobacteria comprised 86% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (27.4 mg/L) was also the 37th highest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses for Central Park Lake are assessed (monitored) as “partially supported” due to violations of Iowa’s Class A1,B(LW) criterion for pH. The ISU and UHL lake surveys data from 2004-2008 show no violations of the Class B(LW) criterion for dissolved oxygen in 25 samples. The data show one violation of the Class B(LW) criterion for ammonia in 24 samples (4%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not constitute an impairment of the Class B(LW) uses. The data for pH, however, show 6 violations of the Class A1,B(LW) criterion for pH in 25 samples (24%). Based on IDNR’s assessment methodology, these violations are significantly greater than 10% of the samples and therefore suggest impairment of the Class B(LW) uses (partial support/monitored) of Central Park Lake. These violations likely reflect the high levels of primary productivity at Central Park 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 at this lake.
