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), (3) information from the IDNR Fisheries Bureau, and (4) results from the IDNR-county voluntary beach monitoring program from 2008, 2009, and 2010.
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 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), (3) information from the IDNR Fisheries Bureau, and (4) results from the IDNR-county voluntary beach monitoring program from 2008, 2009, and 2010.
EXPLANATION: Results of IDNR city/county beach monitoring from 2008 through 2010 suggest that the Class A1 uses are “not supported." Levels of indicator bacteria at Central Park Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2008 (13 samples), 2009 (13 samples), and 2010 (12 samples) as part of the IDNR beach monitoring program. 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) the geometric mean of the samples from each recreation season of 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 sampling season 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).
NOTE: Based on consultation with EPA Region 7 staff in 2011, IDNR’s methodology for assessing impairments based on the geometric mean water quality criterion was changed. Prior to the 2012 listing cycle, IDNR calculated geometric means for lakes based on a 30-day periods within the recreational season. Any violation of one of these 30-day periods within 3 years resulted in an impairment of the Class A1 uses of that lake. Because water quality standards do not identify a 30 day period but instead a recreational season, Region 7 concurred that the approach used for rivers and streams with less frequent bacteria data (seasonal geometric means) would be appropriate for identifying §303(d) impairments at lake beaches. Thus, for the 2012 listing cycle, IDNR identified primary contact recreation impairments for lakes when the geometric mean of all samples from the recreation season of a given year exceeded the geometric mean criterion. This does not impact the way IDNR assesses beaches for closure to protect the recreating public in the short term.
At Central Park Lake beach, the geometric mean from 2008 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 197 E. coli orgs/100 ml in 2008, 123 E. coli orgs/100 ml in 2009, and 30 E. coli orgs/100 ml in 2010. The exceedence of the geometric mean criterion suggests relatively high levels of bacteria at this beach and impairment of the Class A1 uses. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 62% in 2008, 38% in 2009 and 8% in 2010. The percentage of samples exceeding the single-sample maximum criterion was significantly greater than 10% of the samples in 2008 and 2009. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest “nonsupport” of the Class A1 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 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 70, 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 50th lowest median of the 134 lakes sampled by the ISU and UHL surveys.
Data from the 2006-2010 ISU and UHL surveys suggest that a large population of cyanobacteria exists at Central Park Lake that contributes to an impairment due to nuisance aquatic life at this lake. These data show that cyanobacteria comprised 99% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (73.8 mg/L) was also the 9th 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 for the 2006-2010 period. 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 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 2006-2010 show no violations of the Class B(LW) criterion for ammonia in 22 samples or for dissolved oxygen in 22 samples. The data for pH show 4 violations of the Class A1,B(LW) criterion for pH in 22 samples (18%). Based on IDNR's assessment methodology, these results are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class A1,B(LW) uses. However, Central Park Lake was assessed as “partially supporting” in the 2010 assessment/listing cycle due to significant violations of the pH criteria and therefore remains “partially supported” due to the continued violations. Based on IDNR’s assessment methodology 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the criterion are necessary to propose delisting based on pH violations. The pH violations at Central Park 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 at this lake.
