Assessment Comments
Assessment is based on (1) results from the IDNR beach monitoring program in 2000-01 (2) the Rock Creek Lake diagnostic/feasibility study (2000), (3) ISU lake surveys in 2000-01, & (4) information from the DNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "partially supporting." The Class B(LW) aquatic life uses remained assessed (evaluated) as "partially supporting." Fish consumption uses remained assessed as "fully supported." The Class C (drinking water) uses remain "not assessed." Sources of data for this assessment include (1) results of weekly monitoring at Rock Creek beach during the summer recreational seasons of 2000 and 2001 as part of the IDNR beach monitoring program, (2) results from the ISU lake surveys in 2000 & 2001, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), (4) results of fish contaminant monitoring conducted in 1998 as part of the Rock Creek Lake Restoration Diagnostic / Feasibility Study (Downing et al. 2000), and (5) information from the IDNR Fisheries Bureau. EXPLANATION: The Class A (primary contact recreation uses) are assessed (evaluated) as "fully supporting / threatened" based on results of the ISU lake surveys in 2000 and 2001. Results of IDNR beach monitoring in 2000 and 2001, however, suggest "full support" of these uses. Levels of indicator bacteria at Rock Creek beach were monitored once per week during the primary contact recreation seasons (April through October) of 2000 and 2001 as part of the IDNR beach monitoring program. According to U.S. EPA guidelines for determining support of primary contact recreation uses (U.S. EPA 1997b, page 3-35), the geometric mean of fecal coliform bacteria level from at least five samples collected over a 30-day period is compared to the water quality standard of 200 fecal organisms/100ml. If a 30-day geometric mean exceeds 200 orgs/100 ml, the primary contact recreation uses are assessed as "not supported." In addition, the U.S. EPA guidelines state that if more than 10% of the total samples taken during any 30-day period has a bacterial density that exceeds 400 fecal coliform organsims/100 ml, the primary contact recreation uses are assessed as "partially supported." Due to the relatively low numbers of samples collected during any 30-day period (N=5), the use of single-sample maximum values to assess beaches is problematic. With less than 10 samples collected during any 30-day period at Iowa beaches, the occurrence of a single level of bacteria above the single-sample maximum value will result in more than 10% violation of the single-sample maximum value and thus suggest impairment of the primary contact recreation uses. The use of less than 10 samples in an assessment based on a critical value of 10% results in large probabilities (approximately 60%) of incorrectly concluding that an impairment exists. For this reason, the single-sample maximum value is not used to assess support of primary contact recreation uses with data from the IDNR beach monitoring program. At Rock Creek beach, none of the 24 thirty-day periods during summers of 2000 and 2001 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. The maximum 30-day geometric means were 16 orgs/100ml in 2000 and 17 orgs/100 ml in 2001. None of the 35 weekly samples collected during 2000 and 2001 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml (maximum sample values were 120 orgs/100 ml in 2000 and 70 orgs/100 ml in 2001). These results suggest (1) full support of primary contact recreation uses at this beach and (2) that levels of bacteria at this beach are extremely low.
Despite the "full support" of Class A uses suggested by results of beach monitoring, results from the ISU surveys at Rock Creek Lake in 2000 and 2001 suggest that high levels of algal turbidity threaten full support of the Class A uses of Rock Creek Lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 66, 63, and 58, respectively. According to Carlson (1977), the index values for total phosphorus and chlorophyll-a place this lake in the range between eutrophic and hyper-eutrophic lakes; the index value for secchi depth is in the upper range of eutrophic lakes. These index values suggest relatively high levels of phosphorus, moderately high levels of chlorophyll-a (suspended algae), and somewhat better than expected water transparency. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 53, thus suggesting that phosphorus is the nutrient that limits production of suspended algae at this lake. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for impairing designated uses. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Rock Creek Lake was 3.9 mg/l. The relatively high TSI values for total phosphorus and chlorophyll-a suggest potential impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae. Presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a very small portion of the summertime phytoplankton community of this lake (less than 5% of wet mass). Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" (see assessments developed for the 1998 and 2000 reports above). The Class C uses remained "not assessed" due to a lack of water quality for Class C water quality parameters. Fish consumption uses remain assessed as "fully supported" based on results of fish contaminant monitoring conducted in 1998 (see assessment for the 2000 report).
