Assessment Comments
Assessment is based on (1) results from the IDNR beach monitoring program from 2000-02 (2) the 2000 Rock Creek Lake diagnostic/feasibility study (included fish contaminant monitoring, (3) ISU lake surveys from 2000-02, & (4) information from the DNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supporting" due to levels of indicator bacteria at this lake’s beach area that exceed state water quality standards. Levels of chlorophyll-a and inorganic turbidity suggest threats to full support of these uses through reductions in water transparency. The Class B(LW) aquatic life uses remained assessed (evaluated) as "fully supporting / threatened." Fish consumption uses remained assessed (monitored) as "fully supported" based on fish contaminant monitoring conducted in 1998. The Class C (drinking water) uses remain "not assessed" due to lack of information upon which to base an assessment. Sources of data for this assessment include (1) results of weekly monitoring at Rock Creek beach during the summer recreational seasons of 2000, 2001, and 2002 as part of the IDNR beach monitoring program, (2) results from the ISU lake surveys in 2000-02, (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 (monitored) as "not supported" based on results of IDNR beach monitoring from 2000 through 2002. Levels of indicator bacteria at Rock Creek beach were monitored once per week during the primary contact recreation seasons (April through October) of 2000, 2001, and 2002 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. In summer 2002, however, three of the geometric means exceeded this standard. The maximum 30-day geometric means were 16 orgs/100ml in 2000, 17 orgs/100 ml in 2001, and 291 orgs/100 ml in 2002. 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). In summer 2002, however, three of the 30 samples collected exceeded 400 orgs/100 ml: June 18, 2002 (970 orgs/100 ml), July 1, 2002 (1,400 orgs/100 ml), and August 27, 2002 (5,400 orgs/100 ml). These high sample values led to geometric means that exceeded the water quality standard of 200 orgs/100 ml. These occurrence of geometric means greater than 200 orgs/100 ml at Rock Creek Lake beach suggest non-support of the primary contact recreation uses.
Despite the non-support of Class A uses suggested by results of beach monitoring, results from the ISU surveys at Rock Creek Lake from 2000 through 2002 suggest that high levels of algal turbidity only threaten full support of the Class A uses of Rock Creek Lake. Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 65, 63, and 62, respectively. According to Carlson (1977), the index values for all three parameters are in the range between eutrophic and hyper-eutrophic lakes. These index values suggest somewhat elevated levels of phosphorus and chlorophyll-a and marginally good water transparency. The respective TSI values for this lake are similar and thus suggest only a minor non-phosphorus limitation. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 40, thus suggesting that phosphorus (versus nitrogen) is the nutrient that limits production of suspended algae at this lake. Data from Downing et al. (2002) show relatively large populations of zooplankton species (Cladocerans) at this lake that graze on algae. Sampling in 2000 showed that the average summer mass of Cladoceran taxa (e.g., Daphnia) was 23.6 mg/l at Rock Creek Lake; this level was the 53rd highest of the 131 lakes sampled and thus suggests at least the potential for zooplankton grazers to limit algal production. The levels of inorganic suspended solids at this lake are moderately high and suggest a threat to full support of the designated uses. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l; the median level at Rock Creek Lake was 5.1 mg/l; this was the 57th highest of the 131 lakes sampled. Thus, the somewhat elevated TSI values for chlorophyll-a and Secchi depth, and the moderately high levels of inorganic suspended solids, suggest threats to full support of the Class A (primary contact) uses due to lowered water transparency.
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). The summer 2000 average mass of bluegreen algae (0.6 mg/l) was the 8th lowest of the 131 lakes sampled and thus does not suggest the potential to either threaten or impair designated uses.
Based on information from the IDNR Fisheries Bureau, the Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened." The Class C uses remained "not assessed" due to a lack of water quality for Class C water quality parameters. The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU survey from 2000-02 show that nitrate levels are low at this lake (maximum value = 7.6 mg/l; median = 2.0 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses. Fish consumption uses remain assessed (monitored) as "fully supported" based on results of fish contaminant monitoring conducted in 1998 (see assessment for the 2000 report).
