Assessment Comments
Assessment based on results of (1) IDNR beach monitoring program, (2) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1996.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) were assessed as "fully supported." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supported / threatened." The Class C (drinking water) uses remain "not assessed." Fish consumption uses remain assessed as "fully supported." EXPLANATION: Levels of indicator bacteria at Lake Geode 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 Lake Geode beach, none of the 27 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 67 orgs/100ml in 2000 and 64 orgs/100 ml in 2001. One of the weekly samples in 2000 and in 2001 (6% of samples) exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml (maximum sample values were 2,200 orgs/100 ml in 2000 and 960 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 generally very low. The previous assessment of support of the Class B(LW) uses ("fully supported / threatened") was reviewed and approved by the DNR Fisheries Bureau. The Class C (drinking water) uses remained "not assessed" due to the lack of water quality information. Fish consumption uses remained assessed as "fully supported" based on results of EPA/DNR fish tissue (RAFT) monitoring in 1996 (see assessment developed for the 1998 report).
