Lake Darling IA 03-SKU-924

Assessment Comments

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses were assessed as "fully supporting."  The Class B(LW) aquatic life uses remained assessed as "partially supporting"; fish consumption uses were considered "not assessed."  EXPLANATION:  The assessment of support of the Class A uses was changed from "partially supported" to "fully supported."  Levels of indicator bacteria at Lake Darling beach were monitored approximately twice per week during summer 1999 by DNR Parks, Recreation and Preserves Division as part of a beach monitoring program at 11 state-owned lakes.   Results of the 35 samples collected at this beach showed that levels of indicator bacteria (fecal coliforms) were relatively low, with the overall geometric mean (41 orgs/100 ml) below the state water quality criterion of 200 orgs/100 ml.   The maximum level of fecal coliforms in the 35 samples was 5,200 orgs/100 ml on June 1, 1999.   According to U.S.  EPA guidelines for determining "full support" of primary contact uses (U.S.  EPA 1997b, page 3-35), the geometric mean of fecal coliform bacteria levels should not exceed 200 orgs/100ml based on at least five samples in a 30-day period.   In addition, not more than 10% of the total samples taken during any 30-day period should have a density that exceeds 400 orgs/100 ml.   None of the sixteen 30-day periods during summer 1999 had geometric means (N = from 6 to 10 samples per period) greater than the state water quality criterion of 200 orgs/100ml.; the maximum 30-day geometric mean was 72 orgs/100ml.   One sample (5,200 orgs/100 ml on June 1) exceeded the EPA-recommended single sample maximum density for fecal coliform bacteria of 400 orgs/100ml.  One 30-day period contained this sample; the percentage of samples exceeding 400 orgs/100 ml for this period was 11%.   Given that fewer than 10 samples were collected during one of these periods, DNR does not consider this minor variance (11% violation) for the EPA criterion for full support (not more than 10% violation) to suggest an impairment to the primary contact recreation uses.   Thus, the Class A (primary contact recreation) uses were assessed as "fully supported."  The Class B(LW) aquatic life uses remain assessed as "partially supporting" based on review and approval of the previous (1998) assessments by the DNR Fisheries Bureau in 2000.   Fish consumption uses were considered "not assessed" due to the lack of recent fish tissue monitoring at this lake.   The most recent fish tissue monitoring was conducted in 1993 as part of the EPA/DNR fish tissue (RAFT) monitoring progream.   These data are now conisdered too old (greater than five years) for characterizing current water quality conditions.   Thus, the assessment of fish consumption uses was considered "not assessed."