Assessment Comments
Assessment based on 1996 biocriteria: 20 fish spp., 5 fams., Fish IBI= 51(good), BM-IBI= 50(fair); also on TMDL monitoring in 2001.
Basis for Assessment
The Class B(WW) aquatic life uses remain assessed (evaluate) as "partially supported," and fish consumption uses remain assessed as fully supported. Sources of data for this assessment include (1) results of biological monitoring in 1996 and (2) monthly monitoring conducted near 1 mile SW of Hudson from March to November 2001 by IDNR/UHL in support of TMDL development for the Class A (primary contact recreation) segment of Black Hawk Creek (IA 02-CED-0370-1). EXPLANATION: The Class B(WW) uses remain assessed as "partially supported" based on results of biological monitoring conducted in 1996 (see assessment for the 2000 report). The assessment category for this impairment, however, is changed from "monitored" to "evaluated." This assessment was developed for the 1998 report (see above) and was based on results of a 1996 IDNR biocriteria sampling. However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively low confidence). Results of monitoring conducted in 2001 in support of TMDL development for the downstream segment of Black Hawk Creek, however, show no violations of state water quality criteria for conventional parameters in the nine monthly samples collected between March and November 2001. Biological monitoring is better able to reflect cumulative impacts of water quality over time and thus is believed to more accurately represent water quality conditions of this segment of Black Hawk Creek than do results from the monthly TMDL monitoring site. Levels of fecal coliform bacteria in the six non-flow affected samples from the TMDL station 1 mile SW of Hudson (Site 17) were very high, with a six-sample geometric mean for the 2001 recreational season of 1,005 orgs/100 ml; all six samples exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100ml. Fish consumption uses were assessed as "fully supported" based on results of EPA/DNR fish tissue (RAFT) monitoring in 1998 near Hudson that showed levels of contaminants in composite samples of fillets from channel catfish were less than ½ the respective FDA action levels and DNR levels of concern. Levels of the primary contaminants of concern in the 1998 sample were as follows (all values are wet weight): technical chlordane: 0.055 mg/kg; dieldrin: 0.096 mg/kg; PCB-Aroclor 1260: 0.016 mg/kg (Aroclors 1248 and 1254 were not found above detection levels); and mercury: 0.102 mg/kg.
