South Skunk River IA 03-SSK-934

Assessment Comments

Assessment is based on results of IDNR/UHL biological monitoring in 1997-98, and EPA/IDNR fish tissue monitoring in 1998.

Basis for Assessment

The October 1997 bioassessment results from 4 DNR bioassessment sites located in the vicinity of the Story City STP wastewater discharge, and 1 location in the South Skunk River Greenbelt were used to update the assessment for the 2000 report.   Based on an analysis of benthic macroinvertebrate and fish community health indexes, in addition to water and sediment sampling results from the Story City STP mixing zone, the assessment was downgraded from fully supporting (=FS) to fully supporting threatened (=FS/T).   Sampling in the South Skunk River greenbelt approximately 8 miles downstream from the Story City STP indicated aquatic life uses were fully supported.   However, a significant decline in biological integrity was found in the stream reach immediately downstream from the STP mixing zone.   A higher than normal frequency of diseases fish was found in the downstream reach and in the stream mixing zone.   A deposit of organic solids was observed covering the stream bottom in the mixing zone of the stream.  Sediments from the mixing zone contained extremely high levels of ammonia, as well as elevated levels of several metals and synthetic organic compounds.   The organic deposit was caused by mechanical malfunctions during the final stages of the sewage treatment process.   The mechanical problems were fixed and releases of organic solids to the stream were curtailed.   Follow-up sampling in 1998 indicated a reduction in the frequency of diseased fish immediately downstream from the STP discharge.   The deposit of organic solids on the stream bottom was gone, apparently flushed downstream by high flows.   A second follow-up sampling event conducted in September, 2000 confirmed the incidence of diseased fish was remaining within the normal range, and no additional organic solids were accumlating on the stream bottom.  The South Skunk River has very poor flow stability and the biological community is particularly vulnerable to organic waste loading from agricultural and/or point sources during low flow periods.  Fish consumption uses were assessed as "fully supported."  EXPLANATION:  Fish consumption uses were assessed as "fully supported" based on results of EPA/DNR fish tissue (RAFT) monitoring in 1998 near Story City.   This monitoring showed that all contaminant levels in the composite samples of fillets from common carp and smallmouth bass were less than ½ of the respective FDA action levels or DNR levels of concern.