Assessment Comments

Assessment is based on results of (1) IDNR/UHL biological (REMAP) monitoring in 2002 and (2) IDNR/UHL monthly ambient monitoring 4 mi N of Indianola from 2000-02.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life are assessed (monitored) as "not supported" based on results of IDNR/UHL biological monitoring in 2002.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.   The sources of data for this assessment include (1) the results of IDNR ambient monthly monitoring during the 2000-2002 assessment period at STORET station 10910001 (formerly station 200551) at the USGS gaging station approximately 1.5 miles west of Highway 65-69 near Indianola and (2) results of IDNR/UHL biological monitoring conducted in 2002 as part of the REMAP project.  

EXPLANATION:  Based on results of biological monitoring in 2002, the Class B(WW) aquatic life uses are assessed as "not supported."  Results of IDNR/UHL ambient chemical/physical monitoring, however, do not suggest an impairment.   None of the 36 samples collected during the 2000-2002 assessment period at the IDNR monthly station violated Class B(WW) water quality criteria for pH or ammonia-nitrogen; no violations occurred in the nine samples analyzed for pesticides and other toxic organic compounds.   However, one of the 36 samples violated the Class B(WW) criterion for dissolved oxygen:  the sample collected on December 13, 2000 contained 4.8 mg/l of dissolved oxygen.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), the percentage of violations for dissolved oxygen at this station (4.2%) does not suggest a water quality impairment (the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated).  

Despite results from fixed station ambient monitoring that suggest good water quality, results of IDNR/UHL biological monitoring in 2002 as part of the REMAP project suggest an impairment.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data.   The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species that were collected in the stream sampling reach.   The biological metrics were combined to make a fish community index of biotic integrity (F-IBI) and a benthic macroinvertebrate index (BM-IBI).   The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2002 Fish IBIs= 22 (poor), 38 (fair); BM-IBIs= 39 (fair), 24 (poor).  The aquatic life use support was assessed as not supporting (=NS), based on a comparison of the F-IBI and BM-IBI scores with biological assessment criteria established for previous Section 305(b) reports.   The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.   Previous assessments of this stream segment have indicated potential impacts due to alteration of aquatic habitats (see assessments for previous 305(b) reports).   Thus, problems with habitat quality may explain the relatively poor IBI values for fish and aquatic macroinvertebrates.   Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this stream segment.

As part of the REMAP project, diurnal dissolved oxygen/temperature monitoring conducted during July, September, and October 2002 found substantial fluctuation of d.o.  levels and tempererature, including nighttime d.o.  sags (<5 mg/L) and daytime temperatures exceeding 35 C (95 F).   These conditions were associated with very high estimated levels of community respiration and primary production.   Levels of BMIBI metrics that are sensitive to organic enrichment suggest a nutrient enrichment/algal growth-related water quality problem.   Habitat modification has resulted in a wide and shallow stream channel subjected to high solar input.   These physical factors probably contribute to excessive levels of algal growth, and extreme dissolved oxygen and termperature fluctuations.   Additional sampling data are needed to evaluate the extent, causes and sources of this water quality concern.