Assessment Comments

Assessment is based on results of (1) IDNR/UHL monthly water quality monitoring from 2000-02 near Davis City, (2) U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1997, and (3) IDNR/UHL biological (biocriteria) monitoring in 2001.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses are assessed (monitored) as “not supported) due to levels of indicator bacteria that violate state water quality criteria.   The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported / threatened." based on results of IDNR/UHL biological monitoring in 2001.   Fish consumption uses remain assessed (monitored) as "fully supported" based on results of fish contaminant monitoring in 1997.   This assessment is based on results of (1) IDNR/UHL monthly ambient monitoring conducted during the 2000-2002 assessment period at Davis City (STORET station 10270001 (formerly station 784089)), (2) IDNR/UHL biocriteria monitoring in 2001, and (3) results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring in 1997.  

EXPLANATION:  The Class A uses were assessed as "not supported" based on results of ambient monitoring for indicator bacteria (fecal coliforms).   For purposes of Section 305(b) assessments, DNR uses the long-term average monthly flow plus one standard deviation of this average to identify river flows that are materially affected by surface runoff.   According to the Iowa Water Quality Standards (IAC 1990:8), the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) does not apply "when the waters are materially affected by surface runoff."  Twenty of the 21 samples collected from the Davis City station during the 2000, 2001, and 2002 recreational seasons were collected at flows not materially affected by surface runoff.   The geometric mean level of indicator bacteria (fecal coliforms) in these 20 non-runoff-affected samples (214 orgs/100ml) is slightly above the Iowa Class A water quality criterion of 200 orgs/100ml.   However, five of the 20 samples (25%) exceeded the U.S.  EPA-recommended single-sample maximum value of 400 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting, if the geometric mean is greater than 200 orgs/100ml, the primary contact recreation uses are "not supported”; in addition, if more than 10% of the samples exceed the single-sample maximum value of 400 orgs/100 ml, the primary contact recreation uses are "partially supported" (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).   Thus, both the geometric mean of fecal coliforms (212 orgs/100ml) and the percentage of samples that exceed the single-sample maximum value of 400 orgs/100 ml (25%) suggest impairment of the Class A (primary contact recreation) uses of this river segment.  

Monitoring at the IDNR station at Davis City showed no violations of Class B(WW) water quality criteria during the 2000-2002 assessment period for dissolved oxygen (minimum value = 5.8 mg/l), pH (range = 7.3 to 8.6 units), or ammonia-nitrogen (maximum value = 0.90) in the 36 samples analyzed.   In addition, levels of pesticides in the nine samples analyzed, were all below the analytical level of detection.   These results suggest "full support" of the Class B(WW) aquatic life uses.  

Results of IDNR/UHL biocriteria monitoring, however, suggest that the Class B(WW) uses are "fully supported / threatened."  Data were collected in 2001 as part of the DNR/UHL stream biocriteria project.   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 2001 Fish IBI score was 32 (fair) and the BM-IBI score was 42 (fair).  The aquatic life use support was assessed as fully supported/threatened (=FST), based on a comparison of the F-IBI and BM-IBI scores with biological assessment criteria established specifically for the 2002 Section 305(b) report.   The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.   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 the Thompson River than do results from IDNR ambient monthly monitoring.   Thus, despite the results of IDNR ambient water quality monitoring that suggest "full support" of the Class B(WW) uses, these uses are assessed as "fully supported / threatened" based on results of biological monitoring in 2001.  

Fish consumption uses remain assessed (monitored) as "fully supported" based on results of EPA/DNR fish tissue (RAFT) monitoring in 1997 (see assessment for the 1998 report).   This site was again monitored as part of the 2003 RAFT program; the results from this monitoring, however, are not yet available.