Assessment Comments

Assessment is based on (1) IDNR ambient monitoring downstream from Charles City from 2000-02, (2) 2002 evaluated biocriteria data: BM-IBI = 51 (fair), and (3) fish tissue (RAFT) monitoring in 1997.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed (evaluated) as "partially supporting" based on results of biological monitoring in 2002.   The fish consumption uses remain assessed as "fully supporting" based on results of fish contaminant monitoring in 1997.   The primary sources of data for this assessment are (1) results of IDNR ambient monthly monitoring approximately 4 miles southeast of Charles City (STORET station 10340001), (2) results of IDNR/UHL biological (REMAP) monitoring in 2002, and (3) results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring 4 miles southeast of Charles City in 1997.  

EXPLANATION:  The Class B(WW) aquatic life uses were assessed (evaluated) as "partially supporting" due to results of biological monitoring in 2002 conducted 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 2002 BM-IBI score was 51 (fair).   The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the 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.   The assessment type is considered  “evaluated” (of lower confidence) because the size of the segment’s watershed exceeds the range of reference condition watershed sizes used to calibrate the benthic macroinvertebrate and fish bioassessment indexes.   For this assessment, the drainage area for the segment (approximately 1,000 mi2) exceeds the maximum drainage area cutoff (500 mi2) that IDNR has established for use of BMIBI and FIBI data.   Therefore, IDNR considers the aquatic life use impairments indicated by these data as “evaluated” assessments that are not appropriate for Section 303(d) listing (Category 5 of the Integrated Report).   IDNR does, however, consider these impairments as appropriate for listing under either Category 2b or 3b of the Integrated Report (waters potentially impaired and in need of further investigation).

Despite results of biological monitoring that suggest impairment of aquatic life uses, results of ambient water quality monitoring from 2000-2002 suggest "full support" of the Class B(WW) aquatic life uses due to the lack of violations of water quality criteria in (1) the 36 samples analyzed for conventional and toxic parameters (dissolved oxygen and ammonia-nitrogen) and (2) the nine samples analyzed for pesticides at the IDNR monthly monitoring station during the 2000-2002 assessment period.   These results suggest full support of the Class B(WW) uses.   The levels of pH in the sample collected on May 1, 2000, was 9.2 units and thus violated the Class B(WW) criterion of 9.0 pH units.   This violation occurred on a day when dissolved oxygen levels were relatively high (15.4 mg/l) and were at 140% of saturation.   These conditions suggest that high levels of primary productivity resulted in the high level of pH.   Because this violation is more related to natural conditions than to pollution, the occurrence of high levels of pH in this river segment is not seen as a water quality impairment.   This station was monitored for toxic organic compounds and pesticides from March 1996 to September 1998 as part of the USGS National Water Quality Assessment (NAWQA) program in the eastern Iowa river basins study unit (see assessment for the 2000 report).   Results of this monitoring showed that one of 27 samples violated the Class B(WW) chronic water quality criterion for DDE.   This violation led to the assessment of the Class B(WW) uses as "fully supported/threatened" for the 2000 reporting cycle.   IDNR monitoring during the 2000-2002 assessment period, however, showed no violations of the Class B(WW) water quality criterion for DDE in the 9 samples analyzed.   Thus, the 2002 assessment of the Class B(WW) uses was revised to "fully supported" (not threatened); this assessment remains valid for the 2004 reporting cycle.  

Fish consumption uses remain assessed (evaluated) as "fully supported" based on results of EPA/DNR fish tissue (RAFT) monitoring southeast of Charles City in 1997.   Results of this sampling showed that levels of all contaminants were below ½ of the respective FDA action levels and DNR levels of concern (see assessments for the 1998 and 2000 reports).   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 lower confidence).