Des Moines River IA 04-LDM-1002

Assessment Comments

Assessment is based on results of (1) IDNR/UHL ambient monthly monitoring near Keokuk from 2000-02 and (2) U.S. EPA/IDNR fish contaminant monitoring in 1999 near Croton.

Basis for Assessment

SUMMARY:  Note:  this is the same assessment as used for the upriver segment (IA 04-LDM-0010-2).   The Class A (primary contact recreation) uses were assessed (monitored) as "fully supported," and the Class B(WW) aquatic life uses were assessed (monitored) as "fully supported"; both assessments are based on results of IDNR/UHL ambient water quality monitoring from 2000-2002.   Fish consumption remain assessed (monitored) as "fully supported" based on results of fish contaminant monitoring in 1999.   The sources of data for this assessment include (1) results of monthly monitoring from January 2000 through December 2002 at the IDNR ambient station located at the Highway 61 bridge south of Keokuk (STORET station 10560001; formerly station 410085) and (2) results of U.S.  EPA/IDNR fish contaminant (RAFT) monitoring in 1999 near Croton.  

EXPLANATION:  The Class A uses were assessed as "fully supported" based on results of ambient monitoring for indicator bacteria.   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."  Nineteen of the 21 samples collected from the Highway 61 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 19 non-runoff-affected samples (30 orgs/100ml) is well below the Iowa Class A water quality criterion of 200 orgs/100ml.   Only one of the 13 samples (8%) 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 no more than 10% of the samples exceed the single-sample maximum value of 400 orgs/100 ml, the primary contact recreation uses are "fully supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).  

The Class B(WW) aquatic life uses were also assessed as "fully supported" based on results of monitoring from the IDNR ambient station at Highway 61 near Keokuk from 2000 through 2002.   Monitoring at this station showed no violations of Class B(WW) water quality criteria for dissolved oxygen or ammonia-nitrogen in the 36 samples collected or for pesticides in the nine sampled analyzed.   Levels of pH, however, occasionally violated the Class B(WW) criterion of 9.0 pH units.   Three of the 36 samples (8%) had pH values greater than 9.0 units, with a maximum value of 9.2 units.   All violations occurred on days when dissolved oxygen levels were at or above 100% saturation:  March 6, 2000 (pH=9.2; % DO saturation=120%), November 6, 2000 (pH=9.2; DO saturation=100%), and August 1, 2002 (pH=9.2; DO saturation=120%).   These conditions suggest that high levels of primary productivity may have resulted in the high levels of pH observed.   Because these violations are likely 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.   In addition, according to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b:  page 3-17), a violation frequency for conventional parameters (including pH) of 10% or less does not suggest an impairment of aquatic life uses.  

Fish consumption uses remain assessed (monitored) as "fully supported" based on results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring in 1999 near Croton.   This monitoring showed that levels of all contaminants in composite fillet samples from common carp and freshwater drum collected from the Des Moines River near Croton were well below ½ of the respective FDA action levels as well as DNR levels of concern (see also assessment for the 2000 report).