Assessment Comments
Assessment is based on: results of (1) water quality monitoring conducted from 2006-2008 by Iowa State University (under contract with the U.S. Army Corps of Engineers) (ISU/ACOE) upstream from Red Rock Reservoir (ISU/ACOE Station 7 at County Road S35 near Runnells (STORET Station 17770003) as part of the Des Moines River Water Quality Study and (2) IDNR/UHL ambient city monitoring downstream from Des Moines near Runnells from 2006 through 2008 (STORET station 10770003).
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supporting" due to levels of indicator bacteria (E. coli) that exceed state water quality standards. The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient chemical/physical water quality monitoring. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this assessment segment. The assessments of support of the beneficial uses are based on results of (1) water quality monitoring conducted from 2006-2008 by Iowa State University (under contract with the U.S. Army Corps of Engineers) (ISU/ACOE) upstream from Red Rock Reservoir (ISU/ACOE Station 7 at County Road S35 near Runnells (STORET Station 17770003) as part of the Des Moines River Water Quality Study (see Lutz and Francois 2007, Lutz and Francois 2008, and Lutz and Steffen 2009) and (2) IDNR/UHL ambient city monitoring downstream from Des Moines near Runnells from 2006 through 2008 (STORET station 10770003).
Note: A TMDL for bacteria was prepared by IDNR and approved by U.S. EPA in March 2010. Because this TMDL covers all the identified impairments for the assessment segment, this segment is moved to IR Category 4a (TMDL approved).
EXPLANATION: The Class A1 primary contact recreation uses are assessed (monitored) as "not supported" based on results of monitoring for indicator bacteria (E. coli). Monitoring results from both ISU/ACOE and IDNR stations at Des Moines and near Runnells showed that the overall geometric means for E. coli (indicator) bacteria in summer periods of 2006, 2007, and 2008 (183 orgs/100 ml from IDNR/UHL; 387 orgs/100 ml from ISU/ACOE) were greater than the state WQ criterion of 126 organisms/100 ml. In addition, moderately high percentages of samples exceeded Iowa’s single sample maximum criterion for E. coli (235 orgs/100 ml): seven of 22 samples (32%) at the IDNR/UHL station, and five of nine samples (56%) at the ISU/ACOE station, exceeded this value. According to U.S. EPA guidelines for Section 305(b) reporting, and according to IDNR’s assessment/listing methodology, if the geometric mean of E. coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b).
Results from the both the ISU/ACOE and IDNR/UHL monitoring stations at Runnells suggest that the Class B(WW1) aquatic life uses should be assessed (monitored) as "fully supported." No violations of Class B(WW1) water quality criteria for conventional parameters (dissolved oxygen, pH, or ammonia-nitrogen) occurred in the 34 samples collected at the IDNR/UHL station during the 2006-2008 assessment period. Also, none of the four samples analyzed for toxic metals, and neither of the two samples analyzed for pesticides, exceeded the respective Class B(WW1) criteria.
At the ISU/ACOE station, no violations of Class B(WW1) water quality criteria for ammonia or pH occurred in the 66 samples collected during the 2006-2008 assessment period. One of these samples, however, exceeded the Class B(WW1) criterion for dissolved oxygen. The sample collected on August 14, 2006, contained a dissolved oxygen level of 4.96 mg/l, thus very slightly violating the Class B(WW1) criterion of 5.0 mg/l. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), a violation frequency of less than 10 % for conventional parameters such as dissolved oxygen nonetheless suggests "full support" of aquatic life uses. Thus, the percentage of violation of the dissolved oxygen criterion at this station (2%) does not suggest an impairment of aquatic life uses in this stream segment.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this assessment segment.
