Missouri River IA 06-WEM-1709

Assessment Comments

Assessment is based on (1) surveys of Nebraska fisheries biologists, (2) consultation with DNR staff, (3) ambient monitoring by Nebraska DEQ, and (4) results of USGS/ NASQAN monitoring at Omaha (I-80 bridge).

Basis for Assessment

Used monitoring results from the USGS WQ monitoring station at Omaha sampled as part of the National Stream-quality Accounting Network (NASQAN) to develop assessments of support of the Class B(WW) aquatic life uses and the Class C drinking water uses (data were not reported for indicator bacteria to assess Class A primary contact recreation uses).   In the 28 samples collected between January 1996 and September 1997, only one violation of a conventional or toxic contaminant was reported:  on July 18, 1996, the level of dissolved oxygen (4.8 mg/l) violated the Class B(WW) WQ criterion of 5.0 mg/l.   This single violation does not suggest impairment.   Levels of all but one Class C contaminant-- including nitrate+nitrite, alachlor, cyanazine, aresenic, cadmium, chromium, copper, lead, and zinc, were well below MCLs and Class C WQ criteria.   The only violation of a Class C WQ criterion was for atrazine in the sample collected on June 24, 1996; this sample contained 4.00 ug/l of atrazine, thus exceeding the 3.0 ug/l MCL.   The average level of atrazine in the 28 samples was 0.29 ug/l; the median 0.083 ug/l, with a range of  0.017 to 4.0 ug/l; 4% of the samples exceeded the MCL for atrazine.   Based on guidelines described on page 3-18 of the Supplement to the U.S.  EPA Guidelines for preparation of the 1998 Section 305(b) report, with modifications provided by DNR, assess support of the Class C drinking water uses as FST.   Despite the relatively good chemical water quality that fully supports both aquatic life and drinking water uses, the Class B(WW) aquatic life uses of this reach were assessed as PS due to habitat alterations and flow modifications that resulted from development of the river for navigation uses in the mid- Twentieth Century.  This assessment was presented to the Nebraska fisheries biologist for the Missouri River and to DNR/EPD staff with historical involvement with Missouri River issues.   These professionals agreed with the assessment that aquatic life uses are "partially supported."  The habitat alterations are due to channelization of the river, while the flow modifications are due primarily to control of river flows through the system of mainstem dams upriver from Sioux City.