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
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported." The Class B(WW) aquatic life uses were assessed (evaluated) as "partially supported." The Class C (drinking water) uses were assessed (monitored) as "not supported." Fish consumption uses remain "not assessed." Sources of data for this assessment include (1) results of monitoring conducted from October 1999 through September 5, 2001 by the U.S. Geological Survey at station 06610000 at Omaha (part of the USGS NASQAN network) and (2) results of monitoring for indicator bacteria at Omaha from May through October 2000 by the Nebraska Department of Environmental Quality. EXPLANATION: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported." Levels of indicator bacteria at the USGS station at Omaha were monitored once per week during the primary contact recreation seasons (May through September) of 2000 as part of the Nebraska DEQ's ambient water quality monitoring program. According to U.S. EPA guidelines for determining support of primary contact recreation uses (U.S. EPA 1997b, page 3-35), the geometric mean of fecal coliform bacteria levels from at least five samples collected over a 30-day period is compared to the water quality standard of 200 fecal organisms/100ml. If a 30-day geometric mean exceeds 200 orgs/100 ml, the primary contact recreation uses are assessed as "not supported." In addition, the U.S. EPA guidelines state that if more than 10% of the total samples taken during any 30-day period has a bacterial density that exceeds 400 fecal coliform organsims/100 ml, the primary contact recreation uses are assessed as "partially supported." Due to the relatively low numbers of samples collected during any 30-day period (N=5), the use of single-sample maximum values to assess beaches is problematic. With less than 10 samples collected during any 30-day period at Iowa beaches, the occurrence of a single level of bacteria above the single-sample maximum value will result in more than 10% violation of the single-sample maximum value and thus suggest impairment of the primary contact recreation uses. The use of less than 10 samples in an assessment based on a critical value of 10% results in large probabilities (approximately 60%) of incorrectly concluding that an impairment exists. For this reason, the single-sample maximum value is not used to assess support of primary contact recreation uses with data from the IDNR beach monitoring program. At the Omaha station, seven of the 18 thirty-day geometric means for summer 2000 exceeded the Iowa Water Quality Standard of 200 orgs/100 ml (maximum geometric mean = 2,101 orgs/100 ml; maximum sample value = 5,200 orgs/100 ml). All weekly sample values from May 22 through June 28 exceeded the state standard of 200 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, if a 30-day geometric mean based on at least five samples is greater than 200 orgs/100ml, the primary contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, based on multiple geometric mean values greater than 200 orgs/100 ml in summer 2000, the Class A uses of this river segment are assessed as "not supported." The Class B(WW) uses remain assessed as "partially supported" based on the assessment developed for the 1998 and 2000 reports (i.e., habitat alterations and flow modifications that resulted from development of the river for navigation uses in the mid-Twentieth Century). This assessment was developed in consultation with the Missouri River fisheries biologist for the state of Nebraska. Results of USGS monitoring at the Omaha station during the 2000-2001 biennial period suggest relatively good water quality for support of aquatic life uses. Levels of pH, dissolved oxygen, ammonia-nitrogen were all below Class B(WW) criteria in the 29 samples collected, and levels of toxic metals were below Class B(WW) chronic criteria in the five samples analyzed during the 2000-2001 biennial period. Levels of pH ranged from 8.0 to 8.6, the minimum dissolved oxygen level was 5.7 mg/l, and the maximum level of ammonia-nitrogen was 0.84 mg/l. The only violation of a Class B(WW) criterion during this biennial period was for the DDT metabolite DDE. The sample collected on June 13, 2000, contained an "estimated" level of DDE of 0.002 ug/l; this level exceeds the Class B(WW) chronic criterion for DDE of 0.001 ug/l. According to U.S. EPA guidelines for Section 305(b) reporting (U.S. EPA 1997b, page 3-18), if data are collected monthly or more frequently, one violation of water quality criterion for a toxic contaminant does not suggest an impairment of the aquatic life uses. According to DNR's methodology for Section 305(b) reporting, the occurrence of this potential violation indicates that the Class B(WW) uses should be assessed as "fully supporting / threatened." Levels of other pesticides (for example, dieldrin, chlorpyrifos, lindane, and parathion) were all below their respetive Class B(WW) criteria in the 29 samples analyzed during the 2000-2001 biennial period. Although levels of dieldrin exceeded Class B(WW) criteria in two of the 29 samples collected during the 1998-1999 biennial period (thus suggesting "partial support" of aquatic life uses), dieldrin levels were below levels of detection (0.001 ug/l in 2000 and 0.005 ug/l in 2001) in all 29 samples collected during the 2000-2001 biennial period (see assessment for the 2000 report above). Class C (drinking water) uses were assessed as "not supported" due to violations of the human health water quality criterion for arsenic. Violations of the human health criterion for arsenic (0.18 ug/l) occurred in all 29 samples (100%) collected during the 2000-2001 biennial period. Arsenic levels ranged from 1.2 to 4.1 ug/l, with a mean level of 2.6 ug/l and a median level of 2.4 ug/l. According to IDNR guidelines for Section 305(b) assessments, if the mean level of a toxic metal or pesticide is greater than an MCL or human health criterion, "nonsupport" of the drinking water use is indicated. The mean level of arsenic during the biennial period (2.6 ug/l) is well above the human health criterion of 0.18 ug/l; thus, nonsupport of drinking water uses is indicated. Levels of nitrate, alachlor, cyanazine, and toxic metals were well below their respective Class C human health criteria, MCLs and/or MCLGs in the 29 samples collected from the Omaha stattion during the 2000-2001 biennial period. The maximum level of nitrate was 3.0 mg/l. One of the 29 samples, however, contained atrazine above the MCL: the sample collected on June 15, 2001 contained 4.4 ug/l of atrazine, thus exceeding the MCL of 3.0 ug/l. The mean and median levels of atrazine in the 29 samples collected during the 2000-2001 biennial period were 0.31 and 0.059 ug/l, respectively. Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is less than the MCL, but the MCL is exceeded in a single sample, the Class C (drinking water) uses of the source water should be assessed as "fully supported / threatened." Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river segment.
