Assessment Comments
Assessment is based on: (1) results of monitoring conducted from January 2008 through November 2010 by the U.S. Geological Survey at station 06610000 at Omaha (part of the USGS NASQAN monitoring network) and (2) information from Nebraska fisheries biologists.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are considered “not assessed” due to the lack of information upon which to base an assessment. The Class B(WW1) aquatic life uses remain assessed (evaluated) as "partially supported" (IR Category 4c) based on information from local fisheries biologists on impacts related to flow modification and habitat alterations in this segment of the Missouri River. The Class C (drinking water) uses are assessed (monitored) as "not supported" (IR Category 5a) due to levels of arsenic that exceed state water quality criteria to protect human health from arsenic in fish and water. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment. Sources of data for this assessment include (1) results of monitoring conducted from January 2008 through December 2010 by the U.S. Geological Survey at station 06610000 at Omaha (part of the USGS NASQAN monitoring network) and (2) information from Nebraska fisheries biologists.
EXPLANATION: The Class A1 (primary contact recreation) uses are “not assessed” due to the lack of information upon which to base an assessment.
The Class B(WW1) aquatic life uses remain assessed (evaluated) as "partially supported" (IR Category 4c, non-pollutant stressor) based on the assessment developed for previous reporting cycles (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 ambient water quality monitoring conducted by USGS during the 2008-2010 assessment period, however, suggest good water quality in this river segment. No violations of Class B(WW1) criteria occurred in the 37 samples analyzed during this period for ammonia (maximum of 0.78 mg/l), dissolved oxygen (minimum = 6.0 mg/L) or pH (range from 8.0 to 8.7 pH units). Levels of toxic metals (e.g., arsenic) and pesticides (e.g., dieldrin) in the 37 samples collected were below their respective Class B(WW1) criteria.
Similar to previous assessments for this river reach, the Class C (drinking water) uses are assessed (monitored) as "not supported" (IR Category 5a) due to violations of the state human health water quality criterion for arsenic (includes contributions of arsenic from consuming fish as well as from drinking water). Violations of Iowa’s human health criterion for arsenic (0.18 ug/l) occurred in all 37 samples (100% violation) collected during the 2008-2010 assessment period. Arsenic levels ranged from 1.6 to 4.6 ug/l, with a mean level of 2.7 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 2008-2010 period (2.70 ug/l) is well above the human health criterion of 0.18 ug/l; thus, continued “nonsupport” of drinking water uses is indicated. The levels of arsenic seen during the 2008-2010 assessment period are consistent with levels seen in previous assessment periods with mean levels over the last 10 years (2000-2010) ranging from 2.3 to 2.6 ug/l (from 31 to 45 samples analyzed per assessment period) and with 100% of the samples analyzed in violation of the HH criterion of 0.18 ug/l. Levels of nitrate (maximum of 5.5 mg/l), atrazine (maximum 0.3 ug/l), and alachlor (maximum of 0.024 ug/l) were well below their respective Class C human health criteria and MCLs in the 37 samples collected from the Omaha station during the 2008-2010 assessment period. Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is less than the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported." The overall assessment for Class C (drinking water) uses, however, remains “not supported” (monitored) due to levels of arsenic that routinely exceed Iowa’s Class C (i.e., Human Health (fish plus water)) criterion.
Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river segment.
