Assessment Comments
Assessment is based on: (1) results of monitoring conducted from January 2010 through December 2012 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 remain “not assessed” (IR 3a) 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 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 assessment of support of the Class C (drinking water) uses is changed from "not supporting" (IR 5a) to “fully supporting” (IR 2a) due to an error in assessment. This change in assessment represents a de-listing of the Class C impairment. Fish consumption uses remain "not assessed" (IR 3a) 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 2010 through December 2012 by the U.S. Geological Survey at station 06610000 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. As they have in all previous assessment periods, results of ambient water quality monitoring conducted by USGS during the 2010-2012 assessment period, however, suggest good water quality in this river segment. No violations of Class B(WW1) criteria occurred in the samples analyzed during this period for ammonia (38 samples, maximum of 0.35 mg/l), dissolved oxygen (48 samples, minimum = 6.0 mg/L), pH (47 samples, range from 8.0 to 8.7 pH units), chloride (38 samples), or sulfate (38 samples). No violations of the Class B(WW1) criterion for temperature occurred in the 323 samples collected during the 2010-12 period. Levels of toxic metals (e.g., arsenic and selenium) and pesticides (e.g., chlorpyrifos and dieldrin) in the 36 samples collected were below their respective Class B(WW1) criteria.
The assessment of support of the Class C (drinking water) uses is changed from “partially supporting (IR 5a) to “fully supporting” (IR 2a) based on an assessment error. This change represents a de-listing of the Class C impairment for arsenic. Iowa DNR’s previous assessments of support of the Class C (drinking water) uses in this river segment were based on violations of the state human health water quality criterion for arsenic (includes contributions of arsenic from consuming fish as well as from drinking water). Monitoring data from USGS station 06610000 at Omaha have shown that levels of arsenic in this river segment average between 2 and 3 ug/l, thus far exceeding Iowa’s human health criterion (fish + water) of 0.18 u g/l. For the recent (2010-12) period, arsenic levels in the 37 samples collected ranged from 1.5 to 4.6 ug/l, with a mean level of 2.5 ug/l; all samples exceeded Iowa’s human health criterion. These results are consistent with results of past monitoring periods with mean levels of arsenic over the last 12 years (2000-2012) 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. Iowa DNR has incorrectly identified impairments of the drinking water use by comparing USGS data for dissolved arsenic to Iowa’s water quality criterion for arsenic III. Results of monitoring data for arsenic III would be needed in order to identify violations of Iowa’s Human Health criterion for arsenic III: these data do not exist. Thus, this impairment is proposed for de-listing for the current (2014) Integrated Reporting cycle.
Levels of nitrate (maximum of 3.9 mg/l; average = 1.4 mg/l), atrazine (maximum 2.0 ug/l; average = 0.3 ug/l), and alachlor (maximum of 0.024 ug/l; all other values <0.008 ug/l) were well below their respective Class C human health criteria and MCLs in the approximately 37 samples collected from the Omaha station during the 2010-2012 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."
Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river segment.
