Assessment Comments
Assessment is based on results of (1) IDNR/UHL ambient monitoring from 2008-10 upstream from Cedar Rapids near Palo, (2) monitoring of the Cedar River for nitrate by the Cedar Rapids Water Department from 2008-10, and (3) USGS ambient monitoring at Cedar Rapids from 2008-10.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to levels of indicator bacteria. The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient water quality monitoring. The Class C (drinking water) uses are assessed (monitored) as "partially supported” due to levels of nitrate that occasionally exceed the state drinking water criterion of 10 mg/l (=U.S. EPA’s MCL for nitrate). Fish consumption uses are not assessed due to the lack of fish contaminant monitoring in this river segment. The sources of data for this assessment are (1) the results of monthly monitoring from January 2008 through December 2010 at the IDNR ambient city monitoring station located at County Road E36 east of Palo (STORET station 10570002), (2) the results of ambient monitoring of the Cedar River for nitrate by the Cedar Rapids Water Department from 2008 through 2010, and (3) results of USGS chemical/physical water quality monitoring at Edgewood Road (station 05464480) and at Blairs Ferry Road (station 05464420) from May 2008 through October 2010.
Note 1: A TMDL for nitrate in this segment of the Cedar River was prepared by IDNR and approved by EPA in January 2007. Thus, the nitrate impairment in this segment is appropriate for Category 4a of Iowa’s Integrated Report.
Note 2: A TMDL for the bacteria impairment in this segment of Cedar River was prepared and approved by EPA in February 2010. Thus, the bacteria impairment in this segment is appropriate for Category 4a of Iowa’s Integrated Report.
EXPLANATION: The Class A1 uses are assessed (monitored) as "not supported" based on results of monitoring for indicator bacteria (E. coli) at the IDNR/UHL ambient monitoring station on the Cedar River east of Palo. The geometric means of indicator bacteria (E. coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at station 10570002 upstream from Cedar Rapids were as follows: the 2008 geometric mean was 202 orgs/100 ml, the 2009 geometric mean was 63 orgs/100 ml and the 2010 geometric mean was 122 orgs/100 ml. Only the 2008 geometric means exceed the Class A1 criterion of 126 orgs/100 ml, thus suggesting that extreme flooding in the Cedar River in summer 2008 contributed to this impairment. Seven of the 22 samples (32%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”
Since Iowa’s adoption of E. coli as an indicator bacterium in 2003 until summer 2008, geometric mean levels at IDNR monitoring station 10570002 had fully met Iowa’s geometric mean criterion of 126 orgs/100 ml. This segment of river has some of the lowest geometric means of any free-flowing rivers segment in the state of Iowa. Regardless of the recent (2010 and 2012) impairments, levels of indicator bacteria in this segment of the Cedar River have been and continue to be very low relative to levels of bacteria in other Iowa rivers, and thus this bacterial impairment should be considered very slight in magnitude.
The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of water quality monitoring 2008-2010 from (1) the IDNR ambient station near Palo and (2) the USGS stations at Edgewood Road and Blairs Ferry Road. Monitoring at these stations showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the approximately 40 samples analyzed, for toxic metals in the approximately ten samples analyzed, or for the approximately five samples analyzed for pesticides during this assessment period.
Based on results of ambient monitoring conducted by USGS, IDNR/UHL, and by the Cedar Rapids Water Department, the Class C (drinking water) uses are assessed as "partially supported" due to the history of violations of the state water quality standard (= U.S. EPA’s MCL) for nitrate. Results of IDNR and USGS ambient monthly monitoring on the Cedar River in this assessment segment during the 2008-2010 period, however, show no violations of the nitrate MCL in the 33 monthly samples (maximum=9.7 mg/l). According to the IDNR assessment methodology, these results do not suggest that significantly more than 10% of the samples is greater than the Class C criterion for nitrate of 10 mg/l (=U.S. EPA MCL) and thus do not suggest impairment of the Class C drinking water uses. Based on DNR's Section 305(b) assessment methodology, if less than 10% of the samples exceed the nitrate MCL, the drinking water uses should be assessed as "fully supported."
Similarly, monitoring of the Cedar River by the Cedar Rapids Water Department (CRWD) near their water supply intake areas from January 2008 through December 2010 also suggest that levels of nitrate in the Cedar River are typically below the MCL of 10 mg/l. The CRWD data from the Mohawk Park monitoring station show that only three of the 168 samples collected (2% violation) during this three-year period exceeded the 10 mg/l water quality standard. At the Mohawk Park station, the mean, median, and maximum sample values were 6.2, 6.4, and 11.4 mg/l, respectively. All three violations tended occurred during 2008: April 17, April 18, and June 4.
Due to over-sampling by water supply utilities during times of year when nitrate levels tend to be high, the use of a simple percentage of sample violations likely overestimates the percentage of time that nitrate levels actually exceed the MCL. Thus, in order to determine whether the Class C uses are impaired, IDNR staff summarized nitrate data from the Cedar River Water Department as weekly averages and compared these averages to the water quality standard. None of the 141 weekly average nitrate levels for the 2008-2010 period exceeded the MCL at the Mohawk Park station. The mean, median, and maximum weekly average values were 5.9, 6.1, and 9.9 mg/l, respectively.
According to IDNR's assessment guidelines, the IDNR, USGS, and CRWD datasets for the 2008-2010 period all suggest “full support” of the Class C drinking water uses. Due, however, to the history of high levels of nitrate in this assessment segment and due to the tendency of nitrate levels in surface waters to fluctuate with year-to-year changes in precipitation, this segment will remain assessed (monitored) as “partially supported” for the 2012 Integrated Reporting cycle. If results for the next (2010-2012) assessment period continue to show low levels of nitrate, this impairment will be moved to a “fully supporting” category of Iowa’s 2014 Integrated Report.
None of the three samples analyzed for atrazine from the IDNR/UHL station, and none of the 11 samples analyzed for atrazine from the USGS station during the 2008-2010 assessment period, exceeded the Iowa Class C criterion for atrazine of 3 ug/l (=U.S. EPA’s MCL). The maximum level of atrazine in the combined 14 samples from USGS and IDNR/UHL monitoring was 0.67 ug/l; these results do not suggest an atrazine-related impairment of the Class C uses. Also, none of the samples analyzed for metals and/or pesticides violated the respective Class C (drinking water) criteria.
Fish consumption uses remain “not assessed” due to lack of recent fish tissue monitoring in this river segment.
