Assessment Comments
Assessment is based on results of (1) IDNR/UHL ambient monitoring from 2006-08 upstream from Cedar Rapids, (2) monitoring of the Cedar River for nitrate by the Cedar Rapids Water Department from 2004-08, and (3) USGS ambient monitoring at Cedar Rapids from 2006-08.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially 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 2006 through December 2008 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 2006 through 2008, and (3) results of USGS chemical/physical water quality monitoring at Edgewood Road from March 2006 through October 2008.
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, this impairment is appropriate for Category 4a of Iowa’s 2010 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, this impairment is appropriate for Category 4a of Iowa’s 2010 Integrated Report.
EXPLANATION: The Class A1 uses are assessed (monitored) as "partially 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 mean level of indicator bacteria (E. coli) in the 22 samples collected (118 orgs/100ml) is below the Iowa Class A1 water quality criterion of 126 orgs/100ml. Nine of the 22 samples (41%), however, exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, if the geometric mean of E. coli is less than the applicable state criteria, the contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b). Thus, these data suggest that the Class A1 uses should be assessed as “fully supported.” These guidelines, however, also suggest that if more than 10% of samples exceed the single-sample maximum criterion for indicator bacteria, the primary contact recreation uses should be assessed as "partially supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). According to IDNR’s assessment/listing methodology, the monitoring data from 2006-08 suggest that significantly greater than 10% of the samples exceed IDNR’s single-sample maximum criterion, thus suggesting that the Class A1 uses should be assessed as “partially supported/impaired.” Note: monitoring for bacterial indicators was not conducted at USGS station 05464480 during the 2006-2008 period.
Since Iowa’s adoption of E. coli as an indicator bacterium in 2003, geometric mean levels at IDNR monitoring station 10570002 have fully met Iowa’s geometric mean criterion of 126 orgs/100 ml. The three-season geometric means for these past listing cycles are as follow: 2004: 23 orgs/100 ml; 2006: 75 orgs/100 ml; 2008: 56 orgs/100 ml. These geometric means are some of the lowest for any free-flowing rivers segments in the state of Iowa.
The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of water quality monitoring 2006-2008 from (1) the IDNR ambient station near Palo and (2) the USGS station at Edgewood Road. Monitoring at these stations showed no violations of Class B(WW1) water quality criteria for dissolved oxygen or ammonia-nitrogen in the approximately 40 samples analyzed, or for toxic metals in the approximately ten samples analyzed during this assessment period. The level of pH, however, in one of the 34 samples from the IDNR/UHL station violated the Class B(WW1) criterion of 9.0 pH units. Because this violation is more likely related to natural conditions than to a pollutant, the occurrence of the high level of pH in this river segment is not seen as a water quality impairment. In addition, the frequency of pH violations does not suggest impairment: according to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), a violation frequency of less than 10 % for conventional parameters such as pH suggest "full support" of aquatic life uses. Thus, the percentage of violations of the pH criterion at the IDNR/UHL station (3%) does not suggest an impairment of aquatic life uses.
Based on results of ambient monitoring conducted from 2006-2008 by USGS, IDNR/UHL, and by the Cedar Rapids Water Department, the Class C (drinking water) uses are assessed as "partially supported" due to violations of the state water quality standard (= U.S. EPA’s MCL) for nitrate. Results of IDNR ambient monthly monitoring on the Cedar River at Palo station show four violations of the nitrate MCL in the 33 monthly samples (12% violation) collected during the 2006-2008 assessment period (mean=6.9 mg/l; maximum=14.0 mg/l). Results from the USGS station at Edgewood Road show one violation of the nitrate MCL in the 10 samples (10% violation) collected during the 2006-2008 assessment period (mean=7.0 mg/l; maximum=10.1 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." None of the 14 samples analyzed for atrazine from the IDNR/UHL station, and none of the 10 samples analyzed for atrazine from the USGS station during the 2006-2008 assessment period, exceeded the Iowa Class C criterion for atrazine of 3 ug/l (=U.S. EPA’s MCL). The average level of atrazine in the 35 IDNR/UHL samples was 0.12 ug/l (median=0.06 ug/l); the average level in the 10 USGS samples was 0.18 ug/l (median=0.1 ug/l); these results do not suggest an atrazine-related impairment of the Class C uses.
Monitoring of the Cedar River by the Cedar Rapids Water Department (CRWD) near their water supply intake areas from January 2006 through December 2008, however, suggests that the drinking water uses of this segment of the Cedar River should remain assessed as “impaired.” The CRWD data from the Mohawk Park monitoring station show that 87 of the 313 samples collected (28% violation) exceeded the 10 mg/l water quality standard. At the Mohawk Park station, the mean, median, and maximum sample values were 8.2, 8.6, and 13.5 mg/l, respectively. Violations tended to occur during the late spring and early summer.
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. Twenty of the 153 weekly average nitrate levels (13%) for the period 2006-2008 exceeded the MCL at the Mohawk Park station.
According to IDNR's assessment guidelines, while the IDNR and USGS datasets show an MCL violation frequency that is not significantly greater than 10%, thus suggesting “full support” of the Class C drinking water uses, the more robust dataset from the CRWD provides good evidence that levels of nitrate in this segment of the Cedar River exceed the nitrate MCL of 10 mg/l more than 10% of the time. Also, information supplied by the Cedar Rapids Water Department shows evidence of increasing nitrate levels over time. This increasing trend in nitrate also suggests less than full support of the designated drinking water uses. Thus, the drinking water uses remain assessed as “impaired” for the current (2010) Section 303(d) listing cycle. This assessment is consistent with previous assessment and the Section 303(d) listing history for this segment of the Cedar River.
Fish consumption uses remain “not assessed” due to lack of recent fish tissue monitoring in this river segment.
