Assessment Comments
Assessment is based on results of IDNR/UHL ambient water quality monitoring from 2006 through 2008 near Palo.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported”. The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient water quality monitoring. Fish consumption uses remain not assessed due to the lack of fish contaminant monitoring in this river segment. The source of data for this assessment is the results of monthly monitoring from January 2006 through December 2008 at the IDNR ambient city monitoring station located
Note: A TMDL for the bacteria impairments in the two downriver assessment segments of the Cedar River was prepared and approved by EPA in February 2010. This TMDL addressed bacteria impairments in Cedar River segments IA 02-CED-0030_1 and -0030_2 but did not include this segment of the Cedar River (IA 02-CED-0030_3). The bacteria impairments in all three segments, however, are based on the same monitoring data from the same Iowa DNR ambient water quality monitoring station: Cedar River at County Road E36 east of Palo (STORET station 10570002)). As noted in comments from U.S. EPA in May 2011, however, the failure of the TMDL to include segment CED-0030_3 require that this segment remain on Iowa's 2010 Section 303(d) list.
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 river segments in the state of Iowa. Thus, based on the history of low levels of indicator bacteria in this river segment, and based on INDR’s change in listing methodology, this bacterial impairment is proposed for de-listing.
The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of water quality monitoring from the IDNR ambient station near Palo. Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen or ammonia-nitrogen in the approximately 33 samples analyzed, or for toxic metals in the five 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 percentages of violations of the pH criterion at the IDNR station (3%) does not suggest an impairment of aquatic life uses.
Fish consumption uses remain “not assessed” due to lack of recent fish tissue monitoring in this river segment.
