Assessment Comments
Assessment is based on results of (1) IDNR/UHL ambient water quality monitoring from 2000-02 upstream from Cedar Rapids and (2) ambient monitoring of the Cedar River for nitrate by the Cedar Rapids Water Department from 2000-02.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses and the Class B(WW) aquatic life uses were assessed (monitored) as "fully supported" based on results of ambient water quality monitoring. The Class C (drinking water) uses were assessed (monitored) as "fully supported/threatened" with a declining water quality trend 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 were 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 2000 through December 2002 at the IDNR ambient city monitoring station located at County Road E36 east of Palo (STORET station 10570002) and (2) the results of ambient monitoring of the Cedar River for nitrate by the Cedar Rapids Water Department from 2000 through 2002.
EXPLANATION: The Class A uses were assessed as "fully supported" based on results of monitoring for indicator bacteria (fecal coliforms). For purposes of Section 305(b) assessments, DNR uses the long-term average monthly flow plus one standard deviation of this average to identify river flows that are materially affected by surface runoff. According to the Iowa Water Quality Standards (IAC 1990:8), the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) does not apply "when the waters are materially affected by surface runoff." Thirteen of the 16 samples collected from the Palo station during the 2000, 2001, and 2002 recreational seasons were collected at flows not materially affected by surface runoff. The geometric mean level of indicator bacteria (fecal coliforms) in these 13 non-runoff-affected samples (23 orgs/100ml) is well below the Iowa Class A water quality criterion of 200 orgs/100ml (this geometric mean is one of the lowest for any of the monitored Class A river reaches in Iowa). In addition, none of the 13 samples contained a level of fecal coliform bacteria that exceeded the U.S. EPA-recommended single-sample maximum value of 400 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, if the geometric mean is less than 200 orgs/100ml, and less than 10% or less of the samples exceed the single-sample maximum value of 400 orgs/100 ml, the primary contact recreation uses are "fully supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
The Class B(WW) aquatic life uses were assessed as "fully supported" based on results of monitoring from the IDNR ambient station near Palo from 2000 through 2002. Monitoring at this station showed no violations of Class B(WW) water quality criteria for dissolved oxygen, ammonia-nitrogen, or for toxic metals and toxic organic compounds in the 28 samples analyzed during this biennial period; none of the 9 samples analyzed for pesticides exceeded their respective Class B(WW) criteria. The level of pH, however, in one of the 28 samples violated the Class B(WW) criterion of 9.0 pH units in one sample. The sample collected on October 9, 2000 had a pH level of 9.2 units which is greater than the Iowa water quality standard of 9.0 pH units. This violation occurred on a day with an extremely high level of dissolved oxygen (19.2 mg/l) and a percent DO saturation of > 140%. These conditions suggest that the high level of primary productivity resulted in the high level of pH. Because this violation is more related to natural conditions than to pollution, the occurrence of the high level of pH in this river segment is not seen as a water quality impairment.
Based on results of ambient monitoring conducted by IDNR and by the Cedar Rapids Water Department, the Class C (drinking water) uses were assessed as "fully supported/threatened" with a declining water quality trend due to violations of the state water quality standard (= U.S. EPA’s MCL) for nitrate. Results of IDNR ambient monthly monitoring at the Palo station show only one violation of the nitrate MCL in the 28 monthly samples (4% violation) collected during the 2000-2002 assessment period (mean=4.9 mg/l; maximum=11.0 mg/l). Monitoring of the Cedar River by the Cedar Rapids Water Department (CRWD) near their water supply intake areas from January 2000 through December 2002, however, shows that 73 of the 577 sampled collected (13% violation) exceeded the 10 mg/l water quality standard of 10 mg/l (mean=6.0 mg/l; median = 5.7 mg/l; maximum=13.2 mg/l). Violations tended to occur during the spring and early summer with the majority of violations occurring during June in each of the three years. 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 violation likely overestimates actual percentage of time that nitrate levels actually exceed the MCL. Thus, IDNR staff summarized nitrate data from the Cedar River Water Department as weekly averages and compared these averages to the water quality standard. Eight of the 157 weekly average nitrate levels (5%) for the period 2000-2002 exceeded the standard (mean=5.4 mg/l; median=5.4 mg/l; maximum=12.0). According to IDNR's assessment guidelines, both the IDNR and CRWD datasets suggest that full support of the Class C drinking water uses is "threatened." That is, if the MCL is exceeded during the assessment period, but no more than 10% of samples exceed the MCL, the full support of drinking water uses is considered "threatened." Additional information supplied by the Cedar Rapids Water Department shows evidence of increasing nitrate levels over time. This increasing trend in nitrate suggests that less than full support is likely within the next few years. According to IDNR assessment guidelines, waterbodies assessed as "fully supported with a declining water quality trend are considered "impaired" and are candidates for Section 303(d) listing. This assessment is consistent both with previous assessment (see assessments of Class C uses for the 2000 and 1998 reports) and with placement of this river segment on Iowa's 1998 Section 303(d) list of impaired waters.
Fish consumption uses were not assessed due to lack of recent fish tissue monitoring in this river segment.
