Assessment Comments
Assessment is based on results of IDNR/UHL ambient monthly monitoring near Mapleton from 2000-02.
Basis for Assessment
[Note: For the 2002 report, the previous waterbody segment for the Maple River (IA 06-LSR-0070-0), which extended 45 miles from its mouth confluence with the Odebolt Creek at Ida Grove (Ida Co.), was split into two subsegments: (1) mouth (Monona Co.) to confluence with an unnamed tributary 1 mile east of Danbury in Woodbury Co. (IA 06-LSR-0070-1 - this one) and (2) unnamed tributary near Danbury to confluence with Odebolt Creek at Ida Grove (Ida Co.) (IA 06-LSR-0070-2). See previous Section 305(b) assessments from this subsegment for assessments for the original 45-mile river reach.]
SUMMARY: The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported / threatened" due to a single violation of state water quality standards for ammonia-nitrogen. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment. This assessment is based on results of IDNR/UHL monthly ambient monitoring conducted during the 2000-2002 assessment period at the Highway 141-175 bridge at Mapleton (STORET station 10670002 (formerly station 911040)).
EXPLANATION: Monitoring at the IDNR/UHL station at Mapleton showed no violations of Class B(WW) water quality criteria during the 2000-2002 assessment period for dissolved oxygen (minimum value = 6.9 mg/l) or pH (range = 7.7 to 8.7 units). Results of monitoring at the Mapleton station showed that one of the 36 samples collected during the 2000-2002 period (3%) violated the Class B(WW) criterion for ammonia-nitrogen. The sample collected on March 6, 2001 contained 5.7 mg/l of ammonia-nitrogen; this level exceeded the Class B(WW) temperature/pH-dependent chronic criterion of 1.80 mg/l. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-18), however, one violation of a water quality criterion for a toxic pollutant in an abundant data set (at least 10 samples over at three-year period) set does not indicate an impairment of aquatic life uses. Based on Iowa IDNR's assessment methodology for Section 305(b) reporting, this violation suggests that the Class B(WW) aquatic life uses should be assessed as "fully supported/threatened." The next highest levels of ammonia-nitrogen during the assessment period were 0.6 mg/l on February 6, 2001 and 0.62 mg/l on August 6, 2002. Levels of pesticides in the nine samples analyzed were all below the analytical level of detection.
Fish consumption uses remain "not assessed due to the lack of recent fish contaminant monitoring in this river reach.
