Assessment Comments
Assessment is based on results of IDNR/UHL chemical/physical water quality monitoring conducted from May 2004 to September 2008 at the Maple Valley Road crossing (STORET station 15960001) as part of the Yellow River Watershed Project.
Basis for Assessment
[Note: Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(LR) aquatic life uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf), this segment is now also presumptively designated for Class A1 (primary contact recreation) uses. The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses). Thus, for the current (2010) assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to levels of indicator bacteria that far exceed state water quality criteria. The Class B(WW2) aquatic life uses remain assessed (monitored) as "partially supported" due to violations of criteria for dissolved oxygen. The sources of data for this assessment are the results of (1) IDNR/UHL chemical/physical water quality monitoring conducted from May 2004 to November 2006 at the Maple Valley Road crossing (STORET station 15960001) and (2) results of IDNR/UHL bacteria monitoring at station 15960001 from January 2006 through September 2008. Both of these datasets were generated as part of the Yellow River Watershed Project.
EXPLANATION: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to violations of Iowa’s water quality criteria for indicator bacteria. Results of IDNR/UHL monitoring continue to suggest extremely high levels of indicator bacteria in this stream segment. The geometric mean of E. coli in the 38 samples collected at the monitoring station at Maple Valley Road (station 15960001) during the recreational seasons of 2006 through 2008 was 5,308 orgs/100 ml. This geometric mean far exceeds the Class A1 criterion of 126 orgs/100 ml. All 38 samples (100%) exceeded Iowa’s 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 the geometric mean of E. coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b).
The results of the IDNR/UHL water quality monitoring from 2004-2006 suggest impairment of the Class B(WW2) aquatic life uses. Results of this monitoring showed 16 violations of the Class B(WW2) criteria for dissolved oxygen (5.0 mg/l) in the 80 samples collected for a violation frequency of 20%. The minimum level of dissolved oxygen was 2.9 mg/l. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), "partial support" of aquatic life uses is indicated if criteria are exceeded in from 11 to 25% of the samples for conventional parameters (e.g., pH, temperature, or dissolved oxygen). According to the IDNR assessment/listing methodology, the percentage of violations is significantly greater than 10% thus indicating impairment. Although 85 samples were analyzed for ammonia from 2004 through 2008 at station 15960001, none of these samples were analyzed for pH; thus, violations of the temperature/pH dependent ammonia criteria cannot be calculated. Regardless, levels of ammonia during the 2004-08 period were relatively low with a maximum value of 0.4 mg/l. Based on IDNR’s assessment methodology, these results suggest that significantly more than 10 percent of the samples exceed Iowa’s dissolved oxygen criteria and thus suggest an impairment of the Class B(WW2) uses of this stream segment. Note: although IDNR station 15960001 was monitored for E. coli through September 2008, monitoring for dissolved oxygen was not conducted after November 2006.
