Assessment Comments
Assessment remains 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) and is also based on results of monitoring in 2011 at 111th Avenue (STORET station 15960034).
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, 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 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 remain assessed (monitored) as “not supported” (IR 4a) due to levels of indicator bacteria that continue to far exceed state water quality criteria. The Class B(WW2) aquatic life uses remain assessed (monitored) as "partially supported" (IR 5a) due to violations of criteria for dissolved oxygen during the 2004-06 period despite recent (2011) monitoring data suggesting a recovery in levels of 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), (2) results of IDNR/UHL bacteria monitoring at station 15960001 from January 2006 through September 2008, and (3) IDNR monitoring at Site 1 (111th Ave; STORET station 15960034) from April to October 2011.
Note: A TMDL for the bacterial impairment for streams in the Yellow River basin was prepared by Iowa DNR in 2012 and was approved by U.S. EPA in February 2013 (see http://www.iowadnr.gov/Environment/WaterQuality/WatershedImprovement/WatershedResearchData/WaterImprovementPlans/PublicMeetingsPlans.aspx). Due to completion of the TMDL, the Integrated Report category for the Class A1 recreational uses is changed from 5p (impaired; TMDL needed) to 4a (impaired; TMDL approved). Because not all impairments identified for this assessment segment are covered by the TMDL (biological impairment of the aquatic life uses due to low dissolved oxygen), this segment remains in Category 5a of Iowa's Integrated Report (impaired and TMDL required).
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. The geometric mean of E. coli in the 14 samples collected at station 15960034 during the recreation season of 2011 was 7,097 orgs/100 ml. This geometric means far exceed the Class A1 criterion of 126 orgs/100 ml. All 14 of the samples (100%) exceeded Iowa’s Class A1 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 a geometric mean of E. coli is greater than the applicable state criterion, the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b).
This current assessment and the excessively high levels of indicator bacteria are consistent with previous assessments for this stream segment. The presumptive Class A1 (primary contact recreation) uses were initially assessed as "not supported" based on monitoring data from 2006-2008 that showed violations of Iowa’s water quality criteria for indicator bacteria. 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. Similar to results from the 2011 recreation season, this geometric mean far exceeded the Class A1 criterion of 126 orgs/100 ml. Also similar is that 100% of the samples (all 38) from the 2006-08 period exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml. The consistently and excessively high levels of indicator bacteria in this stream segment suggest an ongoing pollution source that deserves further investigation.
The Class B(WW2) aquatic life uses remain assessed (monitored) as “partially supported” due to violations of aquatic life criteria for dissolved oxygen at station 15960001 during the 2004-06 monitoring period. 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. Results of recent (2011) monitoring at station 15960034 do not suggest a continuing problem with low dissolved oxygen in this assessment segment: none of the 14 samples analyzed from April to October 2011 exceeded the Class B(WW1) criterion of 5.0 mg/l; the minimum level of dissolved oxygen was 6.0 mg/l. Based on IDNR's assessment methodology, however, two consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the standard are necessary to justify the de-listing of an impairment due to low dissolved oxygen. Therefore, the impairment for dissolved oxygen will remain in place for the 2014 assessment/listing cycle. Additional monitoring is needed to determine whether chemical water quality of this assessment segment has improved to the point where the IR 5a impairment can be removed.
In addition, there were no violations of Class B(WW2) ammonia criteria in the eight samples collected in 2011. These result suggest full support of the Class B(WW2) uses. One of the 14 samples collected in 2011 at station 15960034 violated the Class B(WW2) criterion for pH. The sample collected on August 9, 2011, contained a pH of 9.1 units, thus slightly exceeding the Class B(WW2) criterion of 9.0 units. According to IDNR’s assessment/listing methodology, however, these results do not indicate that significantly greater than 10% of the samples exceed the Class B(WW2) criterion for pH and thus do not indicate impairment.
