Assessment Comments
Assessment is based on results of monitoring at STORET station 15960023 (Dry Run Creek Watershed Site 13a) conducted as part of a Clean Water Act Section 319 water quality project from August 2010 through October 2011.
Basis for Assessment
[Note: Prior to the 2008 Section 305(b) cycle, this stream segment was classified only for general uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008, this segment is now presumptively designated for Class A1 (primary contact recreation) uses and for Class B(WW1) aquatic life uses. According to the Iowa Water Quality Standards, all perennial rivers and streams and all intermittent streams with perennial pools that are not specifically listed in the Iowa surface water classification are presumptively designated as Class A1 and Class B(WW1) waters. Thus, for the current assessment, perennial flow is presumed, and the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW1) water quality criteria.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” (IR 5p) due to violations of Iowa water quality criteria for indicator bacteria. This is a new impairment for this previously unassessed segment. The presumptive Class B(WW1) aquatic life uses are assessed as "fully supporting" (IR 2a) based on results of chemcial/physical water quality monitoring. The source of data for this assessment is the results of monitoring at STORET station 15960023 (Dry Run Creek Watershed Site 13a) conducted as part of a Clean Water Act Section 319 water quality project from August 2010 through October 2011 [NULL 030].
EXPLANATION: The presumptive Class A1 uses are assessed (monitored) as “not supported" due to levels of indicator bacteria (E. coli) that exceed the Class A1 geometric mean criterion. The geometric means of indicator bacteria (E. coli) in the 21 samples collected at Site 13a during the recreation seasons of 2010 and 2011 were as follows: The 2010 geometric mean was 2,625 orgs/100 ml, and the 2011 geometric mean wa 3,385 orgs/100 ml. Both geometric means far exceed the Class A1 criterion of 126 orgs/100 ml. All 21 samples (100%) exceeded the 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 recreation season geometric mean is greater than the respective criterion (126 orgs/100 ml for Class A1, the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). In addition, the percentage of samples with levels of E. coli that exceeded Iowa single-sample maximum criteria indicates impairment of presumptive Class A1 uses at this monitoring site. Note: The monitoring conducted as part of this watershed study included sampling for indicator bacteria (E. coli) immediately (within 24 hours) following rainfall runoff events. About half the samples in each of 2010 and 2011 were collected under such conditions. See the following journal article for more information: Witttman et al. 2013. Evaluation of land use and water quality in an agricultural watershed in the USA indicates multiple sources of bacterial impairment. Environ. Monit. Assess (2013) 185:10395-10429.
Results of chemcial/physical monitoring at Site 31a during the 2010-12 period suggest "full support" of the presumptive Class B(WW1) aquatic life uses. None of the approximately 20 samples analyzed from August 2010 to October 2011 violated the Class B(WW1) criteria for ammonia, pH or temperature. Two of 23 samples (9%) violated the Class B(WW1) criterion for dissolved oxygen of 5 mg/l. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), a violation frequency of greater than 10% for conventional parameters such as dissolved oxygen suggests impairment of aquatic life uses. Based on IDNR’s assessment methodology, however, these results suggest that the frequency of violations is not significantly greater than 10 percent; thus, these results do not suggest impairment of the Class B(WW1) aquatic life uses.
