Assessment Comments
Assessment is based on results of (1) chemical/physical monitoring from May 2011 to November 2012 at station MIN 10 (STORET station 15220016) and (2) a summary of trout reproduction in Iowa streams prepared by the IDNR Fisheries Bureau in 2012.
Basis for Assessment
[Note: Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(CW) 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 also now presumptively designated for Class A1 (primary contact recreation) and Class A2 (secondary contact recreation) uses. This segment remains designated for coldwater aquatic life use (now termed Class B(CW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption) uses.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses and the presumptive Class A2 (secondary contact recreation) are both assessed as “not supporting” due to levels of indicator bacteria that violate state water quality standards (IR 5p). These are new impairments for this assessment segment. The Class B(CW1) coldwater aquatic life uses are assessed (monitored) as "fully supported" (IR 2a). Fish consumption uses remain “not assessed.” The sources of data for this assessment include results of (1) chemical/physical monitoring from May 2011 to November 2012 at station MIN 10 at Aztec Road (STORET station 15220016) and (2) a summary of trout reproduction in Iowa streams prepared by the IDNR Fisheries Bureau in 2012.
EXPLANATION: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the seven monthly samples collected during the recreational season of 2011 (868 orgs/100 ml), and the geometric mean of the eight samples collected during 2012 (1,631 orgs/100 ml) both exceeded the Class A1 criterion of 126 orgs/100 ml. Fourteen of the combined 15 samples (93%) 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 the geometric mean is greater than 126 orgs/100 ml., the presumptive primary contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
The presumptive Class A2 (secondary contact recreation) uses are also assessed (monitored) as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the seven monthly samples collected during 2011 (868 orgs/100 ml) and the geometric mean of the eight samples collected during 2012 (1,631 orgs/100 ml) both exceed the Class A2 criterion. Only 1 of the combined 15 samples (7%) exceeded Iowa’s Class A2 single-sample maximum criterion of 2,880 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if the geometric mean is greater than 126 orgs/100 ml., the presumptive secondary contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
Results of chemical/physical water quality monitoring in 2011 and 2012 do not suggest impairment of the Class B(WW2) aquatic life uses. Monitoring at station MIN 10 showed no violations of Class B(CW1) water quality criteria for ammonia in the 15 samples analyzed. Three of the 15 samples analyzed for dissolved oxygen and water temperature violated the Class B(CW1) dissolved oxygen criterion of 7 mg/l and the pH criterion of 9.0 pH units. The samples collected on August 3, 2011, September 5, 2012, and October 3, 2012 all contained 6 mg/l of dissolved oxygen and thus violated the Class B(CW1) standard (note: dissolved oxygen values were recorded only to the nearest mg/l). Water temperatures on August 3, 2011 (22.4C), July 2, 2012 (24.1C), and August 1, 2012 (21.1C) violated the Class B(CW1) temperature criterion of 20C. Also, the pH of the sample collected on June 1, 2011 (9.9 pH units) exceeded the Class B(CW1) pH criterion of 9.0 pH units. 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, temperature, or pH suggests impairment of aquatic life uses. Based on IDNR’s assessment methodology, however, these results suggest that the frequencies of violations for these parameters in this segment of Mink Creek are not significantly greater than 10 percent; thus, these results do not suggest impairment of the Class B(CW1) aquatic life uses. Thus, the Class B(CW1) aquatic life uses are assessed (monitored) as “fully supported”.
In addition, this assessment is also based on an updated summary of trout reproduction in Iowa streams prepared by the DNR Fisheries Bureau in February 2012 which suggests that the Class B(CW1) uses should be assessed as "fully supported". According to the updated summary, Mink Creek is in the group of Iowa coldwater streams that supports natural reproduction of trout. Based on results of surveys by the IDNR Fisheries Bureau, this stream is placed in the category of Iowa trout streams that exhibit recent, but inconsistent, reproductive success and are not capable of maintaining a viable population of brown trout at this time (a Category II trout stream).
Fish consumption uses remain "not assessed" due to lack of fish tissue monitoring in this stream reach.
