Mink Creek IA 01-VOL-314

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 25 samples collected during the recreational seasons of 2012 through 2014 at the station MIN-10 (STORET station 15220016) were as follows:  the 2012 geometric mean was 1,631 orgs/100 ml, the 2013 geometric mean was 1,328 orgs/100 ml, and the 2014 geometric mean was 889 orgs/100 ml.  All three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml.  Twenty-two of the combined 25 samples (88%) exceeded Iowa’s Class A1 single-sample maximum criterion of 235 orgs/100 ml.  All three geometric means also exceed the Class A2 criterion of 630 orgs/100 ml, and five of the 25 samples (20%) exceed the 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 primary contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). 

Results of chemical/physical water quality monitoring in from 2012 through 2014 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 (16 samples), pH (24 samples) or chloride (16 samples).  Two of the 24 samples (8%) analyzed for dissolved oxygen and two of the water temperature values violated the Class B(CW1) criteria during the 2012-2014 monitoring period.  The samples collected on September 5, 2012 and October 3, 2012 both contained 6 mg/l of dissolved oxygen and thus violated the Class B(CW1) standard of 7.0 mg/l (note:  all dissolved oxygen values from monitoring during the 2012-2014 period were recorded only to the nearest mg/l).  Water temperatures on July 2, 2012 (24.1C) and August 1, 2012 (21.1C) violated the Class B(CW1) temperature criterion of 20C.  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 during 2012-2014 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 IDNR Fisheries Bureau in January 2016 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.