Grannis Creek IA 01-VOL-322

The presumptive Class A1 (primary contact recreation) uses remain assessed (monitored) as "partially supported" based on levels of indicator bacteria that exceeded state water quality criteria.The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2012 through 2014 at the station GRA-10 (STORET station 15330005) were as follows: the 2012 geometric mean was 267 orgs/100 ml, the 2013 geometric mean was 152 orgs/100 ml, and the 2014 geometric mean was 203 orgs/100 ml. All three recreation season geometric means slightly exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Ten of the combined 24 samples (42%) 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 DNR’s assessment/listing methodology, if the geometric mean is greater than 126 orgs/100 ml., the contact recreation uses should remain assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).

All three geometric means are well-below the Class A2 criterion of 630 orgs/100 ml, and only two of the 24 samples (8%) exceeded the Class A2 single-sample maximum criterion of 2,880 orgs/100 ml. Because all geometric means are less than the Class A2 criterion, and because less than 10% of samples exceeded the Class A2 single-sample maximum criterion, the Class A2 uses remain assessed as “fully supporting”.

Results of chemical/physical water quality monitoring from 2012 through 2014 do not suggest impairment of the Class B(WW2) aquatic life uses. Monitoring at station GRA 10 (STORET station 15330005) showed no violations of Class B(WW2) water quality criteria for ammonia (16 samples), pH (24 samples), or temperature (24 samples) during the 2012-2014 monitoring period. Two of the 24 samples (8%) analyzed for dissolved oxygen violated the Class B(CW1) criterion of 7 mg/l. The samples collected on August 1 and September 5 of 2012 both contained 6 mg/l of dissolved oxygen and thus violated the Class B(WW2) standard (note: all dissolved oxygen data generated during the 2012-2014 were recorded only to the nearest 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 pH and dissolved oxygen suggests impairment of aquatic life uses. Based on DNR’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(CW1) aquatic life uses. Thus, the Class B(CW1) aquatic life uses are assessed (monitored) as “fully supported”.

An updated summary of trout reproduction in Iowa streams prepared by the DNR Fisheries Bureau in January 2019 is consistent with the assessment of the Class B(CW1) coldwater aquatic life uses as "fully supported". According to the updated summary, Grannis Creek is in the group of Iowa coldwater streams that supports natural reproduction of trout. Based on results of surveys by the DNR Fisheries Bureau, this stream is placed in the category of Iowa trout streams that exhibit fairly consistent natural reproduction and maintain a viable population of the listed species (Brown Trout) without any stocking (a Category I trout stream). This assessment is consistent with previous assessments of the ability of this stream to support natural reproduction of trout.

Fish consumption uses remain "not assessed" due to lack of fish tissue monitoring in this stream reach.