Rock River IA 06-BSR-1538

The presumptive Class A1 uses remain assessed (evaluated) as "partially supported" based on results of ambient monitoring for indicator bacteria (E. coli) by Iowa DNR in 2002 and 2003. The geometric mean level of indicator bacteria (E. coli) in the 18 samples collected at the DNR/SHL TMDL monitoring station near the Iowa/Minnesota border during summer recreational seasons of 2002-2003 (104 orgs/100ml) was below, and met, the Iowa Class A1 water quality criterion of 126 orgs/100ml. Four of the 18 samples (22%) exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml. Levels of indicator bacteria were very low in this assessment segment throughout most of the 2002-2003 assessment period. According to U.S. EPA guidelines for Section 305(b) reporting and DNR’s assessment/listing methodology, if the geometric mean of E. coli is less than the applicable state criteria, the contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, these data suggested that the Class A1 uses should be assessed as “fully supported.” These guidelines, however, also suggest that if more than 10% of samples exceed the single-sample maximum criterion for indicator bacteria, the primary contact recreation uses should be assessed as "partially supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). According to DNR’s assessment/listing methodology, the monitoring data from 2002-03 suggest that significantly greater than 10% of the samples exceed DNR’s single-sample maximum criterion, thus suggesting that the Class A1 uses should be assessed as “partially supported/impaired.” Because these data are now more than five years old, and because as data age beyond five years they are less able to represent current water quality conditions, the assessment type is changed from “monitored” (a higher confidence assessment) to “evaluated” (lower confidence assessment.)

Results of more recent bacteria monitoring by the Minnesota Pollution Control Agency approximately 7 miles south of Luverne, MN (and approximately three miles north of the Iowa/MN state line) suggest that this impairment should remain in place. The geometric means of indicator bacteria (E. coli) in the 18 samples collected by MPCA during the recreational seasons of 2010 through 2012 at the Rock River station CSAH-1 were as follows: the 2010 geometric mean was 392 orgs/100 ml, the 2011 geometric mean was 307 orgs/100 ml and the 2012 geometric mean was 781 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Eleven of the 18 samples (61%) 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 DNR’s assessment/listing methodology, if a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses should be assessed as “impaired.” Although the MPCA monitoring station is approximately three miles north of the IA/MN state line, these results suggest that the impairment of the Class A1 uses should remain in place.

Regarding support of the Class B(WW2) aquatic life uses, results of chemical/physical water quality monitoring conducted by DNR/SHL during the 2002-04 period in support of TMDL development, and conducted by MPCA from 2010 through 2012 suggest relatively good water quality in this stream segment. Results of DNR/SHL monitoring from March 2002 through December 2003 show that none of the 21 samples collected had levels of dissolved oxygen or ammonia-nitrogen that violated state water quality standards (these samples were not analyzed for other conventional parameters, toxic metals, or pesticides). One of the 11 samples, however, violated the upper Class B(WW) criterion of 9.0 pH units the sample collected on August 19, 2002 contained a pH of 9.3 units. This violation occurred on a day with an extremely high level of dissolved oxygen (19.9 mg/l) and a water temperature of 25.6 C; these readings correspond to percent DO saturation well in excess of 150%. These conditions suggest that the high level of primary productivity resulted in the high level of pH. Because this violation is more related to natural conditions than to pollution, the occurrence of the high level of pH in this river segment is not seen as a water quality impairment. Regardless, the U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), the percentage of violations at this station during the 2000-2002 period for pH (9%) does not suggest a water quality impairment. These guidelines allow up to 10% violations of conventional parameters such as pH and dissolved oxygen before impairment of water quality is indicated.

Similarly, results of MPCA monitoring show that none of the 19 samples collected from 2010-12 had levels of dissolved oxygen, or temperature that exceeded Iowa water quality criteria. One of the 19 samples violated the upper Class B(WW) criterion of 9.0 pH units (11.5 units on May 4, 2010. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), the percentage of violations at this station during the 2010-2012 period for pH (5%) does not suggest a water quality impairment. These guidelines allow up to 10% violations of conventional parameters such as pH and dissolved oxygen before impairment of water quality is indicated. Samples from the MPCA station were not analyzed for ammonia, toxic metals, or pesticides. Monthly sampling at this station from April to September 2007, however, showed that levels of ammonia in the six samples collected were below the level of detection (0.16 mg/l).