Assessment Comments
Assessment based on results of ambient physical/chemical water quality monitoring conducted by (1) IDNR/UHL in 2002 in support of TMDL development and (2) the Minnesota Pollution Control Agency.
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses are assessed (evaluated) as “fully supported / threatened” (minor impacts) based on results of ambient chemical/physical water quality monitoring conducted near the Iowa/Minnesota state line by (1) IDNR/UHL from March through November 2002 as part of TMDL monitoring (TMDL station 3; STORET station 11600002) and (2) the Minnesota Pollution Control Agency (MPCA) from November 2000 through September 2001 (MPCA STORET station S000-097).
EXPLANATION: Results of chemical/physical water quality monitoring conducted from March through November 2002 by IDNR/UHL in support of TMDL development, and conducted by MPCA from November 2000 through September 2001, suggest relatively good water quality in this stream. Results of IDNR/UHL monitoring show that none of the 11 samples collected had levels of dissolved oxygen or ammonia-nitrogen that violated state water quality standards (these samples were not analyzed for 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.
Results of MPCA monitoring show that none of the 10 samples collected had levels of pH or ammonia-nitrogen that violated state water quality standards (these samples were not analyzed for toxic metals or pesticides). One of the 10 samples, however, violated the Class B(LR) criterion for dissolved oxygen of 5.0 mg/l: the sample collected on July 10, 2001 contained only 2.9 mg/l of dissolved oxygen. 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 2000-2002 period for dissolved oxygen (10%) 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.
Due to the occurrence of the violation of the water quality criterion for dissolved oxygen, and due to the apparently wide fluctuations in levels of dissolved oxygen in this stream (as indicated by levels of pH that exceed state standards), the Class B(LR) uses were assessed as "fully supported / threatened" with minor impacts. The decision to consider this assessment as “evaluated” (lower confidence) versus “monitored” (higher confidence) is based on (1) the relatively few samples collected by each agency (approximately 10) and the relatively poor parameter coverage (i.e., no analysis of samples for toxic metals or pesticides).
