South Skunk River IA 03-SSK-931

The Class A1 uses are assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E. coli). The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2012 through 2014 at IDNR station 10850002 near Cambridge were as follows: the 2012 geometric mean was 1,209 orgs/100 ml, the 2013 geometric mean was 892 orgs/100 ml, and the 2014 geometric mean was 298 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Twenty-one of the 24 samples (88%) 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 IDNR’s assessment/listing methodology, if a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses should be assessed as "impaired" (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 are assessed as “impaired.”

The Class B(WW2) aquatic life uses remain assessed (evaluated) as “partially supported” based on results of biological sampling. Results of biological sampling conducted in 2012 and 2014 as part of the IDNR/SHL stream biocriteria project and the 2002 IDNR Fisheries sampling suggest that these uses are “partially supported.” A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species that were collected in the stream sampling reach. The biological metrics were combined to make a fish community index of biotic integrity (FIBI) and a benthic macroinvertebrate index (BMIBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).  The 2012 and 2014 BMIBI scores were 19 (poor) and 47 (fair). The 2002 Fisheries FIBI score was 48 (fair). The aquatic life use support was assessed (evaluated) as partial supporting (=PS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008. The non-riffle FIBI BIC for this ecoregion is 32 and the BMIBI BIC for this ecoregion is 62. This segment passed the FIBI BIC 1/1 times in 2002 and passed the BMIBI BIC 0/2 times in the last five years. This assessment is considered evaluated because the drainage area (585 and 605 mi2) above the sampling sites was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. Even though this site passed the FIBI BIC (1/1) and failed the BMIBI BIC (0/2), it is uncertain as to whether or not this segment is meeting the aquatic life criteria because the sites used for the assessment don’t fall in the calibrated watershed size.

According to IDNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are thus not appropriate for Section 303(d) listing (Category 5 of the Integrated Report). IDNR does, however, consider these impairments as appropriate for listing under either Category 2b or 3b of the Integrated Report (waters potentially impaired and in need of further investigation).

Despite the results of biological monitoring that suggest impairment of the Class B(WW2) uses, results of ambient chemical/physical monitoring suggest full support of these uses. The results of routine monthly ambient water quality monitoring during the 2012-2014 assessment period at the Cambridge station (located approximately 1/3 mile downstream from the outfall of the Ames wastewater treatment plant) suggest relatively good water quality in this segment of the South Skunk River. Results of this monitoring show no violations of Class B(WW2) criteria for pH, dissolved oxygen, ammonia nitrogen (maximum value = 1.6 mg/l), chloride, or sulfate in the 36 samples analyzed during the 2012-14 assessment period.