Thompson River IA 05-GRA-1351

The Class A1 (primary contact recreation) uses are 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 2014 through 2016 at the Thompson Fork of the Grand River near Davis City (STORET station 10270001) were as follows: the 2014 geometric mean was 905 orgs/100 ml, the 2015 geometric mean was 263 orgs/100 ml, and the 2016 geometric mean was 367 orgs/100 ml. All three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Fourteen of the combined 24 samples (58%) 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 primary 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 "partially supported."

Results of chemical/physical water quality monitoring at the Thompson Fork of the Grand River near Davis City (STORET station 10270001) suggest “full support” of the aquatic life uses. Monitoring showed no violations of Class B(WW1) water quality criteria for 36 Ammonia samples (maximum = 1 mg/L), 36 pH samples (range = 7.2 to 8.6), 36 Temperature samples (maximum = 25.2 °C), 36 Chloride samples (maximum = 22 mg/L), or 36 Sulfate samples (maximum = 49 mg/L) occurred during monitoring from January 2014 to December 2016. One of the 36 samples (3%) analyzed for Dissolved Oxygen (minimum = 4.9 mg/L) violated the Class B(WW1) criteria during the 2014-2016 monitoring period. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17) and Iowa DNR's assessment/listing methodology, a violation frequency of significantly greater than 10% for conventional parameters such as Dissolved Oxygen suggests impairment of aquatic life uses. Because the frequency of violations for this parameter is not greater than 10 percent, these results do not suggest impairment of the Class B(WW1) aquatic life uses.

Contrary to the water quality assessment, the results of DNR/SHL biocriteria sampling suggest that the Class B(WW1) uses are "partially supported." Data were collected in 2011, 2012, 2013 and 2015 as part of the DNR/SHL stream large river sampling project. 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 2011 FIBI score was 43 (fair) and the BMIBI score was 59 (good). The 2012 BMIBI score was 19 (poor), the 2013 BMIBI scores were 30 (poor) and 36 (fair) and the 2015 BMIBI score was 50 (fair). The aquatic life use support was assessed (evaluated) as partially supported (=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 FIBI BIC for this ecoregion is 33 and the BMIBI BIC for this ecoregion is 41. This assessment is considered evaluated because the drainage areas (602 and 701 mi2) above this sampling sites were 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 in 2011 and failed the BMIBI BIC (2/5) in the last six years, 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 DNR’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). DNR 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).

Fish consumption uses remain assessed (evaluated) as “fully supported” based on results of U.S.EPA/DNR fish contaminant monitoring northwest of Davis City in 2003. Because the data upon which these assessment are based are now more than five years old, the assessment type is changed from “monitored” (a higher confidence assessment) to “evaluated” (a lower confidence assessment). The composite samples of fillets from channel catfish and flathead catfish had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.048 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of flathead catfish fillets were as follows: mercury: 0.074 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2003 RAFT sampling northwest of Davis City show that the levels of contaminants do not exceed any of the Iowa advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.