Assessment is based on: (1) IDNR/UHL monthly ambient monitoring conducted during the 2008-2010 assessment period at Davis City (STORET station 10270001 (formerly station 784089)), (2) IDNR/UHL biocriteria monitoring in 2001, and (3) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2003.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported" due to levels of indicator bacteria (E. coli) that violate state water quality criteria. The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of IDNR/UHL chemical/physical and biological monitoring. Fish consumption uses are assessed (evaluated) as "fully supported" based on results of fish contaminant monitoring in 2003. This assessment is based on results of (1) IDNR/UHL monthly ambient monitoring conducted during the 2008-2010 assessment period at Davis City (STORET station 10270001 (formerly station 784089)), (2) IDNR/UHL biocriteria monitoring in 2001, and (3) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2003.
EXPLANATION: The Class A1 (primary contact recreation) are assessed as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at the Thompson Fork of the Grand River near Davis City were as follows: the 2008 geometric mean was 918 orgs/100 ml, the 2009 geometric mean was 1020 orgs/100 ml and the 2010 geometric mean was 512 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Fourteen of the 22 samples (64%) 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 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 are assessed as “impaired.”
Regarding support of the Class B(WW1) aquatic life uses, results of ambient water quality monitoring at the IDNR/UHL station at Davis City showed no violations of Class B(WW1) water quality criteria during the 2008-2010 assessment period for dissolved oxygen (minimum value = 5.8 mg/l), pH (range = 7.4 to 8.6 units), or ammonia-nitrogen (maximum value = 0.7 mg/l) in the 33 samples analyzed. These results suggest "full support" of the Class B(WW1) aquatic life uses.
Similarly, results of IDNR/UHL biocriteria monitoring suggest that the Class B(WW1) uses are "fully supported." Data were collected in 2001 as part of the IDNR/UHL stream biocriteria 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 2001 FIBI score was 32 (fair) and the BMIBI score was 43 (fair). The aquatic life use support was assessed (evaluated) as fully supported (=FS), 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 1994-2004. 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 area (602 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. Even though this site passed both the FIBI and BMIBI BICs (using the FIBI uncertainty adjustment value of 7 points), it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it doesn’t fall in the calibrated watershed size. Also, this assessment is considered of lower confidence (evaluated) as opposed to higher confidence (monitored) because the data from the 2001 biological sampling are now older than five years; thus, these data are considered less able to accurately characterize current water quality conditions than would be more recent data.
Fish consumption uses are assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) 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.