Assessment Comments

Assessment is based on: (1) IDNR/SHL monthly ambient monitoring conducted during the 2010-2012 assessment period at Davis City (STORET station 10270001 (formerly station 784089)), (2) IDNR/SHL biological sampling in 2001, 2011 and 2012, 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" (IR 5a) 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" (IR 2a) based on results of IDNR/SHL chemical/physical and biological sampling.   Fish consumption uses are assessed (evaluated) as "fully supported" (IR 2a) based on results of fish contaminant monitoring in 2003.   This assessment is based on results of (1) IDNR/SHL monthly ambient monitoring conducted during the 2010-2012 assessment period at Davis City (STORET station 10270001 (formerly station 784089)), (2) IDNR/SHL biological sampling conducted in 2001, 2011 and 2012 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 24 samples collected during the recreational seasons of 2010 through 2012 at the Thompson Fork of the Grand River near Davis City were as follows:  the 2010 geometric mean was 512 orgs/100 ml, the 2011 geometric mean was 239 orgs/100 ml and the 2012 geometric mean was 90 orgs/100 ml.   The 2010 and 2011 geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Eleven of the 24 samples (46%) 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/SHL station at Davis City showed no violations of Class B(WW1) water quality criteria during the 2010-2012 assessment period for dissolved oxygen (minimum value = 6.6 mg/l), pH (range = 6.9 to 8.7 units), ammonia-nitrogen (maximum value = 0.4 mg/l), chloride, or sulfate in the 36 samples analyzed.   One of the 36 samples (<1%) violated the Class B(WW1) criterion for water temperature of 32C.  

According to U.S.  EPA guidelines for Section 305(b) reporting, if more than 10% of samples exceed state criteria for conventional parameters such as dissolved oxygen and water temperature, the aquatic life uses should be assessed as "impaired" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).   According to IDNR’s assessment/listing methodology, however, the results from station 10270001, however, do not indicate that significantly greater than 10% of the samples exceed the Class B(WW1) criterion for water temperature.   Thus, the results of monthly ambient water quality monitoring from 2010-12 suggest that the presumptive Class B(WW1) uses should be assessed (monitored) as “fully supported.”

Similarly, results of IDNR/SHL biocriteria monitoring suggest that the Class B(WW1) uses are "fully supported."  Data were collected in 2001, 2011 and 2012 as part of the IDNR/SHL stream biocriteria and 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 2001 FIBI score was 32 (fair) and the BMIBI score was 43 (fair).   The 2011 FIBI score was 43 (fair) and the BMIBI score was 59 (good).   The 2012 BMIBI score was 19 (poor).   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 1994-2008.   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 both the FIBI BIC (2/2) and the BMIBI BIC (2/3), 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.   IDNR normally requires a simple majority of passing scores to indicate "full support."  The 2001 FIBI score failed the BIC by one point; however, given the age of those data combined with the improvement in the 2011 FIBI score, the aquatic life uses of this segment was assessed as "fully supporting" based on the professional judgement of IDNR staff.    

Fish consumption uses remain 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.