Iowa DNR

Water Quality Assessments

Impaired Waters List

Thompson River IA 05-GRA-1351

from the Iowa/Missouri state line to confluence with Long Cr. in SW 1/4, S8, T69N, R26W, Decatur Co.

Assessment Cycle
Release Status
Release Period
2012 - 2014
9/20/2016 1:39:37 PM
Overall IR Category
5 - Water is impaired or threatened and a TMDL is needed.
Last Updated
9/28/2016 12:04:48 PM
Use Support
Class A1
Primary contact
Partially Supporting
Bacteria: Indicator Bacteria, E. coli
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Unknown: Source Unknown
Source Confidence
Cycle Added
Impairment Rationale
Geometric mean criterion exceeded
Data Source
Ambient monitoring: Iowa DNR-rivers
TMDL Priority
Tier III
Class B(WW-1)
Aquatic Life
Warm Water Type 1
Partially Supporting
Biological: low aquatic macroinvertebrate IBI
Impairment Code
3b-u - Use potentially biologically impaired based on uncalibrated IBI metrics.
Cause Magnitude
Unknown: Source Unknown
Source Confidence
Cycle Added
Impairment Rationale
Low Biotic Index
Data Source
Biological monitoring: Iowa DNR WQMA
Class HH
Human Health
Not Assessed
General Use
General Use water
Not Assessed
Impairment Delistings
No delistings for this assessment cycle.
Assessment Summary

The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially 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 (evaluated) as "partially supported" (IR 3b-u) based on results of IDNR/SHL 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 2012-2014 assessment period at Davis City (STORET station 10270001 (formerly station 784089)), (2) IDNR/SHL biological sampling conducted in 2011, 2012 and 2013 and (3) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2003.

Assessment Explanation

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 2012 through 2014 at the Thompson Fork of the Grand River near Davis City were as follows: the 2012 geometric mean was 90 orgs/100 ml, the 2013 geometric mean was 104 orgs/100 ml, and the 2014 geometric mean was 905 orgs/100 ml.  One of the three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml.  Eight of the combined 24 samples (33%) 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 IDNR’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 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 = 6.9 to 8.7), 36 Chloride samples (maximum = 22 mg/L), or 36 Sulfate samples (maximum = 53 mg/L) occurred during monitoring from January 2012 to December 2014. One of the 36 samples (3%) analyzed for Dissolved Oxygen (minimum = 4.9 mg/L) and one of the 36 samples (3%) analyzed for Temperature (maximum = 32.2° c) violated the Class B(WW1) criteria during the 2012-2014 monitoring period. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), a violation frequency of greater than 10% for conventional parameters such as Dissolved Oxygen, or Temperature suggests impairment of aquatic life uses. Because the frequency of violations for these parameters are 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 IDNR/SHL biocriteria sampling suggest that the Class B(WW1) uses are "partially supported." Data were collected in 2011, 2012 and 2013 as part of the IDNR/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) and the 2013 BMIBI scores were 30 (poor) and 36 (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 and failded the BMIBI BIC (1/4), 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).

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.

Monitoring and Methods
Assessment Key Dates
1/4/2012 Fixed Monitoring Start Date
12/1/2014 Fixed Monitoring End Date
7/19/2012 Biological Monitoring
9/3/2013 Biological Monitoring
9/23/2011 Biological Monitoring
8/23/2003 Fish Tissue Monitoring
230 Fixed station physical/chemical (conventional plus toxic pollutants)
260 Fish tissue analysis
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
330 Fish surveys
380 Quan. measurements of instream parms-- channel morphology-- floodplain-- 1-2 seasons-- by prof
420 Water column surveys (e.g. fecal coliform)
150 Monitoring data more than 5 years old