Iowa DNR
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Water Quality Assessments

Impaired Waters List

Shell Rock River IA 02-SHL-783

from south corporate limit of Shell Rock (S12 T91N R15W Butler Co.) to confluence with Flood Cr. in S27 T93N R16W Butler Co.

Assessment Cycle
2018
Release Status
Final
Data Collection Period
Overall IR Category
5 - Water is impaired or threatened and a TMDL is needed.
Trend
Unknown
Created
5/23/2019 12:21:36 PM
Updated
7/11/2019 11:55:08 AM
Assessment conducted in accordance with Iowa's 2018 IR methodology
Use Support
Class A1
Presumptive Use
Partially Supported
Bacteria: Indicator Bacteria- E. coli
Support Level
Partially Supported
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Slight
Status
New
Source
Unknown: Source Unknown
Source Confidence
N/A
Cycle Added
2018
Impairment Rationale
Geometric mean criterion exceeded
Data Source
Ambient monitoring: Iowa DNR-rivers
TMDL Priority
Tier III
Class BWW1
Fully Supported
Class HH
Partially Supported
Fish Consumption Advisory: Mercury
Support Level
Partially Supported
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Moderate
Status
Continuing
Source
Unknown: Source Unknown
Source Confidence
N/A
Cycle Added
2012
Impairment Rationale
Fish consumption advisory in effect: no more than 1 meal/week
Data Source
Fish contaminant monitoring: Iowa DNR
TMDL Priority
Tier IV
General Use
Not Assessed
Impairment Delistings
No delistings for this assessment cycle.
Documentation
Assessment Summary

The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" based on levels of indicator bacteria. The Class B(WW1) aquatic life uses are assessed (evaluated) as "fully supported" based on results of DNR/SHL biological sampling conducted in 2012-2014, 2016.  The fish consumption (Class HH) uses remain assessed (monitored) as "partially supported" due to issuance of fish consumption advisory.  The sources of data for this assessment are (1) the results of monthly ambient water quality monitoring from January 2014 through December 2016 at the DNR/SHL ambient station located at the County Road C45 bridge at Shell Rock, Butler Co. (STORET station 10120001); (2) results of U.S. EPA/Iowa DNR fish contaminant (RAFT) monitoring near Clarksville in September 2011; (3) DNR/SHL biological sampling conducted in 2012-2014, 2016 and (4) DNR Fisheries Bureau fish sampling data collected in 2007 and 2008. 

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 2014 through 2016 at Iowa DNR Station 10120001 Shell Rock River at Shell Rock were as follows: the 2014 geometric mean was 59 orgs/100 ml, the 2015 geometric mean was 44 orgs/100 ml, and the 2016 geometric mean was 129 orgs/100 ml. One of the three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Three of the combined 24 samples (12%) 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."

The aquatic life assessment based on biological sampling suggests the aquatic life uses are "fully supporting". This evaluated biological assessment was based on data collected in 2012-2014, 2016 as part of the DNR/SHL stream nutrient sampling project and fish sampling data collected in 2007 and 2008 by the DNR Fisheries Bureau. A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biological sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species 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 2007 and 2008 FIBI scores were 62 (good) and 74 (excellent). The 2012-2014, 2016 BMIBI scores were 51 (fair), 59, 65, 65, 69, 71 (all good) and 81 (excellent). The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008. The artificial substrate BMIBI BIC for this ecoregion is 52 and the natural substrate BMIBI BIC is 70. This assessment is considered evaluated because the drainage area (1626, 1662 and 1730 mi2) above the sampling sites was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. The riffle/non-riffle FIBI BICs for this ecoregion are 65/44 and the riffle status is unknown at the two sites where fish sampling was conducted. This site passed the artificial substrate BMIBI BIC (52) in 2/3 samples and passed the natural substrate BMIBI BIC (70) in 2/4 samples. Overall, this site passed the BMIBI BIC in 4/7 samples. Even though this site passed the FIBI BIC and passed the BMIBI BIC (4/7), 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.

In agreement with the biological sampling results that suggest "full support" of the Class B(WW1) aquatic life uses, results of chemical/physical water quality monitoring Shell Rock also suggest “full support” of the aquatic life uses. Monitoring showedno violations of Class B(WW1) water quality criteria for 35 Ammonia samples (maximum = 0.3 mg/L), 36 Dissolved Oxygen samples (minimum = 7.9 mg/L), 36 Temperature samples (maximum = 24° c), 36 Chloride samples (maximum = 37 mg/L), or 36 Sulfate samples (maximum = 60 mg/L) occurred during monitoring from January 2014 to December 2016.1 of the 35 samples (3) analyzed for pH (range = 7.8 to 9.1) 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), a violation frequency of greater than 10% for conventional parameters such as pH 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.These results suggest that the Class B(WW1) aquatic life uses should be assessed as “fully supported.”

Fish consumption uses remain assessed (monitored) as “partially supported” based on results of U.S.EPA/DNR fish contaminant (RAFT) monitoring in 2011. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. The fish contaminant data generated from the 2011 RAFT sampling conducted on the Shell Rock River near Clarksville show that levels of mercury are sufficiently high for concern and thus justify issuance of a one meal/week fish consumption advisory that extends from the mouth of the Shell Rock River to its confluence with the Winnebago River near Rockford in Floyd County. Results from the 2011 sampling showed that the level of mercury in the sample of walleye muscle tissue (0.753 ppm) far exceeded the 1 meal/week trigger level (0.30 ppm) as defined in Iowa’s revised (2007) fish consumption advisory protocol. According to the Iowa DNR/Iowa Dept. of Public Health fish advisory protocol, the single occurrence of contaminant above an advisory trigger level does not typically result in issuance of an advisory. Such an advisory is issued only after follow-up monitoring confirms that contaminant levels exceed the advisory trigger level. Because, however, the level of mercury in the 2011 sample of walleye was well above the one meal/week advisory trigger, a one meal/week consumption advisory was issued. Follow-up monitoring was conducted in 2012 to better define the level of mercury in Shell Rock River fish and to provide information for any changes needed to the consumption advisory. Results showed that the average level of mercury in tissue samples form three Walleye was just below the consumption advisory threshold of 0.3 ppm (ave = 0.285, sd = 0.078). Thus, the one-meal per week advisory that was issued in 2013 remained in effect. Because a one meal per week fish consumption advisory covers this waterbody, the fish consumption uses are assessed as “partially supported.”

Monitoring and Methods
Assessment Key Dates
8/23/2011 Fish Tissue Monitoring
8/25/2016 Biological Monitoring
8/6/2012 Biological Monitoring
1/9/2014 Fixed Monitoring Start Date
12/6/2016 Fixed Monitoring End Date
8/2/2012 Fish Tissue Monitoring
9/3/2013 Biological Monitoring
9/8/2014 Biological Monitoring
9/26/2007 Biological Monitoring
9/16/2008 Biological Monitoring
Methods
150 Monitoring data more than 5 years old
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
420 Indicator bacteria monitoring