Iowa DNR

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
Result Period
2008 - 2010
Class A1 Class B(WW-1) Class HH
Assessment Methodology
Assessment Type
Integrated Report
Category 5a
Legacy ADBCode
IA 02-SHL-0010_2
Overall Use Support
Not supporting
Aquatic Life Use Support
Fish Consumption
Primary Contact Recreation
Not supporting
Assessment Comments

Assessment is based on (1) results of monthly (plus) ambient water quality monitoring from January 2008 through December 2010 at the IDNR/UHL ambient station located at the County Road C45 bridge at Shell Rock, Butler Co. (STORET station 10120001) and (2) results of U.S. EPA/Iowa DNR fish contaminant monitoring in 2011.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this river segment was designated only for Class B(WW) aquatic life uses, including fish consumption uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.   EPA in February 2008 and the results of an Use Attainability Analysis, this segment is also now designated for Class A1 (primary contact recreation) uses.   This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" based on results of monitoring for bacteria.   This assessment represents a re-listing of the bacterial impairment previously indentified for this assessment segment.   The Class B(WW1) aquatic life uses were assessed (monitored) as "fully supported" based on results of IDNR/UHL ambient water quality monitoring conducted during the 2008-2010 assessment period.   The fish consumption (Class HH) uses are assessed (monitored) as "partially supported" (IR Category 5a) due to issuance of fish consumption advisory.   The sources of data for this assessment are (1) the results of monthly (plus) ambient water quality monitoring from January 2008 through December 2010 at the IDNR/UHL ambient station located at the County Road C45 bridge at Shell Rock, Butler Co.  (STORET station 10120001) and (2) results of U.S.  EPA/Iowa DNR fish contaminant (RAFT) monitoring near Clarksville in September 2011.   Note:  USGS conducted monitoring in this assessment segment at station 05462000 at Shell Rock in October 2010.   Too few data were produced from this monitoring during the 2008-2010 assessment period (one sample) to be of use for assessment for purposes of either Section 305(b) reporting or Section 303(d) listing.

EXPLANATION:  The Class A1 uses are assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E.  coli) at IDNR/UHL station 10120001.   The geometric means of indicator bacteria (E.  coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at IDNR ambient station 10120001 at Shell Rock were as follows:  the 2008 geometric mean was 254 orgs/100 ml, the 2009 geometric mean was 61 orgs/100 ml and the 2010 geometric mean was 48 orgs/100 ml.   The 2008 geometric mean exceeded the Class A1 criterion of 126 orgs/100 ml; the 2009 and 2001 geometric means were well below the Class A1 criterion.   Three of the 22 samples (14%) 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.”  Despite this impairment, levels of indicator bacteria in this segment have historically been, and continue to be, low relative to other Iowa streams and rivers.   The very low geometric means during the 2009 and 2010 recreation seasons suggest that this river segment typically “fully supports” its designated Class A1 primary contact recreation uses.

Results of chemical/physical monitoring at station 10120001 continue to suggest “full support” of the Class B(WW1) aquatic life uses.   Monitoring at this station from 2008 through 2010 showed no violations of Class B(WW1) water quality criteria for ammonia, dissolved oxygen, or pH in the approximately 30 samples analyzed or in the three samples analyzed for toxic metals.   These results suggest that the Class B(WW1) aquatic life uses should be assessed as “fully supported.”  This assessment of the aquatic life uses is consistent with previous Section 305(b) assessments.  

EXPLANATION: Fish consumption uses are assessed (monitored) as “partially supported” based on results of U.S.EPA/IDNR 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 show that the level of mercury in the sample of walleye muscle tissue (0.75 ppm) far exceeds 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 will be conducted to better define the level of mercury in Shell Rock River fish and to provide information for any changes needed to the consumption advisory.  Because a one meal per week fish consumption advisory was issued for this waterbody, the fish consumption uses are assessed as “partially supported.”

Monitoring and Methods
Assessment Key Dates
8/23/2011 Fish Tissue Monitoring
12/2/2010 Fixed Monitoring End Date
1/8/2008 Fixed Monitoring Start Date
230 Fixed station physical/chemical (conventional plus toxic pollutants)
420 Water column surveys (e.g. fecal coliform)
260 Fish tissue analysis
Monitoring Levels
Biological 0
Habitat 0
Physical Chemistry 3
Toxic 3
Pathogen Indicators 3
Other Health Indicators 0
Other Aquatic Life Indicators 0
# of Bio Sites 0
BioIntegrity N/A
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Pathogens Primary Contact Recreation Slight
  • Source Unknown
  • Slight
Mercury Fish Consumption Moderate
  • Source Unknown
  • High