Assessment Comments

Assessment is based on (1) results of IDNR/SHL monthly ambient monitoring from 2010 through 2012 approximately two miles south of Humboldt (STORET station 10460001 (formerly station 444061), (2) results of IDNR/U.S. EPA fish contaminant monitoring in 2009 and 2010, (3) results of IDNR follow-up fish contaminant monitoring in 2011 and (4) IDNR/SHL biological sampling data conducted in 2012 near Humboldt.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream 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 due to the completion of a Use Attainability Analysis in 2007, 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" (IR 5a) based on results of ambient monitoring for indicator bacteria (E.  coli) during recreational seasons of 2010 through 2012.   The Class B(WW1) aquatic life uses are assessed (evaluated) as "partially supported" (IR 3b-u) based on 2012 IDNR/SHL biological sampling data.   Fish consumption uses are assessed as “partially supporting” (IR 5a) due to issuance of a fish consumption advisory for this river segment due to levels of mercury that exceed the one meal per week consumption advisory threshold.   This is a new impairment for this assessment segment.   The sources of data for this assessment are (1) the results of IDNR/SHL monthly ambient monitoring from 2010 through 2012 approximately two miles south of Humboldt (STORET station 10460001 (formerly station 444061)) and (2) fish contaminant monitoring on the West Fork Des Moines River from 2009 to 2011.  

EXPLANATION:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" based on results of monitoring for indicator bacteria (E.  coli).   The geometric means of indicator bacteria (E.  coli) in the 24 samples collected during the recreational seasons of 2010 through 2012 at IDNR station 10460001near Humboldt were as follows:  the 2010 geometric mean was 160 orgs/100 ml, the 2011 geometric mean was 110 orgs/100 ml, and the 2012 geometric mean was 95 orgs/100 ml.   Only the 2010 geometric mean slightly exceed the Class A1 criterion of 126 orgs/100 ml.   Five of the 24 samples (21%) 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.”  

The water quality monitoring at the IDNR/SHL station south of Humboldt showed no violations of Class B(WW1) water quality criteria during the 2010-2012 assessment period for dissolved oxygen, pH, temperature, ammonia-nitrogen, chloride, or sulfate in the 36 samples analyzed.   These results suggest "full support" of the Class B(WW1) aquatic life uses.  

In contrast to the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the aquatic life uses are "partially supporting".   This evaluated biological assessment was based on data collected in 2012 as part of the IDNR/SHL large river sampling project.   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 collected in the stream sampling reach.   The biological metrics were combined a benthic macroinvertebrate index (BMIBI).   The index rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2012 BMIBI scores were 57 and 70 (both good).  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 BMIBI BIC for this ecoregion is 62.   This assessment is considered evaluated because the drainage area (2308 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 failed the BMIBI BIC (1/2), 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.   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 are assessed (monitored) as “partially supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring in 2009 and 2010.   The fish contaminant data generated from the 2009 RAFT sampling conducted at the West Fork Des Moines River near Bradgate showed that levels of mercury were sufficiently high for concern and thus justified follow-up monitoring.  Results from the 2009 sampling showed that the level of mercury in the sample of walleye fillets (0.328 ppm) exceeded the trigger level of 0.30 ppm for a one meal per week consumption advisory as defined in Iowa’s fish consumption advisory protocol.  According to IDNR’s assessment methodology, the single occurrence of contaminant above an advisory trigger level neither warrants issuance of an advisory nor indicates impairment of the fish consumption uses: two consecutive samplings that show contaminant levels are above the trigger level in fillet samples are needed to justify issuance of an advisory.  Follow-up sampling conducted in 2010 showed that levels of mercury in walleye (0.37 ppm) again exceeded the 1 meal/week trigger level (0.30 ppm).  Thus a one meal per week fish consumption advisory was issued for West Fork Des Moines River in 2011 that extends from the confluence of the East Fork and West Fork of the Des Moines River upriver to Highway 15 south of West Bend, Iowa.  

Additional IDNR monitoring conducted in 2011 on the West Fork Des Moines River at Gotch Park showed that levels of mercury were below the consumption advisory threshold of 0.3 mg/kg.   The average levels of mercury in tissue samples from three walleye was 0.143 mg/kg (SD=0.067 mg/kg).   Even though this level of mercury is well below the advisory threshold, the existence of the consumption advisory for this segment of the West Fork Des Moines River indicates that the fish consumption uses should be assessed as “partially supporting”.