Assessment Comments

Assessment is based on (1) results of monthly monitoring from January 2010 through December 2012 at the IDNR ambient monthly monitoring station located at Hardin County Road D53 approximately 1.8 miles northeast of Gifford (STORET station 10420001); (2) results of IDNR/EPA fish contaminant monitoring at Marshalltown in 2011 and 2012 and (3) IDNR/SHL biological sampling conducted in 2011 and 2012.

Basis for Assessment

NOTE:  The placement of recent water quality assessments for this segment and the downriver segment (IA 02-IOW-0070_2) have been confused.   Segment IA 02-IOW-0070_2 extends from the Hardin/Marshall county line upriver to the South Fork Iowa River; segment IA 02-IOW-0070_3 (this segment) extends from the South Fork Iowa River upriver to Pine Creek near Eldora.   Because IDNR/SHL ambient monitoring station 10420001 is upstream from the confluence with the South Fork Iowa River--and is thus in this segment (i.e., IA 02-IOW-0070_3)--any assessments based on monitoring data from this station should have been applied to this segment.   Thus, any assessments based on data from this station and erroneously placed in the downriver segment (IA 02-IOW-0070_2) were restored to this segment (IA 02-IOW-0070_3) as part of the 2006 Section 305(b) assessment cycle.   Also, recent assessments for the downriver segment (IA 02-IOW-0070_2) are now correctly considered "not assessed" due to the lack of monitoring information upon which to base an assessment.  

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" IR 5a) due to levels of indicator bacteria that slightly exceed state water quality criteria.   The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" (IR 2a) based on results of ambient water quality monitoring from 2010-2012.   Fish consumption uses are assessed as “partially supporting” (IR 5a) due to issuance of a fish consumption advisory in 2013.   This is a new impairment for this assessment segment.   The sources of data for this assessment are (1) the results of monthly monitoring from January 2010 through December 2012 at the IDNR ambient monthly monitoring station located at Hardin County Road D53 approximately 1.8 miles northeast of Gifford (STORET station 10420001) and (2) results of IDNR/EPA fish contaminant monitoring at Marshalltown in 2011 and 2012 and (3) IDNR/SHL biological sampling conducted in 2011 and 2012.

EXPLANATION:  The Class A1 uses are assessed (monitored) as "not supported" due to levels of indicator bacteria (E.  coli) that slightly exceed state criteria.   The geometric means of indicator bacteria (E.  coli) in the 24 samples collected during the recreational seasons of 2010 through 2012 at IDNR station 10420001 were as follows:  the 2010 geometric mean was 292 orgs/100 ml, the 2010 geometric mean was 246 orgs/100 ml and the 2012 geometric mean was 136 orgs/100 ml.   All three geometric means slightly exceed the Class A1 criterion of 126 orgs/100 ml.   Ten of the 24 samples (42%) 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 should be assessed as "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 Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of monitoring from the IDNR/UHL ambient station near Gifford from 2010 through 2012.   Monitoring at this station showed no violations of Class B(WW1) water quality criteria in the 36 samples analyzed for pH, temperature, ammonia, chloride, and sulfate.   One of the 36 samples analyzed (3%) violated the Class B(WW1) criterion for dissolved oxygen.   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, 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 10570001 do not indicate that significantly greater than 10% of the samples exceed the Class B(WW1) criteria for dissolved oxygen.   Thus, these results do not suggest impairment of the Class B(WW1) aquatic life uses.  

In agreement with the water quality aquatic life assessment, 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 2011 and 2012 as part of the IDNR/SHL stream nutrient 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 2011 BMIBI score was 71 (good) and the 2012 BMIBI score was 57 (good).   The aquatic life use support was assessed (evaluated) as fully supporting (=FS), 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 51.   This assessment is considered evaluated because the drainage area (775 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 passed the BMIBI BIC (2/2), 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.  

Fish consumption/human health uses are assessed (monitored) as “partially supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring in 2011 and 2012.   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 Iowa River at Marshalltown showed that levels of mercury were sufficiently high for concern.   The average level of mercury in the tissue plugs from four walleye was 0.323 mg/kg (SD=0.045), thus slightly exceeding the one meal per week advisory threshold of 0.3 mg/kg.   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.   Results of follow-up monitoring in 2012 confirmed that levels of mercury in predator fish (walleye) were above the threshold for issuance of a one meal/week fish consumption advisory.   The 2012 average level of mercury in the tissue plugs from three walleye was 0.308 mg/kg (SD=0.071), thus again slightly exceeding the one meal per week advisory threshold of 0.3 mg/kg.   Because the levels of mercury in both the 2011 and 2012 samplings were above the one meal/week advisory trigger, a one meal/week consumption advisory was issued in 2013 that extends from the upper end of Coralville Reservoir to the dam at Iowa Falls (this new advisory incorporates an existing advisory from the Highway 20 bridge upstream to the dam at Iowa Falls).   The advisory recommends that no more than one meal per week of predator fish from this river segment be eaten.   According to the Iowa DNR assessment/listing methodology, the existence of a one meal per week consumption advisory indicates that the fish consumption uses should be assessed as “partially supported.”