Assessment Comments

Assessment is based on: (1) the results of monthly monitoring from January 2010 through December 2012 at the IDNR/SHL ambient monitoring station located at the Highway 63 bridge north of Oskaloosa (STORET station 10620001; formerly station 821035), (2) the results of U.S. EPA/IDNR fish tissue (RAFT) monitoring near Oskaloosa in 2005 and 2006 and (3) IDNR/SHL biological sampling conducted in 2012 and 2013.

Basis for Assessment

[Note 1:  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 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf) and 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).]

[Note 2:  The assessments for segments IA 03-SSK-0010_2 and IA 03-SSK-0010_3 are the same except for the lack of the Class C (drinking water) assessment and biological aquatic life use assessment for the downstream segment (IA 03-SSK-0010_2).]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" (IR 5a) due to levels of indicator bacteria 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 2012/2013 biological sampling.   The assessment of the Class C (drinking water) is changed from "not supported" (IR 5a) due to violations of state water quality criterion for nitrate to “fully supported” (IR 2a).   This assessment represents a de-listing of the nitrate impairment for this river segment.   Fish consumption uses remain assessed (monitored) as "fully supported" (IR 2a) based on results of fish contaminant monitoring in 2005 and 2006.   The sources of data for this assessment include (1) the results of monthly monitoring from January 2010 through December 2012 at the IDNR/SHL ambient monitoring station located at the Highway 63 bridge north of Oskaloosa (STORET station 10620001; formerly station 821035), (2) the results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring near Oskaloosa in 2005 and 2006 and (3) IDNR/SHL biological sampling conducted in 2012 and 2013.    

EXPLANATION:  The Class A1 uses are assessed (monitored) as "not supported" based on results of ambient 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 10620001 near Oskaloosa were as follows:  the 2010 geometric mean was 426 orgs/100 ml, the 2011 geometric mean was 138 orgs/100 ml and the 2012 geometric mean was 232 orgs/100 ml.   All three geometric means slightly exceed the Class A1 criterion of 126 orgs/100 ml.   Twelve of the 24 samples (50%) 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.”

Based on the results of monitoring from the IDNR ambient station north of Oskaloosa from 2010 through 2012, the Class B(WW1) aquatic life uses should be assessed as "fully supported".    Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, temperature, pH, ammonia-nitrogen, chloride, or sulfate in the 36 samples analyzed.  

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 and 2013 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 2012 BMIBI score was 18 (poor).   The 2013 BMIBI score was 42 (fair).   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 51.   This assessment is considered evaluated because the drainage area (1635 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 (0/2), 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).  

The Class C drinking water uses are assessed (monitored) as “fully supported”.   None of the 36 samples analyzed for nitrite+nitrate, chloride; the 13 samples analyzed for atrazine; or the seven samples analyzed for alachlor, cyanazine, and carbofuran violated the respective Class C water quality criteria.   The previous impairment due to violations of the Class C criterion for nitrate (10 mg/l) is proposed for de-listing.   This impairment was identified for the 2004 Integrated Reporting cycle when results of monitoring at station 15660001 from 2000-2002 showed seven violations of the Class C criterion for nitrate in the 36 samples collected.   According to the IDNR assessment/listing methodology, the percentage of samples in violation of this criterion (19%) suggested that the Class C uses should be assessed as only “partially supported” (i.e., Section 303(d) impaired).   Results of IDNR routine ambient monitoring for the 2006 through 2010 Integrated Reporting cycles continued to suggest impairment of the Class C uses due to nitrate.   For the 2012 cycle, however, levels of nitrate in all 30 samples collected between January 2008 and December 2010 were below the nitrate MCL of 10 mg/l (maximum:  8.7 mg/l).   These results continued a trend toward lower nitrate levels that began in 2007.   For the current (2014) IR cycle, this downward trend in nitrate levels continued:  none of the 36 samples collected from January 2010 to December 2012 exceeded the Class C criterion for nitrate (maximum=8.1 mg/l; average=3.6 mg/l).   According to Iowa’s assessment/listing methodology, data showing “full support” of the assessed use through two consecutive listing cycles (five years) are needed to de-list an impairment such as nitrate.   This requirement is designed to help ensure that lowered contaminant levels do not reflect a transitory (e.g., climate-related) influence on nitrate levels.   Because the trend toward lower nitrate levels has continued through two consecutive IR cycles (2012 and 2014), this impairment is proposed for de-listing from Iowa’s Section 303(d) list.  

Fish consumption uses remain assessed (monitored) as "fully supported" based on results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring near Oskaloosa in 2005 and 2006.   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 levels of one contaminant from the 2005 sampling (mercury) was sufficiently high for concern and justified follow-up monitoring.   The 2005 composite sample of fillets from common carp had generally low levels of contaminants.   (Although sampled as a RAFT “status” site, a sufficient number of predator species was not available, and a predator sample was not submitted).   Levels of primary contaminants in the composite sample of the common carp fillets were as follows: mercury: 0.215 ppm; total PCBs: < 0.09 ppm; and technical chlordane: < 0.03 ppm.   Although assessed as “fully supported” for the 2006 reporting cycle, the level of mercury in the sample of common carp fillets (0.215 ppm) slightly exceeded the interim IDNR/IDPH one meal per week trigger level of 0.20 ppm.   Note:  the IDNR/IDPH trigger level for mercury for a one-meal/week consumption advisory was changed in 2007 to 0.3 ppm to be consistent with U.S.  EPA's recommendation for a tissue-based water quality criterion for mercury.  

According to the IDNR/IDPH advisory protocol, this single result for mercury from the 2005 sampling on the South Skunk River neither warranted issuance of an advisory nor indicated 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 monitoring was conducted in 2006 to better determine (1) levels of mercury in the edible portions (fillets) of fish in this river segment and (2) whether a one-meal-per-week consumption advisory needs to be issued.   The 2006 monitoring showed a lower level of mercury in the composite sample of common carp fillets (0.201 ppm), thus indicating that Iowa’s advisory trigger of 0.3 ppm was not exceeded and that a consumption advisory was thus not justified for this river segment.