South Skunk River IA 03-SSK-927

[Note: 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).]

The Class A1 uses are assessed (monitored) as "partially 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 2012 through 2014 at IDNR station 10620001 near Oskaloosa were as follows: the 2012 geometric mean was 232 orgs/100 ml, the 2013 geometric mean was 132 orgs/100 ml, and the 2014 geometric mean was 269 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 "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 “impaired.”

Results of monitoring from the IDNR ambient station north of Oskaloosa from 2012 through 2014 suggest "full support" of the Class B(WW1) aquatic life uses.  Monitoring at this station showed no violations of Class B(WW1) water quality criteria for temperature, pH, ammonia-nitrogen, chloride, or sulfate in the approximately 36 samples analyzed. One of the 36 samples (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 10620001 do not indicate that significantly greater than 10% of the samples exceed either the Class B(WW1) criteria for dissolved oxygen. Thus, these results do not suggest impairment 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 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) and 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 the site used for the assessment 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 samples analyzed for alachlor (32 samples), atrazine (26 samples) carbofuran (26 samples), chloride (36 samples), or sulfate (36 samples) violated the respective Class C water quality criteria.  Two of the 36 samples (6%) analyzed for nitrate violated the Class C criterion (MCL) of 10 mg/l as nitrogen.  The samples violating the Class C standard were collected on July 1, 2013 (14 mg/l) and on May 1, 2014 (11 mg/l).  According to Iowa DNR’s assessment/listing methodology, drinking water uses should be assessed as impaired for nitrate if more than 10% of the samples exceed the Class C criterion.  Because less than 10% of the samples at monitoring station 10620001 exceeded the nitrate criterion, these violations do not indicate impairment of the Class C uses. 

Fish consumption uses remain assessed (evaluated) 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.