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 (primary contact recreation) uses are assessed (monitored) as "partially supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2014 through 2016 at DNR station 10620001 near Oskaloosa were as follows: the 2014 geometric mean was 269 orgs/100 ml, the 2015 geometric mean was 257 orgs/100 ml, and the 2016 geometric mean was 458 orgs/100 ml. All three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Sixteen of the combined 24 samples (67%) exceeded Iowa’s Class A1 single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and DNR’s assessment/listing methodology, if the geometric mean is greater than 126 orgs/100 ml., the primary 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 "partially supported."

Results of chemical/physical water quality monitoring at DNR station 10620001 near Oskaloosa suggest “full support” of the aquatic life uses. Monitoring showed no violations of Class B(WW1) water quality criteria for 35 Ammonia samples (maximum = 1.6 mg/L), 35 pH samples (range = 7.5 to 8.7), 36 Temperature samples (maximum = 24.3° c), 36 Chloride samples (maximum = 72 mg/L), or 36 Sulfate samples (maximum = 77 mg/L) occurred during monitoring from January 2014 to December 2016.1 of the 36 samples (3%) analyzed for Dissolved Oxygen (minimum = 4.7 mg/L) violated the Class B(WW1) criteria during the 2014-2016 monitoring period. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17) and Iowa DNR's assessment/listing methodology, a violation frequency of significantly greater than 10% for conventional parameters such as Dissolved Oxygen suggests impairment of aquatic life uses. Because the frequency of violations for this parameter is not greater than 10 percent, 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 2012-2013, 2016 as part of the DNR/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) and the 2016 BMIBI score was 51 (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 (1/3), it is uncertain as to whether or not this segment is meeting the aquatic life criteria becausethe site used for the assessmentdoesn’t fall in the calibrated watershed size. According to DNR’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). DNR 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).

Results of Class C(drinking water) monitoring at DNR station 10620001 near Oskaloosa suggest “full support” of thedrinking wateruses. Monitoring showed no violations of Class C(drinking water) criteria for 36 Chloride samples (maximum = 72 mg/Land average = 24.9 mg/L), 14 Alachlor samples (maximum = 0.05 ug/Land average = 0.03 ug/L), 7 Atrazine samples (maximum = 2.5 ug/Land average = 0.74 ug/L), 7 Carbofuran samples (maximum = 0.05 ug/Land average = 0.05 ug/L), or 7 Cyanazine samples (maximum = 0.05 ug/Land average = 0.05 ug/L) occurred during monitoring from January 2014 to December 2016.3 of the 36 samples (8%) analyzed for NO2NO3 (maximum = 14 mg/Land average = 6.81 mg/L) violated the Class C(drinking water) criteria during the 2014-2016 monitoring period.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/DNR 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.

According to the DNR/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.