Assessment Comments

Assessment is based on: (1) the results of monthly monitoring from January 2008 through December 2010 at the IDNR/UHL ambient monitoring station located at the Highway 63 bridge north of Oskaloosa (STORET station 10620001; formerly station 821035) and (2) the results of U.S. EPA/IDNR fish tissue (RAFT) monitoring near Oskaloosa in 2005 and 2006.

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 for the downstream segment (IA 03-SSK-0010_2).]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that violate state water quality criteria.   The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient water quality monitoring from 2008 through 2010.   Although levels of nitrate in this segment were low during the 2008-10 period, the Class C (drinking water) uses remain assessed (monitored) as "not supported" due to violations of state water quality criterion for nitrate.   Fish consumption uses remain assessed (monitored) as "fully supported" 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 2008 through December 2010 at the IDNR/UHL ambient monitoring station located at the Highway 63 bridge north of Oskaloosa (STORET station 10620001; formerly station 821035) and (2) the results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring near Oskaloosa in 2005 and 2006.  

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 22 samples collected during the recreational seasons of 2008 through 2010 at IDNR station 10620001 near Oskaloosa were as follows:  the 2008 geometric mean was 220 orgs/100 ml, the 2009 geometric mean was 473 orgs/100 ml and the 2010 geometric mean was 426 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Fourteen of the 22 samples (64%) 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 Class B(WW1) aquatic life uses are assessed as "fully supported" based on results of monitoring from the IDNR ambient station north of Oskaloosa from 2008 through 2010.   Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the approximately 30 samples analyzed.  

Despite a downward (improving) trend in nitrate levels, the Class C (drinking water) uses remain assessed (monitored) as "not supported" due to violations of the state water quality criterion for nitrate during the previous (2006-08) assessment period.   Results of IDNR ambient monthly monitoring at the Oskaloosa station from 2006 through 2008 showed 11 violations of the nitrate MCL (=state water quality standard) in the 33 samples collected (33% in violation).   According to IDNR's assessment guidelines, this percentage of violations suggested that the Class C uses were "not supported"; thus, this impairment was added to Iowa’s Section 303(d) list (the nitrate impairment in this segment goes back to the 2004 listing cycle).   For the current (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 continue a trend toward lower nitrate levels that began in 2007.   According to Iowa’s assessment/listing methodology, data showing “full support” of the assessed use through two consecutive listing cycles is needed to de-list an impairment such as nitrate.   This requirement is designed to ensure that lowered contaminant levels do not reflect a transitory (e.g., climate-related) influence on nitrate levels.   If the trend toward lower nitrate levels continues through the next (2014) listing cycle (i.e., monitoring years 2010-1012), this impairment will be removed 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.