Assessment Comments

Assessment is based on results of (1) IDNR/UHL ambient monthly monitoring near Oskaloosa from 2002-04 and (2) U.S. EPA/IDNR fish contaminant monitoring near Oskaloosa in 2005.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported " based on results of ambient water quality monitoring from 2002 through 2004.   The Class C (drinking water) uses are assessed (monitored) as "not supported" due to violations of state water quality criteria for nitrate.   Fish consumption uses are assessed (monitored) as "fully supported " based on results of fish contaminant monitoring in 2005.   The sources of data for this assessment include (1) the results of monthly monitoring from January 2002 through December 2004 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.  

EXPLANATION:  The Class B(WW) aquatic life uses were assessed as "fully supported " based on results of monitoring from the IDNR ambient station north of Oskaloosa from 2002 through 2004.   Monitoring at this station showed no violations of Class B(WW) water quality criteria for dissolved oxygen or ammonia-nitrogen in the 36 samples analyzed.   Also, none of approximately 10 samples analyzed for toxic metals, and none of the approximately seven samples analyzed for pesticides, violated the respective Class B(WW) criteria.   One of the 36 samples analyzed for pH, however, exceeded the Class B(WW) criterion of 9.0 pH units (this sample contained a pH of 9.6 pH units).   According to U.S.  EPA guidelines (U.S.  EPA 1997b, page 3-17), however, a violation frequency of less than 10 % for conventional parameters such as pH nonetheless suggest "full support" of aquatic life uses.   Thus, the percentages of violations of the pH criterion at this station (3%) does not suggest an impairment of aquatic life uses in this stream segment.  

Although the Class B(WW) uses were assessed as "threatened" for the 2004 Section 305(b) reporting period due to a single violation of the chronic Class B(WW) criterion for chlorpyrifos (see assessment for the 2004 report), none of the 11 samples analyzed as part of IDNR/UHL ambient monitoring from 2002 through 2004 contained a detectable level of chlorpyrifos.   Thus, the level of support for the Class B(WW) aquatic life uses is upgraded from “fully supported / threatened” to “fully supported”.  

The Class C (drinking water) uses are assessed as "not supported" due to violations of state water quality criteria for nitrate.   Results of IDNR ambient monthly monitoring at the Oskaloosa station from 2002 through 2004 show 11 violations of the nitrate MCL (=state water quality standard) in the 36 samples collected (31% in violation) (mean=7.0 mg/l; maximum=17.0 mg/l).   According to IDNR's assessment guidelines, this dataset suggests that the Class C uses are "not supported."  That is, if more than 25% of the samples exceed the MCL for nitrate,  the Class C uses are assessed as “not supported.”  Compared to the previous (2004) assessment period, the percentage of samples at this monitoring station that exceeded the nitrate MCL increased from 19% (7 of 36 samples) to 31% (11 of 36 samples).   In addition, one of 36 samples contained atrazine at a level above the MCL of 3.0 ug/l.  The average level of atrazine in the 36 samples was 0.5 ug/l.   Based on DNR's Section 305(b) assessment methodology, however, if the average contaminant level in source water is less than the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported".   Thus, based on the percentage of violations of the Class C criterion for nitrate, the Class C uses of this river segment are assessed as "not supported."  

Fish consumption uses are assessed as "fully supported " based on results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring near Oskaloosa in 2005.   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.   Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa.   In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health (IDPH), in cooperation with Iowa DNR, developed a risk-based advisory protocol.   This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol).   Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses.

Although this scenario does not apply to the fish contaminant data generated from the 2005 RAFT sampling conducted in this assessment segment, levels of contaminant are sufficiently high for concern and justify follow-up monitoring.   The levels of at least one contaminant from past monitoring now exceeds one or more advisory trigger levels, thus suggesting the need for additional (follow-up) monitoring to determine whether a consumption advisory should be issued.   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 this reporting cycle, the level of at least one contaminant does, however, now exceed one or more advisory trigger levels:  the level of mercury in the sample of common carp fillets (0.215 ppm) slightly exceeds the one meal per week trigger level of 0.20 ppm.  

According to the IDNR/IDPH advisory protocol, this single result from the 2005 sampling on the South Skunk River neither warrants issuance of an advisory nor indicates 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.   But, this elevated level does indicate a concern and the need to conduct additional monitoring to better define contaminant levels in fish from this river segment.   Thus, follow-up monitoring will be conducted 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.