North Raccoon River IA 04-RAC-1131

Assessment Comments

Assessment is based on results of IDNR monthly ambient monitoring downstream from Sac City.

Basis for Assessment

SUMMARY:  Support of the Class A (primary contact recreation) uses was assessed as "fully supported;" support of the Class B(WW) aquatic life uses was assessed as "fully supported."  Fish consumption uses were "not assessed."  EXPLANATION:  The assessments of support for the Class A and Class B(WW) uses are based on results of DNR monthly monitoring downstream from Sac City during the 1998-1999 biennial period.   This monitoring showed that 12 of the 14 samples analyzed for levels of indicator bacteria (fecal coliforms) during summer periods of 1998 and 1999 were not materially affected by surface runoff.   For purposes of Section 305(b) assessments, DNR uses the long-term average monthly flow plus one standard deviation of this average to identify river flows that are "materially affected by surface runoff."  According to the Iowa Water Quality Standards (IAC 1990:8), the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) does not apply during these conditions of high runoff and river flow.   The geometric mean of fecal coliform bacteria in the 12 non-runoff-affected samples (142 orgs/100) ml was less than the Class A criterion of 200 orgs/100 ml, thus suggesting full support of the primary contact recreation uses.   One of the 12 samples (8% of the samples) contained a level of fecal coliforms greater than U.S.  EPA's recommended single sample maximum density of 400 orgs/100 ml.   Thus, because the geometric mean was less than the WQ criterion of 200 orgs/100 ml, and because less than 10% of the samples exceeded the U.S.  EPA recommended single-sample maximum value of 400 orgs/100 ml., the Class A (primary contact recreation) uses were assessed as "fully supported." See U.S.  EPA guidelines for Section 305(b) reporting (1997b, pages 3-33 to 3-35) for more information on this methodology.   The only violation of Class B(WW) criteria for conventional parameters was in one of the 23 samples analyzed for pH during the biennial period.   The sample collected on October 6, 1997, had a pH level of 9.1 units; this level violated the Class B(WW) (and Class A) criterion of 9.0 units.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), the percentage of violations for pH at this station (4%) does not suggest a water quality impairment (the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated).   Levels of dissolved oxygen and ammonia-nitrogen did not violate the respective Class B(WW) criteria in the 24 samples analyzed during the biennial period, and no violations of Class B(WW) chronic criteria for toxic metals occurred in the two samples analyzed during this period.   Thus, the Class B(WW) aquatic life uses were assessed as "fully supported."  The level of support of the fish consumption uses was changed from "fully supported" to "not assessed" due to the age of the information:  EPA/DNR fish contaminant (RAFT) monitoring was last conducted in this river reach in 1993.   Thus, the data are too old (greater than five years) for developing an assessment of current conditions.