Assessment Comments
Assessment remains based on results of (1) monitoring of water quality monitoring from 1996-98 near Marengo for the USGS NAWQA program and (2) daily IDNR/UHL monitoring for dissolved oxygen and temperature at Marengo from May-August 2000.
Basis for Assessment
[Note: Although additional monitoring data for dissolved oxygen and temperature were collected in this river segment from May through August 2000, none of these data change the assessments developed for the 2000 and 2002 reports. Thus, this (2008) assessment is the largely the same as that developed for the 2000 through the 2006 reporting cycles, and the use support decisions for the primary contact and aquatic life uses are identical to those in the 2000 through 2006 assessments. Due to the age of the data (older than five years), the assessments remain considered “evaluated.” In the context of Section 305(b) reporting, the confidence level of an "evaluated" assessment is low relative to a "monitored" assessment which is based on recent site-specific data. No attempt has been made to re-evaluate support of the Class A1 (primary contact recreation) uses based on the 2003 change in the Iowa Water Quality Standards from fecal coliforms to E. coli as the indicator bacterium: the impairment based on fecal coliforms will remain in effect for this assessment segment.]
SUMMARY: The Class A1 (primary contact recreation) uses remain assessed (evaluated) as "not supported" due to high levels of indicator bacteria that exceed state water quality standards. The Class B(WW1) uses remain assessed (evaluated) as "fully supported" based on results of ambient chemical/physical monitoring. Fish consumption uses are "not asssessed" due to the lack of recent fish contaminant monitoring in this assessment segment. The sources of data for this assessment include (1) results of ambient water quality monitoring conducted by USGS from 1996 to 1998 as part of their National Water Quality Assessment Program (NAWQA) and (2) results of daily IDNR/UHL monitoring for dissolved oxygen and temperature at Marengo (STORET station 10480001) from May to August 2000.
EXPLANATION: Assessments of support of beneficial uses are based on results of monitoring conducted on the Iowa River near Marengo from March 1996 to September 1998 by USGS as part of the National Water Quality Assessment Program (NAWQA) (eastern Iowa river basins study unit, station 05453100).
Class A1 (primary contact recreation) uses were assessed (evaluated) as "not supporting" due to results of the USGS/NAWQA monitoring that showed that levels of indicator bacteria (fecal coliforms) suggest less than full support of the primary contact recreation uses. In summer 1996, the geometric mean of fecal coliform bacteria in the 8 non-runoff-affected samples (431 orgs/100 ml) far exceeded the state Class A criterion of 200 orgs/100 ml. Levels of bacteria in 4 of the 8 samples (50%) exceeded the EPA-recommended single-sample maximum value of 400 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting (pgs 3-33 to 3-35of U.S. EPA 1997b), "nonsupport" of primary contact recreation uses is indicated if geometric mean for fecal coliforms is greater than 200 organisms/100 ml. Despite the lack of sufficient indicator bacteria data points for developing a "monitored" assessment (according to DNR's 305(b) assessment methodology, at least 10 non-runoff-affected samples), the Class A1 uses were assessed as "not supported." In addition, USGS switched to E. coli as the bacteria indicator in summer 1997. The geometric mean of E. coli in the 4 samples (332 orgs/100 ml) was much greater than the EPA-recommended (and, as of 2003, Iowa's newly-adopted) criterion of 126 orgs/100ml, and 2 samples (50%) exceeded the EPA-recommended single sample maximum value (289 orgs/100 ml) for E. coli). According to the U.S. EPA guidelines, these results also suggest "nonsupport" of the Class A1 uses.
Regarding support of the Class B(WW1) aquatic life uses, results of USGS/NAWQA monitoring at Marengo showed that one of 34 samples (3%) violated the Class A1 and Class B(WW1) criterion for pH (6.1 units on January 14, 1997) and that one of 34 samples (3%) violated the Class B(WW1) criterion (5.0 mg/l) for dissolved oxygen (3.3 mg/l on July 7, 1998). 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 and dissolved oxygen criteria at this station (3%) 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). None of the 89 daily samples collected by IDNR/UHL from May to August 2000 violated state criteria for dissolved oxygen (minimum: 6.0 mg/; maximum: 16.3 mg/l). None of the 34 samples collected and analyzed by USGS from 1996-98 contained levels of ammonia-nitrogen above state chronic criteria for Class B(WW1) waters, and none of 23 samples analyzed contained levels of toxic organic compounds or pesticides that exceeded the respective state water quality criteria.
Fish consumption uses are "not assessed" due to the lack of recent fish contaminant monitoring in this river segment. Previous assessments of the fish consumption uses for this segment ("fully supported") were based on results of fish contaminant monitoring conducted by USGS in 1995 as part of the NAWQA project. These data are now considered too old (greater than 10 years) to accurately characterize current water quality conditions.
