Assessment Comments

Assessment is based on: (1) results of IDNR/UHL ambient water quality monitoring conducted from September 2006 through December 2007 at Lone Tree (STORET station 10580002) and (2) results of a statewide survey of freshwater mussels conducted by Iowa State University in 1998 and 1999.

Basis for Assessment

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to violations of water quality criteria for indicator bacteria.   The Class B(WW1) aquatic life uses remain assessed (evaluated) as "not supported" based on results of a 1998-99 statewide assessment of freshwater mussels in Iowa streams.   The fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment.   The sources of data for this assessment include (1) results of IDNR/UHL ambient water quality monitoring conducted from September 2006 through December 2007 at Lone Tree (STORET station 10580002) and (2) results of a statewide survey of freshwater mussels conducted by Iowa State University in 1998 and 1999.   [Note:  IDNR/UHL monthly ambient water quality monitoring at the Lone Tree station began in September 2006 following termination of the previous monitoring location on the Iowa River at Columbus Junction (STORET station 10580001).   Although the data cutoff period for the 2008 assessment/listing cycle is the end of calendar year 2006, the inclusion of the 2007 data from the Lone Tree station was necessary to develop a higher confidence (“monitored”) assessment.]  

EXPLANATION:  The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to violations of Iowa’s water quality criteria for indicator bacteria.   Due to recent changes in Iowa’s Water Quality Standards, Iowa’s assessment methodology for indicator bacteria has changed.   Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A1 criterion for indicator bacteria (fecal coliforms) designed to protect primary contact recreation uses:  the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) did not apply "when the waters [were] materially affected by surface runoff."  Due to a change in the Standards in July 2003, E.  coli is now the indicator bacterium, and the high flow exemption was eliminated and replaced with language stating that the Class A criteria for E.  coli apply when Class A1, A2, or A3 uses “can reasonably be expected to occur.”  Because the IDNR Technical Advisory Committee on WQ Standards could not agree on what flow conditions would define periods when uses would not be reasonably expected to occur, all monitoring data generated for E.  coli during the assessment period, regardless of flow conditions during sample collection, will be considered for determining support of Class A uses for purposes of Section 305(b) assessments and Section 303(d) listings.  

The geometric mean of E.  coli in the 10 samples collected at the Lone Tree station during the recreational seasons of 2006 and 2007 was 281 orgs/100 ml.   This geometric mean exceeds the Class A1 criterion of 126 orgs/100 ml.   Four of the 10 samples (40%) 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 IDNR’s assessment/listing methodology, if the geometric mean of E.  coli is greater than the applicable state criterion, the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).   Thus, these results suggest that the Class A1 uses should be assessed as “not supported.”  The amount of data available for the 2006 and 2007 recreational seasons (10 samples) is the minimum preferred by IDNR for developing a higher-confidence (monitored) assessment.   Additional data for indicator bacterial will be available for the 2010 Section 305(b) cycle to confirm the existence of this impairment.  

The Class B(WW1) aquatic life uses are assessed (evaluated) as "not supported" based on information from the report "Statewide Assessment of Freshwater Mussels (Bivalva, Unionidae) in Iowa Streams" by Arbuckle et al.  (2000).   [Note:  because the data from Arbuckle et al.  (2000) are now older than five years, the assessment category is changed from a “monitored” (i.e., a higher confidence assessment) to “evaluated” (i.e., lower confidence assessment).   Despite this change in assessment category, the impairment indicated by these data remains in IR Category 5 (i.e., Section 303(d) list) until more recent data suggest a good cause for de-listing.]  As part of this study, sampling results from 1998 and 1999 (Arbuckle et al.  2000) were compared to results from stream sites surveyed in 1984 and 1985 by Frest (1987).   In general, this comparison showed sharp declines in the numbers of mussel species ("species richness") from the mid-1980s to the late 1990s in Iowa’s stream and rivers.   Results of this comparison were used by staff of the Iowa DNR to assess the degree to which the aquatic life uses of the sampled stream segments are supported.  

For purposes of Section 303(d) listing, this assessment was based on the percent change in the number of species of freshwater mussels found in the 1984-85 survey versus the 1998-99 survey.   Greater than a 50% decline in species richness from the 1984-85 to the 1998-99 period suggests an impairment of the aquatic life uses.   Species richness of freshwater mussels at the one sample site in this stream segment was 13 in the 1984-85 period and was 1 in the 1998-99 period for an average percent change of minus 92%.   Based on these results, the aquatic life use are assessed (monitored) as "not supported."  As presented by Arbuckle et al.  (2000), the potential causes of declines in species richness of Iowa's freshwater mussels include siltation, destabilization of stream substrate, stream flow instability, and high instream levels of nutrients (phosphorus and nitrogen).   Their study also suggested the importance of stream shading provided by riparian vegetation to mussel species richness.   Additional monitoring is needed to better define the biological status of this stream segment as well as the site-specific causes and sources of impairment of these uses that may exist.   Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).   Despite this change in assessment type, this waterbody remains in IR Category 5b.

Despite the impairment of aquatic life uses suggested by the result of the ISU mussel survey, the results of IDNR ambient monthly chemical/physical water quality monitoring at Lone Tree suggest that the Class B(WW1) aquatic life uses should be assessed (monitored) as "fully supported."  Results from DNR ambient monthly monitoring at this station from September 2006 through December 2007 show no violations of Class B(WW1) water quality criteria in the 16 samples analyzed for dissolved oxygen, pH, or ammonia-nitrogen.   These results suggest relatively good water quality in this segment of the Iowa River.  

Fish consumption uses are "not assessed" due to the lack of fish contaminant monitoring in this river segment.