Assessment Comments

Assessment is based on: (1) results of IDNR/UHL ambient water quality monitoring conducted from January 2008 through December 2010 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" (303(d) impaired; IR Cat.  5b) 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 January 2008 through December 2010 near Lone Tree (STORET station 10580002) and (2) results of a statewide survey of freshwater mussels conducted by Iowa State University in 1998 and 1999.   [Notes:  (1) 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).   (2) USGS conducted monitoring in this assessment segment at station 05454500 near at Iowa City in October 2010.   Too few data were produced from this monitoring during the 2008-2010 assessment period (one sample) to be of use for assessment for purposes of either Section 305(b) reporting or Section 303(d) listing.]

EXPLANATION:  Results of water quality monitoring during the 2008-2010 assessment period for indicator bacteria (E.  coli) at the IDNR ambient monthly station on the Iowa River near Lone Tree show that the Class A (primary contact recreation) uses are "not supported."  The geometric means of indicator bacteria (E.  coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at IDNR station 10580002 near Lone Tree were as follows:  the 2008 geometric mean was 323 orgs/100 ml, the 2009 geometric mean was 218 orgs/100 ml and the 2010 geometric mean was 317 orgs/100 ml.   All three geometric means slightly exceed the Class A1 criterion of 126 orgs/100 ml.   Ten of the 22 samples (45%) 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 the criterion for Class A1 uses, these uses are assessed as “impaired.”  

The Class B(WW1) aquatic life uses remain assessed (evaluated) as "not supported" (IR Category 5b) 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 four sample site in this river segment were 8, 13, 4, and 10 in the 1984-85 period and were 1, 1, 0, 0, respectively, in the 1998-99 period for an average percent change of minus 94%.   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 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 near Lone Tree during the 2008-2010 assessment period show no violations of Class B(WW1) water quality criteria in the approximately 25 samples analyzed for dissolved oxygen, pH, chloride, and sulfate.   One of 30 samples analyzed for ammonia exceeded the Class B(WW1) chronic criterion.   According to U.S.  EPA guidelines and the IDNR assessment/listing methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s chronic criterion for ammonia and thus do not suggest an impairment of the Class B(WW1) aquatic life uses.  

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