Assessment Comments
Assessment is based on: (1) the results of ambient water quality monitoring conducted at station 15660007 on Deer Creek at Dancer Avenue from May 2006 through November 2007 as part of the Cedar River/Mitchell County study, (2) results of a statewide survey of freshwater mussels conducted by Iowa State University in 1998 and 1999 and (3) results of 2001 IDNR/UHL biological sampling near Carpenter.
Basis for Assessment
[Note: Prior to the current (2008) Section 305(b) cycle, this stream segment was designated only for Class B(LR) aquatic life uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf), this segment is now presumptively designated for Class A1 (primary contact recreation) uses. The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses). Thus, for the current (2008) assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]
SUMMARY: The presumptive Class A1 primary contact recreation uses are assessed (monitored) as “not supported” due to high levels of indicator bacteria. The Class B(WW2) aquatic life uses remain assessed (monitored) as “fully supported” based on results of IDNR/UHL biological monitoring in 2001. Results from the 1998-99 statewide survey of freshwater mussels also suggest "full support" of aquatic life uses. The sources of data for this assessment are (1) the results of ambient water quality monitoring conducted at station 15660007 on Deer Creek at Dancer Avenue from May 2006 through November 2007 as part of the Cedar River/Mitchell County study, (2) results of a statewide survey of freshwater mussels conducted by Iowa State University in 1998 and 1999 and (3) results of 2001 IDNR/UHL biological sampling near Carpenter.
EXPLANATION: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to violations of Iowa’s water quality criteria for indicator bacteria. [Note: monitoring data for the Mitchell County project station from 2007 were combined with data from 2006 for this assessment. Although the data cutoff period for the 2008 assessment/listing cycle is the end of calendar year 2006, the inclusion of the 2007 data is necessary to develop a higher confidence (“monitored”) assessment and to confirm the existence of a Section 303(d) impairment suggested by the relatively few data values available for 2006.]
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 15 samples collected at station 15660007 during the recreational seasons of 2006 and 2007 was 155 orgs/100 ml. This geometric mean slightly exceeds the Class A1 criterion of 126 orgs/100 ml. Four of the 15 samples (27%) 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).
The Class B(WW-2) aquatic life uses are assessed (evaluated) as “fully supported.” The 2001 biological sampling data are now more than five years old; therefore, this assessment is now considered "evaluated" instead of "monitored". The assessment was based on data collected in 2001 as part of the DNR/UHL stream biocriteria project. The 2001 FIBI score was 68 (good) and the BMIBI score was 78 (excellent). The aquatic life use support was assessed (evaluated) as Fully Supporting (=FS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports. The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004. The riffle habitat FIBI BIC for this ecoregion is 65 and the natural substrate BMIBI BIC for this ecoregion is 70.
This segment of Deer Creek was sampled as part of the 1998-99 statewide study of freshwater mussels in Iowa streams and rivers (Arbuckle et al. 2000). 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). On a statewide basis, this comparison showed sharp declines in the numbers of mussel species ("species richness") in Iowa streams and rivers from the mid-1980s to the late 1990s. For stream segments having four or more species reported for the 1984-95 survey, results of this comparison were used by staff of the Iowa DNR Water Quality Bureau to assess the degree to which the aquatic life uses of the sampled stream segments are supported. The results of this sampling on Deer Creek, however, showed an increase in species richness between the 1984-98 and 1998-99 survey periods. Species richness of freshwater mussels at the one sample site in this segment was 7 in the 1984-85 period and was 12 in the 1998-99 period, respectively, for a percent change of plus 71%. Relative to other sites in the 1998-99 ISU survey, the freshwater mussel community of Deer Creek is exceptional: this stream tied with Buffalo Creek in Jones and Linn counties (IA 01-WPS-0110_2) for the highest mussel species richness (12) of the 118 survey sites used for purposes of Section 305(b) assessments, and this stream tied for the biggest increase in mussel species richness between the 1984 and 1998-99 survey periods (increase of 7 species). These results suggest "full support" of the aquatic life uses.
