Assessment Comments
Assessment is based on results of chemical/physical/bacterial water quality monitoring conducted on Dry Run Creek from June 2005 through June 2008 at four stations sampled as part of the Dry Run Creek Watershed Project: Site 1 at Cedar Falls (STORET station 1107006), at Site 4 at Cedar Falls (STORET station 11070007), Site 6D1 at Cedar Falls (STORET station 11070008), and Site DRC3 at 18th Street (STORET station 15070002) and IDNR/UHL biological monitoring conducted in 1999 and 2005.
Basis for Assessment
[Note: Prior to the 2008 Section 305(b) cycle, this stream segment was designated for Class A (primary contact recreation) uses and for Class B(LR) aquatic life uses. This segment remains designated for both primary contact recreation and aquatic life uses. Due, however, 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), the primary contact recreation uses are now termed Class A1 uses, and the warmwater aquatic life uses are now termed Class B(WW2) uses. The Class A1 and B(WW2) designations are equivalent to the former Class A and B(LR) designations.]
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported” due to levels of indicator bacteria that exceed state water quality criteria. The Class B(WW2) aquatic life uses are assessed (evaluated) as "partially supported" based on results of biological monitoring in 1999 and 2005. This stream was added to Iowa’s 2002 Section 303(d) list of impaired waters based on results of IDNR/UHL biological (biocriteria) sampling in 1999 that showed a failure of aquatic life present in Dry Run Creek to meet regional expectations. The sources of data for this assessment include the results of chemical/physical/bacterial water quality monitoring conducted on Dry Run Creek from June 2005 through June 2008 at four stations sampled as part of the Dry Run Creek Watershed Project: Site 1 at Cedar Falls (STORET station 11070006), at Site 4 at Cedar Falls (STORET station 11070007), Site 6D1 at Cedar Falls (STORET station 11070008), and Site DRC3 at 18th Street (STORET station 15070002). The assessment of “partial support” of the Class B(WW2) aquatic life uses are based on results of IDNR/UHL biological monitoring conducted in 1999 and 2005.
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. The geometric means of E. coli in the samples collected at all four monitoring stations in this assessment segment exceeded Iowa’s Class A1 criterion of 126 orgs/100 ml. These geometric means and the number of samples collected during recreation seasons from June 2005 to June 2008 are as follows: 502 orgs/100 ml at Site 1 (50 samples); 296 orgs/100 ml at Site 4 (48 samples); 356 orgs/100 ml at Site 6D1 (45 samples), and 411 orgs/100 ml at Site DRC3 (58 samples). The percentages of samples at these stations that exceeded Iowa’s single-sample maximum criterion are as follows: Site 1: 35 of 50 samples (70% violated); Site 4: 28 of 58 samples (58% violated), Site 6D1: 31 of 45 samples (69% violated); and Site DRC3: 41 of 58 samples (71% violated). 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 state criterion of 126 orgs/100 ml., 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(WW2) aquatic life uses are assessed as “partially supported” based on data collected in 1999 and 2005 as part of the IDNR/UHL stream biocriteria project. A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species that were collected in the stream sampling reach. The biological metrics were combined to make a fish community index of biotic integrity (FIBI) and a benthic macroinvertebrate index (BMIBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 1999 FIBI score was 50 (fair) and the BMIBI score was 48 (fair). The 2005 FIBI scores were 44 (fair), 37 (fair). The 2005 BMIBI scores were 42 (fair), 38 (fair). The aquatic life use support was assessed as partially supporting (=PS), 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 non-riffle FIBI BIC for this ecoregion is 44 and the natural substrate BMIBI BIC for this ecoregion is 70. Although one site should be considered a riffle site (FIBI BIC = 65) both sites failed to even meet the non-riffle site FIBI BIC of 44. This segment passed the FIBI BIC 2/3 times and passed the BMIBI BIC 0/3 times in the past 10 years.
This aquatic life assessment is now considered "evaluated" based on a change in the 2010 IDNR assessment methodology. IDNR now requires a segment have two or more biological samples collected from the segment in multiple years between 2004 and 2008 to be considered “monitored”. This segment had multiple samples collected in the previous 10 years (1999-2008); however, the multiple samples were not collected during 2004-2008 and were not collected in multiple years. According to IDNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are thus not appropriate for Section 303(d) listing (Category 5 of the Integrated Report). IDNR does, however, consider these impairments as appropriate for listing under either Category 2b or 3b of the Integrated Report (waters potentially impaired and in need of further investigation). However, despite this change in assessment methodology and type, this waterbody remains on Iowa’s 2010 Section 303(d) list of impaired waters.
Somewhat in contrast to the results of biological monitoring, results of chemical/physical monitoring conducted for the Dry Run Creek Watershed project from June 2005 through June 2008 suggest generally good water quality. Levels of dissolved oxygen do, however, occasionally violate Class B(WW2) criterion (5.0 mg/l). From 53 to 64 samples were collected at each of the four sample sites monitored during this period. None of the samples contained violations of Class B(WW2) water quality criteria for ammonia, pH, or temperature. Four of the 25 samples (8%) collected from station 6D1 contained levels of dissolved oxygen that violated the Class B(WW2) criterion of 5.0 mg/l; all these violations occurred from June to October, 2005. One of the 55 samples (2%) collected at sites 1 and 4 also violated the criterion for dissolved oxygen. None of the 64 samples from DRC3 violated the dissolved oxygen criterion. According to U.S. EPA assessment guidelines, if more than 10% of samples exceed state criteria for conventional parameters such as dissolved oxygen, the aquatic life (Class B) uses should be assessed as "impaired" (see pgs 3-17 of U.S. EPA 1997b). Based on IDNR’s assessment methodology, however, the results from Dry Run Creek do not suggest that significantly more than 10 percent of the samples violated Iowa’s dissolved oxygen criteria and thus do not suggest an impairment of the Class B(WW2) uses of this stream segment.
