Assessment Comments
Assessment is based on results of (1) 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 (2) Section 319 monitoring from January 2008 to November 2010, and (3) IDNR/UHL biological monitoring conducted in 2009-2011.
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, 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” (IR 5a) due to levels of indicator bacteria that exceed state water quality criteria. The Class B(WW2) aquatic life uses are assessed (monitored) as "partially supported" (IR 5b) based on results of biological monitoring in 2009-2011. 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 (1) the results of chemical/physical/bacterial water quality monitoring conducted on Dry Run Creek from January to 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) and (2) the results of Section 319 bacteria monitoring at three stations (DRC-4, DRC-1, and DRC-3) from January 2008 through November 2010. The assessment of “partial support” of the Class B(WW2) aquatic life uses remains based on results of IDNR/UHL biological monitoring conducted in 2009-2011.
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 indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2008, 2009, and 2010, respectively, at each of the three Section 319 monitoring stations in this assessment segment were as follows:
1. DRC-4 geometric means were 194, 255, and 348 orgs/100 ml;
2. DRC-1 geometric means were 165, 411, and 416 orgs/100 ml;
3. DRC-3, geometric means were 126, 311, and 383 orgs/100 ml.
From 50 to 54% of the samples at each of the Section 319 sites exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml.
The geometric means of indicator bacteria (E. coli) in the 7 samples collected during the recreational season of 2008 at each of the three IDNR/TMDL monitoring stations in this assessment segment were as follows:
4. IDNR/TMDL-11070007: 194 orgs/100 ml;
5. IDNR/TMDL-11070006: 165 orgs/100 ml;
6. IDNR/TMDL-11070008: 64 orgs/100 ml.
From 29 to 43% of samples at each of the IDNR/TMDL sites 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 criteria for Class A1 uses, these uses are assessed as “impaired.”
The Class B(WW2) aquatic life uses are assessed as “partially supported” based on data collected in 2009-2011 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 2009-2011 FIBI scores from DRC1 were 76, 74, and 74 (all excellent); the 2000-2011 BMIBI scores at DRC1 were 47, 48 and 49 (all fair). The 2009-2011 FIBI scores at DRC4 were 44, 36, and 38 (all fair); the 2000-2011 BMIBI scores at DRC4 were 43 (fair), 57 and 60 (both good). 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 site FIBI BIC for this ecoregion is 44, the riffle site FIBI BIC is 65 and the natural substrate BMIBI BIC for this ecoregion is 70. Site DRC1 passed the non-riffle site FIBI BIC 3/3 times and passed the natural substrate BMIBI BIC 0/3 times. Site DRC4 passed the riffle site FIBI BIC 0/3 times and passed the natural substrate BMIBI BIC 0/3 times. Overall, this segment passed the FIBI BIC 3/6 times and passed the BMIBI BIC 0/6 times in the past five years.
This assessment is considered "monitored" because there were two or more samples collected from this segment in multiple years from 2006-2011.
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.
Similarly, none of the 10 samples collected at each of the Section 319 samples sites in 2008 in this assessment segment (DRC-4, DRC1, and DRC-3) contained violations of Class B(WW2) criteria for ammonia, DO, pH, or temperature.
