Assessment Comments
Assessment based on 2008-2010 IDNR/UHL biological monitoring data: FIBI = 25 (poor), 26, 29, 31 (fair), 54 (good); BMIBI = 40, 54 (fair), 57, 62, 63 (good). FIBI BIC = 36, BMIBI BIC = 51. Assessment also based on results of bacterial monitoring in 2009 and 2010 as part of a Section 319 water quality project.
Basis for Assessment
[Note: Prior to the 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, 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 (2012) 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 levels of indicator bacteria (E. coli) that exceed state water quality criteria. This is a new impairment for this assessment segment. The Class B(WW2) aquatic life uses remain assessed (evaluated) as “partially supported” based on results of biological sampling in 1999 and 2008. The sources of data for this assessment include (1) results of bacteria monitoring from the following Section 319 project monitoring stations: Site 200, Site WC2M, Site WC4M, Site WC1, and Site WC2 and (2) results of IDNR/UHL biological (biocriteria) sampling in 1999 and 2008.
EXPLANATION: The presumptive Class A1 (primary contact recreation) uses of this segment of Walnut Creek 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 27 samples collected during the recreational seasons of 2009 and 2010, respectively, at each of the four Section 319 project monitoring stations in this assessment segment were as follows:
1. Station WC2M: 519 and 727 orgs/100 ml;
2. Station WC4M: 459 and 541 orgs/100 ml;
3. Station WC1: 367 and 750 orgs/100 ml;
4. Station WC2: 78 and 57 orgs/100 ml;
All recreation season geometric means exceeded the Class A1 criterion of 126 orgs/100 ml except the geometric means at Station WC1. From 21 to 74% of the samples at each of the four Section 319 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 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". Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”
The assessment of the Class B(WW2) aquatic life uses was based on data collected in 2008-2010 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 2008 FIBI score was 31 (fair) and the BMIBI score was 40 (fair). The 2009 FIBI scores were 26, 29 (fair) and 54 (good) and the 2009 BMIBI scores were 57, 62, 63 (good). The 2010 FIBI score was 25 (poor) and the BMIBI score was 54 (fair). The aquatic life use support was assessed (evaluated) 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 FIBI BIC for this ecoregion is 36 and the BMIBI BIC for this ecoregion is 51. This segment passed the FIBI BIC 1/5 times and passed the BMIBI BIC 4/5 times in the past five years.
