Walnut Creek IA 02-IOW-1916

The presumptive Class A1 uses remain assessed (monitored) as “partially supported” due to levels of indicator bacteria (E. coli) that exceed the Class A1 geometric mean criterion.  A total of 25 samples were collected at each of the two monitoring stations:  15 samples were collected during the recreation season of 2010, and 10 samples were collected during the recreation season of 2011.  The geometric means of indicator bacteria (E. coli) in the 25 samples collected during the recreational seasons of 2010 and 2011 at Section 319 monitoring stations 5m and 6 were as follows: 

  1.  Site 5m geometric means in 2010 and 2011 were 395 and 912 orgs/100 ml, respectively; 20 of 25 samples (80%) exceeded the Class A1 single-sample maximum criterion; 

  2.  Site 6 geometric means in 2010 and 2011 were 334 and 484 orgs/100 ml, respectively; 15 of 25 samples (60%) exceeded the Class A1 single-sample maximum criterion. 

All of these recreation season geometric means exceed Iowa’s Class A1 geometric mean criterion of 126 E. coli orgs/100 ml, thus indicating nonsupport of the Class A1 primary contact recreation uses.  According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean is greater than the respective criterion (126 orgs/100 ml for Class A1, the primary contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).  Thus, the presumptive Class A1 uses remain assessed as “impaired” (partially supporting). 

The results of the limited chemical/physical monitoring in 2010 and 2011 suggest the Class B(WW2) aquatic life uses be assessed as “fully supporting”.  None of the combined 10 samples collected in 2010 and 2011 at the monitoring stations 5m and 6 violated the Class B(WW2) criteria for dissolved oxygen, ammonia-nitrogen (8 samples), pH, or temperature.

In contrast to the limited chemical/physical monitoring, the biological data collected in 2009 as part of a IDNR/SHL TMDL sampling project suggest the presumptive Class B(WW1) aquatic life uses be assessed (evaluated) as "partially supporting".  A series of biological metrics that 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 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 FIBI score was 23 (poor) and the BMIBI score was 53 (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 from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008.   The FIBI BIC for this ecoregion is 36 and the BMIBI BIC for this ecoregion is 51.   Even though this site failed the FIBI BIC and passed the BMIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it is a small headwater stream and doesn’t fall in the calibrated watershed size.  

Additionally, the observed FIBI score of 23 is at, or near, the predicted Ecoregion Fish Habitat Index (EFHI) and within expectations based on physical habitat characteristics of the segment and ecoregion.

This biological assessment is considered “evaluated” (of lower confidence) due to the lack of a biological assessment protocol for headwater streams.   Prior to the changes in Iowa’s surface water classification that were approved by U.S.  EPA in February 2008, this stream was classified only for general uses due to the inability of the stream to support a viable aquatic community at low-flow conditions.   IDNR’s existing biological assessment protocol, however, was calibrated to aquatic life reference conditions in larger wadeable streams with perennial flow.   The aquatic environment of most headwater streams is one of extremes in water quality.  Consequently, intermittent headwater streams tend to have biological diversity that is low relative to more stable aquatic environments of larger streams with perennial flow.   Thus, the use of biological assessment methods developed for the ecologically more stable and diverse streams will likely overstate the existence of impairment in headwater streams.   For these reasons, headwater stream reaches that show a failure to meet regional expectations for aquatic biota (fish or aquatic macroinvertebrates) will not be added to Iowa’s Section 303(d) list of impaired waters (Category 5 of the Integrated Report).   The assessment type for these assessed waters will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to “monitored" (indicating an assessment with relatively high confidence).   Such waters are typically placed in either Category 2b or 3b of the IR and are added to the state’s list of “waters in need of further investigation”.