Marrowbone Creek IA 04-RAC-2034

[Note: A TMDL for nitrate and pathogen indicators (E. coli) for the Raccoon River basin was prepared by IDNR and approved by EPA in June 2008. Because this TMDL covers the primary Section 303(d) impairment identified for this segment for the 2012 assessment/listing cycle (E. coli), this waterbody is placed in IR Category 4a (impaired; TMDL approved). The following is excerpted from the 2008 TMDL: “TMDLs for pathogen indicator E.coli have also been determined for all other Class A1 streams in the Raccoon River watershed not previously identified as impaired. Although these segments have not been listed on the state’s 303(d) list, TMDLs have been determined for these segments in the event that monitoring data is collected in the future that could be used for listing purposes.”]

The presumptive Class A1 (primary contact recreation) uses are assessed (evaluated) as “partially supporting” due to the high levels of indicator bacteria (E. coli). The geometric means of indicator bacteria (E. coli) in the 13 samples collected during the recreational seasons of 2008 and 2009 at IDNR station 11140003 near Lanesboro (Site MaB2) were as follows: the 2008 geometric mean was 313 orgs/100 ml and the 2009 geometric mean was 310 orgs/100 ml. Both geometric means slightly exceed the Class A1 criterion of 126 orgs/100 ml. Seven of the 11 samples (64%) 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 should be assessed as "impaired" (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 presumptive Class B(WW1) aquatic life uses remain assessed (evaluated) as “partially supporting” based on results of biological sampling in 2007. 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 2007 FIBI score was 57 (good) and the 2007 BMIBI score was 22 (poor). 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 non-riffle habitat FIBI BIC for this ecoregion is 32 and the BMIBI BIC is 62. Even though this site passed the FIBI BIC using the FIBI UAV (uncertainty adjustment value = 7 points) and failed to meet 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.

This assessment is considered “evaluated” (of lower confidence) due to the lack of a biological assessment protocol for general use-only streams. IDNR’s existing protocol has been calibrated to aquatic life reference conditions in wadeable Class B(LR) and Class B(WW) streams. The aquatic environment of most general use streams is one of extremes in water quality. Consequently, general use-only streams tend to have biological diversity that is low relative to more stable aquatic environments of Class B(LR) and Class B(WW) streams. Thus, the use of biological assessment methods developed for the ecologically more stable and diverse Class B(LR) streams to assess general use reaches will likely overstate the existence of impairment. For these reasons, general use-only stream reaches that show a failure to meet regional expectations for aquatic biota (fish or aquatic macroinvertebrates) of Class B(LR) streams, 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 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 will be placed in either Category 2b or 3b of the IR and thus will be added to the state’s list of “waters in need of further investigation”.

In contrast to the results of the biological assessment, the results of chemical/physical water quality monitoring at IDNR TMDL station 11140003 in 2008 and 2009 suggest “full support” of the presumptive Class B(WW1) aquatic life uses. Results of this monitoring show no violations of Class B(WW1) water quality criteria for conventional parameters dissolved oxygen or pH) or for ammonia in the 13 samples collected.