Mud Creek IA 02-CED-519

The presumptive Class A1 (primary contact recreation) uses are assessed (evaluated) as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 11 samples collected at station 15060001 during the recreational season of 2010 (655 orgs/100 ml), and the geometric mean of the 12 samples collected during the recreation season of 2011 (638 orgs/100 ml) both exceeded the Class A1 criterion of 126 orgs/100 ml. Twenty-one of the combined 23 samples (91%) exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and DNR’s assessment/listing methodology, if the geometric mean is greater than 126 orgs/100 ml., the primary contact recreation uses should be assessed as ”impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).Additionally, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). According to DNR’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). DNR 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 in IR Category 5p and remains on Iowa’s 2018 Section 303(d) list of impaired waters.

The Class B(WW2) aquatic life uses are assessed (evaluated) as "fully supporting" based on data collected in 2005 and 2015 as part of the DNR/SHL stream REMAP and Biocriteria sampling projects. A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biological 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 2005 FIBI score was 49 (fair) and the BMIBI score was 68 (good).The 2015 FIBI score was 62 (good) and the BMIBI scores were 57 and 63 (both good).The aquatic life use support was assessed as fully supporting (=FS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports. The non-riffle substrate FIBI BIC for this ecoregion is 44, the artificial substrate BMIBI BIC for this ecoregion is 52 and the natural substrate BMIBI BIC is 70. The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008. This segment passed the FIBI BIC 2/2 times and passed the BMIBI BIC 2/3 times in the last 12 years (the 2015 BMIBI score of 63 failed the natural substrate BMIBI BIC of 70).This aquatic life assessment is considered "evaluated" because there were not two or more samples collected from this segment in multiple years over a five-year period.

The results of ambient water quality monitoring at station 15060001 in 2010 and 2011 indicate good chemical water quality in this stream segment, are consistent with the results of biological monitoring, and thus suggest “full support” of the Class B(WW2) aquatic life uses. No violations of Class B(WW2) water quality criteria for dissolved oxygen, pH or temperature occurred in the 23 samples analyzed from May 2010 to July 2011.