Silver Creek IA 01-TRK-192

Notes:  The Iowa DNR TMDL for biology, sediment, and ammonia for this segment of Silver Creek was approved by U.S. EPA in July 2012.  Because not all the impairments identified for this assessment segment were covered by the TMDL (i.e., indicator bacteria and dissolved oxygen), this segment remains impaired and in IR Category 5.  Because this TMDL was approved in 2012, the biological impairment of the Class B(WW2) uses should have been de-listed for the 2014 IR cycle.  This TMDL approval, however, was inadvertently overlooked during preparation of Iowa's 2014 IR and Section 303(d) list.  In addition, there is no Section 303(d) listing history of an aquatic life impairment for ammonia in this assessment segment (IA 01-TRK-0381_0; aka, IA 01-TRK-192).  The presumptive Class B(WW1) aquatic life use of an upstream segment in the Silver Creek watershed (IA 01-TRK-03817_0; aka IA 01-TRK-2058), however, remains impaired (IR 5p) due to ammonia.  

The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to continued 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 2012, 2013, and 2014 at station 11220014 were as follows:  the 2012 geometric mean was 1,559 orgs/100 ml, the 2013 geometric mean was 1,667 orgs/100 ml, and the 2014 geometric mean was 699 orgs/100 ml.  All three geometric means far exceed the Class A1 criterion of 126 orgs/100 ml.  Sixteen of the combined 24 samples (67%) exceeded Iowa’s 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 is greater than 126 orgs/100 ml., the primary contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). 

The assessment of the Class B(WW2) aquatic life uses is, in part, based on data collected in 2013 & 2014 as part of the IDNR/SHL 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 2013 FIBI score was 38 (fair) and the BMIBI score was 61 (good).  The 2014 FIBI scores were 41, 46 (both fair) and the BMIBI scores were 44, 55 (both fair).  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 1994-2008.  The FIBI BIC for this ecoregion is 52 and the BMIBI BIC for this ecoregion is 61.  This segment passed the FIBI BIC 0/3 times and the BMIBI BIC 1/3 times in the last five years (2010-2014).

This aquatic life assessment is now also considered "monitored" based on a change in the 2010 IDNR assessment methodology.  IDNR now requires a segment have two or more biological samples collected from the segment in multiple years over a recent five-year period to be considered “monitored”.  This segment had multiple samples collected in the previous five years (2010-2014).

In contrast to the previous assessment period (2010-2012), water quality data collected during the 2012-2014 period do not suggest impairment of the aquatic life uses of this stream.  No violations of Class B(WW2) criteria for ammonia (14 samples), pH (22 samples), dissolved oxygen (24 samples), chloride (19 samples), or temperature (24 samples) occurred in during the 2012-2014 monitoring period.  During the previous (2010-2012) monitoring period, however, four of the 13 samples analyzed violated the Class B(WW2) criterion for dissolved oxygen.  According to Iowa DNR’s listing methodology, if significantly more than 10% of the samples exceed water quality criteria for conventional parameters such as dissolved oxygen, the aquatic life uses should be assessed as impaired.  The percentage of samples during the 2011-2012 period that violated the Class B(WW2) dissolved oxygen criterion (31%) was significantly greater than 10% and thus suggested impairment of the aquatic life uses.  This was a new impairment for this assessment segment for the 2014 IR cycle.  According to Iowa’s assessment/listing methodology, removal of an existing impairment due to violation of Iowa’s numeric water quality criteria requires that monitoring data show full support of the previously impaired beneficial use for two consecutive Integrated Report cycles.  Although results of monitoring for the current (2016) IR cycle do not suggest an impairment of the Class B(WW2) uses due to dissolved oxygen, this impairment will remain in effect.  If monitoring results for the next (2018) IR cycle also show non-impairment levels of dissolved oxygen, then this impairment can be removed.