Milford Creek IA 06-LSR-1666

[Note:  A TMDL for phosphorus was prepared by IDNR and approved by EPA in December 2008 (http://www.iowadnr.gov/water/watershed/tmdl/files/final/milford.pdf).   As noted in this TMDL, IDNR’s 2004 stressor identification document for Milford Creek identified excessive aquatic vegetation (both algae and macrophytes) and low levels of dissolved oxygen as the primary stressors of the fish and aquatic macroinvertebrate communities of this stream segment.   The high level of phosphorus in this stream, as documented as part of previous Section 305(b) assessments and as part of the 2008 TMDL, was identified as the causal factor behind the responses of excessive vegetation and low levels of dissolved oxygen.   Because the 2008 TMDL covers these causal factors, the biological impairment identified for the 2004 and 2006 listing cycles is moved from IR Category 5b to Category 4a (impaired; TMDL approved).]

The Class A3 (primary contact recreation) uses remain not assessed due to the lack of data for indicator bacteria (E. coli) upon which to base an assessment.  The TMDL-related monitoring conducted in this stream segment from 2001 through 2005 did not included monitoring for indicator bacteria.    

The assessment of the Class B(WW2) aquatic life uses remains partially based on data collected in 2001 and 2005 as part of the IDNR/SHL stream biocriteria project.   Results of biological sampling in 2001 and 2005 suggest that the Class B(WW2) uses are only “partially supported.”  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 2001 FIBI score was 50 (fair) and the BMIBI score was 43 (fair).   The 2005 BMIBI score was 13 and fish were not collected.   The aquatic life use support was assessed (evaluated) as partially supported (=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 riffle site FIBI BIC for this ecoregion is 53 and the BMIBI BIC for this ecoregion is 62.   This segment passed the FIBI BIC 1/1 times (using the FIBI uncertainty adjustment value of 7 points applied to single samples) and passed the BMIBI BIC 0/2 times in the last 14 years.  

This aquatic life assessment is now considered "evaluated" 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 BMIBI samples collected in the previous 14 years; however, the multiple samples were not collected during a recent five year period.   According to IDNR’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).   IDNR 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 4a based on the prepared and approved TMDL for this segment.    

Results of chemical monitoring conducted in this stream segment as part of TMDL-related monitoring during the period from May 2004 to September 2005 suggest water quality problems in this stream reach but do not suggest impairment of the Class B(WW2) aquatic life uses.   Two of 20 samples violated the Class B(WW2) criterion for dissolved oxygen of 5 mg/l; these samples were collected on August 4 (3.9 mg/l) and August 10 (4.2 mg/l) of 2004.   One of 17 samples collected at this station (6%) violated the Class B(WW2) criterion for pH.  The pH of the sample collected on June 7, 2004 was 9.3 units, thus exceeding the criterion of 9.0 pH units.   According to U.S.  EPA guidelines (U.S.  EPA 1997b, page 3-17), a violation frequency of between 10 and 25% for conventional parameters such as dissolved oxygen suggests "partial support" of aquatic life uses.   Based on IDNR’s assessment methodology, however, these results do not suggest that the violation frequency for dissolved oxygen is significantly greater than 10% and thus do not suggest impairment of Class B(WW2) uses of this segment of Milford Creek.    

Two of 14 samples collected from 2004-2005 violated the Class B(WW2) chronic criteria for ammonia; these samples were collected on July 6, 2004 (4.5 mg/l) and July 12, 2004 (0.74 mg/l).   Based on IDNR’s 2008 assessment/listing methodology, however, these results do not suggest that the violation frequency of chronic criteria for ammonia is significantly greater than 10% and thus do not suggest impairment of Class B(WW2) uses of this segment of Milford Creek.   For a previous (2006) assessment/listing cycle, however, chronic violations of ammonia criteria were treated identical to violations of acute criteria: more than one violation in a three-year period suggested impairment of the aquatic life uses.   This assessment approach resulted in identification of an impairment of the aquatic life uses of this stream segment for the 2006 listing cycle due to violations of chronic criteria for ammonia.   In recent (Fall 2007) discussions with U.S.  EPA, however, Region 7 states questioned the issue of using chronic criteria for toxics to identify impairments.   Because no EPA Region 7 state collects ambient data at a frequency that can be used to calculate the short-term (e.g., 4-day or 30-day) averages required to accurately identify violations of chronic criteria, states suggested that data for toxics be used with the 10% rule to identify chronic criteria-based impairments of aquatic life uses.   EPA did not object to this proposed change in the states’ assessment/listing methodology.   Thus, for the 2008 listing cycle, IDNR used the 10% rule as described by Lin et al.   (2000) to determine impairment of aquatic life uses due to violations of chronic criteria for toxics.   Based on this change in assessment/listing methodology, the ammonia impairment for this segment of Milford Creek is proposed for de-listing.  [Methodology follow-up:  in 2012, U.S.  EPA Region 7 informed Iowa DNR that, because ammonia is not a priority pollutant (i.e., a "toxic" in the sense of the Clean Water Act), the 10% rule can be used for ammonia data to determine support of aquatic life uses.]

The TMDL-related monitoring at Site 49 (11300002) from 2004-2005 did not include analysis for either pesticides or toxic metals.    

Limited monitoring at TMDL Site 2 (Station 11300015; 1 mile SSE of Milford) from July to September 2005 did not show violations of Class B(WW2) criteria  no violations occurred for ammonia (maximum = 0.21 mg/l), pH (range of 7.9-8.4), or dissolved oxygen (minimum = 6.8 mg/l) in the four samples collected during this period.   Although these results suggested good water quality, the number of samples and period of time covered was limited and does not provide for development of a higher confidence (monitored) assessment.  

This stream has historically shown elevated levels of phosphorus.   The mean, median, and maximum total phosphorus levels for the 24 sampling events from May 2004 through September 2005 at TMDL monitoring station 49 in lower Milford Creek were 1.4, 1.5, and 3.0 mg/l, respectively.   Summary statistics for dissolved phosphorus, measured as orthophosphate, for these 24 sampling events (mean, median, and maximum) were 1.2, 1.3, and 3.1 mg/l, respectively.