Assessment Comments
Assessment is based on results of (1) IDNR/UHL biological (biocriteria) sampling in 2001 upstream from the Milford WWTP and (2) IDNR/UHL TMDL monitoring in 2001.
Basis for Assessment
SUMMARY: The general aquatic life uses remain assessed (evaluated) as "partially supported" based on results of biological monitoring conducted in 2001 for fish and aquatic macroinvertebrates. The sources of data for this assessment include results of (1) biological monitoring conducted by IDNR/UHL in September 2001 and (2) monthly monitoring conducted by IDNR/UHL upstream from the Milford wastewater treatment plant at one location (STORET station 11300001) from April to November 2001 as part of monitoring in support of TMDL development.
Although this segment of Milford Creek is classified only for general uses in the Iowa Water Quality Standards, results of water quality monitoring were compared to Class B(LR) aquatic life criteria for purposes of estimating potential water quality impacts. The decision to consider this assessment as “evaluated” (lower confidence; used for Section 305(b) reporting only) versus “monitored” (higher confidence; used for Section 303(d) listing) is based on (1) the relatively few samples collected (10), (2) the relatively poor parameter coverage (i.e., no analysis of samples for toxic metals or pesticides), (3) the difficulty of using water quality criteria developed for designated use waters to assess general use waters to which these criteria do not apply, and (4) difficulty of assessing general use waters using biological assessment protocols based on reference conditions in designated use (Class B(LR) and wadeable Class B(WW)) streams. Despite this rationale, U.S. EPA Region VII considered this assessment segment as pollutant-impaired in their May 23, 2006 transmittal of Iowa's final 2004 Section 303(d) list and thus added this segment of Milford Creek to Category 5a of Iowa's 2004 Integrated Report as "monitored/partially supported". This assessment is the same assessment that was developed for the previous (2004) Section 305(b) reporting and Section 303(d) listing cycle. See the narrative for the 2004 assessment for an expanded description of EPA’s decision to consider this general use segment as “impaired”.
EXPLANATION: This assessment was based, in part, on data collected in 2001 as part of the DNR/UHL 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 (F-IBI) and a benthic macroinvertebrate index (BM-IBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The F-IBI scores were 38 (fair) and 50 (fair); the BM-IBI scores were 14 (poor) and 44 (fair). The aquatic life use support was assessed as partially supported (=PS), based on a comparison of the F-IBI and BM-IBI scores with biological assessment criteria established specifically for the 2000 Section 305(b) report. The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-1998.
This assessment is considered “evaluated” (of lower confidence) in part 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 impairment based on a comparison of monitoring data to Class B(LR) water quality criteria, or that show 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”.
Despite the results of biological monitoring that show impairment, results of IDNR/UHL chemical water quality monitoring in this stream segment show relatively good water quality. None of 8 samples collected from April through November 2001 violated Class B(LR) water quality criteria for dissolved oxygen (minimum value = 5.2 mg/l), pH (range of 7.6 to 8.7 units), or ammonia-nitrogen (maximum value = 0.60 mg/l). Because biological monitoring is better able to reflect cumulative impacts of water quality over time and thus more accurately represents water quality conditions, the results of biological monitoring in 2001 were use to assess support of the general aquatic life uses as "partially supported."
