Assessment Comments
Assessment is based on (1) IDNR/UHL REMAP sampling in 2004 and 2005, (2) IDNR/UHL ambient water quality monitoring in 2002 and 2003 at two locations in support of TMDL development and (3) results of a fish kill investigation in September 2003.
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses are assessed (monitored) as “partially supported” based 2004 & 2005 IDNR/UHL REMAP biological monitoring and the occurrence of a fish kill in this stream segment in September 2003. Results of ambient water quality monitoring, however, suggest relatively good water quality conditions in this stream. In addition to results of the September 2003 fish kill investigation and 2004 DNR/UHL REMAP biological data, the sources of data for this assessment include results of ambient chemical/physical water quality monitoring conducted by IDNR/UHL in support of TMDL development during the 2002-2004 assessment period at two locations on Mud Creek: (1) near Doon, Iowa (TMDL station 5; STORET station 11600004) and (2) near the Iowa/Minnesota state line (TMDL station 4; STORET station 11600003).
EXPLANATION: Results of chemical/physical water quality monitoring conducted by IDNR/UHL during the from March 2002 through December 2003 in support of TMDL development suggest relatively good water quality in this stream. None of the combined 44 samples collected from these two stations had levels of dissolved oxygen or ammonia-nitrogen that violated state water quality standards (these samples were not analyzed for toxic metals or pesticides). Monitoring results for pH, however, showed that one of the combined 44 samples analyzed (3%) violated the Class B(LR) water quality criterion for dissolved oxygen: the level of pH in the sample collected on May 20, 2003 at TMDL station 5 (STORET No. 11600004) (10.3 pH units) violated the Class B(LR) criterion of 9.0 units. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), however, if 10% or less of samples exceed criteria for conventional parameters (e.g., dissolved oxygen and pH), aquatic life uses should be assessed as “fully supported”. These results, although somewhat limited in terms of parameter coverage, suggest "full support" of the Class B(LR) aquatic life uses.
Despite results of ambient water quality monitoring that suggest “full support” of the Class B(LR) uses, the results of the 2004 & 2005 biological monitoring in this river segment suggests that these uses should be assessed (monitored) as only “partially supported”. The aquatic life assessment was based on data collected in 2004 & 2005 as part of the DNR/UHL stream REMAP 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 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 2004 FIBI score was 20 (poor) and the 2004 BMIBI score was 55 (fair). The 2005 FIBI score was 13 (poor) and the 2005 BMIBI score was 26 (poor). The FIBI average was 16.5 (poor) and the BMIBI average was 41 (fair). The FIBI BIC for this ecoregion is 43 and the BMIBI BIC for this ecoregion is 54. 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 1994-2004.
In addition, a fish kill in this river segment in September 2003 also suggests that these uses should be assessed (evaluated) as only “partially supported”. This kill occurred on or before September 10, 2003. The cause of the kill was attributed to low levels of dissolved oxygen, potentially related to a rainfall event and high levels of turbidity that resulted. An estimated 130 fish were killed, including common carp (20), channel catfish (10), and “minnows” (100). No estimate of the length of this stream affected by the kill was provided. No responsible party was identified. The location point for this kill is the SW ¼ of the SE ¼ of Section 15, T98N, R46W, Sioux County. According to the IDNR investigation, the kill occurred at a road construction site at the county road A42 bridge. Due to the construction the water was diverted through a culvert which caused the stream to form a pool; the kill took place in the pooled area. The pooled water became very turbid following the heavy rains, thus potentially killing the fish.
According to IDNR’s assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2002-2005) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired”. If a cause of the kill was not identified during the IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.” Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5). Waterbodies affected by such fish kills will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation.
