Assessment Comments
Assessment based on results of (1) IDNR/UHL biological sampling in 2000 and 2002: FIBI ave = 49.5, BMIBI ave = 59.5, FIBI BIC = 52, BMIBI BIC = 61; and (2) an IDNR investigation of a fish kill in August 2005.
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses are assessed (monitored) as "partially supported" based on results of IDNR/UHL biological sampling in 2000 and 2002. Occurrence of a fish kill in August 2005 due to hog manure also suggests impairment of the Class B(LR) uses of this stream. The party responsible for this kill was identified, and IDNR has sought restitution for the value of the fish killed and the costs incurred by IDNR during investigation of the kill. In the absence of the existing imairment based on IDNR/UHL biological monitoring in 2002 and 2002 (in Category 5b of the Integrated Report), this fish kill-related impairment would be appropriate for Category 4d of Iowa’s 2006 Integrated Report.
EXPLANATION: This assessment was based on data collected in 2000 and 2002 as part of the DNR/UHL stream biocriteria and REMAP projects, respectively. 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 2002 FIBI score was 48 (fair) and the BMIBI score was 61 (good). The 2000 FIBI score was 51 (good) and the BMIBI score was 58 (good). The FIBI average was 49.5 and the BMIBI average was 59.5. 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. The FIBI BIC for this ecoregion is 52 and the BMIBI BIC for this ecoregion is 61.
As part of the 2002 REMAP sampling project, stream physical habitat assessment found adverse conditions that are thought to contribute to the biological impairment. These include, bank instability, excessive silt deposition, and removal of riparian vegetation associated with livestock grazing. High silt levels were also observed during physical habitat assessment of the 2000 sample site.
A fish kill occurred in this stream segment on or before August 1, 2005 and was caused by a release of hog manure from an over-full storage pit. According to the IDNR investigation, an outdoor storage pit lacked sufficient capacity to handle the amount of manure delivered to it from an indoor storage pit; thus, an overflow occurred. Approximately 2.25 miles of Roberts Creek were affected by the resulting kill. The kill began in an unnamed tributary to Roberts Creek (waterbody IA 01-TRK-0361-0) in Section 8 of T95N, R6W, Clayton County, and proceeded downstream through Section 9, 15, and 15 of T95N, R6W. An estimated 8,000 fish were killed. Based on IDNR records, the value of the fish killed was estimated at $4,893. Other than a notation that the kill affected “mostly non-game species", no detailed information on species killed is available. The party responsible for the kill was identified. IDNR has sought restitution for the value of the fish killed and the costs incurred by IDNR during investigation of the kill.
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 is identified, and the cause is either known, or suspected, to be a “pollutant”, the assessment type is considered “monitored” and the affected waterbody is a candidate for Section 303(d) listing. If, however, a consent order has been issued to the party responsible for the kill and monetary restitution has been sought for the fish killed, the affected waterbody will be placed in IR Category 4d (impaired but TMDL not required). IDNR feels that (1) TMDLs should not be required for kills caused by a one-time illegal or unauthorized release of manure or other toxic substance where enforcement actions were taken and (2) enforcement action is more appropriate, efficient, and effective for addressing a spill-related impairment than is the TMDL process.
