Assessment Comments
Assessment is based on: (1) results of IDNR/UHL routine monthly ambient monitoring conducted near the mouth of the Rock River during the 2008-2010 assessment period at the County Road B-40 bridge north of Hawarden (STORET station 10840001 (formerly station 975005)), (2) results of an IDNR investigation of a fish kill in August 2002, (3) results of biological monitoring by IDNR/UHL in 2003 as part of the REMAP project and (4) IDNR Fisheries biological monitoring in 2000.
Basis for Assessment
[Note: Prior to the 2008 Section 305(b) cycle, this river segment was designated only for Class B(WW1) aquatic life uses, including fish consumption uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf) and the results of an Use Attainability Analysis, this segment is also now designated for Class A1 (primary contact recreation) uses. This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that violate state water quality criteria. The Class B(WW1) aquatic life uses are assessed (monitored) as “partially supported” based on occurrence of a pollutant-caused fish kill in August 2002. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment. The sources of data for this assessment include (1) results of IDNR/UHL routine monthly ambient monitoring conducted near the mouth of the Rock River during the 2008-2010 assessment period at the County Road B-40 bridge north of Hawarden (STORET station 10840001 (formerly station 975005)), (2) results of an IDNR investigation of a fish kill in August 2002, (3) results of biological monitoring by IDNR/UHL in 2003 as part of the REMAP project and (4) IDNR Fisheries biological monitoring in 2000.
EXPLANATION: The Class A1 (primary contact recreation) are assessed as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 21 samples collected during the recreational seasons of 2008 through 2010 at the Rock River near Hawarden were as follows: the 2008 geometric mean was 853 orgs/100 ml, the 2009 geometric mean was 275 orgs/100 ml and the 2010 geometric mean was 803 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Twelve of the 21 samples (57%) exceeded the Class A1 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 a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”
Regarding support of the Class B(WW1) aquatic life uses, monitoring at the IDNR/UHL ambient monitoring station near Hawarden showed no violations of Class B(WW1) water quality criteria for dissolved oxygen (minimum = 6.0 mg/L), pH (range = 7.8 to 8.5) or ammonia-nitrogen (maximum = 0.43 mg/L) in the 29 samples analyzed during the 2008-2010 assessment period.
Note: USGS conducted monitoring in this assessment segment at station 6483500 near Hawarden in August 2009. Too few data were produced from this monitoring during the 2008-2010 assessment period (one sample) to be of use for assessment for purposes of either Section 305(b) reporting or Section 303(d) listing.
Results of biological monitoring in 2000 and 2003 also suggest “full support” of the Class B(WW1) aquatic life uses of this river segment. This assessment is based on biological monitoring conducted in 2003 as part of the IDNR/UHL stream REMAP project and on data collected in 2000 by the IDNR Fisheries bureau. A series of biological metrics that 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 2000 Fisheries FIBI score was 50 (fair). The 2003 FIBI score was 57 (good) and the 2003 BMIBI score was 70 (good). The aquatic life use support was assessed (evaluated) as fully supporting (=FS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004. The FIBI BIC for this ecoregion is 43 and the BMIBI BIC for this ecoregion is 54. This segment passed the FIBI BIC 2/2 times in the last 11 years and passed the BMIBI BIC 1/1 times in 2003. This assessment is considered evaluated because the drainage area (1591 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. Even though this site passed both the FIBI and BMIBI BICs, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it doesn’t fall in the calibrated watershed size.
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 between 2006 and 2010 to be considered “monitored”. This segment had multiple FIBI samples collected in the previous 11 years (2000-2010); however, the multiple samples were not collected during 2006-2010. Additionally, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
Despite results of ambient water quality and biological monitoring that suggest “full support” of the Class B(WW1) uses, the occurrence of a fish kill in this river segment in August 2002 suggests that these uses should remain assessed (evaluated) as only “partially supported.” A fish kill occurred on this stream on August 8, 2002. The kill followed heavy rainfall; no specific cause was determined, but runoff from feedlots was suspected as the cause and source of this kill. Approximately 1.75 miles of river was affected, and an estimated 930 fish were killed. According to IDNR's assessment methodology for Section 305(b) reporting, occurrence of a single pollution-caused fish kill indicates an impairment of the aquatic life uses. Because the data upon which these assessment are based are now more than five years old, the assessment type is changed from “monitored” (a higher confidence assessment) to “evaluated” (a lower confidence assessment). As water quality data age, they are less able to represent current water quality conditions. Despite the age of the data and the change from a “monitored” to an “evaluated” assessment type, any impairments previously identified for this assessment segment remain on Iowa’s section 303(d) list.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment.
