Assessment Comments
Assessment is based on the results of IDNR ambient monthly monitoring east of Grimes from 2010 through 2012 and on 2012 IDNR/SHL biological sampling conducted near Grimes.
Basis for Assessment
[Note: Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(LR) aquatic life uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008, this segment is now presumptively designated for Class A1 (primary contact recreation) uses. The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses). Thus, for the current assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" (IR 5p) due to levels of indicator bacteria that violate state water quality criteria. The Class B(WW2) aquatic life uses are assessed (evaluated) as "not supported" (IR 3b-c) based on results of IDNR/SHL biological sampling in 2012. The sources of data for this assessment are the results from the IDNR/SHL ambient monthly monitoring station on Beaver Creek east of Grimes (STORET station 10770001 (formerly station 420307) from 2010 through 2012 and 2012 IDNR/SHL biological (benthic macroinvertebrate) sampling conducted near Grimes.
EXPLANATION: The presumptive Class A1 uses are assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E. coli). The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2010 through 2012 at IDNR station 10770001 were as follows: the 2010 geometric mean was 1,484 orgs/100 ml, the 2011 geometric mean was 249 orgs/100 ml, and the 2012 geometric mean was 264 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Thirteen of the 24 samples (54%) 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.”
Monitoring at the IDNR/SHL station east of Grimes showed no violations of Class B(WW2) water quality criteria during the 2010-2012 assessment period for dissolved oxygen, pH, ammonia-nitrogen, chloride, or sulfate in the 36 samples analyzed. One of the 36 samples collected (3%) exceeded the Class B(WW1) criterion for temperature of 32C (violation was 32.8C on July 2, 2012). According to U.S. EPA guidelines for Section 305(b) reporting, if more than 10% of samples exceed state criteria for conventional parameters such as water temperature, the aquatic life uses should be assessed as "impaired" (see pgs 3-33 to 3-35of U.S. EPA 1997b). According to IDNR’s assessment/listing methodology, however, the results from IDNR station 10770001 do not indicate that greater than 10% of the samples exceed either the Class B(WW1) criteria for temperature. Thus, these results do not suggest impairment of the Class B(WW1) aquatic life uses.
In contrast to the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the aquatic life uses are "not supporting". This evaluated biological assessment was based on data collected in 2012 as part of the IDNR/SHL large river/stream nutrient sampling project. A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biological sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa collected in the stream sampling reach. The biological metrics were combined a benthic macroinvertebrate index (BMIBI). The index rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2012 BMIBI score was 21 (poor). The aquatic life use support was assessed (evaluated) as "not supporting" (=NS), based on a comparison of the BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008. The BMIBI BIC for this ecoregion is 62.
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 during a recent five year period to be considered “monitored”. This segment had a single sample collected in 2012. 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).
