Assessment Comments
Assessment is based on results of monitoring at two stations on Lime Creek sampled in 2010 and 2011: at 240th Street (STORET station 15100007) and at 250th Street (STORET station 15100008).
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 uses are assessed (monitored) as “not supported” (IR 5p) due to levels of indicator bacteria that violate Class A1 criteria. This is a new impairment for this previously unassessed segment. The Class B(WW2) aquatic life uses are assessed (monitored) as "fully supported" (IR 2a) based on results of IDNR-sponsored monitoring in 2010 and 2011. The sources of data for this assessment are the results of monitoring at two stations on Lime Creek sampled from May 2010 to July 2011: STORET station 15100007 at 240th Street and STORET station 15100008 at 250th Street.
EXPLANATION: The presumptive Class A1 uses are assessed (monitored) as "not supported" based on results of monitoring for indicator bacteria (E. coli) at the two stations. The geometric mean of indicator bacteria (E. coli) in the 12 samples collected at Station 15100007 at 240th Street during the recreational season of 2010 (637 orgs/100 ml), and the geometric mean of the 12 samples collected during 2011 (946 orgs/100 ml) both exceeded the Class A1 criterion of 126 orgs/100 ml. Twenty of the 24 samples (83%) collected at this station exceeded Iowa’s Class A1 single-sample maximum criterion of 235 orgs/100 ml. Similarly, the geometric mean of indicator bacteria (E. coli) in the 12 samples collected at Station 15100008 at 250th Street during the recreational season of 2010 (677 orgs/100 ml), and the geometric mean of the 11 samples collected during 2011 (957 orgs/100 ml) both exceeded the Class A1 criterion of 126 orgs/100 ml. Twenty-one of the 23 of the samples (91%) from this station exceeded Iowa’s 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 the geometric mean is greater than 126 orgs/100 ml., the primary contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
The Class B(WW-2) aquatic life uses are assessed (monitored) as "fully supported" based on results of chemical/physical water quality monitoring from 2010 to 2011 at stations 1510007 and 15100008. No violations of Class B(WW2) water quality criteria for temperature occurred in the combined 48 samples analyzed from May 2010 to July 2011. One of the 24 samples (4%) collected at station 15100007 violated the Class B(WW2) criterion for dissolved oxygen. The sample collected on August 3, 2010, contained 4.0 mg/l of dissolved oxygen, thus violating the criterion of 5.0 mg/l. Similarly, one of the 24 samples (4%) collected at this station violated the Class B(WW2) criterion for pH. The sample collected on June 8, 2011, contained a pH of 6.3 units, thus violating the Class B(WW2) criterion of 6.5 units. 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 dissolved oxygen or pH, 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 station 15100007, however, do not indicate that significantly greater than 10% of the samples exceed the Class B(WW2) criterion. Thus, the results of chemical/physical water quality monitoring in 2010 and 2011 in this segment suggest that the Class B(WW2) aquatic life uses should be assessed (monitored) as “fully supported.”
