Assessment Comments
Assessment is based on results of ambient water quality monitoring conducted at Site ICLM (STORET station 15570011) at Linn Mar from May 2010 to July 2011.
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 exceed Class A1 criteria. The Class B(WW2) aquatic life uses are assessed (monitored) as “fully supported” (IR 2a) based on results of ambient water quality monitoring conducted at Site ICLM (STORET station 15570011) at Linn Mar in 2010 and 2011.
EXPLANATION: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to violations of Iowa’s water quality criteria for indicator bacteria. The geometric means of indicator bacteria (E. coli) in the 12 samples collected during the recreational season of 2010 (1,962 orgs/100 ml), and the geometric mean of the 12 samples collected during the recreation season of 2011 (2,284 orgs/100 ml) both far exceeded the Class A1 criterion of 126 orgs/100 ml. All 24 of the samples exceeded Iowa’s 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).
Despite impairment of the Class A1 uses, results of ambient water quality monitoring at station 15570011 in 2010 and 2011 indicate good chemical water quality in this stream segment and “full support” of the Class B(WW2) aquatic life uses. No violations of Class B(WW2) water quality criteria for pH or temperature occurred in the 24 samples analyzed from May 2010 to July 2011. One of the 24 samples (4%) violated the Class B(WW2) criterion for dissolved oxygen. The sample collected on July 18, 2011, contained 4.5 mg/l of dissolved oxygen, thus slightly violating the criterion of 5.0 mg/l. 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, 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 15570011, however, do not indicate that significantly greater than 10% of the samples exceed the Class B(WW1) criterion for dissolved oxygen. Thus, the results of chemical/physical water quality monitoring in 2010 and 2011 at station 15570011 suggest that the Class B(WW2) aquatic life uses should be assessed (monitored) as “fully supported.”
