Mosquito Creek IA 02-CED-6489
from mouth (SE 1/4 S20 T87N R15W Grundy Co.) to headwaters in NE 1/4 S36 T87N R17W Grundy Co.
- Cycle
- 2018
- Release Status
- Final
- Overall IR
- 5 - Water is impaired or threatened and a TMDL is needed.
- Trend
- Unknown
- Created
- 7/19/2019 11:56:40 AM
- Updated
- 7/19/2019 11:57:57 AM
The presumptive Class A1 (primary contact recreation) uses remain assessed as “not supported” due to violations of Iowa water quality criteria for indicator bacteria. The Class B(WW1) aquatic life uses remain “not assessed” due to the lack of water quality information upon which to base an assessment. The source of data for this assessment remains the results of monitoring at three sites (BHC-11, BHC-8, and BHC-9) conducted as part of a Clean Water Act Section 319 water quality project from June 2009 through November 2010. This is the same assessment as developed for previous IR cycles. Additionally, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment categories are considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
The presumptive Class A1 uses remain assessed (evaluated) as “not supported” due to levels of indicator bacteria (E. coli) that exceed the Class A1 geometric mean criterion. The geometric means (GM) of indicator bacteria (E. coli) in the 22 samples collected at these three stations during the recreational seasons of 2009 and 2010 were as follows:
N 2009 GM 2010 GM No. & percent of samples > single sample max criterion:
BHC-11 22 1,840 637 20; 91% violation
BHC-8 23 733 205 16; 70% violation
BHC-9 22 1,400 669 20; 91% violation
The geometric means at all three stations exceeded Iowa’s Class A1 geometric mean criterion of 126 E. coli orgs/100 ml, thus indicating nonsupport of the Class A1 primary contact recreation uses. From 70% to 90% 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 DNR’s assessment/listing methodology, if a recreation season geometric mean is greater than the respective criterion (126 orgs/100 ml for Class A1, the primary contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). In addition, the percentage of samples with levels of E. coli that exceeded Iowa single-sample maximum criteria indicate impairment of Class A1 uses at this monitoring site.