Rock Creek IA 02-CED-585
mouth (S24 T97N R17W Floyd Co.) to confluence with unnamed tributary in NW 1/4 SE 1/4 S17 T97N R17W Mitchell Co.
- Cycle
- 2016
- Release Status
- Final
- Overall IR
- 5 - Water is impaired or threatened and a TMDL is needed.
- Trend
- Unknown
- Created
- 8/17/2016 2:03:36 PM
- Updated
- 9/29/2016 10:43:26 AM
The presumptive Class A1 uses are assessed (monitored) as “partially supported” (IR 5p) due to levels of indicator bacteria that exceed state water quality criteria. The Class B(WW1) aquatic life uses are assessed (monitored) as “fully supported” (IR 2a) based on results of chemical/physical water quality monitoring from 2012-2014. Fish consumption uses remain not assessed (IR 3a) due to the lack of fish contaminant monitoring in this assessment segment. Sources of data for this assessment include the results of ambient chemical/physical water quality monitoring conducted at two monitoring stations: (1) STORET station 15340005 at County Road T38 monitored from May 2012 through November 2013 as part of the Cedar River/Mitchell County study and (2) STORET station 14000125 at Kirkwood Avenue monitored from April to November 2013. Bacterial monitoring was conducted at station 15340005 from May 2012 to October 2014 and was conducted at STORET station 14000125 from April 2013 to October 2014.
The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" due to violations of Iowa’s water quality criteria for indicator bacteria. The geometric means of indicator bacteria (E. coli) in the 20 samples collected during the recreational seasons of 2012 through 2014 at site 15340005 at County Road T38 were as follows: the 2012 geometric was 175 orgs/100 ml, the 2013 geometric mean was 144 orgs/100 ml, and the 2014 geometric mean was 321 orgs/100 ml. All three geometric means slightly exceed the Class A1 criterion of 126 orgs/100 ml. Eleven of the 20 samples (55%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml. At station 14000125 at Kirkwood Avenue, the geometric means of indicator bacteria (E. coli) in the 14 samples collected during the recreational seasons of 2013 and 2014 were as follows: the 2013 geometric was 172 orgs/100 ml, and the 2014 geometric mean was 208 orgs/100 ml. Both geometric means slightly exceed the Class A1 criterion of 126 orgs/100 ml. Nine of the 14 samples (64%) 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 should be assessed as "impaired". Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”
The Class B(WW1) aquatic life uses are assessed as “fully supported” based on results of chemical/physical monitoring at Cedar River/Mitchell County station 15340005 and station 14000125. Results of ambient water quality monitoring at these stations during the 2012-2014 assessment period showed no violations of Class B(WW) water quality criteria for ammonia or dissolved oxygen in the 23 combined samples collected or for chloride in the seven samples collected. One of the 15 samples (7%) collected at station 15340005 from May 2012 to November 2013 violated the Class B(WW1) criterion for pH: the sample collected on September 19, 2012, contained a pH of 9.5 units, thus violating the Class B(WW1) criterion of 9.0 units. In addition, one of 15 samples (7%) at this station violated the Class B(WW1) criterion for temperature: the sample collected on June 19, 2012, had a temperature of 34.6C, thus exceeding the criterion of 32C. 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 pH or temperature, the aquatic life uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Because, however, less than 10% of the samples contained violations of criteria for pH or water temperature, these violations do not indicate impairment of the Class B(WW1) aquatic life uses.
Fish consumption uses remain not assessed due to the lack of fish contaminant monitoring in this assessment segment.