Cedar River IA 02-CED-479

[Note: A TMDL for indicator bacteria in this segment of Cedar River was prepared and approved by EPA in February 2010. The approval of this TMDL moves the bacterial impairment for this segment from Iowa's list of Section 303(d) waters (Category 5a of the Integrated Report) to IR Category 4a (impaired; TMDL not required). Because, however, the impairment due to the existence of the mercury-related fish consumption advisory was not covered by this TMDL, this segment of the Cedar River remains in Category 5a (impaired and TMDL required) of Iowa's Integrated Report.]

The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 18 samples collected during the recreational seasons of 2014 through 2016 at Iowa DNR station 15660010 (Cedar River at State Line St.) were as follows: the 2014 geometric mean was 86 orgs/100 ml, the 2015 geometric mean was 356 orgs/100 ml, and the 2016 geometric mean was 120 orgs/100 ml. One of the three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Four of the combined 18 samples (22%) 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 Iowa DNR’s assessment/listing methodology, if the geometric mean is greater than 126 orgs/100 ml., the primary contact recreation uses should be assessed as "impaired" (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 "partially supported."

The 2006 DNR/SHL stream REMAP biological sampling at two sites also suggested (evaluated) "full support" of the Class B(WW1) aquatic life uses. This evaluated biological assessment was based on data collected in 2006 as part of the DNR/SHL stream REMAP project. A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species collected in the stream sampling reach. The biological metrics were combined to make a fish community index of biotic integrity (FIBI) and a benthic macroinvertebrate index (BMIBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2006 FIBI scores were 46 (fair) and 74 (excellent) and the BMIBI scores were 76 (excellent), 75 (good). The aquatic life use support was assessed (evaluated) as fully supporting (=FS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008. The non-riffle habitat FIBI BIC for this ecoregion is 44 and the natural substrate BMIBI BIC for this ecoregion is 70. This segment passed the FIBI BIC 2/2 times and passed the BMIBI BIC 2/2 times in 2006. This assessment is considered evaluated because the drainage areas (675 & 862 mi2) above the sampling sites were greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. Even though this site passed both the FIBI and BMIBI BICs, it is uncertain as to whether or not this segment is meeting the aquatic life criteria becausethe sites used for the assessmentdon’t fall in the calibrated watershed size.

Results of chemical/physical water quality monitoring at Iowa DNR station 15660010 (Cedar River at State Line St.) suggest “full support” of the aquatic life uses. Monitoring showed no violations of Class B(WW1) water quality criteria for 5 Dissolved Oxygen samples (minimum = 8 mg/L), 11 pH samples (range = 7.4 to 8.6), or 11 Temperature samples (maximum = 20.5°C) occurred during monitoring from July 2015 to October 2016.According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated. Thus, these results thus suggest “full support” of the Class B(WW1) aquatic life uses.

Follow-up sampling was conducted in 2007, 2008, and 2009 to better assess the amount of mercury in fish in this river segment. The composite samples of smallmouth bass fillets in 2007 and 2008 had low levels of mercury (2007: 0.219 ppm, 2008: 0.19 ppm). These levels were below the advisory trigger level (0.30 ppm). Based on DNR's assessment methodology two consecutive samples with levels below the advisory trigger level are necessary to suggest "full support" of the fish consumption uses. While this is the case for this segment of the Cedar River, there were no samples of walleye collected, therefore it is not known if levels of mercury in walleye are also below the advisory trigger level. The composite samples of smallmouth bass fillets in 2009 had a level of mercury (0.351 ppm) that again exceeded the advisory trigger level. Therefore the fish consumption advisory will remain and the fish consumption uses will remain assessed as "partially supported". Additional follow-up monitoring will be conducted to determine if the levels of mercury in walleye are also below advisory trigger levels.