Cedar River IA 02-CED-477

[Note: A TMDL for indicator bacteria impairments in eight segments of Cedar River was prepared and approved by EPA in February 2010. Because the bacterial impairment in this segment (IA 02-CED-0110_1) was not included in the EPA TMDL, this impairment is considered appropriate for Category 5a 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 24 samples collected during the recreational seasons of 2014 through 2016 at DNR station 10340001 near Charles City were as follows: the 2014 geometric mean was 134 orgs/100 ml, the 2015 geometric mean was 98 orgs/100 ml, and the 2016 geometric mean was 150 orgs/100 ml. Two of the three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Four of the combined 24 samples (17%) 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."

In contrast to the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the Class B(WW1) aquatic life uses are "partially supporting". This evaluated biological assessment was based on data collected in 2011-2014as part of the DNR/SHL stream nutrient sampling project. A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biological sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa collected in the stream sampling reach. The biological metrics were combined a benthic macroinvertebrate index (BMIBI). The index rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2012-2016 BMIBI scores were 86 (excellent), 50 (fair) and 27, 27 (both poor). The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008. The artificial substrate BMIBI BIC for this ecoregion is 52 and the natural substrate BMIBI BIC is 70. This assessment is considered evaluated because the drainage area (1095 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. The BMIBI score of 86 was a natural substrate sample and passed the natural substrate BIC of 70. The 27, 27 and 50 BMIBI scores were artificial substrate scores and they failed the artificial substrate BIC of 52. Even though this site failed the BMIBI BICs (1/4), it is uncertain as to whether or not this segment is meeting the aquatic life criteria because the site used for the assessmentdoesn’t fall in the calibrated watershed size. According to DNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are thus not appropriate for Section 303(d) listing (Category 5 of the Integrated Report). DNR does, however, consider these impairments as appropriate for listing under either Category 2b or 3b of the Integrated Report (waters potentially impaired and in need of further investigation).

Fish consumption uses are assessed (monitored) as “fully supported/threatened” (impaired) based on results of U.S.EPA/DNR fish contaminant (RAFT) monitoring near Midway in 2012. This monitoring showed an increase in levels of mercury in predator species in this river segment.The composite samples of fillets from channel catfish continued to have low levels of contaminants: mercury: 0.172 ppm; total PCBs: 0.09 ppm; and technical chlordane: < 0.03 ppm. Levels of mercury in tissue plug samples of smallmouth bass, however, were much higher than in the 2003 composite samples of smallmouth bass fillets. The average level of mercury in the tissue plugs from five smallmouth bass was 0.464 ppm (SD=0.089 ppm; maximum=0.572; minimum=0.344). The average and minimum levels of mercury were well below Iowa’s “do not eat” advisory threshold of 1.0 ppm but were above Iowa’s threshold for issuance of a one meal per week fish consumption advisory. According to the DNR/DPH advisory protocol, two consecutive samplings showing that contaminant levels are above the advisory threshold level are needed to justify issuance of a consumption advisory.

Because, however, levels of mercury in the 2012 sampling of smallmouth bass were well-above the one-meal/week advisory threshold, the fish consumption uses of this segment are assessed as “fully supported/threatened”. Waters assessed as “threatened” demonstrate an adverse water quality trend such that impairment is anticipated within the next two years. Waters assessed as “fully supported/threatened” are thus candidates for addition to a states list of Section 303(d) (impaired) waters (IR Category 5).

Follow-up monitoring in 2014 showed that the average concentration of mercury in tissue plugs from four smallmouth Bass (0.27 ppm) was below the one meal/week consumption advisory threshold of 0.3 ppm thus indicating lower levels of mercury and the lack of need to issue a consumption advisory for this river segment. According to Iowa DNR assessment/listing guidelines, two consecutive fish tissue samplings showing that average levels of mercury are below the advisory threshold are needed to de-list the existing impairment. Follow-up fish tissue monitoring will be conducted in this assessment segment to determine whether this impairment should be removed.