Assessment Comments
Assessment is based on: (1) results of monthly monitoring from 2010 through 2012 at the IDNR/SHL ambient city monitoring station located downstream from Mason City (STORET station 10170003); (2) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring east of Mason City in 2012, and (3) IDNR/SHL biological sampling conducted in 2012.
Basis for Assessment
[Note: Prior to the 2008 Section 305(b) cycle, this river segment was designated only for Class B(WW) aquatic life uses, including fish consumption uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008, and due to results of a Use Attainability Analysis, this segment is also now designated for Class A1 (primary contact recreation) uses. This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" (IR 5a) due to levels of indicator bacteria that violate state water quality criteria. The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported” (IR 2a) based on results of ambient water quality monitoring from 2010 through 2012 and 2012 IDNR/SHL biological sampling. Fish consumption uses are assessed (monitored) as "fully supported" (IR 2a) based on results of fish contaminant monitoring in 2012. The sources of data for this assessment include (1) results of monthly monitoring from 2010 through 2012 at the IDNR/UHL ambient city monitoring station located downstream from Mason City (STORET station 10170003); (2) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring east of Mason City in 2012 and (3) IDNR/SHL biological sampling conducted in 2012.
EXPLANATION: The Class A1 primary contact recreation uses are assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E. coli). The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2010 through 2012 at station 10170003 downstream from Mason City were as follows: the 2010 geometric mean was 255 orgs/100 ml, the 2011 geometric mean was 171 orgs/100 ml and the 2012 geometric mean was 387 orgs/100 ml. All three geometric means slightly exceed the Class A1 criterion of 126 orgs/100 ml. Thirteen of the 24 samples (54%) 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 are "not supported" (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 “impaired.”
The Class B(WW1) aquatic life uses are assessed as “fully supported” based on results of IDNR/SHL water quality monitoring downstream from Mason City (STORET station 10170003). Monitoring at this station from January 2010 through December 2012 showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, ammonia-nitrogen, chloride, or sulfate in the 36 samples collected.
In agreement with the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the aquatic life uses are "fully supporting". This evaluated biological assessment was based on data collected in 2012 as part of the IDNR/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 BMIBI scores were 70 (good) and 72 (good). The aquatic life use support was assessed (evaluated) as fully supporting (=FS), 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 (654 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 70 was a natural substrate sample and passed the natural substrate BIC of 70. The 72 score was an artificial substrate score and passed the artificial substrate BIC of 52. Even though this site passed the BMIBI BIC (2/2), it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it doesn’t fall in the calibrated watershed size.
Fish consumption uses are assessed as "fully supported" based on results of U.S. EPA/IDNR fish tissue (RAFT) monitoring east of Mason City in 2012. The composite sample of fillets from common carp had low levels of contaminants: mercury: 0.122 ppm; total PCBs: 0.09 ppm; and technical chlordane: < 0.03 ppm. The average level of mercury in the tissue plugs from five walleye was 0.218 ppm (SD=0.082); this level is below the one-meal-per-week advisory threshold of 0.3 ppm. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The levels of contaminants in the tissue samples from fish collected east of Mason City in 2012 do not exceed any of Iowa’s advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.
