Assessment Comments
Assessment is based on: (1) the results of ambient monthly water quality monitoring from January 2010 through December 2012 at the IDNR/SHL ambient city monitoring station located upstream from Mason City (STORET station 10170002) and (2) IDNR/SHL biological sampling conducted in 2012 and 2013 near Mason City.
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 on 2012 and 2013 IDNR/SHL biological sampling. Fish consumption uses remain "not assessed" (IR 3a) due to the lack of recent fish contaminant monitoring in this river segment. The sources of data for this assessment include the results of ambient monthly water quality monitoring from January 2010 through December 2012 at the IDNR/SHL ambient city monitoring station located upstream from Mason City (STORET station 10170002) and (2) IDNR/SHL biological sampling conducted in 2012 and 2013 near Mason City.
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 10170002 upstream of Mason City were as follows: the 2010 geometric mean was 118 orgs/100 ml, the 2011 geometric mean was 113 orgs/100 ml and the 2012 geometric mean was 279 orgs/100 ml. The 2010 and 2011 geometric means are low and meet the Class A1 criterion of 126 orgs/100 ml. Only the 2012 geometric mean slightly exceeds the Class A1 criterion. Eight of the 24 samples (33%) 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.” Despite this impairment, results of bacteria monitoring in recent years show that this segment of the Winnebago River has very low levels of indicator bacteria relative to other Iowa rivers and relative to the Class A1 geometric mean criterion.
The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient water quality monitoring from 2010-2012. Results of monthly ambient water quality monitoring at the IDNR/SHL station upstream from Mason City from January 2010 through December 2012 showed no violations of Class B(WW1) water quality criteria for either conventional or toxic parameters. No violations occurred in the 36 samples analyzed for dissolved oxygen, pH, and ammonia-nitrogen, chloride, or sulfate.
In agreement with the water quality aquatic life assessment, the monitored aquatic life assessment was based on data collected in 2012 and 2013 as part of the IDNR/SHL stream large river sampling project also suggests "full support". A series of biological metrics which 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 to make a benthic macroinvertebrate index (BMIBI). The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2012 BMIBI scores were 81 and 83 (both excellent). The 2013 BMIBI scores were 73 (good) and 77 (excellent). The aquatic life use support was assessed as fully supporting (=FS), based on a comparison of the BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports. The natural substrate BMIBI BIC for this ecoregion is 70 and the artificial substrate BMIBI BIC for this ecoregion is 52. The biological impairment criteria were determined from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008. THis segment passed the BMBIBI BIC 4/4 times in the last five years. This assessment is considered "monitored" because there were two or more samples collected in multiple years during a recent five year period.
Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river segment.
