Assessment Comments

Assessment is based on: (1) USGS ambient water quality monitoring conducted on the Nishnabotna River at Hamburg from January 2010 to December 2012, (2) IDNR monitoring from October 2010 to June 2012, (3) IDNR/SHL biological sampling in 2003 and 2012, and (4) EPA/IDNR fish contaminant (RAFT) monitoring in 2011.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream 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 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf) and results of an 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 presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" (IR 5p) due to violations of the state criteria for indicator bacteria.   The Class B(WW1) aquatic life uses remain assessed (evaluated) as “partially supported” (IR 3b-u) based on results of biological sampling in 2003 and 2012.   The fish consumption uses remain assessed (evaluated) as “fully supported” (IR 2a) based on results of fish contaminant monitoring in 2002 and based on comparison of USGS ambient water quality data to Iowa’s human health (HH) criteria.   The sources of data for this assessment include the results of (1) USGS ambient water quality monitoring conducted on the Nishnabotna River at Hamburg (station 06810000) from 2010-2012, (2) IDNR ambient monitoring from October 2010 to June 2012, (3) IDNR/SHL biological sampling in 2003 and 2012, and (4) EPA/IDNR fish contaminant (RAFT) monitoring in 2011.  

EXPLANATION:  The presumptive Class A1 (primary contact recreation) are assessed as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria.   The geometric means of indicator bacteria (E.  coli) in the 25 samples collected at the USGS station during the recreational seasons of 2010 through 2012 at the Nishnabotna River above Hamburg were as follows: the 2010 geometric mean was 2,600 orgs/100 ml, the 2011 geometric mean was 675 orgs/100 ml, and the 2012 geometric mean was 900 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Sixteen of the 25 samples (64%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml.  

The geometric means of indicator bacteria (E.  coli) in the 13 samples collected at the Iowa DNR station during the recreational seasons of 2010 through 2012 at the Nishnabotna River near Hamburg were as follows: the 2010 geometric mean was 379 orgs/100 ml, the 2011 geometric mean was 271 orgs/100 ml, and the 2012 geometric mean was 2,615 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Eight of the 13 samples (62%) 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 remain assessed (evaluated) as "partially supported" based on results of IDNR/sHL biological sampling in 2003 and 2012.   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 2003 FIBI score was 5 (poor) and the BMIBI score was 54 (fair).   The 2012 BMIBI score was 23 (poor).    The aquatic life use support was assessed (evaluated) as partially supporting (=PS), 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 FIBI BIC for this ecoregion is 31 and the BMIBI BIC for this ecoregion is 54.   This assessment is considered evaluated because the drainage areas (2,807 and 2,818 mi2) above this 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 failed the FIBI (0/1) and BMIBI (1/2) BICs, 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.   According to IDNR’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).   IDNR 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).  

In contrast to the results of biological monitoring that suggest only “partial support” of the Class B(WW1) uses, results of water quality monitoring by the U.S.  Geological Survey above Hamburg and by IDNR near Hamburg from 2010-2012 suggest “full support” of these uses.   No violations of state water quality criteria occurred in the combined 56 samples analyzed for ammonia (maximum = 0.66 mg/L), pH (range = 7.3-8.7), and dissolved oxygen (minimum = 6.6 mg/L) during this period.   No violations of Class B(WW1) criteria occurred in the 35 samples analyzed by USGS for the pesticides chlorpyrifos and dieldrin.   These results suggest "full support" of the Class B(WW1) aquatic life uses.  

Fish consumption uses remain assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Hamburg in 2011.   The composite samples of fillets from common carp had low levels of contaminants.   Levels of primary contaminants in the composite sample of common carp fillets were as follows: mercury: 0.06 ppm; total PCBs: <0.09 ppm; and technical chlordane: <0.03 ppm.   The average level of mercury in the tissue plug samples from three black crappie was 0155 (SD=0.041).   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 fish contaminant data generated from the 2011 RAFT sampling conducted in this assessment segment show that the levels of contaminants do not exceed any of Iowa’s advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.   In addition, none of the 35 samples analyzed by USGS for dieldrin in water exceeded the state human health/fish criterion of 0.0054 ug/l.