Assessment Comments
Assessment is based on results of monthly monitoring from 2010 through 2012 at the IDNR/SHL ambient city monitoring station located upstream from Fort Dodge (STORET station 10940002) and on 2011 and 2012 IDNR/SHL biological sampling conducted near Fort Dodge.
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, this segment is also now presumptively 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) based on results of ambient monitoring for indicator bacteria (E. coli). The Class B(WW1) aquatic life uses are assessed (evaluated) as "partially supported" (IR 3b-u) based on results of 2011/2012 IDNR/SHL biological sampling. Fish consumption uses remain "not assessed" (IR 3a) due to the lack of fish contaminant monitoring in this river segment. The sources of data for this assessment are the results of monthly monitoring from 2010 through 2012 at the IDNR/UHL ambient city monitoring station located upstream from Fort Dodge (STORET station 10940002) and 2011 and 2012 IDNR/SHL biological sampling conducted near Fort Dodge. .
EXPLANATION: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" based on results of monitoring for indicator bacteria (E. coli). The geometric means of indicator bacteria (E. coli) in the 25 samples collected during the recreational seasons of 2010 through 2012 at IDNR station 10940002 upstream from Fort Dodge were as follows: the 2010 geometric mean was 120 orgs/100 ml, the 2011 geometric mean was 150 orgs/100 ml, and the 2012 geometric mean was 48 orgs/100 ml. Only the 2011 geometric mean slightly exceeded the Class A1 criterion of 126 orgs/100 ml. Six of the 25 samples (24%) 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.” This impairment is slight given that two of the three recreation season geometric means meet the Class A1 criterion and that the one geometric mean violation was slight.
Basd on the water quality data alone, the Class B(WW1) aquatic life uses should be assessed (monitored) as "fully supported.” Monitoring at the IDNR city station upstream from Fort Dodge showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, temperature, ammonia-nitrogen, chloride, or sulfate in the 36 samples collected during the 2010-2012 assessment period. These results suggest "full support" of the Class B(WW1) aquatic life uses.
In contrast to the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the aquatic life uses are "partially supporting". This evaluated biological assessment was based on data collected in 2011 and 2012 as part of the IDNR/SHL large river 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 2011 BMIBI score was 52 (fair) and the 2012 BMIBI scores were 51 and 55 (both fair). The aquatic life use support was assessed (evaluated) as partially 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 BMIBI BIC for this ecoregion is 62. This assessment is considered evaluated because the drainage area (4001 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. Even though this site failed the BMIBI BIC (0/3), 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).
This segment of the Des Moines River was sampled as part of the 1998-99 statewide study of freshwater mussels in Iowa streams and rivers (Arbuckle et al. 2000). As part of this study, sampling results from 1998 and 1999 (Arbuckle et al. 2000) were compared to results from stream sites surveyed in 1984 and 1985 by Frest (1987). On a statewide basis, this comparison showed sharp declines in the numbers of mussel species ("species richness") in Iowa streams and rivers from the mid-1980s to the late 1990s. For stream segments having four or more species reported for the 1984-95 survey, results of this comparison were used by staff of the Iowa DNR Water Quality Bureau to assess the degree to which the aquatic life uses of the sampled stream segments are supported. The results of this sampling on the Des Moines River, however, do not meet IDNR guidelines for developing an assessment of support for the aquatic life uses. Species richness of freshwater mussels at the five sample sites in this segment was 0, 0, 1, 1, and 2 in the 1984-85 period and was 0, 0, 0, 0, and 3 in the 1998-99 period, respectively, for an average 1984-85 species richness of approximately 1 and an average percent change of minus 30%. Based on these results, the aquatic life uses are considered “not assessed” due (1) to IDNR’s assessment methodology in which assessment decisions are developed only for those stream segments having an average of four or more species reported in the 1984-85 (Frest) survey and (2) the difficulty of interpreting status of mussel communities showing relatively low species richness during the both the historical (1984-85) and current (1998-99) surveys.
Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river segment.
