Walnut Creek IA 02-IOW-709
from confluence with North Walnut Cr. (S7 T81N R13W Poweshiek Co.) to confluence with unnamed tributary in NW 1/4 S24 T81N R15W Poweshiek Co.
The presumptive Class A1 (primary contact recreation) uses remain assessed (evaluated) as “not supported” due to levels of indicator bacteria (E. coli) that exceed state water quality criteria. The Class B(WW2) aquatic life uses are assessed (evaluated) as “partially supported” based on results of biological sampling in 2009 and 2010. The sources of data for this assessment are (1) results of DNR/SHL biological sampling in 2009 and 2010 and (2) the results of monitoring as part of the Bear Creek Section 319 Watershed Monitoring Project during 2010-2011 at three stations: (moving from downstream to upstream): (1) WLNT-1 near Hartwick (STORET ID 13790008), (2) Site 2m at 328th Avenue (STORET ID 13790002) and (3) Site 4m near 150th Street (STORET ID 13790004). Note: no additional water quality monitoring was conducted in this segment since 2011 and this is the same assessment as that developed for the 2014 IR cycle.
The presumptive Class A1 uses are assessed (evaluated) as "not supported" based on results of bacteria monitoring from March 2010 to September 2011. A total of 25 samples were collected at each of the three monitoring stations: 15 samples were collected during the recreation season of 2010, and 10 samples were collected during the recreation season of 2011. The geometric mean levels of indicator bacteria (E. coli) at all three stations exceed the Class A1 geometric mean criterion of 126 orgs/100 ml. The geometric means at Site WLNT-1 were 750 orgs/100 ml in 2010 and 1,105 orgs/100 ml in 2011 with 22 of the 25 samples (88%) exceeding the single-sample maximum criterion of 235 orgs/100 ml. The geometric means at Site 2m were 727 orgs/100 ml in 2010 and 977 orgs/100 ml in 2011 with nine of the 21 samples (84%) exceeding the single-sample maximum criterion of 235 orgs/100 ml. The geometric means at Site 4m were 541 orgs/100 ml in 2010 and 917 orgs/100 ml in 2011 with 21 of the 25 samples (84%) exceeding the single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and according to DNR’s assessment/listing methodology, if the geometric mean level of E. coli is greater than the state criterion, the primary contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). This aquatic life assessment is now considered "evaluated" based on a change in the 2010 DNR assessment methodology. DNR now requires a segment have two or more biological samples collected from the segment in multiple years between 2010 and 2014 to be considered “monitored”. This segment had multiple samples collected in the previous eight years (2009-2016). 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). However, despite this change in assessment methodology and type, this waterbody remains in IR Category 5b but moves from sub-category 5b-v (verified) to sub-category 5b-t (tentative) and remains on Iowa’s Section 303(d) list of impaired waters.
The assessment of the Class B(WW2) aquatic life uses was based on data collected in 2009 and 2010 as part of the DNR/SHL stream biocriteria project. A series of biological metrics which 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 that were 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 2009 FIBI scores were 26, 29 (fair) and 53 (good) and the 2009 BMIBI scores were 57, 62, 63 (good). The 2010 FIBI score was 25 (poor) and the BMIBI score was 54 (fair). 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 for previous Section 305(b) reports. The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008. The FIBI BIC for this ecoregion is 36 and the BMIBI BIC for this ecoregion is 51. This segment passed the FIBI BIC 1/4 times and passed the BMIBI BIC 4/4times in the past eight years.
Despite the results of biological monitoring that suggest impairment of the Class B(WW2) uses, results of chemical/physical water quality monitoring conducted during the 2010-2011 suggest good water quality conditions in this assessment segment. None of the combined 17 samples collected in 2010 and 2011 at the three monitoring stations (WLNT1, 2m, and 4m) violated the Class B(WW2) criteria for dissolved oxygen, ammonia-nitrogen, pH, or temperature.