Wolf Creek IA 05-CHA-1339

Assessment Comments

Assessment is based on results of (1) water quality monitoring conducted from 2002-04 at two stations as part of the Rathbun Lake Water Quality Project; (2) IDNR/UHL biological (biocriteria) monitoring in 1999, and (3) biological monitoring conducted by IDNR Fisheries Bureau from 1999-2002.

Basis for Assessment

SUMMARY:  The Class B(LR) aquatic life uses are assessed (monitored) as "fully supported" based on results of chemical/physical water quality monitoring and on results of biological monitoring.   The assessment of support of the Class B(LR) uses is based on (1) results of chemical/physical water quality monitoring conducted from 2002 through 2004 on Wolf Creek near Chariton (station RA-41; lower segment) and near Humeston (station RA-44; upper segment) by Iowa State University under contract with the U.S.  Army Corps of Engineers-Kansas City District as part of the Rathbun Water Quality Project, (2) results of IDNR/UHL biocriteria monitoring in 1999, and (3) results of biological monitoring by the IDNR Fisheries Bureau from 1999-2002.   This assessment differs from the previous (2004) assessment ("impaired") due to apparent improvements in water quality conditions over the last four years.

EXPLANATION:  None of the 30 samples collected at (downstream) station RA-41 during the 2002-2004 period violated Class B(LR) criteria for ammonia-nitrogen (maximum level = 0.79 mg/l).   Two of the 30 samples collected (7%) at station RA-41 from 2002 through 2004 violated the Class B(LR) water quality criterion for dissolved oxygen; these violations occurred just prior to, and just following, a period of dry-weather no flow in Wolf Creek.   These violations are as follows:  are as follows:  4.5 mg/l on July 23, 2002; 1.9 mg/l on December 11, 2002.   Only one of 30 samples violated the Class B(LR) criterion for low pH (6.5 units).  

Similarly, one of the 30 samples collected at (upstream) station RA-44 during the 2002-2004 period violated Class B(LR) criteria for ammonia-nitrogen (maximum level = 0.57 mg/l).   Three of the 30 samples collected (10%) at station RA-44 from 2002 through 2004 violated the Class B(LR) water quality criterion for dissolved oxygen.   Two of these violations were associated with dry-weather low flow conditions in Wolf Creek.   These violations are as follows:  are as follows:  1.9 mg/l on December 11, 2002; 4.7 mg/l on June 17, 2003, and 4.7 mg/l on June 15 2004.   None of 30 samples violated the Class B(LR) criterion for pH at station RA-44.   (Note:  station RA-44 is within the Class B(LR) segment of Wolf Creek.)

The isolated violations of the criteria for dissolved oxygen and pH that occurred during the 2002-04 period do not suggest an impairment:  according to U.S.  EPA assessment guidelines, if 10% or less of the samples exceed state criteria for conventional parameters such as dissolved oxygen and pH, the aquatic life uses should be assessed as "fully supported" (see pgs 3-17 of U.S.  EPA 1997b).   Although this stream segment has a history of problems with low levels of dissolved oxygen and pH, levels of these parameters over the last four years of monitoring appear to have improved.   The bulk of the violations of Class B(LR) water quality criteria that led to this waterbody being assessed as impaired occurred in the dry summer of 2000 and were thus likely related to naturally-occurring low flow conditions.   Since 2000, violations of the Class (BL) criteria for dissolved oxygen and pH have been relatively rare, although they have continued to be associated with dry weather and low flow conditions.   Thus, based on results of recent monitoring this stream segment does not currently show impairments of the Class B(LR) uses related to either dissolved oxygen or pH.  

Results of IDNR/UHL biocriteria and IDNR Fisheries monitoring also suggest that the Class B(LR) uses are "fully supported".   This biological assessment was based on data collected in 1999 as part of the DNR/UHL stream biocriteria project and on data collected in 1999-2002 as part of a DNR Fisheries stream sampling project of the IDNR’s Chariton research station.   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 (F-IBI) and a benthic macroinvertebrate index (BM-IBI).   The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 1999 IDNR/UHL Fish IBI score was 33 (fair) and the BM-IBI score was 41(fair).  The 1999-2002 evaluated (IDNR Fisheries Bureau) Fish IBI average score (n=10) was 26 (fair).   The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.   Biological monitoring is better able to reflect cumulative impacts of water quality over time and thus is believed to more accurately represent water quality conditions of this segment of Wolf Creek than do results of chemical water quality monitoring from 2000 through 2002.  

Similar to results of previous monitoring conducted in the Rathbun Watershed, results from the 2002-2004 period continued to show high levels of nutrient parameters.   While levels of total nitrogen in Wolf Creek and other streams in the Rathbun watershed are relatively low for Iowa, levels of phosphorus tend to be relatively high.   The 2002-2004 mean and median levels of total phosphorus at Station RA-41 were 0.27 mg/l and 0.19 mg/l, respectively (N=27); the mean and median levels of total Kjeldahl nitrogen for this period were 0.9 mg/l and 0.7 mg/l, respectively.   At station RA-44, the 2002-2004 mean and median levels of total phosphorus were 0.24 mg/l and 0.17 mg/l, respectively (N=27); the mean and median levels of total Kjeldahl nitrogen for this period were 0.8 mg/l and 0.7 mg/l, respectively.   The Iowa Water Quality Standards do not currently have aquatic life criteria for these nutrient parameters or for other nonpoint source-related parameters (for example, total suspended solids).   The Iowa DNR, however, continues to participate in a nationwide program, led by U.S.  EPA, for states to develop and adopt surface water criteria for total phosphorus, total nitrogen, chlorophyll, and some measure of turbidity.  

Although this stream reach is not designated for Class C drinking water uses, Wolf Creek does flow into Rathbun Reservoir which is used as a source of drinking water for a public water supply.   The seasonal elevation of pesticide levels in the Chariton River and other tributaries of Rathbun Reservoir presents a continuing concern for full support of the Class C (drinking water) uses designated for the Chariton River immediately downriver from Rathbun Reservoir.   For example, the mean level of atrazine in the 21 samples collected at Station RA-41 from 2002 through 2004 (3.07 ug/l)  is approaching the atrazine MCL of 3 ug/l (due to rounding convention, the atrazine MCL is not technically exceeded until the comparison value exceeds 3.5 mg/l).   At station RA-44, the mean level of atrazine in the 21 samples collected from 2002 through 2004 (3.8 ug/l) exceeded the atrazine MCL of 3 ug/l.