Walker Branch IA 05-CHA-1329

Assessment Comments

Assessment is based on results of monitoring conducted from 2002-04 as part of the Rathbun Lake Water Quality Project.

Basis for Assessment

SUMMARY:  The Class B(LR) aquatic life uses are assessed (monitored) as "fully supported".   The results of chemical/physical water quality monitoring conducted from 2002-2004 suggest improving water quality conditions in Walker Branch.   The assessment of support of these uses is based on results of water quality monitoring conducted from 2002 through 2004 on Walker Branch near Confidence (station RA-38) by Iowa State University/Limnology under contract with the U.S.  Army Corps of Engineers as part of the Rathbun Water Quality Project.   This assessment differs from that for the 2004 reporting cycle (“fully supported / threatened”) due to the lack of violations of Class B(LR) criteria in the 2002-04 period versus the 2000-02 period.   The very dry year of 2000—and the violations of state water quality standards that resulted from these dry conditions—was the primary factor affecting the assessments of “impairment” for the tributaries of Rathbun reservoir monitored as part of the Rathbun Water Quality Project.

EXPLANATION:  None of the approximately 21 samples collected during the 2002-2004 assessment period violated Class B(LR) criteria for dissolved oxygen, ammonia-nitrogen, or pH.   Because previous assessment cycles have shown impairments in this waterbody related to low levels of dissolved oxygen (see assessments developed for the 2002 and 2004 assessment cycles), the 2002-04 monitoring results suggest improving water quality conditions in this segment of Walker Branch.  

Similar to results of previous monitoring, results from the 2002-2004 period continued to show high levels of nutrient parameters.   While levels of total nitrogen in Walker Branch 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 for total phosphorus at Station RA-38 were 0.20 mg/l and 0.14 mg/l, respectively (N=16); the mean and median levels of total Kjeldahl nitrogen for this period were 1.0 mg/l and 0.8 mg/l, respectively (N=19).   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 is not designated for Class C drinking water uses, Walker Branch does flow into Rathbun Reservoir which is used as a source of drinking water for a public water supply.   Thus, the seasonal elevation of pesticide levels in Walker Branch 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.   Fortunately, however, the levels of atrazine in this segment of the South Fork Chariton River during the 2002-04 period do not suggest a serious threat to support of drinking water uses in Rathbun Reservoir.   For example, the mean level of atrazine in the 16 samples collected at Station RA-38 from 2002 through 2004 (2.1 ug/l) was below the atrazine MCL of 3.0 ug/l; the maximum level of 18.8 ug/l of atrazine occurred in the sample collected on May 14, 2002.   This maximum level was the only level above the MCL during the 2002-2004 monitoring period.