Jackson Creek IA 05-CHA-1332

Assessment Comments

Assessment is based on results of monitoring conducted in 2000 & 2001 as part of the Rathbun Lake Water Quality Project.

Basis for Assessment

SUMMARY:  The Class B(LR) aquatic life uses were assessed as "fully supported / threatened."  EXPLANATION:  The assessment of support of the Class B(LR) uses is based on results of water quality monitoring conducted on Jackson Creek near Bridgeport (station RA-39) by the U.S.  Geological Survey (1998) and by Iowa State University/Limnology (1999) under contract with the U.S.  Army Corps of Engineers as part of the Rathbun Water Quality Project.   The results of this monitoring are summarized in the "Rathbun Lake Water Quality Reports" for 1998 and 1999 (see ACOE 1999 and Kirsh and Leonard 2000).   Results of this monitoring show no violations of Iowa Class B(LR) water quality criteria for pH in the 10 samples analyzed in 1999 or for ammonia-nitrogen in the 21 samples analyzed during 1998 and 1999.   Although these results suggest "full support" of the Class B(LR) uses, the one violation of the Class B(LR) criterion for dissolved oxygen (5 mg/l) in the nine samples (11%) analyzed in 1999 suggests a threat to the full support of these uses.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), "full support" of beneficial uses is indicated if criteria are exceeded in from 10% or less of the samples analyzed for conventional parameters (pH, temperature, and dissolved oxygen); if more than 10% of samples exceed the criterion, the aquatic life uses are "impaired."  Due to the lack of sufficient data points for dissolved oxygen during the biennial period for developing a "monitored" assessment (according to DNR's 305(b) assessment methodology, at least 10 samples of conventional parameters are needed), the Class B(LR) uses were "evaluated" as "fully supported / threatened."  The violation of the criterion for dissolved oxygen occurred in the sample collected on October 13, 1999 (2.9 mg/l); this sample was collected at 12:30 PM.   The level of dissolved oxygen in this sample (2.9 mg/l) is unusually low for an Iowa surface water.   Additional monitoring is needed to better define the water quality conditions in this stream reach.   This additional monitoring is being conducted in 2000 as part of the ongoing Rathbun Water Quality Project.   Monitoring conducted during 1998 and 1999 showed high levels of both nutrient parameters (total nitrogen and total phosphorus) and pesticides at station RA-39 and at stations on other reservoir tributaries, with the highest levels often associated with storm runoff.   The Iowa Water Quality Standards does not currently have aquatic life criteria for these nutrient parameters or for other nonpoint source-related parameters (for example, total suspended solids).   Although this stream reach is not designated for Class C drinking water uses, Jackson Creek is a tributary of Rathbun Reservoir which is designated for Class C uses.   The seasonal elevation of pesticides levels in the tributaries of the Rathbun watershed presents a continuing threat to full support of the Class C (drinking water) uses designated for Rathbun Reservoir.   For example, of the 21 samples collected at Station RA-39 in 1998 and 1999, the three samples that exceeded the atrazine MCL of 3.0 ug/l were collected in the month of June following application of herbicides to farm fields and during a month with typically high levels of rainfall.   The mean levels of atrazine at Station RA-39 were 4.08 ug/l in 1998 (N=11; maximum of 37.5 ug/l) and 2.39 ug/l in 1999 (N=10; maximum of 17.6 ug/l).