OCCURRENCE, DISTRIBUTION, AND EVALUATION OF REGIONAL SOURCES OF PERCHLORATE IN THE LOWER UMATILLA BASIN OREGON GROUNDWATER MANAGEMENT AREA
Perchlorate was originally detected in regional groundwater resources in 2003 as part of the concurrent groundwater nitrate investigations. A number of well types were tested including monitoring, domestic, irrigation, industrial, community, and stock watering wells, with perchlorate concentrations ranging from 1 ug/l to 24.8 ug/l in groundwater. The frequency of occurrence based on major well types for monitoring, domestic, and irrigation wells were 59%, 46%, and 75%, respectively from a total of 135 wells tested.
A number different land uses within the LUB GWMA may potentially contribute to perchlorate occurrence, including two Department of Defense munitions sites, a formerly used defense site (FUDS), a commercial aerospace rocket motor engine test facility, and agricultural application of Chilean nitrate fertilizers. Collectively, however, these potential sources do not account for the complete range of occurrence of perchlorate in the LUB GWMA, and a naturally occurring component may exist. The current hypothesized Conceptual Site Model (CSM) for regional groundwater perchlorate occurrence is facilitated transport of irrigation recharge in areas that may exhibit salt build-up due to evapotransporation in excess of precipitation. This is supported by a general correlation of total dissolved ions concentrations to perchlorate concentrations in groundwater. Due to the use of groundwater resources for crop irrigation, stock watering for dairy cattle, and food processing industries, further investigations of other exposure pathways are also planned.