TECHNIQUES FOR VISUALIZATION OF HISTORICAL RADIOACTIVE CONTAMINANT DATA TO DETERMINE GROUNDWATER PATHWAYS AND TRAVEL TIME

Radioactive contaminants in groundwater can impose significant risk to human health and to the environment. However, short-half life isotopes, with today's computational technology, can also provide a unique opportunity to trace groundwater flow systems. Groundwater contamination at the Hanford Nuclear Reservation, Washington has been monitored extensively since the early 1960s. Site monitoring has created a large collection of tritium data containing valuable information about groundwater movement under the Hanford Site.

Tritium concentrations from 1974 to 2000 were used to visualize groundwater movement at Hanford. Radioactive decay was calculated with respect to well data from 1974, resulting in maps of tritium distributions for each subsequent year until 2000. These calculated "decay" maps were then subtracted from their respective year maps created from measured data. The result of this raster-based calculation is maps for the time period 1975 to 2000 that represent increased or decreased concentrations relative to the ideal case of radioactive decay being the only process that influences the fate of tritium. Positive (elevated) values indicate inflow into the area and/or new contaminant source, whereas negative (depleted) concentrations are the result of outflow and/or extraction. The pathway and the travel times of tritium were estimated by examining these differential concentration maps. Tritium travel times were also determined by a comparative study of concentration patterns among potential source area wells and wells along several pathways to the Columbia River. The study looked at concentration variations from 1973 until 2000 in wells of the source area from where contaminants have been traveling eastward to the Columbia River. A bimodal pattern, representing two phases of plutonium production from WWII until 1973 and from 1983 until 1989, was identified from the graphs. This pattern was compared to other wells between the source area and the Columbia River. Tritium travel times were determined from the shift of the pattern to later dates. The study estimated travel times between two and six years and identified two dominant groundwater pathways between the source area and the Columbia River.