From Ground Water Currents, July 1995, Issue No. 12

Tracers Detect Aquifer Contaminatoin

By Carl Enfield, EPA National Risk Management Research Laboratory, Ada, Oklahoma

The EPA National Risk Management Laboratory (NRMRL) at Ada, Oklahoma, along with the University of Florida and the University of Texas, have developed a tracer procedure to detect the amount of contamination in aquifer formations. The tracer procedure has been successfully applied in a highly controlled field experiment in a contaminated cobbly, sandy gravel aquifer at Hill Air Force Base in Layton, Utah. The tracer procedure should substantially improve our ability to remediate aquifers and is an improvement over the traditional reliance upon water samples from monitoring wells and core samples. To date, the extent and distribution of ground water contaminants have been decided by chemical analysis performed on water samples from monitoring wells. The hydraulic gradient and direction of water flow is determined by carefully analyzing water depth data from the monitoring wells. Although monitoring well data effectively show the existence and the extent of the contamination, they provide little of the data needed to design remedial alternatives. Core samples have been used to determine the amount of contamination in the sediment at the measurement point(s). However, due to heterogeneities, cores are not effective in deciding the total amount of contamination without taking many samples and doing many expensive chemical analyses.

This is where the development of the tracer fits in. Tracer principles build upon the work of the past two decades in which researchers have learned how organic chemicals move in aquifers. Chemicals, dissolved in water, move at speeds equal to, or less than, the speed of water. The speed of those chemicals that move slower than water is directly proportional to the amount of organic matter associated with the sediments. Most uncontaminated geologic sediments which are low in organic matter do not retard the movement of solubilized chemical contaminants like benzene or trichloroethene. However, when soils are contaminated with nonaqueous phase liquids (NAPLs), like gasoline or fuel oil, the amount of organic matter increases dramatically. NAPLs, like naturally occurring organic matter, retard the movement of chemicals and also serve as a source of contamination.

The tracer procedure involves injecting non-retarded tracers (like bromide) that move with the water along with tracers retarded by the NAPL (but which are not a component of the NAPL) making it possible to compute the amount of NAPL in the formation. The computation is based on the ratio of the travel times of the retarded and non-retarded tracer. Travel time is determined by measuring the tracer's concentration as a function of time at the monitoring well. The travel time is the time required for the tracer's center of mass to move from the injection well to the monitoring well (the larger the travel time ratio, the more NAPL in the formation). The computed amount NAPL present is based on a water to NAPL partitioning for the selected tracer(s). Analysis of known tracer compounds in water is easy, compared to the measurement of all the different contaminants in soil core samples. Knowing the NAPL distribution in the subsurface will reduce the amount of remedial fluid/energy needed to achieve remedial objectives by directing remedial activities only at contaminated locations. Current approaches treat an aquifer as a homogeneously contaminated region.

On the test site at Hill Air Force Base, interpretation of tracer data provided logical descriptions of the contaminant distribution which were reproducible. In contrast, the interpretation of the core data is extremely difficult because representative core samples cannot be obtained due to the size and amount of cobbles in the formation. The researchers hope to provide a proven, cost-effective method of characterizing the amount and distribution of NAPL in a contaminated site. Additional tests will be run over the next year to perfect the procedure.

For more information, call Carl Enfield at EPA's NRMRL at 405-436-8530.