SENSITIVITY MODELS AND DESIGN PROTOCOL FOR PARTITIONING TRACER TESTS IN ALLUVIAL AQUIFERS

Zones of dense, nonaqueous phase liquids (DNAPLs) are difficult to cha racterize as to their volume, composition, and spatial distribution us ing conventional ground-water extraction and soil-sampling methods. Su ch incompletely characterized sites have negative consequences for tho se responsible for their remedial design, e.g., the uncertainties in t he optimal placement of ground-water extraction wells and in the durat ion of remediation. However, the recent use of the partitioning interw ell tracer test (PITT) to characterize DNAPL zones at sites in New Mex ico [unsaturated alluvium] and in Ohio, Texas, and Utah [saturated all uvium] demonstrates that the volume and spatial distribution of residu al DNAPL can be determined with accuracy. The PITT involves injection of a suite of tracers which reversibly partition to different degrees between the DNAPL and the ground water or soil air resulting in the ch romatographic separation of the tracer signals observed at the extract ion well(s). The design of a PITT requires careful consideration of th e hydrostratigraphic, hydraulic, and certain geochemical properties of the alluvium being tested. A three-dimensional, numerical model of a heterogeneous alluvial aquifer containing DNAPL has been developed for use with the UTCHEM simulator to demonstrate partitioning tracer test ing and to address questions that are frequently raised in its applica tion. The simulations include (1) the estimation of DNAPL volume for t he simple case where only residual DNAPL is present in heterogeneous a lluvium, (2) sensitivity studies to demonstrate the effect of increasi ngly low residual DNAPL saturation on the tracer signal, and (3) the e ffect of free-phase DNAPL on the estimation of the volume of DNAPL pre sent. Furthermore, the potential interference of sedimentary organic c arbon as a DNAPL surrogate on the tracer signal is considered and show n to be readily resolved by the careful choice of tracers. Finally, a protocol for the use of PITTs in alluvial aquifers is presented.