Laboratory Studies of Aqueous Partitioning Tracer Tests for Measuring Nonaqueous Phase Liquid Volumes

Abstract

The partitioning tracer test (PTT) is an innovative method for measuring nonaqueous phase liquid (NAPL) volume distributions in contaminated aquifers. During a PTT, a tracer pulse containing nonpartitioning and partitioning tracers is injected to an aquifer and extracted from downgradient wells. The appearance of the tracers at downgradient locations is analyzed to determine tracer mean residence times, for example, using the methods of moments and retardation factors based on residence times. The NAPL volume present in the tracer flow path is then determined based on retardation factors and independently measured tracer partition coefficients. Column experiments were performed to assess the validity of PTTs conducted at a former chemical disposal pit at Hill Air Force Base, Utah. The NAPL contamination was a complex mixture of volatile and semivolatile organic chemicals. Up to four alcohol tracers were used in PTTs: methanol; n‐hexanol; 2,2‐dimethyl‐3‐pentanol; and 6‐methyl‐2‐heptanol and batch test, conducted at 10 °C yielded partition coefficients of 0, 2.07, 7.42, and 18.8, respectively. Partition coefficients increased between 53 and 60% with a 12 °C increase in temperature. Column experiments were used to evaluate the validity of using a PTT for field test conditions of temperature and average flow velocity. Measured NAPL volumes based on comparing three replicate column tests using two to three partitioning tracers and different flow rates were within 84.3 to 101% of the NAPL volume used to contaminate the column (overall, 8.91% of the pore volume contained NAPL). The average NAPL saturations from all the tracers yielded estimates between 89 to 99% of the known value. Afterwards, the column was sparged with 5800 pore volumes of air, and the subsequent PTT yielded an estimate of the reduction in NAPL volume of 16%.