Interpretation of LNAPL Thickness Measurements under Unsteady Conditions

Abstract

In hydrocarbon-contaminated sites, it is common practice to estimate the total hydrocarbon volume based on the hydrocarbon thickness measurements made in monitoring wells. In most cases, the hydrocarbon thickness measurement data obtained from a well field and the volume estimate analysis based on these data show significant variations over time. In this paper we consider an important cause of this variation, which is neglected in most field applications, i.e., the unsteady ground-water table fluctuations and its effect on monitoring well measurements. The analytical solutions we present indicate that, under fluctuating ground-water table conditions, if the hydrocarbon volume estimates in the formation are based on monitoring well measurements made at one point in time, or if these estimates are based on a sequence of measurements made over a period of time, which then are evaluated independently, then the volume estimate results will be in significant error. The analysis discussed in this paper is based on the analytical solution of volumetric equilibrium equations for constant residual saturation levels. Thus, results discussed in this paper only may represent the best estimates of hydraulic equilibrium conditions at a contaminated site. For a complete analysis, in addition to the effect of ground-water table movement, the effect of entrapment on residual saturation, capillarity, hystereses, threshold pressures, and non-Darcian behavior should be included in the analysis. However, because of inherent nonlinearities introduced, such cases cannot be solved using analytical methods and the correlation between light nonaqueous phase liquid (LNAPL) thickness in the aquifer and the monitoring well cannot be determined easily. Based on our findings, we conclude that present-day volume estimation techniques, which are based solely on physical equilibrium models, are not reliable tools to estimate light nonaqueous phase liquid volume at contaminated sites under unsteady conditions.