This paper describes a field experiment involving the release of 230.9 liters of tetrachloroethylene (PCE) below the water table in a naturally occurring, unconfined sand aquifer. The release was executed in a 3 m X 3 m X 3.4 m deep, scalable‐joint steel sheet‐pile cell anchored into an underlying clay aquitard. After allowing 28 days for redistribution, excavation of the upper approximately 0.9 m of the cell revealed PCE pools and residual to be present in relatively coarser grained horizons, with substantial degrees of lateral flow having taken place. This lateral flow was observed in laminations and lenses ranging in thickness from a few mm to a few cm, with only subtle variations in texture separating individual migration pathways. Detailed sampling during the excavation procedure and subsampling of three cores extended down to the clay aquitard revealed a spatially variable distribution of PCE with saturations ranging from 1% to 38% of pore space. Laboratory measurement of a fully hysteretic capillary pressure curve demonstrated that the degree of nonwetting phase residual is a function of the maximum saturation attained along main drainage during the initial infiltration process. Various models for consolidated petroleum reservoir materials did not fit the experimental data well. The theory governing pool formation in heterogeneous porous media is also presented, and it is demonstrated that pools can form in homogeneous media exhibiting a distinct entry pressure.

Abstract. Ground-penetrating radar (GPR) was used to measure the distribution of soil water content below a wastewater trench. Two household septic system trenches were installed in a sandy soil. Vertical time domain reflectometry (TDR) probes were installed through the bottom of one of the trenches. The TDR probes were used to measure soil water content for comparison to the GPR data. The GPR measurements were performed in zero-offset gather (ZeG) and multiple-offset gather (MEG) survey modes with the antennas in horizontal boreholes located on opposite sides and below the wastewater trench. The ZeG survey mode gave an estimate of the average water content in the horizontal plane below the trench at a prescribed interval along the trench. The MeG survey mode produced a tomographic image of the distribution of soil water content in the same horizontal plane below the trench. A similar range of values of water contents was measured by TDR and GPR methods. The main objective of this study is to test a borehole GPR system for measuring soil water content distribution beneath an experimental wastewater trench. A secondary objective is to examine the utility of surface and borehole GPR methods for wastewater trench site investigations. For comparison to the GPR data, TDR measurements of soil water content were made below a second experimental wastewater trench.To our knowledge, the idea of using borehole GPR to measure the spatial variability of soil water content in a horizontal plane is new. Although in this case the specific application of the technique is to investigate the spatial variability of water content below a wastewater trench, there are many other potential applications of the method. For instance, as we demonstrate, the method appears to be able to detect preferential pathways for water flow through the soil profile. Quantification of the degree of preferential flow of water through a soil profile using a nonintrusive method such as GPR is potentially useful in quantifying agricultural leaching, groundwater recharge, and infiltration through landfill covers.

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