Observations Of Seasonal Dynamics In The Vadose Zone Using Borehole Radar And Resistivity

Abstract

Understanding the processes controlling recharge to aquifers is critical if accurate predictions are to be made of the fate of contaminants in the subsurface environment. This is particularly important for the case of non-point surface substances such as fertilizers and pesticides. In order to fully understand the hydrochemical mechanisms in the vadose zone it is essential that the dynamics of the hydrology can be suitably characterized. The correlation between moisture content and both bulk dielectric and resistivity properties of porous media is well established. Using suitably placed sensors in boreholes detailed depth profiles of dielectric and resistive behaviour have been monitored in conjunction with surface hydrological properties over a two year period at a site at Hatfield, England in the Sherwood Sandstone aquifer. The borehole-borehole transmission radar and borehole resistivity profiles show a significant correlation. Through appropriate petrophysical relationships, derived from core samples, seasonal dynamics of the vadose zone are seen to illustrate the migration of wetting and drying fronts over the monitoring period. Travel times of seasonal wetting fronts to the 12m deep water table appear to be typically several months. The results have important consequences to existing groundwater modelling programmes which are being utilized to predict transfer of agricultural chemicals through the vadose zone. Introduction Accurate modelling of vadose zone flow and solute transport is essential if reliable assessments of the vulnerability of aquifer resources are to be made. The Sherwood Sandstone is a major groundwater resource in the UK and is subject to increasing demand for public and private water supply. After the Chalk, the Permo-Triassic Sandstone aquifer group is the UK's second most important aquifer for groundwater resources. Licensed abstraction accounts for approximately 25% of all UK groundwater abstraction (Allen et al., 1997). Heavy chemical loading from agricultural and industrial practices threaten this resource. Despite this, studies of unsaturated processes in the Sherwood Sandstone are rare. Geophysical methods have been widely employed for many years as an aid to hydrogeological characterization. Because of the sensitivity of resistivity and dielectric permittivity to volumetric moisture content, resistivity and ground penetrating radar (GPR) tools have shown great value in aiding hydrological studies of the vadose zone. Investigations of vadose zone processes have typically used surface deployed surveys, for example using DC resistivity (Kalinski et al., 1993; Fohlich and Parke, 1989), or GPR profiling (van Overmeeran et al., 1997; Greaves et al., 1996). A number of studies have utilized cross-borehole tomographic imaging resistivity (for example, Daily et al., 1992) or radar (for example, Alumbaugh et al, 2000; Eppstein and Dougherty, 1998; Hubbard et al., 1997) tools due to the