Seasonal variation of moisture content in unsaturated sandstone inferred from borehole radar and resistivity profiles

Abstract

Understanding the processes controlling recharge to aquifers is critical if accurate predictions are to be made on the fate of contaminants in the subsurface environment. In order to understand fully the hydrochemical mechanisms in the vadose zone it is essential that the dynamics of the hydrology can be suitably characterised. 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 resistivity behaviour have been monitored over a period of two years at a Triassic Sherwood Sandstone aquifer field site at Hatfield, England. 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. At a second field site in Eggborough, located 17km from Hatfield, similar temporal changes in moisture content in the sandstone were observed using borehole radar profiles. Travel times of seasonal wetting fronts through the sandstone at both sites appear to be approximately 2m per month. The retardation of this front propagation in the top 3m is also common to both sites, suggesting that pollutant transport may be principally controlled by near surface sediments. The results have important consequences to existing groundwater modelling programmes that are being utilised to predict transfer of agricultural chemicals through the vadose zone.