Recharge Assessment Using Surface Geophysics and Cone Penetrometer Testing

Abstract:

In many locations throughout the world, we are seeing declining groundwater levels, indicating an imbalance in the water budget, i.e. “water” exceeds “water in”. One approach to understanding and addressing this imbalance is to focus on recharge, both natural and managed. Identifying sites of natural recharge will assist in quantifying that component in the water budget, and can inform land-use planning, so as to limit any changes that could adversely impact recharge. Identifying sites well-suited for managed recharge make it possible to enhance the natural recharge processes, by directing excess surface water to these sites. Effective recharge at a site requires that water applied at the ground surface will be able to infiltrate rapidly into the subsurface. After initial wetting, coarse-grained materials are the key infiltration pathways from the surface. Characterizing the amount and distribution of these coarse-grained materials in the subsurface is a key step in assessing the quantity of natural recharge at a site and in assessing a potential site for managed recharge.

Geophysical methods provide a way of assessing recharge potential at a site by capturing the spatial variation in sediment texture. One of these methods is tTEM, a time domain electromagnetic method, which can be used to rapidly acquire data need to make a 3D model of subsurface electrical resistivity. Interpretation of resistivity models in terms of sediment texture requires a transform between the two, which can be difficult to establish, as changes in resistivity can result from changing water content, sediment texture, or water quality.

In this work we developed a methodology for creating a transform from tTEM-derived resistivity to fraction coarse-dominated material in the unsaturated zone, using cone penetrometer testing (CPT) data. This methodology was established using data from a study of an almond grove outside of Tulare, California. The purpose of the study was to assess the suitability of the grove as a location for managed recharge. The data from this site included 43 line-km of tTEM data, and five CPT points where both resistivity and soil behavior type data were collected. Using these data, and our new transform, we created a 3D model of sediment texture that can be used to identify infiltration pathways from the ground surface to the water table.