Assessment of managed aquifer recharge sites using a new geophysical imaging method

In many places around the world, much attention is focused on managed aquifer recharge (MAR) because of reduced groundwater levels due to droughts. To assess the suitability of a site for MAR, detailed three-dimensional (3D) information about the subsurface materials and their hydraulic properties is needed. In areas where the groundwater level is at an intermediate depth (e.g., 20-40 m), such information is needed from the ground surface down to a minimum depth of ~50 m. To achieve this goal, we used a new geophysical imaging system: a towed time-domain electromagnetic system that is efficient for acquiring data at a significantly improved resolution and a scale needed for MAR. During a 2-d period, we acquired ~92 line-kilometers of data in one almond [Prunus dulcis (Mill.) D.A. Webb] grove, one pistachio (Pistacia vera L.) grove, one open field, and two active recharge basins in the Tulare Irrigation District in the Central Valley of California. At each site, a detailed 3D resistivity model with a resolution down to the 10- by 10-m scale is presented in terms of resistivity distribution plots, which are then used to assign a saturated-unsaturated boundary. In addition, we used a resistivity-lithology transform to interpret the resistivity models and create lithology maps at each site. We used this information to assess the suitability of each site for MAR.