Airborne Electromagnetics for Characterizing Water Quality Due to Saltwater Intrusion

Abstract:

Airborne electromagnetic (AEM) surveying for mapping saltwater intrusion has, in the last few years, become well established. Water agencies, municipalities, and farmers need to know where the various modalities of water quality are within the zones of groundwater extraction. The questions typically asked are “Can you locate the saltwater intrusion horizontally and vertically?”, “Can you tell me the water quality?”, “Estimate TDS?”, “Estimate the chloride concentration?”. Often the typical follow up question is “How sure are you of your water quality characterization?” To address these questions additional information besides the AEM inverted resistivity earth model is required. This additional information is in the form of borehole water quality measurements including the electrical conductivity (EC) of the water (in situ to the well) and the lab-measured chloride concentrations. The water quality is critical to understand the mixing of the earth materials with the fluid, as porosities can vary from location to location. Clays can also complicate a simple mixing relationship. Borehole sampling depths and screened intervals along with sampling procedure records are deterministic in evaluating the water sample results. This process has been applied to a site along the central California coast. Regressive correlations have been made between the borehole water EC and chloride concentrations and then between the AEM inverted resistivities and the borehole water EC’s. Once this is accomplished, 2D maps and 3D visualizations of chloride concentrations can be developed and locations and the character of various water qualities determined. To address the accuracy and resolution of the chloride concentrations, forward modeling of various chloride concentrations (after converting the concentrations back to AEM resistivities) followed by inverse modeling to determine inversion sensitivities has been performed on a sample sounding (AEM model in Figure 1) and sensitivity to different regulatory thresholds in two layers have been examined. Modeled chloride concentrations range from 200 mg/L up to 20,000 mg/L (Figure 1). The results demonstrate remarkable sensitivity of the AEM technique via water resistivity and chloride concentration regressions to characterize saltwater intrusion and water quality.