Comparative analysis of geophysical techniques to characterize shallow groundwater conditions

Measuring depth to groundwater is one of the most fundamental and basic observations in hydrogeology. Traditionally, this is done in piezometers or boreholes. Using interpolation (and extrapolation) techniques the point measurements are generally used to create a water table surface, which can result in significant error due to sparse measurements and also subtle changes in the low lying topography. In semi-arid regions assessing zones with shallow groundwater depths is crucial, especially when agricultural development could result in rising water tables. However, limited availability of and/or high costs of installing piezometers means that we still lack methods of fast and reliable estimation of depth to groundwater. An obvious interesting opportunity is to use nonintrusive hydrogeophysical sensing techniques. The arid plains north of Adelaide, South Australia (~450 mm/yr rainfall) are an important food growing area. Recent proposals to extend the irrigation area (sourced from treated sewage plant effluent) needs to be evaluated closely, as groundwater quality and depth to groundwater are variable and not well known. High risk areas are zones where the groundwater is shallower than 3 to 4 m. In these areas even slight over-irrigation will result in rising groundwater tables and soil salinisation. We have collected geophysical data sets at three study sites within the Northern Adelaide Plains (NAP), South Australia. Techniques evaluated include: a frequency domain, shallow terrainconductivity meter (CMD); a fast-sampling time domain electromagnetics system (TEM); a resistivity system (ERT); and a shallow reflection seismic system where first arrival times and surface waves were processed together to estimate depth to groundwater. Additionally, to provide further information on soil variability, as well as groundwater depth and quality, 47 geoprobe boreholes were drilled to 6 to 8 m depth. If water was encountered these holes were extended and logged using a shallow borehole nuclear magnetic resonance (NMR) system. Comparison of the techniques and evaluation of the results suggest that: a) while much of the study area is flat, subtle features in the landscape (predominantly ephemeral streams) appear to be important conduits to supply water to shallow and perched water tables; b) the seismic survey appeared to provide useful information about depth to groundwater; c) Geophysical techniques that measure the ground conductivity (i.e. the electricallyand electromagnetically-based techniques TEM, ERT, CMD) can determine the depth to groundwater when there is a sufficient contrast between the conductivity of the groundwater and the background soils; d) In-situ results of water content and water boundness (soil texture) obtained using the NMR downhole tool were generally consistent with those obtained from samples collected and evaluated in the laboratory; and e) Borehole NMR has the potential to identify the presence of heavy clays that could impede infiltration.