Mapping localised freshwater anomalies in the brackish paleo-lake sediments of the Machile–Zambezi Basin with transient electromagnetic sounding, geoelectrical imaging and induced polarisation

A recent airborne TEM survey in the Machile-Zambezi Basin of south western Zambia revealed high electrical resistivity anomalies (around 100 Omega m) in a low electrical resistivity (below 13 Omega m) background. The near surface (0-40 m depth range) electrical resistivity distribution of these anomalies appeared to be coincident with superficial features related to surface water such as alluvial fans and flood plains. This paper describes the application of transient electromagnetic soundings (TEM) and continuous vertical electrical sounding (CVES) using geoelectrics and time domain induced polarisation to evaluate a freshwater lens across aflood plain on the northern bank of the Zambezi River at Kasaya in south western Zambia. Coincident TEM and CVES measurements were conducted across the Simalaha Plain from the edge of the Zambezi River up to 6.6 km inland. The resulting TEM, direct current and induced polarisation data sets were inverted using a new mutually and laterally constrained joint inversion scheme. The resulting inverse model sections depict a freshwater lens sitting on top of a regional saline aquifer. The fresh water lens is about 60 m thick at the boundary with the Zambezi River and gradually thins out and deteriorates in water quality further inland. It is postulated that the freshwater lens originated as a result of interaction between the Zambezi River and the salty aquifer in a setting in which evapotranspiration is the net climatic stress. Similar high electrical resistivity bodies were also associated with other surface water features located in the airborne surveyed area

Anthropogenic wetlands due to over-irrigation of desert areas: A challenging hydrogeological investigation with extensive geophysical input from TEM and MRS measurements

During the last century, many large irrigation projects were carried out in arid lands worldwide. Despite a tremendous increase in food production, a common problem when characterizing these zones is land degradation in the form of waterlogging. A clear example of this phenomenon is in the Nubariya depression in theWestern Desert of Egypt. Following the reclamation of desert lands for agricultural production, an artificial brackish and contaminated pond started to develop in the late 1990s, which at present extends for about 2.5 km^2. The available data provide evidence of a simultaneous general deterioration of the groundwater system. An extensive hydrogeophysical investigation was carried out in this challenging environment using magnetic resonance sounding (MRS) and ground-based time-domain electromagnetic (TEM) techniques with the following main objectives: (1) understanding the hydrological evolution of the area; (2) characterizing the hydrogeological setting; and (3) developing scenarios for artificial aquifer remediation and recharge. The integrated interpretation of the geophysical surveys provided a hydrogeological picture of the upper 100-m sedimentary setting in terms of both lithological distribution and groundwater quality. The information is then used to set up (1) a regional groundwater flow and (2) a local density-dependent flow and transport numerical model to reproduce the evolution of the aquifer system and develop a few scenarios for artificial aquifer recharge using the treated water provided by a nearby wastewater treatment plant. The research outcomes point to the hydrological challenges that emerge for the effective management of water resources in reclaimed desert areas, and they highlight the effectiveness of using advanced geophysical and modeling methodologies

Application of time domain induced polarization to the mapping of lithotypes in a landfill site

Abstract. A direct current (DC) resistivity and time domain induced polarization (TDIP) survey was undertaken at a decommissioned landfill site situated in Hørløkke, Denmark, for the purpose of mapping the waste deposits and to discriminate important geological units that control the hydrology of the surrounding area. It is known that both waste deposits and clay have clear signatures in TDIP data, making it possible to enhance the resolution of geological structures compared to DC surveys alone. Four DC/TDIP profiles were carried out crossing the landfill, and another seven profiles in the surroundings provide a sufficiently dense coverage of the entire area. The whole dataset was inverted using a 1-D laterally constrained inversion scheme, recently implemented for TDIP data, in order to use the entire decay curves for reconstructing the electrical parameters of the soil in terms of the Cole-Cole polarization model. Results show that it is possible to resolve both the geometry of the buried waste body and key geological structures. In particular, it was possible to find a silt/clay lens at depth that correlates with the flow direction of the pollution plume spreading out from the landfill and to map a shallow sandy layer rich in clay that likely has a strong influence on the hydrology of the site. This interpretation of the geophysical findings was constrained by borehole data, in terms of geology and gamma ray logging. The results of this study are important for the impact of the resolved geological units on the hydrology of the area, making it possible to construct more realistic scenarios of the variation of the pollution plume as a function of the climate change