Electrical Resistivity Tomography (ERT) and resistivity depth sounding surveys were undertaken at two sites located along the Chamelecòn Valley (Honduras). The objective of the surveys was to determine the occurrence, geometry and lithological properties of the Chamelecòn River's shallow paleochannels. Following the interpretation of aerial photograms which indicated the likely presence of paleochannels, two sites were selected for the surveys. Resistivity depth soundings were used to achieve electro-stratigraphic sections obtained by interpolating 1D inverted data that would assist with assessing the occurrence of paleochannels at a large scale. The presence of paleochannels at the two sites was assessed through interpretation of the electro-stratigraphic sections. At Site 2, the abundant 1D data, the small distance between the soundings and the shallow target, allowed determining the course of the paleoriver as shown by resistivity and electrical transmissivity maps. The results from the 1D data, although affected by strong heterogeneities, are congruent with the 2D inverted models. ERT surveys confirmed the presence of paleochannels and allowed achieving detailed imaging of the alluvial geological bodies. Paleochannel deposits show resistivity values ranging between 70 and 100 ohm m at Site 1, while resistivity values range between 90 and 120 ohm m at Site 2. The latter resistivity values were affected by the presence of coarser deposits. Resistivity values suggest that at both sites paleochannel deposits have a clay content that is lower than 3 %. Other alluvial deposits in the area show resistivity values that range between 19 and 70 ohm m at Site 1, while resistivity values range between 30 and 90 ohm m at Site 2. These lower values suggest that clay content can be as high as 29 %. The presence of paleochannels assessed by means of 1D surveys and the lithological characterisation assessed by means of 2D surveys were confirmed by logs from boreholes drilled at the sites.
ERT surveys were undertaken at the Sunceri test site located in the city of San Pedro Sula (Honduras). Four deep pumping wells are located at the site, together with 26 shallow and 12 deep piezometers drilled through alluvial deposits. These deposits are 100-150 m thick and overlie intrusive and low grade metamorphic rocks. The site is a major public water supply (PWS) for the city with a total groundwater abstraction of 160-200 l/s. The main objective of this paper is to identify if ERT surveys can determine the occurrence and continuity of a clay layer with a variable thickness and an average resistivity of 17 Ω·m. Based on borehole logs, the clay layer is located at a depth of approximately 25 m. This layer is widespread across the entire area. It separates an upper unconfined aquifer from a deeper confined aquifer. The aquifers have different piezometric levels and hydro-chemical features. It is essential to correctly assess the thickness and paramatise the aquiclude layer so that a correct vulnerability assessment of the groundwater resource can be undertaken. Indeed, pollution of the heavily abstracted deep aquifer from contaminated shallow groundwater should be prevented. The inverted resistivity sections reveal the presence of the clay layer. However, they fail to show that the clay layer is discontinuous across the area. In particular, the clay layer doesn't appear in the middle portion of the surveyed area where its thickness falls below 4.9 m as revealed by 2D synthetic dataset modelling. 1D modelling indicates a slightly lower value of 3 m. The fact that the thinning of the clay layer (that has a resistivity consistent with a 37 % clay content, i.e. 4 ·10 -3 m/d hydraulic conductivity) occurs in an area affected by a severe drawdown of the piezometric level due to the interference between the depression cones of the pumping wells, increases the vulnerability of the deep aquifer under the current abstraction rates.
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