Too thin to be detected: When ERT surveys can fail to assess an aquiclude layer interposed between two aquifers: The Sunceri test site (Honduras)

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.