Using geoelectrical models for evaluating subsurface conditions for sustainable aquaculture in the Niger-delta region of Nigeria

Abstract:

Locating suitable sites for aquaculture is a major challenge for fish farmers within the Niger-Delta region of Nigeria who rely on earthen ponds. When these ponds are not able to retain water, they are eventually abandoned resulting in loss of revenue and environmental degradation. To prevent this, there is need to identify sites with appropriate hydrological properties for siting earthen ponds. Geophysical methods including electrical resistivity and induced polarization have been used to delineate subsurface lithology and hydraulic properties. However, there has been no studies using geophysics to guide the selection of earthen fish pond sites. This study focuses on using subsurface electrical resistivity and chargeability models to guide aquaculture site selection. To do this, we first developed a conceptual model of an earthen fish pond site using known geology of the Niger-Delta region. We further converted the conceptual model into a block geophysical model of the subsurface and conducted a forward simulation of apparent resistivities and chargeability which were later inverted after adding noise with known statistical properties. The forward simulation informed our field experimental settings and guided interpretation of field results. Field assessments were carried out at Ugono-Abraka and Agbara-Otor areas of Delta State, Nigeria. Acquired data include ten vertical electrical sounding and five 2D electrical resistivity tomography and Induced polarization profiles using both schlumberger and dipole – dipole configurations. The field data were inverted using WinResist, IPI2win, and Diprowin software. The geophysical models were combined with lithological data to determine the clay content, filtration coefficient and transmissivity. The delineated subsurface properties at Ugbono-Abraka (clay content: 0.3 - 1.6 %; infiltration coefficient: 0.05 - 9.51 m/day; transmissivity: 204 - 657 m²/day) and Agbarha-Otor (cay content:1.1 - 9.8 %; infiltration coefficient: 4.63 - 304.29 m/day; transmissivity: 7.8 - 2,537 m²/day) show higher variation than assumed by practitioners. This implies variable water loss in the earthen ponds hence, we recommend characterizing these variations using simple hydrogeophysical methods prior to establishing medium to large scale fish ponds in the area.