Abstract
Seawater intrusion is a major challenge in many coastal areas all around the world, mainly caused by over-exploitation of freshwater resources, climate change, and sea-level rise. Consequently, seawater intrusion reaches several kilometers inland, thus making the freshwater resources polluted and unsuitable for human use. Conventionally, the fresh-saline water interface is delineated by the number of laboratory tests obtained from boreholes. However, such tests suffer from efficiency in terms of data coverage, time, and cost. Hence, this work introduces Dar-Zarrouk (D-Z) parameters, namely transverse resistance (Tr), longitudinal conductance (Sc), and longitudinal resistivity (ρL) computed from non-invasive vertical electrical sounding (VES). Two-dimensional (2D) imaging of D-Z parameters provides a clear distinction of fresh-saline aquifers. Such techniques remove ambiguities in the resistivity interpretation caused by overlapping of fresh and saline aquifers during the process of suppression and equivalence. This study was carried out by 45 VES along five profiles in the coastal area of Bela Plain, Pakistan. D-Z parameters delineate fresh, brackish, and saline aquifers with a wide range of values such as freshwater with Tr > 2000 Ωm2, Sc < 3 mho, and ρL > 20 Ωm; saline water with Tr < 1000 Ωm2, Sc > 25 mho, and ρL < 5 Ωm; and brackish water with Tr between 1000–2000 Ωm2, Sc from 3 to 25 mho, and ρL between 5–20 Ωm. The D-Z results were validated by the physicochemical analysis using 13 water samples and local hydrogeological setting. The obtained results propose that D-Z parameters can be used as a powerful tool to demarcate the fresh-saline aquifer interface with more confidence than other traditional techniques. This geophysical approach can reduce the expensive number of borehole tests, and hence contributes to the future planning and development of freshwater resources in the coastal areas.
Highlights
In Pakistan, groundwater is an essential resource for sustaining the population and economy [1]
Saline, brackish, and fresh groundwater zones revealed by D-Z parameters of the vertical electrical sounding (VES) method are revealed by D-Z parameters of the VES method are validated by the physicochemical analysis, which suggests that this approach can be successfully used as an alternate to the expensive laboratory tests analysis in order to assess seawater intrusion into fresh aquifers for coverage of large areas
The calibration between the formation resistivity computed from 45 VES models and lithological logs constructed from 13 boreholes constrained subsurface units into six layers, such as clay having resistivity
Summary
In Pakistan, groundwater is an essential resource for sustaining the population and economy [1]. Demarcation of the fresh-saline groundwater boundary is very important for extraction and management of freshwater reserves in the coastal areas. A clear difference in resistivity values of freshwater and saltwater is useful to assess the fresh-saline water interface in the coastal areas [22,23]. Based on local geological settings and groundwater conditions, D-Z parameters were acquired for each distinct layer of fresh or saline water with various arrangements of resistivity and thickness These parameters delineated saline and fresh aquifers with a wide range of specific values depending on local hydrogeological conditions and wells information. The monitoring boreholes data of physicochemical parameters were integrated with a geophysical interpretation to improve and validate the results of the D-Z parameters for the assessment of saline and fresh aquifers
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