Abstract
Delineation of fresh/saline groundwater is essential for sustainable water quality management, especially in the coastal areas all around the globe. Seawater intrusion causes substantial degradation in quality of freshwater resources in the coastal areas. The main reason for saltwater intrusion is the changing environment in terms of sea-level rise, climate change, and over-extraction of freshwater resources to meet the growing demands. In this study, an integrated approach of geophysical and geochemical methods was used to assess saltwater intrusion in the coastal areas of Bela Plain, Pakistan. The inverted electrical resistivity computed from 50 vertical electrical sounding (VES) constrained the subsurface into five layers and two aquifers through 3D imaging, such as silty clay and sandy clay containing saline water, and sand, sandy gravel, and gravel containing freshwater. However, the narrow range of resistivity values shows an overlap of saline/fresh groundwater. Such ambiguity in the resistivity interpretation was removed by Dar-Zarrouk (D-Z) parameters. D-Z parameters, namely transverse unit resistance (Tr), longitudinal unit conductance (Sc), and longitudinal resistivity (ρL) estimated from VES, marked a clear distinction between saline and fresh aquifers with a wide range of values. The geochemical method was performed using 20 water samples for the main cations (K+, Ca2+ Na+, and Mg2+), anions (SO42−, HCO3−, Cl−, and NO3−), and other parameters (TDS, EC, and pH). Fresh/saline aquifers revealed by D-Z parameters are in good agreement with those delineated by physicochemical parameters and local hydrogeological conditions. This study delineates seawater intrusion of about 13–42 km from Sonmiani Bay in the Arabian Sea towards the inlands of Bela Plain. Therefore, it is expected that this investigation will be helpful in future planning for the management and exploitation of freshwater resources in the study area. Our study suggests that D-Z parameters can be used as the most inexpensive alternative to the traditional geotechnical and environmental tests for the demarcation of fresh/saline groundwater with a large coverage in any coastal or contaminated area under a homogeneous or heterogeneous setting.
Highlights
Due to their dense population, many coastal areas around the world have become a hub of economic activities, such as metropolitan development, tourism, trade, and food development [1,2]
Based on the calibration between the well logs and nearby vertical electrical sounding (VES) models, the first layer above water table was interpreted as topsoil cover or dry strata, having a resistivity of > 30 Ωm; the second layer below the water table was delineated as clay or silt, having a resistivity of < 10 Ωm; the third layer was revealed as sandy clay, having a resistivity of 5–25 Ωm; the fourth layer was interpreted with sand as the main lithology, having a resistivity of 20–40 Ωm; sandy gravel was delineated as the fifth prominent layer, having a resistivity of 35–55 Ωm; and the last layer was revealed as gravel, having a resistivity of >50 Ωm
The results suggest that the fresh/saline aquifers revealed by D-Z parameters of the VES method match with those delineated by physicochemical parameters (Figures 5–10)
Summary
Due to their dense population, many coastal areas around the world have become a hub of economic activities, such as metropolitan development, tourism, trade, and food development [1,2]. Groundwater is being excessively exploited, and the over-extraction of this precious resource has resulted in seawater intrusion in most of the coastal regions around the world [7,8,9,10]. A freshwater resource overlies a saltwater aquifer because seawater is denser than freshwater, and brackish water is formed in the zone of contact between saltwater and freshwater [11]. The distance of the fresh–saline groundwater boundary from the sea is not constant, and it varies from one area to another depending on the environmental conditions that cause the increase in seawater intrusion. The study of sustainable water quality management requires delineating saltwater intrusion, especially in the coastal aquifers, in order to avoid such global issues and to remediate the coastal regions efficiently for sustainable management of groundwater resources
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