Active saltwater intrusion of shrinking Bakhtegan -Thask Lakes in South Iran threatens the freshwater resources of coastal aquifers

Abstract

Study region: Eight Coastal aquifers (CA) of Bakhtegan and Tashk lakes (BTL) and salt marsh, southern Iran Study focus: BTL, Wetlands of International Importance, have been shrinking due to reduction in surface discharge, groundwater overexploitation, and drought. We show that this resulted in an increase of BTL total dissolved solids (TDS) from 45400 to 256000 mg/l. To characterize the hydrogeological behavior of the coastal aquifers after shrinkage, major ions, TDS and water level were measured along a transect from inside the BL to one of the aquifers. The spatial distribution of electrical conductivity (EC), discharge and iso-potential maps was determined. A support vector regression technique was used to forecast EC and water level in CA for the next decade. New hydrological insights for the region: Five zones based on EC changes and hydraulic gradient are recognized, namely: brine, highly saline, brackish, transition, and freshwater. The groundwater flow direction is from both the BTL and Fresh Water Zone, converging towards the intermediately positioned Transition Zone. Saltwater intrusion is active based on the hydraulic gradient towards the land and the gradient of water density. A schematic flow model of CA was proposed based on active SWI, groundwater flow direction, and chemical signature. Forecasting reveals a significant further deterioration of water quality and drop in groundwater levels, which emphasizes the need for systematic and sustainable water management.

Keywords

• Saltwater intrusion
• Hydrochemistry
• Coastal aquifers
• Bakhtegan and Tashk Lakes
• Iran