Transient Investigation of the Critical Abstraction Rates in Coastal Aquifers: Numerical and Experimental Study

Abdelrahman M Abdelgawad,G Hamill,Ashraf A Ahmed,Salissou Moutari,Antoifi Abdoulhalik

doi:10.1007/s11269-018-1988-3

Abstract

This research investigated the transient saltwater upconing in response to pumping from a well in a laboratory-scale coastal aquifer. Laboratory experiments were completed in a 2D flow tank for a homogeneous aquifer where the time evolution of the saltwater wedge was analysed during the upconing and the receding phase. The SEAWAT code was used for validation purposes and to thereafter examine the sensitivity of the critical pumping rate and the critical time (the time needed for the saltwater to reach the well) to the well design and hydrogeological parameters. Results showed that the critical pumping rate and the critical time were more sensitive to the variations of the well location than the well depth. The critical time increased with increasing the location and depth ratios following a relatively linear equation. For all the configurations tested, the lowest critical pumping rate was found for the lower hydraulic conductivity, which reflects the vulnerability of low permeability aquifers to salinization of pumping wells. In addition, higher saltwater densities led to smaller critical pumping rate and shorter critical time. The influence of the saltwater density on the critical time was more significant for wells located farther away from the initial position of the interface. Moreover, increasing the dispersivity induced negligible effects on the critical pumping rate, but reduced the critical time for a fixed pumping rate.

Highlights

Seawater intrusion (SWI) is generally characterized by quantitative indicators, which include the external metrics that delineate the outward boundary of the saline plume and the freshwater-saltwater interface, outlining the wedge-like shape of the plume
As saline water intrudes deeper into the aquifer and approaches the location of the pumping wells, large withdrawal of groundwater leads to the rise of the freshwater-saltwater interface below or within the vicinity of the well, a process known as saltwater upconing mechanism (Reilly and Goodman 1987)
The results show that numerical model predicted very well the seaward motion of the saltwater wedge observed in the physical model

Summary

Introduction

Seawater intrusion (SWI) is generally characterized by quantitative indicators, which include the external metrics that delineate the outward boundary of the saline plume and the freshwater-saltwater interface, outlining the wedge-like shape of the plume. Incorrect management of freshwater pumping may eventually lead to the salinization of pumping wells, which would occur following the mixing of only 250 mg/l of chloride after which freshwater is considered undrinkable as per WHO regulations (WHO 2011), which is equivalent to less than 1% of seawater. As saline water intrudes deeper into the aquifer and approaches the location of the pumping wells, large withdrawal of groundwater leads to the rise of the freshwater-saltwater interface below or within the vicinity of the well, a process known as saltwater upconing mechanism (Reilly and Goodman 1987). Groundwater pumping is considered one of the most important challenges that promotes SWI (Werner et al 2013), and was addressed in many investigations. Groundwater pumping is considered one of the most important challenges that promotes SWI (Werner et al 2013), and was addressed in many investigations. Ferguson and Gleeson (2012) shows that coastal aquifers are more vulnerable to wells extraction than the predicted sea level rise

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Conclusion