Assessing impacts of climate change, sea level rise, and drainage canals on saltwater intrusion to coastal aquifer

P Rasmussen,K Hinsby,G Goncear,T O Sonnenborg

doi:10.5194/hess-17-421-2013

Abstract

Abstract. Groundwater abstraction from coastal aquifers is vulnerable to climate change and sea level rise because both may potentially impact saltwater intrusion and hence groundwater quality depending on the hydrogeological setting. In the present study the impacts of sea level rise and changes in groundwater recharge are quantified for an island located in the Western Baltic Sea. The low-lying central area of the investigated part of the island was extensively drained and reclaimed during the second half of the 19th century by a system of artificial drainage canals that significantly affects the flow dynamics of the area. The drinking water, mainly for summer cottages, is abstracted from 11 wells drilled to a depth of around 20 m into the upper 5–10 m of a confined chalk aquifer, and the total pumping is only 5–6% of the drainage pumping. Increasing chloride concentrations have been observed in several abstraction wells and in some cases the WHO drinking water standard has been exceeded. Using the modeling package MODFLOW/MT3D/SEAWAT the historical, present and future freshwater-sea water distribution is simulated. The model is calibrated against hydraulic head observations and validated against geochemical and geophysical data from new investigation wells, including borehole logs, and from an airborne transient electromagnetic survey. The impact of climate changes on saltwater intrusion is found to be sensitive to the boundary conditions of the investigated system. For the flux-controlled aquifer to the west of the drained area only changes in groundwater recharge impacts the freshwater–sea water interface whereas sea level rise does not result in increasing sea water intrusion. However, on the barrier islands to the east of the reclaimed area, below which the sea is hydraulically connected to the drainage canals, and the boundary of the flow system therefore controlled, the projected changes in sea level, groundwater recharge and stage of the drainage canals all have significant impacts on saltwater intrusion and the chloride concentrations found in abstraction wells.

Highlights

The objectives of this study are to investigate the following questions: (a) what is the effect of climate change, including sea level rise and changed groundwater recharge, on an aquifer where the groundwater head is partly controlled by drainage canals? (b) What are the most important factors for seawater intrusion to a coastal aquifer; sea level rise, changes in groundwater recharge or water level in the drainage canals? (c) What are the dynamics of increased seawater intrusion in combination with increased recharge? (d) Will the water works have to move some of their wells in the area in the 21st century due to salinization unless measures are taken to control seawater intrusion?
In this study seawater intrusion (SWI) modeling of a real-world case demonstrates the importance and effect of changes in sea level and groundwater recharge on saltwater intrusion into confined and unconfined coastal aquifers, where the groundwater head is controlled by drainage canals
The model studies of four transition phases showed that the chalk aquifer at the eastern coast of the island of Falster is affected by seawater intrusion and that the system had reached a new equilibrium before the time where extensive groundwater abstraction started in the 1960s

Summary

Introduction

Climate change impacts especially sea level rise and changed precipitation will challenge the current water supply management and groundwater abstraction from well fields close to the coast, globally. Previous studies of seawater intrusion (SWI) and saltwater distribution in coastal aquifers have focused on mapping saltwater occurrence, fluid-density aspects of numerical flow modeling, effects of drainage in a polder context, effects of autonomous salinization, tidal effects, and parameter estimation (Essink, 2001; Post, 2005; Carrera et al, 2010; de Louw et al, 2011; Tran et al, 2012). Focus is very much on climate change effects on seawater intrusion and saltwater distribution in coastal aquifers (Werner and Simmons, 2009; Essink et al, 2010; Webb and Howard, 2011; Chang et al, 2011).

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Discussion

Conclusion