Vulnerability of coastal aquifers to groundwater use and climate change
Grant Ferguson & Tom Gleeson
Journal name:Nature Climate Change
Year published: 2012
Received 21 April 2011
Accepted 18 January 2012
Published online 19 February 2012
Climate change and human population growth are expected to have substantial impacts on global water resources throughout the twenty-first century1, 2. Coastal aquifers are a nexus3 of the world’s oceanic and hydrologic ecosystems and provide a water source for the more than one billion people living in coastal regions4, 5. Saltwater intrusion caused by excessive groundwater extraction is already impacting diverse regions of the globe5, 6, 7. Synthesis studies8, 9 and detailed simulations10, 11, 12, 13 have predicted that rising sea levels could negatively impact coastal aquifers through saltwater intrusion and/or inundation of coastal regions. However, the relative vulnerability of coastal aquifers to groundwater extraction and sea-level rise has not been systematically examined. Here we show that coastal aquifers are more vulnerable to groundwater extraction than to predicted sea-level rise under a wide range of hydrogeologic conditions and population densities. Only aquifers with very low hydraulic gradients are more vulnerable to sea-level rise and these regions will be impacted by saltwater inundation before saltwater intrusion. Human water use is a key driver in the hydrology of coastal aquifers, and efforts to adapt to sea-level rise at the expense of better water management are misguided.
Figures at a glance
- Figure 1: Coastal aquifers are affected both by groundwater extraction and sea-level rise. a,b, Conceptual model used for simulating the impact of groundwater extraction (a) and sea-level rise (b) including both saltwater intrusion and saltwater inundation. The simulation variables include discharge per unit coastline (q), groundwater extraction rate (Q), aquifer thickness (b), the difference in hydraulic head between the inland boundary of the flow system and the coast before sea-level rise (Δho), and the distance from the coastline to the well (xi)and the toe of the saltwater wedge (xt). The grey area shows the distribution of the fresh aquifer water before extraction or sea-level rise.
- Figure 2: Present saltwater intrusion areas have a high population density and/or low hydraulic gradients. a, For coastal watersheds in the contiguous United States, hydraulic gradients >0.001 are mapped in blue and those with hydraulic gradients <0.001 are mapped in red. Insets showing watershed boundaries in the Pacific northwest and the Florida panhandle are provided as more detailed examples. b,c, The distribution of hydraulic gradients and population densities along the west coast (b) and east coast (c) of the contiguous United States. Documented saltwater intrusion locations7 are labelled with black squares in a and grey bars in b and c.
- Figure 3: The impact of groundwater extraction on coastal aquifers is more significant than the impact of sea-level rise. a, Saltwater intrusion for various hydraulic gradients with extraction from a well 1 km from the coast in a 10-km-long watershed (Supplementary Table S1). Grey area denotes changes in recharge of ±30%. Inset histogram summarizes coastal hydraulic gradients (Fig. 2a)20. b, Saltwater inundation and associated infiltration of sea water will be more important than saltwater intrusion from sea-level rise assuming equal topographic gradient and hydraulic gradients. c, Hydraulic gradients for watersheds adjacent to the US Gulf Coast with red indicating greater vulnerability.
- Figure 4: The uncertainty owing to different aquifer types is significant as shown by simulation of these types over a range of coastal populations.