Protecting wells from brine infiltration

Many coastal cities are facing water shortages not because there is not water down their wells, but because brine infiltration has made the water undrinkable. Brine infiltration occurs when fresh and salty ground water interface. Normally the balance of pressure keeps the salty water away from shore and underneath the fresh water. However, when you disturb this balance by pumping the fresh water, first the salt water will mix with the fresh water making the water briny. This is the warning, if you keep pumping eventually the salty water will actually be able to push its way inland, and the well will produce salt water. Once salt water intrudes on the fresh even if the pumping stops, this can take years for the salty water to be flushed out and the well become drinkable again.

Now what do you do if cannot stop withdrawing groundwater? Well if you have an excellent hydrological model, you could strategically place wells out at sea and begin withdrawing the salty water to rebalance the pressure and give the fresh water a chance to push back out to sea. It wouldn’t be that hard, your city would model the fresh withdraw cone, and rent near shore oil drilling equipment, and drill several wells (preferably with horizontal drilling equipment so all the wells can share a single platform.) Once you have the wells you start pumping, if you want to be green about it you use methane from the landfill or wind power to run the pumps. You create several withdraw cones in the salt water that completely shields your fresh water wells, and wait for things to stabilize.

In case someone who could actually do what I am suggesting reads this (an extremely unlikely event), there are a lot of things you need to think about before attempting to shield your freshwater wells from brine inflitration. The big one is subsidence of the seafloor caused by the massive amount of water you would need to withdraw to make this work. It doesn’t sound like it should be a big deal if the seafloor sinks, but it could be. It could cause a change in way the currents flow, which might cause silting or alter navigation features. If there are methane hydrates in the seafloor it could destabilize them, which would be a HUGE problem. Think massive cloud of deadly and flammable gas and huge tsunami. Even if you don’t have methane hydrates if you are causing subsidence on a slope, you might trigger an underwater landslide, and accompanying the tsunami… Assuming you can prove to the insurance people this won’t happen, you will still have to prove to the EPA that the water you will be pumping up and dumping in the sea which will have a different temperature and level of salinity won’t be harmful.

If you manage to do all this, and can afford the energy, feel free to use this idea to save your cities fresh water supply. Perhaps it would be a better idea to leave the water in the ground and just build desalination plants.