WHY AND HOW SEA SHORE DWELLERS GET FRESH WATER FROM AQUIFERS?
By Durgadas Shetti
Groundwater occurs beneath the surface in the form of Aquifers. Aquifers are geologic formations that can store and supply fresh water amidst huge salt water body of the sea.
During rain runoff water from rivers and lakes percolate and charge aquifers. Aquifers with potable water interact with sea water, atmosphere and gravity across a impermeable web of salt gradients. A complex geophysical formations determine the aquifer’s property such as miscibility and exchange of desolved solids and salts.
Groundwater is not stagnant but dynamic. The flow is described by Darcy’s law. Groundwater can be confined within strata and maintained under equilibrium unless disturbed. The level of groundwater is demaked as water table confined by impervious or semi impervious strata.
On the coasts, groundwater exists in “dynamic and often transient equilibrium” on the denser sea water in floating state as it is less saturated with salts. Groundwater tend to flow and discharge into the sea – as long as the water table level is at a higher pressure gradient. Sea water in turn will try to push the contaminant into fresh groundwater. There is trade off between seawater-freshwater at interface zone that is transient with rainfall, river or lake discharge, tides and evaporation.
As sea shore dweller try to pump out water from the coast, sea water will rush and salineate fresh groundwater. The Gyben-Herzberg equation governs the fresher groundwater aquifer-sea water interface. For every cubic feet of water harvest about 40 cubic feet of groundwater is lost to sea to maintain the equilibrium.
Here is a citation that explains why this well gives freshwater -This natural movement of fresh water towards the sea prevents salt water from entering freshwater coastal aquifers (Barlow, 2003).
There are 22 wells in the Rameswaram temple – on a shallow coastal aquifer (unconfined) and are close to the sea. The dissolved solids vary in each that provides water which is not salty and can be consumed. This is a holy place and according to Hindu mythology Lord Rama shot an arrow into the sea to get water to drink for his wife Sita. This is known as the Villundi Tīrtham well.