Salt water gain and fresh water loss

[Hard@it]

How far inland especially on small islands does sea water permeate and is there a degree of filtration that makes it into near fresh ? Furthermore is there a large amount of fresh water lost to the sea via underground fissures etc. ?

[newtron]

If you go deep enough, sea water will permeate the whole land mass. The depth of the boundary between salt water and fresh water depends on many factors such as the hydrogeology (rock types, permeability, porosities, hydraulic gradients, structural geology (including faults and fractures), etc.), precipitation rates over the island, and the relative densities of the sea water and fresh water. For an aquifer characterised by a homogeneous, isotropic porous medium, the 3-dimensional shape of the aquifer can be visualised as a lens of fresh water that sits above the salt water. The transition zone between the fresh water and salt water is usually characterised as a mixing zone, where salinity increases with depth. For more complex groundwater systems, the relationship between fresh water and salt water can vary greatly. For example, on volcanic islands, it is not uncommon to have confined aquifers that are isolated from the sea water by dikes and layers of volcanic rock.

The fresh water originates as precipitation on the island, which recharges the aquifer(s). Groundwater flow paths largely depends on hydraulic gradients and the hydrogeologic characteristics of the aquifer. It is certainly possible for groundwater to discharge into the sea via preferential flow paths such as faults and fractures.

I hope this helps.

[newtron]

I was thinking about your question about whether there is a degree of filtration of the sea water as it flows through the land mass. Again it really depends on geology. In general, I would say that filtration resulting in a significant decrease in salinity is unlikely. However water/mineral interactions such as dissolution, mineral precipitation, ion exchange, sorption, etc., can definitely change the water chemistry as it flows through the land mass.

[newtron]

Hi again, I just wanted to clarify that the conceptual model presented above applies to relatively small islands.

[Hard@it]

Hi newtron, thanks for taking up your time and giving an excellent answer. I was prompted to ask this question after being on Madeira for a holiday and it is all vertical from the shoreline. We were told that there is no problem with fresh water as it all comes from springs up in the high level,but bearing in mind the population, ( which is heavily increased by holidaymakers), must have a high water usage. When we were there, the rivers were at their lowest , but I have to say even with the heat there was lots
of condensation up at the high level.
It is not a massive island, I suppose you could drive round in 2.5 hours or so.
I wondered re the amount of fresh water lost apart from the rivers, and knowing it is volcanic imagined that there would be a huge loss of fresh through any underground blowholes, fissures etc.
Thanks again. Cheers Hard@it

[Catso]

Perhaps not an answer to your Q, but you might be interested...

The St Austell brewery uses water from its own well for brewing. This is pretty much under the brewery. It is said that during times of exceptional drought, the well water becomes slightly salty, and taints the beer. St Austell is about 1� miles from the nearest sea, although well(!) above sea level.

[Hard@it]

Hi catso - thanks for that, I do think there is a fair amount of sea to fresh and we do not notice it ! cheers Hard@it