Factors to be taken into account when considering sub-sand abstraction:
Basic site factors:- (i) depth (ii) extent (iii) texture of the bed: i.e. its particular size pattern and stratification, which may vary widely both up and down and along the beach, Considerable modification may be possible, especially
of (iii), but these three, separately or together, normally set a limit to the volume that can be drawn from any site.
Tidal amplitude and recession may also be limiting factors, especially in north temperate zones, while on beaches with recession of above, say 100 m, the beach may serve as both
filter and reservoir. In the Mediterranean, the Middle East, and in the tropics, the rise and fall
are seldom excessive.
Stability of beach. In areas subject to heavy tidal currents, severe storms, typhoons etc., it is
seldom possible to use shallow beaches (i.e. 1 - 2 m depth). Where the sand is deep, screen
wells can be installed and the suction line buried deeply for security.
Underground fresh water channels may be tapped. Where full salinity water is required for marine aquaria, laboratories etc., such dilution must be avoided. For desalination etc., such a source is an advantage, while for cooling water and general purposes it is immaterial.
The possible presence of iron in the bed. Most seas have negligible iron content (0.002 mg/l), but iron may occur in the bed, especially in temperate and northern zones, often in the form of organic pollution for example when algal material is buried by storms or tidal action. Anaerobic decay is so slow that this may remain unchanged for years, but the passage of oxygenated water, preceded by thorough jetting of the bed, quickly breaks down & evacuates the iron. If the presence of iron is noted during feasibility survey its origin should be checked.
Sewage. It is generally stated that intakes should not be within 1km of a sewage outfall. The
biological activity of a developed sub-sand system makes such a distance completely safe.