Page 87 - Arabian Studies (V)

Water Resources and Agriculture in Qatar 77
approximately 100 m. below sea level. The lens is now in a non-
steady state condition brought about by over-extraction as evi­
denced by declining water levels and a shrinking volume. The
hydraulic behaviour of the lens is governed by the Ghyben-
Herzberg principle relating to floating lens type aquifers whereby a
lowering of the water table will cause a rise of the freshwater/saline
water interface at the base by a factor ranging from 25 to 40,
depending upon the salinity concentration of the underlying saline
water, leading to a situation where over-extraction will cause a
concomittant rise of the interface and upwelling of saline water.
The southern groundwater province encompasses slightly more
than half of the land area of Qatar where hydrogeological conditions
are considerably more complex than those of the north. Differences
in structure, geomorphology and vegetational pattern are thought
to be important factors in reducing the amount of groundwater
recharge with the result that no extensive freshwater lens have been
developed. At the same time there appears to be upward leakage of
deeper, more saline, water under a strong positive head along
major structural features, giving rise to an anomalous situation
whereby the piezometric surface appears to be related to the main
structural features, rather than any recharge-induced type of
configuration. With a reduced quantity of water being recharged,
and hence less circulation, the porosity/permeability is also consi­
derably less than in northern Qatar.
The chemical quality of groundwater reflects, in general, the
composition of the host rock and its mixing ratio with connate or
sea water. Water from the northern freshwater lens has a total
dissolved solids concentration ranging from 400 ppm. to 2,000
ppm. and is, in general, of marginal quality for irrigation with a
low to medium sodium (alkali) hazard and a high to very high
salinity hazard. Except for small isolated lenses of water of 2,000-
3,000 ppm., the salinity of the groundwater in the southern areas
ranged form 3,000 ppm. to 6,000 ppm. In the extreme southwest, at
Abu Samrah, free-flowing wells with a tds. value of 3,000 ppm.
have been developed from a complex, but probably limited,
confined aquifer with a recharge source in Saudi Arabia. Since
1971 over one hundred wells have been regularly monitored for
water quality and these data show that there has been an overall 22
per cent increase in total dissolved solids since then. This increase is
a feature of the continued overdraft of the groundwater reservoir
and encroachment of brackish and salt water from the coastal
margins.