Infiltration water problem

Letters to the Editor (The Guardian)
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Editor: As regards confined aquifers (CA), consider the specific yields of unconfined aquifers are much larger than the storativities of CAs.

Thus, from a hydraulic standpoint, unconfined aquifers are generally preferable to CAs for water supply, because for the same rate of water extraction there is less draw down over a smaller area with an unconfined aquifer than with a CA.

The Winter River abstraction well fields have been drilled into a Permian CA with gravel beds of high lateral conductivity. Over the past 50 years, the area of depressurization has increased to such an extent that the CA has consumed the Stanhope ponds and bogs and has induced anoxia on Covehead, Brackley, Rustico and Winter bays. The area of infiltration may now be greater than 200 square kilometres, extending as far west as Hunter River.

In the sandstone layers of the aquitard, which separate the CA from the water table (unconfined) aquifer, the vertical permeability of sandstone is two to three times less than its horizontal permeability and this suggests that there is more joint permeability than inter-granular permeability.

However, as water pressure is reduced (de-watering) in the CA, stresses between solid grains of the aquitard matrix will increase because of the overburden pressure of land above and compaction of the aquitard sandstone will occur; hence, its permeabilities will decrease; hence, the area of infiltration must increase.

Compaction is an irreversible and permanent change to the sandstone matrix. The CA was primed during the last ice age. Now man has pumped the CA to a steady state standoff and the land and ocean are crying out. The recharge time of the upper CA may be rated in centuries while deeper CAs may be rated in millennia and for this reason CA waters are often classed as a non-renewable resource.

The water table is stable but its lateral flow, waters destined for marine environments, now have a large downward component into the CA. The horizontal (lateral) flow, now largely stopped, of such waters are a renewable resource and are necessary for the survival of many marine plants and animals.

Tony Lloyd,

Mount Stewart

Geographic location: Mount Stewart

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