Land Subsidence and Aquifer Storage Change from a Declining Water Table
An agricultural basin of 1,250 km2 relies on a confined aquifer overlain by a compressible clay aquitard. Water levels fell 22 m over 15 years. InSAR shows 4 to 11 cm/year of subsidence concentrated over the pumping area. Specific storage of the aquifer is estimated at 4 x 10^-5 m^-1, and the aquitard thickness is 18 m with a vertical hydraulic conductivity of 0.002 m/day. Groundwater abstraction is about 210 million m3/year. The water authority must decide whether to declare a moratorium or to allow continued pumping with managed aquifer recharge.
1. Estimate the order of magnitude of storage release from aquifer compaction and compare it with abstraction. What does the comparison imply?
Read the worked answer
Elastic-plus-inelastic storage from the aquifer alone: S = Ss x b x area x head change = 4 x 10^-5 x 22 x 1,250 x 10^6 m2, which is roughly 1.1 x 10^9 m3 over 15 years, or about 7 x 10^7 m3/year. That is a third of the 2.1 x 10^8 m3/year abstraction; the remainder must come from aquitard compaction and from capture/reduction in natural discharge.
The aquitard contribution is dominated by inelastic compaction of the clay, which is largely irreversible. The subsidence rates of 4 to 11 cm/year confirm that a substantial part of the abstracted water is being mined from permanent storage.
The implication: continued pumping is not sustainable at this rate; the storage term is being depleted, and the irreversible compaction means the aquifer will not recover its storage even if heads recover.
2. Contrast the moratorium and the MAR option in terms of what each can and cannot restore, and recommend a decision rule.
Read the worked answer
A moratorium stops further head decline and slows subsidence, but it cannot recover the inelastic storage already lost or reverse compaction; recovery is limited to the elastic component. It is the more certain protection for the aquitard.
MAR can recharge the aquifer and slow decline, but its effectiveness depends on source-water availability, infiltration capacity, and water quality; it does not prevent subsidence if heads continue to fall in the pumping area, and it may not reach the confined aquifer efficiently through the low-K aquitard.
Decision rule: if subsidence exceeds a defined threshold or head decline continues despite MAR, impose the moratorium; otherwise allow continued pumping only with a binding abstraction cap, mandatory MAR with performance monitoring, and subsidence triggers that automatically tighten limits. The rule should tie the permit to measured head and InSAR trends, not to the proposed volume.