Well Efficiency

Well efficiency is one number that says how much of a well's drawdown is the aquifer's doing and how much is the well's own fault: $E = BQ/(BQ + CQ^{2})$, which simplifies to $E = B/(B + CQ)$. At 85 per cent, six-sevenths of the drawdown is the formation and the rest is turbulence through the well's own structure. At 50 per cent, half the drawdown, and therefore roughly half the pumping energy attributable to lift, is being spent moving water through a screen that is not doing its job.

Read the reduced form carefully, because it contains the whole design lesson. Efficiency depends on $Q$. It is not a fixed property of the well; it falls as the rate rises, always, because the well-loss term grows with the square of the rate while the aquifer term grows only linearly. A well quoted at "90 per cent efficient" without a rate attached is not telling you anything. Double the rate on the well in this shard's anchors and the efficiency drops from 85 per cent to 74 per cent, on the same well, the same day, with nothing changed but the throttle.

What causes a low $C$-driven efficiency, in rough order of how often it turns out to be the answer: too little screen open area, which shows up directly on the entrance-velocity page; incomplete development after drilling, leaving drilling mud and fines in the pack; chemical incrustation of the slots by carbonate or iron; and biological fouling, which in iron-bearing groundwater can halve a well's capacity in a few years. All four are maintenance problems with maintenance answers. None of them is the aquifer, which is exactly why the separation is worth making.

The economic argument is simple enough to make on the back of an envelope. Pumping cost scales with total lift, and well loss is pure added lift with nothing to show for it. A well at 60 per cent efficiency running continuously is paying for a great deal of head that produces no water at all, and rehabilitation frequently pays for itself in energy alone inside a couple of years, before you count the extra yield. The one thing efficiency will not tell you is whether the aquifer is any good. A superb well in a poor aquifer can run at 95 per cent efficiency and still barely produce.