Drawdown data from an observation piezometer near a well used for a pumping test with constant pumping rate is plotted on a semilog plot vs. time (the time axis, on the horizontal, is on a log scale). After early time, the drawdown assumes a linear trend, rising smoothly on the semilog plot. But then as time goes on the rate of drawdown increase slows and slows and eventually goes to zero so that the drawdown is no longer changing with time. What is happening?