Assessment of Methods and Conditions to Locate Boundaries: I. One or Two Straight Impervious Boundaries

Abstract

AbstractA straight impervious boundary within an ideal confined aquifer may be detected in the semilog drawdown graph of an observation well. This paper gives and assesses two known and one proposed equations that may help to determine the location and orientation of an impervious boundary. Then the two conditions for an adequate detection are established. First, if the distance of the pumping well to the boundary is called “a,” only those observation wells at a distance smaller than a/5 from the pumping well can detect the boundary. Second, data must be collected for approximately two time log cycles after the slope has changed in the drawdown graph. Similar equations, and time or distance conditions, are given for the case of two intersecting straight impervious boundaries. Frequently, the two conditions are not met. In this case, the drawdown graphs give incorrect estimates of aquifer parameters. In most field conditions, a combination of drawdown data, geophysical surveys, and exploratory boreholes is mandatory to locate one or two impervious boundaries. Available methods and a proposed one for recharge boundaries are assessed in a companion paper.