SALT-WATER UPCONING IN AN AQUIFER OVERLAIN BY A LEAKY CONFINING BED

Salt-water upconing that occurs in an aquifer overlain by a leaky confining bed is described in terms of an analytical model that assumes the existence of a sharp interface between the fresh water and salt water and the occurrence of a critical rise in the interface, above which only an unstable cone can exist. Drawdown is calculated along the salt-water/fresh-water interface due to pumping from a well that partially penetrates the fresh-water zone, and then the Ghyben-Herzberg relation is used to calculate the steady-state rise in the interface, which is assumed to be small compared to the thickness of the aquifer. The interface rise and the critical pumping rate (Q(c)) are determined in terms of aquifer and confining bed properties and the degree of penetration of the pumping well into the fresh-water zone. The critical rise is assumed to occur when the interface rise is equal to 0.3 times the distance from the original interface location to the bottom of the well. Based on the analytical model, the nondimensional critical pumping rate increases as the ratio of vertical to horizontal hydraulic conductivity (K(z)/K(r)) is decreased, and it decreases as the degree of well penetration (l/b) is increased. As an example application, Q(c) was calculated using site-specific data from a test well in Pasco County, Florida. A value of Q(c) = 4.28 x 10(5) cubic feet per day was determined by assuming that K(z)/K(r) = 1.0. If K(z)/K(r) were subsequently determined to be less than or greater than 1.0, then the estimated value of Q(c) would be increased or decreased, respectively.