Quantitative analysis of saltwater-freshwater relationships in groundwater systems—A historical perspective

Seawater and fresh groundwater were made to flow through columns filled with samples from a ‘good aquifer’ that contained 5% or less clay minerals. Hydraulic conductivities decreased very sharply whenever seawater was flushed by fresh groundwater. The extent of the decrease depended mainly on the percentage of clays and reached maximum values of 10−1 to 10−3 of the original hydraulic conductivity. Subsequent flushing with seawater restored hydraulic conductivity only in a slight measure, if at all. The mechanism of this process was investigated by using CaCl2 and NaCl solutions with the ionic strengths of seawater and fresh groundwater. It appears that the decrease of hydraulic conductivity is caused by an incipient stage of gel formation. The small water-clay configurations that are formed during this stage behave as practically rigid particles and close the bottlenecks between adjacent pores. The major implications of these findings for groundwater hydrology are as follows: (1) In natural coastal aquifers the seawater-freshwater interface is likely to undergo shifts on a semigeologic time scale. If the aquifer contains even a small percentage of clays, these shifts may create an almost impermeable boundary in the zone of the interface. Hitherto existing theories on movements of the interface regard permeability as constant. (2) The attempt to repel a fresh water-seawater interface by the injection of fresh water may create a practically impermeable boundary within a relatively short time, and thus defeat its own purpose.

Experimental investigation on irreversible changes of hydraulic conductivity on the seawater-freshwater interface in coastal aquifers

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 (Qc) 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 (Kz/ Kr) is decreased, and it decreases as the degree of well penetration (t/b) is increased. As an example application, Qc was calculated using site-specific data from a test well in Pasco County, Florida. A value of Qc= 4.28 × 105 cubic feet per day was determined by assuming that Kz/Kr= 1.0. If Kz/Kr were subsequently determined to be less than or greater than 1.0, then the estimated value of Qc would be increased or decreased, respectively.

Salt-Water Upconing in an Aquifer Overlain by a Leaky Confining Bed

Sustainability of thermal energy production at the flooded mine workings of the former Caphouse Colliery, Yorkshire, United Kingdom

In this contribution, we describe the formation and evolution of the upconing process in a Mediterranean coastal aquifer. The study area has experienced severe salinization over the last 40 years because of intensive exploitation of groundwater. We used historical and current records of piezometric levels and chloride concentrations to trace the development of the salinization of the aquifer. We defined the 3D shape of the saline wedge from the spatial distribution of chloride concentrations and vertical well logs of electrical conductivity using monitoring network data. Upconing first appeared in the early 90s and has continued until the present day. In this study, we examined the intensity of the upconing process. Dry periods and the associated increases in pumping caused the advance of seawater intrusion. The sharp reduction in groundwater withdrawals over the last 10 years has caused the saline wedge to move backwards, although the ongoing pumping and the climate conditions mean that this retreat is quite slow.

/pdf/spatial-characterization-of-the-seawater-upconing-process-in-4qs2fo9rup.pdf

Spatial characterization of the seawater upconing process in a coastal Mediterranean aquifer (Plana de Castellón, Spain): evolution and controls

The upconing of saline water in response to pumping from an overlying layer of fresh water is investigated by numerical integration of the governing differential equation. The transition zone between the fresh and saline water is idealized as an abrupt interface. Full consideration of the nonlinear boundary conditions on the water table and interface surfaces is included for steady flow toward partially penetrating pumping wells in both isotropic and anisotropic aquifers. There exists an optimum well penetration into the fresh-water layer which permits maximum discharge without salt-water entrainment. The optimum penetration increases as the vertical permeability is reduced relative to the horizontal permeability. The maximum well discharge obtainable without salt-water entrainment is greater for aquifers with a reduced vertical permeability than for isotropic aquifers, a result that contrasts with previously published conclusions. Previous analyses which linearize the boundary condition on the interface overestimate the critical discharge.

R. L. Chandler

Papers

Upconing of the Salt-Water–Fresh-Water Interface Beneath a Pumping Well