Chapter 5 Sources and Sinks

“Among the many problems in heat-conduction analogous to those in ground-water hydraulics are those concerning sources and sinks, sources being analogous to recharging wells and sinks to ordinary discharging wells.” --- C.V. Theis, 1935

Water may enter or leave a model in one of two ways – through the boundaries, as determined by the boundary conditions, or through sources and sinks within the interior of the grid.

5.1 Injection and Pumping Wells Finite Difference Models In finite difference models, the node represents the finite difference cell. Hence, a point source or sink of water is injected or extracted over the volume of aquifer represented by the cell that contains the point source or sink.

In a two-dimensional areal model, however the node represents the thickness of the aquifer. Hence, it is implicitly assumed that the well penetrates the full thickness of the aquifer. In three-dimensional models it is possible to simulate the effects of partial penetration because pumping or injection nodes can be placed in any layer. The user specifies the pumping rate and location of the well screen.

Finite Element Models In a finite element model, the pumping or injection rate is assigned to the node itself if the well is located at the node. If the source or sink is not located at a node, the flow is divided among the nodes of the element that contains the well.

5.2 Flux across the Water Table No one has yet devised a universally applicable method for estimating groundwater recharge. For lack of a way to quantify the spatial distribution of recharge and discharge, modelers have traditionally assumed a spatially uniform recharge rate across the water table equal to some percentage of average annual precipitation. The recharge rate is often adjusted during calibration.

5.3 Leakage Leakage refers to the movement of water through a layer of material that has a vertical hydraulic conductivity lower than that of the aquifer. Leakage may enter or leave the aquifer depending on the relative difference in heads between the aquifers and the source reservoir on the other side of the leaky layer. The source reservoir may be an unconfined aquifer, a river, or a lake.

Drains can be broadly interpreted to represent dewatering from the operation of underground mines, and quarries, agricultural drainage tiles, and drains around landfills. Springs and seeps also can be treated as drains. Input of leakage terms in MODFLOW is handled through the River Package, the Drain Package, or the General Head Boundary Package.