Development of a variably saturated groundwater flow model

A two dimensional variably saturated MODFLOW-SURFACT model was developed to help conceptualise how shallow groundwater moves across and through discontinuous clay layers within a more sandy system.  Site observations suggest that extensive vertical hydraulic gradients exist between shallow and deeper groundwater and there was uncertainty in how these gradients were maintained and whether the bulk of the groundwater flow moved laterally within the shallow groundwater system or downwards into the deeper system.

Figure 1 Plot of hydraulic head and water saturation

One conceptualisation was that discontinuous clay layers acted to support the vertical hydraulic gradient.  The model was built to investigate what clay layer spacing would be required to prevent the vertical movement of groundwater through the gaps between these layers.  Figure 1 shows a model with 2 discontinuous clay lense (blue rectangles at top of model) and the unsaturated pressures that develop when a recharge flux is applied to the upper surface.  At the bottom of the model (solid blue area), saturated groundwater conditions can be seen, with the capillary zone above this.  Within the unsaturated zone higher pressures develop within and above the clay lenses and lower pressures develop below the clay  lenses.  The effect of this is to draw water down through the holes between the lenses as is shown on Figure 2.

Figure 2 Plot of velocity vectors and water saturation around lens

The conclusion of the study was that, for even small holes between clay lenses, the bulk of the shallow groundwater will be drawn downwards rather than flow laterally.

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