B Ground water flow direction

A desirable site for a waste storage pond or treatment lagoon is in an area where ground water is not flowing from the vicinity of the site toward a well, spring, or important underground water supply.

The direction of flow in a water table aquifer generally can be ascertained from the topography. In most cases the slope of the land indicates the ground water flow direction. In most humid regions the shape of the water table is a subdued reflection of surface topography. Unconfined ground water moves primarily from topographically higher recharge areas down gradient to withdrawal areas at lower elevations. Lower areas serve as discharge points where ground water rises and merges with perennial streams and ponds, or flows as springs. However, radial flow paths and unusual subsurface geology can too often invalidate this assumption. Consider the case where secondary porosity governs the flow. A common example is rock in upland areas where the direction of ground water flow is strongly controlled by the trend of prominent joint sets or fractures. Fracture patterns in the rock may not be parallel to the slope of the ground surface. Thus, assuming ground water flow is parallel to the ground slope can be significantly misleading in terrain where flow is controlled by bedrock fractures.

Appendix 7A gives a method of calculating ground water flow direction in a water table aquifer.

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