Potentiometric surface is the level to which water in a tightly encased well will rise by hydrostatic pressure or ‘head’. 'Potentiometric surface’, ‘water level altitude’, and ‘hydraulic head’ are often used interchangeably as different expressions of hydrostatic pressure. (9)
Direction of Groundwater Movement Through the Sparta Aquifer
The Sparta is exposed at the surface (outcrop area). It becomes confined as it dips toward the east. In a confined aquifer, water flows downgradient from potentiometric high areas to potentiometric lows. Heavy pumping causes cones of depression in the potentiometric surface and alters the direction of ground-water flow. In 1900 (before widespread pumping, sometimes referred to as ‘pre-development times'), groundwater flow was eastward, indicated by Sparta water level elevations that decreased from 300 feet above mean sea level in Claiborne Parish to 100 feet above mean sea level in Ouachita and Morehouse Parishes. Since development, pumping has induced cones of depression, reversing the flow in Ouachita and Morehouse Parishes westward. (9) (Figure 6)
Speed of Groundwater Movement Through the Sparta Aquifer
Louisiana Department of Environmental Quality researchers estimated 53.2 years as the typical time of travel for one mile in the Sparta, that is, 99.3 feet per year. (10) In calculating typical velocity, they used average values for hydraulic conductivity (the ease with which water flows through the aquifer), hydraulic gradient (head loss per distance of flow), and porosity (percentage of volume through which water can move).
The values are averages and do not take into account the facts that discontinuous units differ in flow rates and direction; or that localized areas differ in hydraulic conductivity, hydraulic gradient, and porosity; or that pumping results in increased ground water velocity in areas where (and to the extent that) the hydraulic gradient has been increased by that pumping. Figure 7 is a Louisiana Geological Survey graph depicting typical travel time in Louisiana aquifers.