POTENTIOMETRIC MAPPING OF DEEP AQUIFERS USING AN ENVIRONMENTAL HEAD

A scalar fluid potential is strictly applicable only in isothermal systems with constant fluid salinity. These conditions are not met in deep (>2 km) aquifers, yet potentiometric maps are routinely constructed, and subsurface flow patterns are sometimes interpreted from head variations. As a result of this misapplication, potentiometric maps of deep aquifers may indicate anomalous flow patterns, such as flow toward the center of a deep, freshwater basin. An index such as the DFR indicates where head maps may mislead, but DFR does not adjust the data to show the correct flow pattern.

Environmental head maps approximate deep aquifer fluid flow patterns better than freshwater head maps in basins where density changes are mainly caused by geothermal heating. Environmental head is calculated using water density averaged from the datum to the elevation of measurement instead of using a constant density for data at all depths. Where thermal expansion is the main cause of density variation, the environmental head assumes that the density change is a function of elevation, much like a negative compressibility. This is possible because thermal gradient is sufficiently low and basin stratigraphy is sufficiently stratified to prevent or minimize thermally driven, free convection in most sedimentary basins. Lateral thermal gradient variations cause environmental head ambiguity of approximately the same magnitude as uncertainty in many subsurface pressure data.

Where aquifers have significant salinity variations, environmental head can be calculated to include the effects of both temperature and salinity. Such environmental heads are valid only in hydrostatic basins, where calculated environmental heads at all locations will be similar. These maps are used only to determine if a basin is essentially hydrostatic. Density is not a simple function of elevation in hydrodynamic, variable-salinity aquifers, so the environmental head approximation cannot be applied.