Effect of natural gas exsolution on specific storage in a confined aquiferundergoing water level decline

The specific storage of a porous medium, a function of the compressibility of the aquifer material and the fluid within it, is essentially constant un der normal hydrologic conditions. Gases dissolved in ground water can incre ase the effective specific storage of a confined aquifer, however, during w ater level declines. This causes a reduction in pore pressure that lowers t he gas solubility and results in exsolution. The exsolved gas then displace s water from storage, and the specific storage increases because gas compre ssibility is typically much greater than that of water or aquifer material. This work describes the effective specific storage of a confined aquifer ex solving dissolved gas as a function of hydraulic head and the dimensionless Henry's law constant for the gas. This relation is applied in a transient simulation of ground water discharge from a confined aquifer system to a co llapsed salt mine in the Genesee Valley in western New York. Results indica te that exsolution of gas significantly increased the effective specific st orage in the aquifer system, thereby decreasing the water level drawdown.