INTRODUCTION

Below the soil water in many continental areas not covered by snow and ice are porous lithosphere formations more or less saturated with groundwater. Those that occur in well-defined forms are commonly referred to as aquifers. These groundwater formations differ tremendously from place to place in at least nine different respects that are discussed below. Together, groundwaters account for a substantial proportion of all freshwater on the globe. They are used in various degrees from either springs or wells by a wide variety of technologies, and their wise use and protection calls for a large range of types of public measure.

It is estimated by Russian hydrologists that approximately 29 percent of the world's freshwater is stored at any one time in aquifers (Shiklomanov, 2000). About 33 percent of that volume is on the Asian continent, 23 percent in Africa, 18 percent in North America, 13 percent in South America, 6 percent in Europe, and 5 percent in Australia. Roughly one-half of the stored groundwater is estimated to be at depths of less than 200 m, and the remainder at depths up to 2000 m.

Around the globe, the actual net withdrawal of groundwater is affected by the conjunctive use of groundwater and surface water, and by the particular purpose for which it is used. In areas where the withdrawal exceeds either natural or artificial recharge, aquifers have been drawn down significantly. In some areas they have been largely or entirely exhausted.