Groundwater

Earth’s land surface is less than monolithically solid. The joints, faults, and weathered rocks described elsewhere in the book allow rainwater to penetrate the crust, sometimes to considerable depths and become groundwater. Stream channels that intersect subsurface layers containing groundwater can add water to or take water from the groundwater supply.

Far from being an isolated source of stored water, the shallower groundwater supplies are intimately related to the rest of the hydrologic cycle in that they are recharged by precipitation. It should be noted that most soil moisture is distinct from groundwater (although there are times and places at which the top of the groundwater supply can be within the soil and so be one in the same). In that groundwater is out of sight and moves much slower than streams, its nature is not appreciated by most people. Yet, its importance looms large in that there is twoand-a-half times more groundwater than in all the streams and lakes on the planet.

Not all groundwater is potable. The contact of underground water with rock leads to the solution of salts into the water. In some cases, the saltiness is much higher than that found in the ocean. In other places with moderately “hard” water containing positively charged ions (cations) of calcium and magnesium, the salts can be dropped out of solution by using water softener, thus making it usable for human purposes.

Groundwater can be found underneath most of Earth’s surfaces but not in consistently developable amounts by either depth or geographical region. Most groundwater is within a kilometer of the surface, but some water has been found as deep as 10 km.With depth, however, the pore spaces in rocks and sediments become considerably smaller because of the pressure of overlying materials, and groundwater is essentially trapped in place. The deeper groundwater is connate or “fossil water” that was trapped as the rock layer was laid down. This deep groundwater is usually brought to the surface as an unavoidable consequence of drilling for oil and natural gas. It is almost invariably salty because of the immense amounts of time the water has had to dissolve surrounding materials. Sometimes these brines can be economically tapped and important materials such as iodine extracted.

Groundwater is sometimes conceived as a gigantic underground lake or stream, but this is not close to the truth. Near Earth’s surface, there are four zones differentiated with respect to groundwater: the zone of aeration, the zone of saturation, the zone of confined water, and the waterless zone. The depths, amount of water, and flow characteristics have immense variations according to the type and structure of the underground materials.