Geology is considered a relatively esoteric science. It isn't. It is the science of the earth and earth processes, many of which affect homes.

The homeowner needs to understand these processes in order to prevent or remedy such problems as wet basements and structural damage, not to mention financial erosion.

Montgomery County residents, for example, may need to know why a wet basement, a dry well and pollution of their water supply may go hand in hand. Prince George's County residents may be faced with extremely heavy, swelling clay, which may damage their homes. Understanding how something works vastly improves one's chances of being able to solve the problem.

The Washington metropolitan area is built over two geologic provinces: the Piedmont (Northwest Washington, Montgomery and Fairfax counties) and Coastal Plain (Southeast Washington, Prince George's County and Alexandria). Arlington is in both provinces.

The Piedmont is characterized by relatively thin soil and shallow bedrock. The Coastal Plain is composed of sands, gravels and clays that thicken to the east; bedrock at Ocean City, Md., is a mile deep. The geologic processes that take place in the two provinces are essentially the same, but there are differences that pose specific problems for residents of these areas.

Many believe that water problems are caused by "underground springs" or "underground rivers" and that "nobody knows how ground water works." In actuality, there are no "underground" springs or rivers except in cave-forming areas.

The science of the behavior of water, known as hydrology, is based on a simple principle: Water flows downhill -- whether it is on the ground or under it.

An aquarium or other glass-sided container may be used to make a model of a simple ground-water system. Pour in several inches of gravel, then a couple of inches of water. The container is impermeable to water and so serves as a "confining layer" that excludes or inhibits the movement of water. The gravel serves as an "aquifer," a "water-bearing" material that is permeable to water, allowing it to move more or less freely. A hole punched into the gravel makes a well.

The top of the water layer is known as the "water table." Add more water and it will rise. If water is removed from the well or evaporates, the water table will drop and the well eventually will go dry.

Springs, rivers and lakes are formed where the water table intersects the ground surface. Springs are of two kinds: "contact" and "artesian." Tilting the aquarium until the water table contacts the gravel surface produces a momentary "contact spring," which, because it has no continuing supply or destination, will quickly settle down into a lake. "Artesian" water rises above the ground surface and spurts out under pressure, rather than just seeping out.

Springs are not magical, nor is spring water necessarily pure. In limestone-karst (cave-forming) areas such as Florida, true underground rivers are mapped by dumping dye and bran, which eventually emerge unaltered at springs. Given the opportunity, cows and Volkswagens could pass through these springs. There is no filtering. If large, lumpy things are not filtered out, little things such as bacteria and chemicals won't be either. In the Washington area's terrain, more filtering occurs, but a spring that is hydrologically below a dump may be polluted, while a well that is hydrologically above the dump may be perfectly safe.

A corollary to the law that Water Flows Downhill is that under certain circumstances, Water Flows Up. This occurs in artesian wells because of pressure; it also may occur through capillarity, a property that manufacturers of thermal underwear call "wicking." To demonstrate, insert a very thin straw, such as a coffee stirrer, into the gravel or into a bottle of ink.

In coarse-grained materials, capillary movement of water is rapid, but very small, no more than a few inches above the water table. In fine-grained materials, such as clay, water may rise as high as 40 feet. This causes special problems. If the water table is at 30 feet (apparently well below basements), but overlain by clay with 30 or 40 feet of potential capillary rise, the ground will be continuously saturated. The problem can be remedied only by filling with material of low capillarity, such as sand or gravel.

In the Piedmont, soils are relatively thin, so water from this source is easily depleted. Solid rock gives no water; the only reliable aquifers in this area are fracture zones, which are broken, intensely weathered areas in the bedrock. A basement in this area may be wet from surface water or because it is in contact with a water-bearing fracture, while a well drilled into a nonfractured area will be dry.

Fractures are rarely considered in well drilling, but when they are, the contrast is startling. The amount of water from wells in the Clarksburg area of Montgomery County averages about two to three gallons per minute. But a test well drilled in the same area several years ago at the intersection of several fracture zones yielded 110 gallons per minute.

This hydrologic system causes some serious problems with water supply and waste disposal. Residents who need wells usually also need septic fields. Because of their relatively high permeability, fracture zones are the best well sites, but also the areas that will "perk," that is, pass the percolation test requirement for septic fields. Failure to recognize the existence or properties of fracture aquifers commonly results in the unwitting combination of water supply and wastes. The result is a contaminated well, condemned water supply and trucked-in water.

The equivalent error in the Coastal Plain is the use of gravel pits for dumps and waste sites. Gravels and sands in the Washington area are the remains of old river beds. These materials are extremely permeable to water and so form aquifers. Wastes dumped or leached into gravels travel easily, with little filtering, into areas where the aquifer is tapped for a water supply.

In theory, there is no difference between a well and a basement. Both are holes in the ground; one is just wider than the other, which may be deeper. Ideally, a hole below the water table will be wet and a hole above the water table will be dry. Ideally, holes intended for basements and for wells are sited accordingly. But this does not always happen in practice.