Figure 2. Saturated and unsaturated zones
WATER WORKS/from page 58 or percolates downward from the surface into the underlying geologic formations is the principal source of groundwater. Groundwater is defined as naturally occurring water located in and traveling through the various cracks, fissures, and pore spaces contained within the underlying soil and geologic structures. These regions of geologic formations that store and transmit groundwater are individually referred to as waterbearing formations, water-bearing zones, water stratas or, to be more technically correct, aquifers. In addition to aquifers, there are additional like-sounding terms but with wholly different definitions. An aquitard is a confined aquifer with leaking properties. Unlike a typical confined aquifer, an aquitard is fairly limited in production capability and can yield only small amounts of water, usually not enough to satisfy even a single residence. An aquifuge is a confining layer that is essentially impermeable and unable to yield any reasonable volume of water, a “dry” formation. Most clay and claystone (shale) formations are virtually impermeable and unable to yield water for all practical purposes; these types of formations are called aquicludes. Contrary to popular misconceptions, groundwater doesn’t occur or reside in underground lakes, rivers, channels, or veins but within the pore spaces between grains of certain geologic materials such as sand and gravel or within the fractures of others, such as basalt rock. Groundwater typically moves very slowly through an aquifer, usually at velocities as slow as inches or up to a few feet per day. Although the actual definition and location of groundwater can vary between states and jurisdictions, for our purposes we will assume groundwater is always that water found at a depth exceeding 20 feet or more below the immediate ground surface. Any water found above that depth is technically defined as surface water. For extraction purposes, groundwater is typically allowed to flow into and be retrieved through the use of a water well. The strict definition of a water well is an artificial opening in the earth, generally circular in shape, intended and used for locating and extracting groundwater from a viable aquifer to the ground surface. Many states define groundwater as that water available at a depth of or exceeding 18 feet below ground surface, but I pre60/ November 2013 Water Well Journal
Figure 3. Types of aquifers
fer to use 20 feet since that figure presumes even the shallowest water well and adjacent sanitary seal will extend to at least 20 feet. In all cases, the minimum depth of the sanitary seal will be assumed to be no less than 18 feet. A sanitary seal—also referred to as a cement seal, bentonite seal, or surface seal—is also created from an artificial opening in the ground, with a diameter generally no less than 4 inches greater than the well casing diameter. This seal prevents surface water and contaminants from migrating into the water well. Hence, a typical 6-inch-diameter water well would be provided with no less than a 10-inch-diameter surface seal. Once again, certain jurisdictions may dispute this definition. But for the sake of discussion, we must have an agreed upon and uniform value. Groundwater typically enters and flows through the ground through various types of rocks. Groundwater begins often in the higher elevations of mountain ranges from snowpack melt or runoff which then travels overland and downhill. The force of gravity causes the slow percolation of the water through the upper regions of geologic formations. This is referred to as overburden or unsaturated zones on its path to join with other water within an aquifer. This action of groundwater replenishment is referred to as recharge. As water enters the upper soil strata and flows downward from the force of gravity, it will eventually run into a region overlying the aquifer where an air-water intermix or vapor resides. This zone is referred to as a vadose zone, part of the unsaturated zone. Immediately under the vadose zone is situated the saturated zone, where all of the pore spaces are filled with water (Figure 2). Depending on the resistance to water flow afforded from the soil and rock formations, groundwater continues to flow downward until it encounters one of the two types of aquifers, confined or unconfined (Figure 3). A confined aquifer, often referred to as an artesian aquifer, is one where the water is sealed within the water-bearing zone and situated under an impermeable layer or layers of soil or formations such as clay, shale, or sandstone which separates and provides a relatively impermeable barrier for the confined layer from the effects of atmospheric pressure. In addition, the term flowing artesian well is often applied to all confined wells too. But this is often a misnomer as an artesian head is one caused from the higher head generated in a confined layer, but this higher head does not necessarily translate to a well that flows at the surface. Depending on the waterwelljournal.com