MAC Educational Materials/ 3: Water Wells
Water Wells and Well Interference
Water wells allow groundwater to be pumped from aquifers. People pump water from the Mahomet aquifer for various purposes-- domestic water supplies, municipal water supplies, and irrigation. Water wells (Figure 1) are generally installed using a drilling rig, which allows a driller to bore a hole to the desired depth, install a well screen and casing, and backfill the boring. A well screen permits the water to enter the well while preventing the aquifer material from entering the well. The casing provides access for the well pump, which removes water from the well and transports it to the surface. The borehole is larger than the well screen and casing, so this remaining space must be filled. Near the well screen, the borehole is filled with sand and gravel, while bentonite or cement grout is used to backfill opposite the casing. Bentonite or cement grout is used to prevent water flow along the borehole. Well screens are commonly constructed of PVC plastic or stainless steel. Well casing is commonly PVC plastic or steel. The Illinois Department of Public Health (IDPH) regulates water well construction in Illinois. The Illinois Water Well Construction Code is available at http://www.idph.state.il.us/rulesregs/lawsrules.htm. A permit from IDPH or the county health department is needed to drill a water well in Illinois.
Figure 1. Illinois Department of Public Health guidelines for a well completed in a glacial drift aquifer such as the Mahomet aquifer (modified from the Illinois Water Well Construction Code).
When water is pumped from a well in an unconfined aquifer, the water level in the well declines and causes the water table near the well to drop. Drawdown is the decrease in the water level measured in the well (Figure 2). In three dimensions, the drawdown looks like an upside-down cone and is called the cone of depression. Pumping water from a well completed in a confined aquifer causes a drop in the groundwater pressure or a drop in the potentiometric surface. In three dimensions, this pressure reduction also has a cone shape and is known as the cone of depression. Compared to an unconfined aquifer, the drawdown in a confined aquifer is less intense but spreads out over a wider area. In other words, the cone would be shorter and have a wider base.
Figure 2. Drawdown in a well completed in an unconfined aquifer
When two wells are close, the cones of depression may intersect as shown in Figure 3. This intersection is known as well interference, which increases the drawdown in both wells. Well interference is possible in confined and unconfined aquifers. For some wells, this additional drawdown may not affect well yield, but will lead to higher pumping costs because the water must be lifted a higher distance. For other wells, the additional drawdown may lower the water level in the well below the pump intake causing the well to go “dry”. A simple remedy for this problem would be to lower the pump in the affected well.
Figure 3. Cones of depression develop around 2 wells completed in a confined aquifer. Well interference is the intersection of the two cones of depression, which increases the drawdown and changes the shape and extent of the cones of depression.