Tapping Groundwater Supplies
We can obtain groundwater at wells or springs. Wells are holes that people dig or drill to obtain water. Springs are natural outlets from which groundwater ﬂows. Wells and springs provide welcome sources of water but must be treated with care if they are to last.
|Pumping groundwater at a normal well affects the water table.|
In an ordinary well, the base of the well penetrates an aquifer below the water table (figure above a). Water from the pore space in the aquifer seeps into the well and ﬁlls it to the level of the water table. Drilling into an aquitard, or into rock that lies above the water table, will not supply water, and thus yields a dry well. Some ordinary wells are seasonal and function only during the rainy season, when the water table rises. During the dry season, the water table lies below the base of the well, so the well is dry.
To obtain water from an ordinary well, you either pull water up in a bucket or pump the water out. As long as the rate at which groundwater ﬁlls the well exceeds the rate at which water is removed, the level of the water table near the well remains about the same. However, if users pump water out of the well too fast, then the water table sinks down around the well, in a process called drawdown, so that the water table becomes a downward-pointing, cone-shaped surface called a cone of depression (figure above b, c). Drawdown by a deep well may cause shallower wells that have been drilled nearby to run dry.
|Artesian wells, where water rises from the aquifer without pumping.|
An artesian well, named for the province of Artois in France, penetrates conﬁned aquifers in which water is under enough pressure to rise on its own to a level above the surface of the aquifer. If this level lies below the ground surface, the well is a nonﬂowing artesian well. But if the level lies above the ground surface, the well is a ﬂowing artesian well, and water actively fountains out of the ground (figure above a). Artesian wells occur in special situations where a conﬁned aquifer lies beneath a sloping aquitard.
We can understand why artesian wells exist if we look ﬁrst at the conﬁguration of a city water supply (figure above b). Water companies pump water into a high tank that has a signiﬁcant hydraulic head relative to the surrounding areas. If the water were connected by a water main to a series of vertical pipes, pressure caused by the elevation of the water in the high tank would make the water rise in the pipes until it reached an imaginary surface, called a potentiometric surface, that lies above the ground. This pressure drives water through water mains to household water systems without requiring pumps. In an artesian system, water enters a tilted, conﬁned aquifer that intersects the ground in the hills of a high-elevation recharge area (figure above c). The conﬁned groundwater ﬂows down to the adjacent plains, which lie at a lower elevation. The potentiometric surface to which the water would rise, were it not conﬁned, lies above this aquifer. Pressure in the conﬁned aquifer pushes water up a well.
Many towns were founded next to springs, places where groundwater naturally ﬂows or seeps onto the Earth’s surface, for springs can provide fresh, clear water for drinking or irrigation, without the expense of drilling or digging. Some springs spill water onto dry land. Others bubble up through the bed of a stream or lake. Springs form under a variety of conditions:
|Geological settings in which springs form.|
- Where the ground surface intersects the water table in a discharge area (figure above a); such springs typically occur in valley floors, where they may add water to lakes or streams.
- Where flowing groundwater collides with a steep, impermeable barrier, and pressure pushes it up to the ground along the barrier (figure above b).
- Where a perched water table intersects the surface of a hill (figure above c).
- Where downward-percolating water runs into a relatively impermeable layer and migrates along the top surface of the layer to a hillslope (figure above d).
- Where a network of interconnected fractures channels groundwater to the surface of a hill (figure above e).
- Where the ground surface intersects a natural fracture (joint) that taps a confined aquifer in which the pressure is sufficient to drive the water to the surface; such an occurrence is an artesian spring.
Springs can provide water in regions that would otherwise be uninhabitable. For example, oases in deserts may develop around a spring. An oasis is a wet area, where plants can grow, in an otherwise bone-dry region.
Figures credited to Stephen Marshak.
Figures credited to Stephen Marshak.