Groundwater True/False Quiz

This is false. You might be confusing land subsidence with sinkholes, and this statement is truer for sinkholes (not for the "granite" part). Sinkholes can occur when water, sometimes a bit acidic in nature, dissolves underground rock, often limestone or dolomite. The land surface can collapse, often dramatically, into the void space underneath.

Land subsidence takes place on a larger scale and is usually a much slower process, but it still involves land that collapses. Actually, "sinks" is a more proper term. Land subsidence is a gradual settling or sudden sinking of the Earth's surface owing to subsurface movement of earth materials. The basic cause of land subsidence is a loss of support below ground. In other words, sometimes when water is taken out of the soil, the soil collapses, compacts, and drops. This depends on the type of soil and also on the type of rock below the surface.

More information: Land subsidence in the United States | Land subsidence from groundwater pumping | Land subsidence

Lucky for us this is true! If rivers dried up every time there was drought, we (and the fish) would be in trouble. Although we only see surface water on the Earth's surface, there is a strong connection between nature's surface-water and groundwater systems.

Groundwater contributes to streams in most geographic areas and climatic settings. The proportion of stream water that comes from groundwater inflow varies according to a region's geography, geology, and climate. Water scientists (hydrologists) can determine the amount of water that groundwater contributes to streams by analyzing streamflow hydrographs. This groundwater component of a stream's flow is called "base flow."

In a USGS study, streams in the United States were studied to see how much of the streamflow came from groundwater flow. The Forest River Basin in North Dakota is underlain by poorly permeable (water moves through it relatively slowly) silt and clay deposits, and only about 14 percent of its average-annual flow comes from groundwater. In contrast, the Sturgeon River Basin in Michigan is underlain by highly permeable (water moves through it relatively quickly) sand and gravel, and about 90 percent of its average-annual flow comes from groundwater. The median value for 54 streams was 55 percent from groundwater.

True, this is one way of using the same groundwater again and again. Sure, it costs money and takes time to do this, but if the groundwater is valuable enough (probably because enough surface water is scarce) it may make sense to artificially inject groundwater back into the same aquifers it came from for use on another day.

In places where the water table is close to the land surface and where water can move through the aquifer at a high rate, aquifers can be replenished artificially. For example, large volumes of groundwater used for air conditioning are returned to aquifers through recharge wells on Long Island, New York. In Orlando, Florida, water is spread across small basins, sinks into the ground, and recharges the shallow surficial aquifer to be used for irrigation of local citrus crop fields.

This is false, even if temperatures do increase the further down you go from the land surface. You do not have to get to the center of the Earth before things get too hot for comfort. In some deep mines, about 3,000 feet down, temperatures can be as hot as in a desert. Water coming from these depths is hot, too, but not near the boiling point. Boiling water would be found at much deeper depths.

Besides, it is a lot cheaper to just add some chlorine to water to kill bacteria rather than bear the cost of drilling a well a mile deep. Most aquifers are much closer to the land surface; many are just meters below the ground.

This is false. While it is true that artesian water, or even just "plain" well water, can sometimes be used directly for bottled water, this statement is false, because artesian water is not defined as being naturally filtered. A simple definition of artesian water is that it is water in the ground that is under pressure.

Groundwater occurring in aquifers between layers of poorly permeable rock, such as clay or shale, may be confined under pressure. If such a confined aquifer is tapped by a well, water will rise above the top of the aquifer and may even flow from the well onto the land surface, as in a spring. Water confined in this way is said to be under artesian pressure, and the aquifer is called an artesian aquifer. The word artesian comes from the town of Artois in France, the old Roman city of Artesium, where the best-known flowing artesian wells were drilled in the Middle Ages.

This is true. The two main characteristics of rocks that affect the presence and movement of groundwater are porosity (size and amount of void spaces) and permeability (the relative ease with which water can move through spaces in the rock). You probably know what a porous material is—it has lots of void spaces and openings, like a sponge. The rocks under our feet are not totally solid. They are full of cracks, fractures, and void spaces. For water to exist underground there must be void spaces to hold it.

However, the rock also must be permeable enough to allow water to move (due mainly to gravity). Rock that is highly permeable has connections between the fractures and openings. These pathways acts as the highways along which water travels underground, and in the case of the owner of a well, hopefully towards his/her well.

This one is false, but some of the concepts are true. The water level in wells can be affected by tides and if the well depth is at the same level as the area where saline and fresh water are somewhat mixed (brackish water), then the tides might have a small influence on the salinity of the brackish water. But, for water-supply wells, you won't find many that are tapping water at the point where saline water and freshwater mix; hopefully the well would be tapping the freshwater above the saline-water layer. In that case, freshwater would be always be accessed, despite the tides.

Wells are drilled along the coasts and they do yield great amounts of freshwater. For example, there are huge paper mills on the coast of Georgia, and they use a lot of fresh groundwater. Since aquifers exist in generally horizontal layers below the land surface, that means freshwater aquifers can extend underneath the oceans. Drilling a well near the coast can still tap a freshwater aquifer.

Saline aquifers also exist both underneath the oceans and under the land surface. If a well happens to be drilled into a saline or brackish aquifer, then the well can yield saline water (which neither you nor an orange tree would like to drink). Saltwater intrusion also can be a problem along the coasts. This can occur if a freshwater well is pumped too intensively for natural freshwater recharge from the surface to replenish it. In this case, salty water then can be drawn toward the well opening in the aquifer, thus yielding a mix of freshwater and saline water.

Find out more: Saltwater contamination at Brunswick, Georgia

This is false. Any water users will tend to use the water they can get to easier, cheaper, and with the least impact on the environment. In terms of water use, public supply refers to water used by organized groups of people—such as towns, cities, and communities. During 2010, the Nation withdrew about 26,300 million gallons per day (Mgal/d) of surface water for public-supply uses as compared to about 15,700 Mgal/d of groundwater. Chances are that the water in that water tower on top of the hill near your house is full of water from a river, lake, or reservoir rather than groundwater.

Now, it is true that if you dipped a jar into a creek and compared the water to water from a well, the groundwater would look a lot cleaner. The water probably would be a lot clearer (unless there is a lot of dissolved iron, which would turn the water brown) and you would not find floating leaf particles in groundwater. Actually, however, groundwater usually has more dissolved minerals and substances in it than surface water. Groundwater spends a lot of time moving through rocks underground—sometimes thousands of years. Water is also the top dog when it comes to being able to dissolve substances. Groundwater will often have more dissolved substances than surface water will.

Source: USA Economics and Statistics System (2001); USGS Estimated Use of Water in the United States in 2000

This is true! You might think that in comparison to a mighty river, a well is a small and insignificant thing, but that well can have a noticeable effect on a river's flow. There is more of an interaction between the water in lakes and rivers and groundwater than most people think. Some, and often a great deal, of the water flowing in rivers comes from seepage of groundwater into the streambed. Groundwater contributes water to streams in most physiographic and climatic settings. The proportion of stream water that comes from groundwater inflow varies according to a region's geography, geology, and climate.

Groundwater pumping can alter how water moves between an aquifer and a stream, lake, or wetland by either intercepting groundwater flow that discharges into the surface-water body under natural conditions or by increasing the rate of water movement from the surface-water body into an aquifer. A related effect of groundwater pumping is the lowering of groundwater levels below the depth that vegetation along the stream needs to survive. The overall effect is a loss of vegetation and wildlife habitat alongside the river.

For more information: Sustainability of Ground-Water Resources

This is false. Most wells are indeed "shallow," although shallow is a relative term. Wells that produce water for peoples' uses are generally from dozens to hundreds of feet deep—you will not find many production wells that go down 5 miles!

It can be done, though. Water can indeed be pumped from below 500 feet, even if multiple pumps at different levels have to be used. It is true that it will cost a lot more to drill and maintain a deep well compared to a shallow well, so there is more incentive to find aquifers closer to the land surface. But, it comes down to economics. If water is valuable (and scarce) enough, then it can make economic sense to spend the money to pump deep water to the surface.

This is false. Have you ever heard this myth about groundwater? "There are rivers of water flowing below ground." For the most part, it really is a myth. Of course, it is true that there are caverns, lava tubes, and large fissures in the ground and some of these spaces have water in them ... ever hear of "cave diving"? A river can indeed disappear into the ground.

These hydrogeologic formations, however, are not often used to supply well water for large uses, such as irrigation and water for households and drinking supplies. Why do all the work to find a cave full of water when there is plenty of water in the aquifers all over (under, actually) the Earth? The most productive wells tap highly porous and highly permeable aquifers that have a reliable source of recharge. Think of a swimming pool filled with a huge sponge (highly porous and permeable), with a garden hose constantly keeping the pool full. If you put a big straw into the sponge, you could drink water out of it indefinitely, as long as you didn't drink faster than the garden hose refilled the pool.

This is false, although a cone of depression is an actual hydrologic term. In a different sense, this is true, remembering how my young daughter complained when her ice cream fell off her cone onto the pavement once.

All pumped wells, to varying degrees, cause cones of depressions to form around the well casing at the water-table (the altitude, below ground, where below it the ground is saturated with water). If large cones of depressions form then the level of the water table can decline below the depth of the water intake for the well, and the well will pump less water and possibly go dry. If this happens, it will take time for the aquifer to recharge enough to raise the water level back to previous levels. That is why it is important to study the recharge characteristics of the aquifer that is tapped by a well—the well operator should not pump a well faster than it is recharged, as a cone of depression could form.