PrepTest 90+, Section 1, Question 2

A major problem facing industrial societies is their exponentially increasing production of toxic waste. Environmental regulations and expenses for landfills and incinerators have increased significantly in recent years. In an effort to save time and money, many industries have turned to alternative methods of hazardous-waste disposal, including increased use of deep-well injection. In this method, wells are drilled into porous and permeable rock strata that are already saturated with salt water. Liquid wastes are then injected into the rock strata. Most of these wells are drilled to a depth of at least 300 meters—the minimum depth that generally puts the injected waste at a safe distance below any aquifer, in this case a rock stratum containing drinkable water. Such wells are rarely deeper than 1,800 meters, because below this depth it is more cost-effective to consider an alternative method of disposal. Deep-well injection, which has been used to some extent since the 1930s, has become a matter of controversy as growing numbers of communities come to rely on underground sources of drinking water. The controversy arises because there are three serious problems with this method of waste disposal.

Under the best conditions, wastes are injected into rock strata saturated with salt water and separated by impermeable rock strata from aquifers containing drinkable water. However, injection wells may leak, allowing significant amounts of noxious chemicals to mix with supplies of drinking water. In other cases, mistakes by personnel working on the wells may lead to the pollution of aquifers. In one such case, workers installing a 500-meter-deep well left a gap along approximately 30 meters of its steel casing. This allowed waste to escape at a depth of only 200 meters, threatening a regional aquifer supplying water to 100,000 people. Because such accidents take place deep within the earth, people may be exposed to dangerous levels of waste materials for long periods of time before the problem is even discovered.

The third problem associated with deep-well injection arises from the fact that it is nearly impossible to predict how the injected wastes will be acted on by the geological features of the injection area. Unlike surface water, the water in underground rock strata does not flow entirely under the influence of gravity. Moving along subterranean pressure gradients, it can flow in any direction and, in some cases, can be transported thousands of meters per year through geologic faults, porous rock, or other geologic formations.

The significant uncertainty about where injected wastes will flow, along with the possibilities of mechanical failure and human error, makes deep-well injection a risky means of managing hazardous wastes. Unfortunately, as societies produce more toxic waste, industry will rely increasingly upon this relatively cheap, efficient means of disposal.

A major problem facing industrial societies is their exponentially increasing production of toxic waste. Environmental regulations and expenses for landfills and incinerators have increased significantly in recent years. In an effort to save time and money, many industries have turned to alternative methods of hazardous-waste disposal, including increased use of deep-well injection. In this method, wells are drilled into porous and permeable rock strata that are already saturated with salt water. Liquid wastes are then injected into the rock strata. Most of these wells are drilled to a depth of at least 300 meters—the minimum depth that generally puts the injected waste at a safe distance below any aquifer, in this case a rock stratum containing drinkable water. Such wells are rarely deeper than 1,800 meters, because below this depth it is more cost-effective to consider an alternative method of disposal. Deep-well injection, which has been used to some extent since the 1930s, has become a matter of controversy as growing numbers of communities come to rely on underground sources of drinking water. The controversy arises because there are three serious problems with this method of waste disposal.

Under the best conditions, wastes are injected into rock strata saturated with salt water and separated by impermeable rock strata from aquifers containing drinkable water. However, injection wells may leak, allowing significant amounts of noxious chemicals to mix with supplies of drinking water. In other cases, mistakes by personnel working on the wells may lead to the pollution of aquifers. In one such case, workers installing a 500-meter-deep well left a gap along approximately 30 meters of its steel casing. This allowed waste to escape at a depth of only 200 meters, threatening a regional aquifer supplying water to 100,000 people. Because such accidents take place deep within the earth, people may be exposed to dangerous levels of waste materials for long periods of time before the problem is even discovered.

The third problem associated with deep-well injection arises from the fact that it is nearly impossible to predict how the injected wastes will be acted on by the geological features of the injection area. Unlike surface water, the water in underground rock strata does not flow entirely under the influence of gravity. Moving along subterranean pressure gradients, it can flow in any direction and, in some cases, can be transported thousands of meters per year through geologic faults, porous rock, or other geologic formations.

The significant uncertainty about where injected wastes will flow, along with the possibilities of mechanical failure and human error, makes deep-well injection a risky means of managing hazardous wastes. Unfortunately, as societies produce more toxic waste, industry will rely increasingly upon this relatively cheap, efficient means of disposal.