Contamination of aquifers
Aquifers are critical geological formations that store and transmit groundwater, which is essential for drinking water and agricultural irrigation in many regions. However, the contamination of aquifers poses significant environmental and health challenges. Contaminants can persist in groundwater for long periods, complicating restoration efforts. There are two primary types of aquifers: unconfined aquifers, where water levels fluctuate with precipitation and well pumping, and confined aquifers, which are completely saturated and separated from surface influences by impermeable rock layers.
Groundwater flow within aquifers is governed by principles such as Darcy's law, which describes how water movement is affected by the aquifer's hydraulic conductivity and gradient. The variability in permeability among different rock types leads to diverse groundwater yields. Effective management of aquifer contamination often emphasizes prevention as the most cost-effective strategy, as remediation can be complex and expensive. Restoration techniques may involve natural treatment methods, engineered systems for containment, or isolation of the contamination source. Understanding these dynamics is vital for protecting aquifer integrity and ensuring a sustainable water supply for future generations.
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Subject Terms
Contamination of aquifers
DEFINITION: Water-bearing geological formations that can store and transmit significant amounts of groundwater to wells and springs
Aquifers are important because groundwater supplies a substantial amount of the water available in many localities. The contamination of aquifers is thus a matter of concern, and so a variety of aquifer restoration techniques have been developed.
All rocks found on or below the earth’s surface can be categorized as either aquifers or confining beds. An is a rock unit that is sufficiently permeable to allow the transportation of water in usable amounts to a well or spring. (In geologic usage, the term “rock” also includes unconsolidated such as sand, silt, and clay.) A confining bed is a rock unit that has such low hydraulic (or poor permeability) that it restricts the flow of into or out of nearby aquifers.
There are two major types of groundwater occurrence in aquifers. The first type includes those aquifers that are only partially filled with water. In those cases, the upper surface (or water table) of the saturated zone rises or declines in response to variations in precipitation, evaporation, and pumping from wells. The water in these formations is then classified as unconfined, and such aquifers are called unconfined or water-table aquifers. The second type occurs when water completely fills an aquifer that is located beneath a confining bed. In this case, the water is classified as confined, and the aquifers are called confined or artesian aquifers. In some fractured rock formations, such as those that occur in the west-central portions of New Jersey and eastern Pennsylvania, local geologic conditions result in semiconfined aquifers, which, as the name indicates, have hydrogeologic characteristics of both unconfined and confined aquifers.
Wells that are drilled into water-table aquifers are simply called water-table wells. The water level in these wells indicates the depth below the earth’s surface of the water table, which is the top of the saturated zone. Wells that are drilled into confined aquifers are called artesian wells. The water level in artesian wells is generally located at a height above the top of the but not necessarily above the land surface. Flowing artesian wells occur when the water level stands above the land surface. The water level in tightly cased wells in artesian aquifers is called the potentiometric surface of the aquifer.
Water flows very slowly in aquifers, from recharge areas in interstream zones at higher elevations along boundaries to areas along streams and adjacent floodplains at lower elevations. Aquifers thus function as pipelines filled with various types of earth material. Darcy’s law governing groundwater flow was developed in 1856 by Henry Darcy, a French engineer. In brief, Darcy’s law states that the amount of water moving through an aquifer per unit of time is dependent on the hydraulic conductivity (or permeability) of the aquifer, the cross-sectional area (which is at a right angle to the direction of flow), and the hydraulic gradient. The hydraulic conductivity depends on the size and interconnectedness of the pores and fractures in an aquifer. It ranges through an astonishing twelve orders of magnitude. Very few other physical parameters exhibit such a wide range of values. For example, the hydraulic conductivity ranges from an extremely low 107 to 108 meters per day in unfractured igneous rock such as diabase and basalt to as much as 103 to 104 meters per day in cavernous limestone and coarse gravel. Typical low-permeability earth materials include unfractured shale, clay, and glacial till. High-permeability earth materials include lava flows, coarse sand, and gravel.
In addition to this wide range of values, hydraulic conductivity varies widely in place and directionality within the same aquifer. Aquifers are isotropic if the hydraulic conductivity is about the same in all directions and anisotropic if the hydraulic conductivity is different in different directions. As a result of all of these factors, groundwater yield is extremely variable both within the same aquifer and from one aquifer to another when they are composed of different rocks.
Because groundwater flows slowly in comparison with surface water, any that gets into the groundwater could be around for a long time, perhaps hundreds or thousands of years. It is thus simpler and much more cost-effective to prevent groundwater contamination than it is to try to correct a problem that has been in existence for years.
Restoration of a contaminated aquifer may be accomplished, albeit at a price, through one or more of the following procedures: inground treatment or containment, above ground treatment, or removal or isolation of the source of contamination. The first approach involves natural treatment based on physical, chemical, or biological means, such as adding nutrients to existing subsurface to help them break down hazardous compounds into nonhazardous materials. The second approach uses engineered systems such as pumping wells or subsurface structures, which create hydraulic gradients that make the contaminated water stay in a specified location, facilitating removal for later treatment. Regardless of the restoration method selected, the source that is continuing to contaminate the aquifer must be removed, isolated, or treated.
Bibliography
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