MODULE-1 Aquifers and How They Work


Aquifers are subsurface areas that hold groundwater and from which significant quantities of groundwater can be extracted. There are three main types of aquifers: sand and gravel, fractured rock, and limestone or karst limestone formations.

Aquifers can be either unconfined or confined, based on the permeability of the soil where the groundwater system is contained. Groundwater movement through an aquifer is determined by the permeability of the soil as well as the slope of the water table, flowing from highest to lowest points beneath the surface. The flow of groundwater can be calculated using a mathematical Flow Velocity formula. Understanding the flow of groundwater through an aquifer helps to ensure that a safe yield, the amount of water that can "safely" be withdrawn from an aquifer, is maintained. Sustaining safe yields is increasingly a challenge for Washington's drinking water supply as development pressures continue to rise. In order to protect the quantity and quality of drinking water supplies, Washington has set up Wellhead Protection Area (WHPA) districts around its supply areas. These WHPAs are defined by wellhead management zones, which are established by the time it takes for groundwater to travel through the aquifer. A variety of WHPA delineation methods are available to Washington communities. Selecting a WHPA delineation method is based on the resource area constraints and available data. We will explore each of these aquifer protection elements in the sections that follow.

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