 Water is the foundation of our society. A key component of our daily lives, it supports our agriculture and municipalities, feeds our industrial sectors, is the basis for recreational activities, and nurtures our wildlife and water ecosystems. Water is the basis for our very existence, and as our population grows, so do our demands on this resource. Texas is blessed with an abundant but limited supply of groundwater. Aquifers supply approximately 60% of our water, or 9.9 million acre-feet annually. The majority of groundwater is presently used for agricultural irrigation. However, the greatest growth in demand is for cities and industries. Texas taps more than 30 different aquifers for groundwater. However, 97% comes from only 9 major aquifers. These aquifers are similar in their ability to store water, but they differ in the quantity of water available and how easily water can be tapped and managed. How we manage an aquifer is an intricate decision. Management strategies can range from sustainability to water mining or overdrafting. Because each aquifer is unique in its composition and in the demands we place on it, each requires specific technical knowledge and a community willing to support a management program. Just as one shoe does not fit every foot, one management goal will not fit every aquifer. Texas has a diverse geological history. Beneath the surface, Texas largely consists of layers of limestone, gravel, sand, clay, and shale. Fortunately, the limestone, gravel, and sand materials hold water and serve as aquifers where water enters, is stored, moves underground, and is retrieved through a well. The subsurface clay and shale layers serve as confining layers, which keep the water in the permeable layers separated. Aquifers are reservoirs of water below the land surface. They are not underground pools, lakes, or streams, but rather layers of limestone, gravel, and sand saturated with water. Water is stored in the small spaces between soil particles. Water naturally enters an aquifer through a recharge zone, which is usually the land over and adjoining the aquifer. Rainfalling on the recharge zone will percolate into the aquifer. Some water may also seep into the aquifer from a stream or river flowing over a recharge zone. We predict an aquifer recharge rate as a portion of the annual rainfall for the area. Because eastern Texas has more rainfall than the western part of the state, recharge rates vary greatly. In general, less than 5% of annual rainfall makes it into an aquifer. And in the dry western part of the state, recharge rates may be as low as 1%. Water is primarily removed from an aquifer through springs and wells. Springs are naturally occurring seeps connected to the aquifer, which discharge water. Springs may be large, such as Komal and San Marcos springs, or small, such as seeps along a stream or river. Spring flow may increase or decrease based on the water pressure in the aquifer. Wells allow us to capture groundwater for our use. Water is removed through the well column, which functions much like a straw, removing water from a glass. Storage or water levels of an aquifer increase or decrease based on the rate water is being added and removed. In periods of greater rainfall, the water level may rise in the aquifer. In periods of drought, the water level may drop. The long-term ability of an aquifer to supply water is related to the size of the recharge zone and rainfall in the region, as well as the rate at which water is removed over time. Water moves through and is stored in an aquifer in the openings of the geologic material, such as limestone, gravel, or sand. How water moves is controlled by the size of the openings and how the openings are interconnected. The speed water moves is somewhat controlled by forces pushing the water from one location to another. For example, small clay particles have small openings that restrict water movement. Larger materials such as gravel and sand have open spaces between the material so water can move easily and quickly. Limestone aquifers may be fractured or partially dissolved to create caves, making it very easy for water to move from one place to another. However, aquifers may have openings filled with water that are not connected together. These aquifers may have large quantities of water, but you will not be able to remove the water because it cannot easily flow to a well. Even aquifers with good interconnected openings need some force to move the water. We rely on gravity or a difference in pressure to provide this force. Gravity functions when there is an elevation difference in the aquifer's water. Pressure difference exists when there is water movement toward a well or spring. Aquifers are typically characterized as confined or unconfined. An unconfined aquifer is a layer of water bearing material with a confining layer on the bottom, but has a permeable soil layer on top extending to the ground surface. Rain can percolate through the permeable material and recharge the aquifer. The top of an unconfined aquifer is referred to as the water table. The wells drilled into an unconfined aquifer will have a water level at the same elevation as the water table. Water movement toward wells in unconfined aquifers is caused by an elevation difference. In contrast, a confined aquifer is a layer of water-filled material that has a confining layer of clay on the bottom and the top. These aquifers have an unconfined section where the permeable materials extend up to the ground surface. This section functions as the recharge zone. Water in a confined aquifer is stored under pressure and called artesian water. Wells drilled into a confined aquifer will have water rise into the well column equal to the pressure in the aquifer. If the water is under sufficient pressure, water may rise to the ground surface and flow out of the well. These wells are generally referred to as free-flowing artesian wells. Water movement toward wells in confined aquifers is caused by a pressure difference. The Ogallala aquifer, for example, is a huge, unconfined aquifer underlying most of the Texas High Plains. It receives limited natural recharge because of the limited rain in the area. More water is pumped from the Ogallala than from all other Texas aquifers combined. It is slowly being depleted. In contrast, the Edwards Aquifer is a confined limestone aquifer that is easily rechargeable. It can be quickly replenished by rainfall. However, if much water is pumped from it, especially during drought, the water level in the aquifer can drop quickly. The Edwards Aquifer also supports major spring flows into the San Marcos and Comal rivers. Well location and spacing can affect the ability to retrieve water. When a pump starts removing water, the surrounding water flows toward the well. This flow is developed by decreasing either the water level or pressure around the well. The water table in unconfined aquifers will decline as water flows to a well. This is seen as a dewatering of the aquifer material around the well. Confined aquifers will have a decline of pressure as water flows to the well. This is seen as a decrease in water levels in surrounding wells. The area where the water table or pressure declines around a well is called a cone of depression. You will not observe large water table or pressure decreases around wells with small pumps. However, large pumps can develop large cones of depression. If two wells are located close together, their cones of depression will overlap and both wells will compete with each other for the water. The combined effect of the two wells causes an even greater reduction in the water table or pressure. The water table and pressure will recover when pumping is stopped. However, if the overall water removal rate from the aquifer is greater than the recharge rate, the aquifer will be depleted. This depletion is an overall lowering of the water table or pressure. Subsidence is a side effect of depletion in aquifers with clays such as the Gulf Coast aquifer. The water prevents the clay from compressing. However, when the water is removed, the clay can compress and cause the land to sink or subside. Wells can be a safe and effective method of tapping into groundwater. However, improper well construction can allow contaminants to enter the aquifer. Different geological layers can contain naturally occurring contaminants and other pollutants can come from human activity. These contaminants can dissolve into the groundwater. Assuming this groundwater layer is confined by clay layers, this low-quality groundwater will remain separate from other aquifers. But improperly constructed active or abandoned wells can allow this low-quality groundwater to mix with good groundwater and contaminate the aquifer. Proper drilling and abandoned well closure programs are key to protecting the quality of the groundwater resource. Aquifers are a valuable source of water. An understanding of aquifers can help us to make informed decisions on groundwater management. We can choose a management goal to protect or deplete the aquifer. Based on the unique characteristics of the aquifer and the appropriate management goal, we can implement management strategies for the resource, guaranteeing a strong foundation of water for our future.