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Geology - Groundwater

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    Study Guide Geology – Groundwater 1. Permeability Permeability describes how easily fluids, such as water, oil, or gas, can move through a rock. It depends on how well the tiny spaces inside the rock are connected to each other. It’s important to know that porosity and permeability are not the same thing . A rock can have many pore spaces (high porosity) but still not allow fluids to flow easily if those spaces are poorly connected. 1.1 How permeability works • Permeability measures connectivity , not just empty space. • If pore spaces link together well, fluids can flow freely. • If pore spaces are small or isolated, fluid movement is slow or blocked. 1.2 Examples of different rocks • Sandstone usually has both high porosity and high permeability . Its grains are relatively large, and the pore spaces connect well, allowing fluids to pass through easily. • Shale can be quite porous , but it has low permeability . This is because its very fine grains create tiny pore spaces that do not connect well. • Igneous rocks generally have low porosity and low permeability . However, if they are heavily cracked or fractured by tectonic forces, fluids can move through those fractures more easily. 2. The Water Table Groundwater does not just appear underground — it gets there by moving slowly downward through soil and rock. Gravity pulls water down through the ground until it reaches a depth of about 5

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    Study Guide kilometers (3 miles) . At that depth, the rocks no longer have pore spaces that can hold water. Above this level, the pore spaces gradually fill with groundwater. 2.1 The Saturated Zone The saturated zone (also called the zone of saturation ) is the part of the ground where all the pore spaces in soil and rock are completely filled with water . As more water moves downward and accumulates, the top of this saturated zone rises upward. Eventually, it reaches a balance with the material above it that is not fully filled with water. 2.2 The Unsaturated Zone Above the saturated zone is the unsaturated zone , also known as the zone of aeration . In this zone: • Pore spaces contain mostly air and some water • The ground is not fully saturated • This zone usually starts at the surface and extends downward The boundary between the unsaturated zone and the saturated zone is called the water table . 2.3 Why Air Can Exist Underground You might wonder how air can exist below the surface. This happens because water clings to soil particles due to a force called surface tension . As a result, water does not completely fill the pore spaces in the unsaturated zone.

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    Study Guide Surface tension also causes water to move upward from the saturated zone into the unsaturated zone. This upward movement is called capillary action . The Capillary Fringe • The capillary fringe is the lower part of the unsaturated zone • It pulls water upward from the water table • It is usually only a few feet thick 2.4 Perched Water Tables A perched water table is a small, localized accumulation of groundwater that sits above the main water table within the unsaturated zone. This happens when: • Water is trapped on top of an impermeable layer , such as clay • A small, lens - shaped saturated area forms Important things to know about perched water tables: • They usually cannot supply enough water for household use • Wells drilled into them often run dry • If they intersect a hillside or slope, they may appear as springs or seeps 2.5 Movement of Groundwater Groundwater moves very slowly compared to surface water. Its speed can range from: • As slow as 1 inch per day • As fast as 1,000 feet per day Groundwater movement occurs in two main directions: • Downward , often for hundreds of feet • Sideways (laterally) , generally following the slope of the water table

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    Study Guide 2.6 What Controls Groundwater Flow? Several factors affect how fast groundwater moves: • Slope of the water table ○ Steeper land surface → steeper water table ○ Steeper water table → faster groundwater flow • Permeability of the rock or sediment ○ High permeability → faster water movement ○ Low permeability → slower water movement 3. Streams and Springs Groundwater and surface water are closely connected. Water moving underground can add to stream flow , but stream water can also seep into the ground , reducing how much water remains in the stream. Which one happens depends on the position of the stream relative to the water table. 3.1 Gaining Streams A gaining stream is a stream that receives water from groundwater . • Groundwater flows from the saturated zone into the stream channel • The stream channel is usually at or below the water table • Stream flow increases as groundwater adds water When the water table intersects the land surface over a wide, flat area, lakes, wetlands, or marshes may form. 3.2 Losing Streams A losing stream behaves in the opposite way. • The stream channel lies above the water table

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    Study Guide • Water leaks downward into the unsaturated zone • This reduces the amount of water flowing in the stream As water moves downward, it eventually reaches the water table and may even cause the local water table to rise . In dry climates , losing streams can shrink or even disappear underground as water is lost downstream. 3.3 Springs A spring forms when groundwater naturally flows out onto the land surface . This happens when: • The water table intersects a sloping surface • Water escapes through cracks or openings in rock Springs can be seasonal . During wet seasons, increased rainfall raises the water table closer to the surface, often creating more active springs . 3.4 Aquifers An aquifer is a porous, permeable, and saturated layer of rock or sediment that allows groundwater to move easily. 3.4.1 Types of aquifers • Best aquifers : coarse materials like sand and gravel • Unconfined aquifer : ○ No layer separating it from the unsaturated zone above • Confined aquifer : ○ Trapped beneath a less permeable layer ○ Water is under pressure

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