Geology - Groundwater - Quick Guides | Geology

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Study GuideGeology–Groundwater1. PermeabilityPermeabilitydescribes how easily fluids, such as water, oil, or gas, can move through a rock. Itdepends on how well the tiny spaces inside the rock are connected to each other.It’s important to know thatporosity and permeability are not the same thing. A rock can havemany pore spaces (high porosity) but still not allow fluids to flow easily if those spaces are poorlyconnected.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•Sandstoneusually has bothhigh porosity and high permeability. Its grains are relativelylarge, and the pore spaces connect well,allowing fluids to pass through easily.•Shalecan be quiteporous, but it haslow permeability. This is because its very fine grainscreate tiny pore spaces that do not connect well.•Igneous rocksgenerally havelow porosity and low permeability. However, if they areheavily cracked or fractured by tectonic forces, fluids can move through those fractures moreeasily.2. The Water TableGroundwater does not just appear underground—it gets there by moving slowly downward throughsoil and rock.Gravitypulls water down through the ground until it reaches a depth of about5

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Study Guidekilometers (3 miles). At that depth, the rocks no longer have pore spaces that can hold water. Abovethis level, the pore spaces gradually fill with groundwater.2.1 The Saturated ZoneThesaturated zone(also called thezone of saturation) is the part of the ground whereall the porespaces 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 ZoneAbove the saturated zone is theunsaturated zone, also known as thezone of aeration. In this zone:•Pore spaces containmostly airandsome water•The ground isnot fully saturated•This zone usually starts at the surface and extends downwardTheboundary between the unsaturated zone and the saturated zoneis called thewater table.2.3 Why AirCan Exist UndergroundYou might wonder how air can exist below the surface. This happens becausewater clings to soilparticlesdue to a force calledsurface tension. As a result, water does not completely fill the porespaces in the unsaturated zone.

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Study GuideSurface tension also causes water to moveupwardfrom the saturated zone into the unsaturatedzone. This upward movement is calledcapillary action.The Capillary Fringe•Thecapillary fringeis thelower part of the unsaturated zone•It pulls waterupward from the water table•It is usuallyonly a few feet thick2.4 Perched Water TablesAperched water tableis a small, localized accumulation of groundwater that sitsabove the mainwater tablewithin the unsaturated zone.This happens when:•Water is trapped on top of animpermeable layer, such as clay•A small, lens-shaped saturated area formsImportant things to know about perched water tables:•They usuallycannot supply enough waterfor household use•Wells drilled into them oftenrun dry•If they intersect a hillside or slope, they may appear assprings or seeps2.5 Movement of GroundwaterGroundwater movesvery slowlycompared to surface water. Its speed can range from:•As slow as 1 inch per day•As fast as 1,000 feet per dayGroundwater 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 Guide2.6 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 movement3.Streams and SpringsGroundwater and surface water are closely connected. Water moving underground canadd tostream flow, but stream water can alsoseep into the ground, reducing how much water remains inthe stream. Which one happens depends on theposition of the stream relative to the water table.3.1 Gaining StreamsAgaining streamis a stream thatreceives water from groundwater.•Groundwater flows from thesaturated zone into the stream channel•The stream channel is usuallyat or below the water table•Stream flow increases as groundwater adds waterWhen thewater table intersects the land surfaceover a wide, flat area,lakes, wetlands, ormarshesmay form.3.2 Losing StreamsAlosing streambehaves in the opposite way.•The stream channel liesabove the water table

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Study Guide•Water leaksdownward into the unsaturated zone•This reduces the amount of water flowing in the streamAs water moves downward, it eventually reaches the water table and may even cause thelocal watertable to rise.Indry climates, losing streams can shrink or evendisappear undergroundas water is lostdownstream.3.3 SpringsAspringforms whengroundwater naturally flows out onto the land surface.This happens when:•Thewater table intersects a sloping surface•Water escapes through cracks or openings in rockSprings can beseasonal. During wet seasons, increased rainfall raises the water table closer to thesurface, often creatingmore active springs.3.4 AquifersAnaquiferis aporous, permeable, and saturated layer of rock or sedimentthat allowsgroundwater to move easily.3.4.1 Types of aquifers•Best aquifers: coarse materials likesand and gravel•Unconfined aquifer:○No layer separating it from the unsaturated zone above•Confined aquifer:○Trapped beneath aless permeable layer○Water is underpressure

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