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EARTHSC 1G03 (139)

15 - Ground Water

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Earth Sciences
Maureen Padden

15 – GROUND WATER Ground water: fresh water beneath the ground surface, filling pore space b/c grains in bodies of sediment and clastic sedimentary rock, and filling cracks/crevices; roughly 1% of water on Earth ZONES Divided into 3 “zones” in the ground: unsaturated, water table, saturated zone 1. Unsaturated Zone: air and water fill pores collectively; surface tension causes water to be held above water table 2. Water table: The line that divides the unsaturated and saturated zones – this is also the upper limit of the saturated zone; capillary forces create a transition zone above the WT 3. Saturated Zone: water fills the pores of rocks/sediment ACCUMALATION: Source of groundwater is snow and rain which percolates down to the water table (~15%). Position of the water table depends on seasonal factors - only water in the saturated zone is “ground water.” Capillary fringe: transition zone where water is located higher than the water table due to capillary effects. In rocks with smaller pore spaces (e.g. clay) we see a higher water level than rocks with larger pore spaces (i.e. gravel). AQUIFERS Porosity: percentage of rock or sediment that consists of voids or opening for water to be held  High %: gravel, sand, silt, clay  Low %: glacial till, crystalline rock, shale Permeability: capacity of a rock to transmit a fluid such as water or petroleum through pores or fractures  Excellent: gravel, sand,  Poor: clay, glacial till, crystalline rock, shale, conglomerate Aquifers: body of saturated rock that allows water to move through easily – conglomerate, well- sorted little cement sand stone, well-jointed lime stone. Further subdivide into:  Unconfined aquifer: access to the surface and rainfall can percolate freely into the aquifer o Quick recharge of water  Confined aquifer: bound by low permeability beds above and below, and can only get water from surface exposure. o Susceptible to pollution; slow recharge of water Aquitard: rocks that do not allow water to transfer through it easily – shale, crystalline rocks (gabbro, particular lime stone) RECHARGING AND DISCHARGING Recharge is the water goes into the saturated zone – typically upland; discharge is where the water comes out of the surface – lakes, ponds, rivers. Groundwater generally moves from recharge to discharge. WATER FROM WELLS Water wells are cylindrical holes dug or drilled into the ground penetrating an aquifer; must be deeper to access water during dry season. Pumping results in the initially straight water tabl
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