1 Groundwater Lecture Notes What is Groundwater? • Groundwater is water that: – lies beneath the earth’s surface – occupies the space between grains in sediments and sedimentary rocks – fills fractures and voids in igneous, sedimentary, and metamorphic rocks • Porosity = a measurement of a rock’s or sediment’s ability to store water • Permeability = a measure of a rock’s or sediment’s ability to transmit water The Water Table • There are 3 major “zones” that describe the distribution of water in the earth’s subsurface (see Fig. 11.1) – Vadose Zone: unsaturated, relatively dry material – Capillary Fringe: moist (but not saturated) material in between the vadose zone and the saturated zone – Saturated Zone: saturated material in which all pores/ fractures are filled with water • The top of the saturated zone is called the “Water Table”. • A “Perched Water Table” occurs where saturated materials are separated from the main water table by an unsaturated zone (Fig. 11.2) The Movement of Groundwater • Most groundwate moves relatively slowly through earth materials – Clay rich soils cm-m/yr – Sandy soils 10’s-100s m/yr – Rock Fractures up to km/day • In porous media (e.g. soils), groundwater moves down the slope of the water table Hydraulic Head • Hydraulic head = elevation of water + pressure on water Hydraulic Gradient • Hydraulic Gradient = change in head from a to b / distance from a to b. • HG = ∆ head / distance = ∆ h/L
2 Rules of Groundwater Flow • Groundwater flows from regions of high head to regions of low head • Groundwater flows from regions of high fluid potential to regions of low fluid potential Water table Water table A B L ∆ h
3 Groundwater Flow Velocity • Flow velocity depends on: – Hydraulic gradient ( ∆ h / L) – Permeability (K) – Porosity (n) • Velocity = (K / n)( ∆ h / L) • This equation essentially shows that groundwater will flow faster where the
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