Ground Waterg110 Lecture prepared by Bob Hall Revised 8/6/2000
Points to Be Covered on Ground Water • How is ground water formed? • What are the zones of subsurface water and their characteristics? • What is a water table, and how is it configured in humid vs arid climates? • How does ground water move? What is porosity? permeability? Primary vs seondary p and p?
Points to Be Covered (cont.) • What is an aquifer, and what kinds are there? • Be able to discuss the use of wells in ground water, including potential problems. • What is karst topography?
Origin of Ground Water • When precipitation falls on the land surface, what happens to it? • Evaporates and/or is transpired back to atmosphere • runs over land surface • infiltrates the land surface and • percolates downward and • becomes ground water
Zone of Aeration pores filled with both air and water Water held against gravity by surface tension Soil water Zone of Saturation pores filled only with water Water drained through soil under influence of gravity. Ground Water Zones of Subsurface Water
What is the configuration of the water table? Humid Area Arid Area R = Recharge Area R = Recharge area
Configuration of Water Table in a Humid climate
Porosity: % by volume of an earth material that is pore space. Primary porosity depends upon: - shape of grains - arrangement of grains - size distribution - compaction/cement’n Permeability: ability of an earth material to transmit water Depends upon - porosity - degree and size of interconnecting pores between larger pores How does ground water move?
What is secondary porosity and permeability? • Secondary porosity developed after the material was formed . Depends upon: • Primary porosity and permeability were created by the same processes that formed the material. - degree of fracturing - amount of solution (for limestone)
What are some typical values of porosity and permeability? • Porosity clay 45-55 % sand 30-40 sandstone 10-20 shale 1-2 limestone 1-10 (or larger) • Permeability: varies over several orders of magnitude. Expressed as a rate, e.g. ft/day
What is Darcy’s Law? An explanation of the factors determining the rate (velocity) of ground water flow. V = K (h/l) where K = coefficient of permeability dh/dl = hydraulic gradient
Configuration of Water Table in a humid climate Et
How does the geology control the existence of ground water? • What is an aquifer? A permeable, water-containing unit. - Water enters from recharge. - Temporarily stored. - Leaves by flow to streams (baseflow) or springs, or to wells
What is an unconfined aquifer? • They are not sealed off at any point. • Recharge can occur anywhere. • Water at w.table under atm pressure. • Must pump.
What is a confined (or artesian) aquifer? • Sealed off • Transmits water down from R.A. • Water confined in aquifer unless drilled. - Water under hydrostatic pressure. - Water rises; well may flow.
What is a cone of depression? • Steepens local hydraulic gradient, increases flow rate. • Created by drawing down water table by pumping a well. • Overdraft occurs where pumping is too rapid, well goes dry.
What are some other problems associated with the use of wells? • Lack of filtering in karst regions. • Limited amounts of g.w. in some rock types. • Subsidence • Salt-water invasion.
How do springs occur? E E E E E E E e j • Lateral diversion of flow • Perching • Fracture zones
What is karst topography? • Topography with features relating to underground solution. • Collapse may also be involved. • Surface waters diverted underground. • Features: sinkholes, sinking streams, rises, caves.
Cone of depression Confined aquifer Darcy’s Law Ground water Karst Permeability Porosity Secondary p & p Sinkhole Soil moisture Spring Unconfined aquifer Water table Zone of aeration Zone of saturation Terms Used in the Study of Ground Water
Student Responsibilities on Groundwater Know how groundwater occurs and moves. Know the factors that determine the direction and rate of movement. Know how we can use groundwater safely, as well as the dangers of using it unwisely.