Geology: Water as a Resource

• 1. Chapter 10

2. FLUID STORAGE ANDMOBILITY: POROSITYAND PERMEABILITY 3. Fluid Storage and Mobility:Porosity and PermeabilityPorosity and permeability are partof basic rock properties (along withMineralogic Composition, Texture,Fabric, and Sedimentary Structures) 4. Fluid Storage and Mobility:Porosity and PermeabilityPorosity-the proportion of a void space in the material.-a measure of how much fluid the material canstore.Permeability-the measure of how readily fluids passthrough the material.-it is related to the extent to which pores orcracks are interconnected, and to their size;is, larger pores have a lower surface-to-volumeratio so there is friction al drag to slow thefluids down. 5. Fluid Storage and Mobility:Porosity and Permeability 6. SUBSURFACEWATERS 7. Subsurface WatersSubsurface water-is all of the water occupying pores space below theground surface.-is the flow of water beneath earth's surface as partof the water cycle.Groundwater-is the water located beneath the earth's surface insoil pore spaces and in the fractures of rockformations. 8. Subsurface WatersSaturated zone or Phreatic zone-it is a part or layer of the Earths crust, excluding thecapillary zone, in which all voids are filled with water.-it is the area in an acquifer, below the water table, inwhich relatively all pores and fractures are saturated with water.- the Phreatic zone size and depth may fluctuate withchanges of season, and during wet and dry periods.Unsaturated zone or Vadose zone-it is the portion of the subsurface above the groundwatertable. The soil and rock in this zone contains air as well as water in itspores.-it is the part of the subsurface between the land surface andthe groundwater table.-the water in unsaturated soil is soil moisture. 9. Subsurface Waters 10. Subsurface WatersRecharge Areas and Discharge AreasThe Earth's surface can be divided into areas wheresome of the water falling on the surface seeps into the saturatedzone and other areas where water flows out of the saturated zoneonto the surface. Areas where water enters the saturated zoneare called recharge areas, because the saturated zone isrecharged with groundwater beneath these areas. Areas wheregroundwater reaches the surface (lakes, streams, swamps, &springs) are called discharge areas, because the water isdischarged from the saturated zone. Generally, recharge areasare greater than discharge areas. 11. Subsurface Waters 12. AQUIFERGEOMETRY ANDGROUNDWATERFLOW 13. Aquifer Geometry andGroundwater flowConfined and Unconfined AquifersUnconfined Aquifers - the most common type of aquifer, where thewater table is exposed to the Earth's atmosphere through the zone ofaeration. Most of the aquifers depicted in the drawings so far havebeen unconfined aquifers.Confined Aquifers - these are less common, but occur when anaquifer is confined between layers of impermeable strata. A specialkind of confined aquifer is an artesian system, shown below. Artesiansystems are desirable because they result in free flowing artesiansprings and artesian wells. 14. Aquifer Geometry andGroundwater flow 15. Aquifer Geometry andGroundwater flowArtesian System-if a well is drilled into a confined aquifer, the water canrise above its level in the aquifer because of this extrahydrostatic(fluid) pressure.Potentiometric Surface-represents the height to which the waters pressure would raisethe water if the water were unconfined. This level will besomewhat higher than the top of the confined aquifer where itsrocks are saturated and it may be above the ground surface. 16. DARCYS LAWANDGROUNDWATER FLOW 17. Darcys Law andGroundwater FlowDischarge and VelocityThe rate at which groundwater moves through the saturated zonedepends on the permeability of the rock and the hydraulic gradient.The hydraulic gradient is defined as the difference in elevationdivided by the distance between two points on the water table.Velocity, V, is then:V = K(h2 - h1)/Lwhere K is the coefficient of permeability.If we multiply this expression by the area, A, through which the wateris moving, then we get the discharge, Q.Q = AK(h2 - h1)/L 18. Darcys Law andGroundwater Flow 19. Darcys Law andGroundwater FlowHow readily ground water can move through rocksand soil is governed by permeability, but where and how rapidlyit actually does flow is also influenced by differences inhydraulic head (potential energy)from place to place. Groundwater flows spontaneously from areas of higher hydraulic head(potential energy) to lower hydraulic head.Hydraulic head or piezometric head -is a specificmeasurement of liquid pressure above a geodeticdatum. 20. CONSEQUENCESOFGROUNDWATERWITHDRAWAL 21. Consequences of GroundwaterWithdrawal Lowering the Water Table Compaction and Surface Subsidence Saltwater Intrusion 22. Consequences of GroundwaterWithdrawal Lowering the Water TableThe most severe consequence of excessivegroundwater pumping is that the water table, belowwhich the ground is saturated with water, can belowered. For water to be withdrawn from the ground,water must be pumped from a well that reaches belowthe water table. If groundwater levels decline too far,then the well owner might have to deepen the well,drill a new well, or, at least, attempt to lower thepump. Also, as water levels decline, the rate of waterthe well can yield may decline. 23. Consequences of GroundwaterWithdrawal Compaction and Surface SubsidenceThe basic cause of surface subsidence is a loss ofsupport below ground. In other words, sometimeswhen water is taken out of the soil, the soil collapses,compacts, and drops. This depends on a number offactors, such as the type of soil and rock below thesurface. Land subsidence is most often caused byhuman activities, mainly from the removal ofsubsurface water 24. Consequences of GroundwaterWithdrawal Saltwater IntrusionSalt water intrusion occurs in coastalfreshwater aquifers when the different densities ofboth the saltwater and freshwater allow the oceanwater to intrude into the freshwater aquifer. 25. Consequences of GroundwaterWithdrawal 26. OTHER IMPACTS OFURBANIZATION ONGROUNDWATERSYSTEMS 27. Other Impacts of Urbanizationon Groundwater Systems Loss of Recharge Artificial RechargeAreas where water enters the saturated zone are called recharge areas. 28. Other Impacts of Urbanizationon Groundwater Systems Loss of RechargeRecharge are loss due to covering of impermeablecover over the recharge areas of confined aquifers, then,recharge can be considerably reduced, thus aggravating thewater-supply situation.Examples of impermeable cover are the following:Buildings, asphalt and concrete roads, sidewalks, parking lots and airport runways. 29. Other Impacts of Urbanizationon Groundwater Systems Artificial RechargeIn steeply sloping areas or those with low-permeabilitysoils, well-planned construction that includes artificial rechargebasin can aid in increasing ground water recharge. 30. Other Impacts of Urbanizationon Groundwater Systems 31. Other Impacts of Urbanizationon Groundwater SystemsIn the shown figure, the basin acts similarly to aflood-control retention pond in that it is designed to catchsome of the surface runoff during high-runoff events (heavyrain or snow melt). Trapping the water allows from which thefreshwater might be otherwise be quickly lost to streams andcarried away. 32. OTHER FEATURESINVOLVING SUBSURFACEWATER 33. Other Features InvolvingSubsurface Water Karst Sinkholes 34. Other Features InvolvingSubsurface Water Karst-a distinctive terrainthat is caused by thedissolution ofsoluble rocks (suchas limestone andother carbonaterocks) by subsurfacewater . 35. Other Features InvolvingSubsurface Water Sinkholes-is produced by thedissolving of largevolume (over a periodof time) of solublerocks in which slowlyenlarging undergroundcaverns can erodesupport for the landabove. 36. WATER QUALITY 37. Water QualityMost of the water in the hydrosphere is in the verysalty oceans, and almost all of the remainder is tied up in ice.That leaves relatively little surface or subsurface water forpotential freshwater sources. Rainwater contains dissolvechemicals of various kinds, especially in industrialized areaswith substantial air pollution. Thus, water quality must be aconsideration when evaluating water supplies. 38. Water QualityMeasure of Water Quality1. Based on present amount of dissolved chemical; expressed by the following: parts per billion (ppb) for very dilute substances parts per million (ppm) parts per hundred (percentage)-for lower concentrationsExample: If water contains 1 (one) weight percent salt, itcontains 1(one) gram of salt per 100 (hundred) grams of water.2. Based on total dissolved solids (TDS), the sum of the concentrationsof all dissolved solid chemicals in the water.Example: 500 or 1000 ppm TDS is needed for drinking water. 39. Water Quality3. Based on acidity or alkalinity of the water.-the pH of water is inversely related to acidity: the lower the pH, themore acid the water. Water that is neither acid nor alkaline has a pH of 7. 40. Water QualityHard Water-water supplies that have passed through soluble carbonaterocks, like limestone.-contains substantial amounts of dissolve calcium andmagnesium.Groundwater quality is highly variable. It may benearly as pure as rainwater or saltier than the oceans. 41. EXTENDING THE WATERSUPPLY 42. Extending the Water Supply Conservation InterbasinWater Transfer Desalination 43. Extending the Water Supply Conservation-the preservation, management, and care of water resources.For example, the raising of crops that require a greatdealof water could be shifted, in some cases at least, toareas where natural rainfall is adequate to supportthem. InterbasinWater Transfer-the moving of water from one river basin where it isavailable, to another basin where water is less available orcould be utilized better for human development. 44. Extending the Water Supply Desalination-improving the quality of waters that are not used, bypurifying them sufficiently to make them usable. Filtration the wateris passed through finefilters or membranes toscreen out dissolvedimpurities. 45. Distillation involves heating orboiling water full ofdissolved minerals.The water vapor drivenoff is pure water, whilethe minerals staybehind in whatremains of the liquid.Extending the Water Supply 46. END.