1Groundwater and the Hydrologic CycleChapter 17Vasey’s Paradise, GCNPOceans Cover >70% of SurfaceOceans are only 0.025% of MassGroundwater•Groundwater is liquid water that lies in the subsurface in fractures in rocks and in pore space between grains in sedimentary rocks. •The Hydrologic Cycle is the evaporation of water from the oceans, transport over the land, precipitation, and return of the water to the ocean.Hydrologic Cycle(x 1000 km3/y)Hydrologic Cycle(x 1000 km3/y)Total ocean mass = 1.4 x 1021kgEvaporation = 4.3 x 1017kg/y= 0.03% / y2ReservoirsThe reservoir of hydrogen in oxygen minerals is many times that of the oceans.Porosity and Permeability•Porosity is the percentage of open space in a rock. •Permeability is the ability of fluids to flow through rock.Porosity and PermeabilityPorosity•Porosity can be as high as: •50% in loose sand to •5% in cemented, lithified sandstone, to•near zero in unfractured igneous rocks. Porosity•Porosity in close-packed identical spheres is about 22%.Permeability•Permeability depends on the connectivity of the pore space.•Permeable rocks include sandstone and fractured igneous and metamorphic rocks and karst limestone. •Impermeable rocks include shales and unfractured igneous and metamorphic rocks.3The Water Table•The water table is the natural level of liquid ground water in an open fracture or well. •The water table follows topography. •The unsaturated zone is the region above the water table.•The saturated zone is the region below the water table.The Water TableUnsaturated Zone•The unsaturated zone is the region above the water table where pores and fractures are partially filled with water and partly by air. •The pressure in the unsaturated zone is atmospheric. •Conditions are usually highly oxidizingdue to the presence of free oxygen in the air and highly reactive due to the presence of water. Unsaturated Zone•The unsaturated zone is the region above the water table where pores and fractures are partially filled with water and partly by air. •The pressure in the unsaturated zone is atmospheric. •Conditions are usually highly oxidizingdue to the presence of free oxygen in the air and highly reactive due to the presence of water. Saturated Zone•The saturated zone is the region below the water table, where all fractures and pores are filled with water. •The pressure in the saturated zone is hydraulic (= that of the overlying water column). It increases at the rate of 100 atm / km (0.1 atm/m) •At very great depths ( > 2km ) the pores are sealed, and the pressure is that of the overlying rock column (~330 atm / km).Clicker Question• That subsurface region above the standing water level (water table) where the pressure is that of the atmosphere is called the:– A. Saturated zone– B. Unsaturated zone– C. Oxidation zone– D. Reduction zone– E. Reactive zone.4Clicker Question• That subsurface region above the standing water level (water table) where the pressure is that of the atmosphere is called the:– A. Saturated zone– B. Unsaturated zone– C. Oxidation zone– D. Reduction zone– E. Reactive zone.Clicker Question• That subsurface region below the standing water level (water table) where the pressure is that of the water column is called the:– A. Saturated zone– B. Unsaturated zone– C. Oxidation zone– D. Reduction zone– E. Reactive zone.Clicker Question• That subsurface region below the standing water level (water table) where the pressure is that of the water column is called the:–A. Saturated zone– B. Unsaturated zone– C. Oxidation zone– D. Reduction zone– E. Reactive zone.Rock Units of Differing Permeabilities•An aquifer is a porous and permeable layer through which water can flow easily.–Sandstone–Alluvium•An aquitard is a layer that retards the movement of water.–Shale–Unfractured igneous or metamorphic rockUnusual Conditions•A perched water table is isolated from the main water table by a layer of impermeable rock.•An artesian basin is one that is under pressure due to hydraulic head. –The artesian aquifer is usually confined between aquitards. –An artesian well is one that will flow without pumping. Clicker Question• Those rocks that commonly make the best aquifers are:– A. Granite– B. Schist– C. Shale– D. Sandstone– E. Salt5Clicker Question• Those rocks that commonly make the best aquifers are:– A. Granite– B. Schist– C. Shale–D. Sandstone– E. SaltArtesian SystemOgallala AquiferOgallala Aquifer Depletion:Ground-waterIrrigationOgallala Aquifer Depletion•Aquifer underlies former dust bowl•Aquifer was artesian when first tapped.•Drilling and pumping greatly increased in last half of 20th century.•The economy is still largely agricultural•Pumping greatly exceeds recharge.•Aquifer is depleted in some regions and nearly depleted in most of the rest.•Agriculture will largely cease in 20 years.•Currently 10 to 15% of US production. Clicker Question• A groundwater system where the pore pressure exceeds that of the water column to the surface so that water can flow spontaneously to the surface is called:– A. Artesian– B. Confined– C. Karst– D. Depleted– E. Running6Clicker Question• A groundwater system where the pore pressure exceeds that of the water column to the surface so that water can flow spontaneously to the surface is called:–A. Artesian– B. Confined– C. Karst– D. Depleted– E. RunningArtesian System Contamin-ation:Ogallala Aquifer, TexasOgallala Aquifer Contamination•Pantax Site near Amarillo, TX•Nuclear weapons facility (DoE)•Used TCE was burned in unlined pits•Solvents leaked down to aquifer (~400ft down)•What can be done?•Use surface water.Limestone (calcite) is soluble in water.Karst•Karst topography refers to a surface topography marked by sinkholes, disappearing streams, and small closed drainage basins. –It is an indication of underground drainage in caverns in limestone. –Limestone (Calcite, CaCO3) is dissolved by acids in soil and rainwater to form caverns. Karst7Thunder River,Grand CanyonNPClicker Question• The rock type underlying most karst drainage systems is:– A. Sandstone– B. Shale– C. Salt– D. Limestone– E. GraniteClicker Question• The rock type underlying most karst drainage systems is:– A. Sandstone– B. Shale– C. Salt–D. Limestone– E. GraniteCave Formations•Precipitation of calcite from dripping
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