Published on January 17, 2008
Groundwater: Groundwater Groundwater is among our most precious natural resources. The main points in this lecture are the behavior of water under the Earth’s surface, and how we deal with its perturbation by natural and unnatural means. Is groundwater important? According to U.S. estimates, groundwater provides: 34% of agricultural use (mostly for irrigation) 40% of the public water supply withdrawals 53% of all drinking water for the total population 97% of drinking water for the rural population Global Water Budget: Global Water Budget Although groundwater is not very globally significant in volume, it is a critical source of domestic water, because it is part of the limited budget of fresh (non-saline) water. It can be viewed as a non-renewable resource: It is possible to withdraw it faster than nature replenishes it. p.254 The Hydrologic Cycle: The Hydrologic Cycle < 5% of Earth’s water is fresh water. Of this, 74% is tied up in glacial ice. Hence, groundwater is a precious commodity. p.255 Porosity and Permeability: Porosity and Permeability porous sediment: < 40% porosity hard rock: <1% porosity porosity: volume proportion made up of voids permeability: connectedness of voids, dictating capacity to transmit flow The Water Table: The Water Table Material saturated with water lies below the water table. Materials that conduct water (are porous and permeable) are aquifers. Materials that do not conduct water (are well-cemented, unfractured, etc.) are aquicludes or confining layers. One inch of rain on one acre of ground results in 27,192 gallons of water (~100,000 L). Mines and Quarries Typically Work Below the Water Table: Mines and Quarries Typically Work Below the Water Table When worked, below-surface mines and quarries have to be actively pumped. Stop working, and they fill up with water to the height of the water table. Perched Water Table: Perched Water Table Perched aquifers are common, since the geology of the near-surface can be fairly complex. Parts of an aquifer can become perched as a result of slight variations in the clay content of sediments. Clay-rich sediments tend to be impermeable. Springs in Cross Section: Springs in Cross Section Springs in Cross Section: Springs in Cross Section Springs Along Lithological Boundaries: Springs Along Lithological Boundaries Groundwater Moves (often very slowly): Groundwater Moves (often very slowly) Water infiltrating the ground is recharge. Groundwater flowing out from the ground is discharge. p.264 Influent and Effluent: Influent and Effluent When groundwater flows into streams they are called effluent. When the water table drops, streams become influent, and water leaks from the stream bed into the ground. p.264 Confined and Unconfined Aquifers: Confined and Unconfined Aquifers Artesian Flow: Artesian Flow p.265 In some areas groundwater is under pressure in a confined aquifer. This results in artesian flow, where water will rise to the surface without pumping. Characteristic US Aquifers: Characteristic US Aquifers Great Plains: water derived from the Colorado Front Range Atlantic Coast: water moves through young, poorly lithified sediment Basin and Range: waters concentrate in sands of alluvial fans Effect of Groundwater Pumping: Effect of Groundwater Pumping p.266 Salt Water Intrusion: Salt Water Intrusion One consequence of overpumping in coastal regions is the encroachment of saline seawater into fresh aquifers. To combat this, many coastal communities re-inject wastewater into coastal aquifers. Subsidence: Subsidence In areas of long-term overpumping, and where recharge is slow, subsidence can be a major problem. Groundwater takes up pore space in sediments and rocks, and acts as support. Withdraw the water and aquifer material compacts irregularly. When the surface subsides, construction (buildings, pipe systems, etc.) may suffer huge damage. This is equally the case in any area where material is withdrawn from the ground (oil, mines, etc.). It can be combatted by injecting water back into the aquifer as groundwater is withdrawn. Solution of Rock by Groundwater: Karst Features: Solution of Rock by Groundwater: Karst Features Carbonate rocks are subject to chemical weathering, especially if water is abundant. Karst features are those produced by the dissolution and reprecipitation of carbonate rocks. What is required for this process? Karst Topography: Karst Topography Limestone dissolution produces a number of characteristic landforms, known collectively as karst topography. Karst topography typifies much of the landscape in Florida and eastern Kentucky, among others. A Fine Sinkhole: A Fine Sinkhole among the largest US sinkholes: 130 meters long, 100 meters wide, 45 meters deep note the classic mass wasting features Sinkhole: Mass Wasting, Karst Style: Sinkhole: Mass Wasting, Karst Style May 8, 1981, Winter Park, FL (a parking lot with two Porsches was destroyed) Karst Taken to the Extreme: Karst Taken to the Extreme When caves are eroded through and roofs collapse, terrains such as this in south China are developed. Flowing Spring in a Karst Region,Oregon County, MO: Flowing Spring in a Karst Region, Oregon County, MO 222 million gallons of water flows from this spring each day. Hot Springs and Geysers: Hot Springs and Geysers Sedimentary Deposition from Hot Springs: Sedimentary Deposition from Hot Springs The minerals that form here are essentially the same as what forms in karst environments: carbonates. The rock travertine is a variety of limestone that forms in cave and hot spring environments. Formation of Geodes: Formation of Geodes rhythmic precipitation of microcrystalline quartz (chalcedony, a.k.a. agate) macroscopic quartz crystals Solutions rich in dissolved Si first precipitate SiO2 as a non-crystalline water-rich gel. The gel loses water over time, forming opal. Opal loses water and becomes more crystalline over time, making chalcedony.