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Lecture Groundwater 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 1 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 A Water table h L B 2 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 hydraulic gradient is steeper (this is the same as saying a stream will flow faster where the stream slope is steeper) Permeability/Hydraulic Conductivity in Unconsolidated Sediments Material Permeability (darcys) 10-6 10-3 Hydraulic Conductivity (cm/sec) 10-9 10-6 Clay Silt, Sandy Silt, 10-3 10-1 Clay-rich Sands, Glacial Till Silty Sands, Fine 10-2 - 1 Sands Well-sorted Sand and Outwash Well-sorted Gravel 1-100 10-6 10-4 10-5 10-3 10-3 10-1 1 - 1000 10-2 - 1 3 Aquifers Aquifer = body of saturated rock or sediment through which water can move easily Good aquifers include: Sandstones, conglomerates, sand, gravel, highly fractured rocks Aquitard = body of rock or soil that resists water movement Good aquitards include Shales, mudstones, clay horizons Unconfined Aquifers An unconfined aquifer is only partially filled with water Unconfined aquifers have water tables Confined Aquifers A confined aquifer is a completely filled aquifer that is usually under pressure, and is separated from the surface by an aquitard Confined aquifers produce "artesian wells" 4 Aquitard Confined aquifer Wells A "well" is a deep, generally cylindrical hole that is commonly drilled in the ground to penetrate the saturated zone in an aquifer Drawdown is the lowering of water in a well due to pumping When we pump a well, the water table is depressed in an inverted "icecream cone" shape that we call the "cone of depression". Recharge occurs when water is added to a well (generally by recovery of the cone of depression when pumping is ceased). Balancing Drawdown & Recharge If we use groundwater at a rate faster than it can recharge, we will eventually cause the aquifer to dry up BAD, BAD, BAD!!! Effects When Withdrawl > Recharge May run out of water in Drop water table yield some wells useless Ground surface settling may occur 5 Compaction may lead to permanent porosity loss Springs A "spring" is a place where groundwater naturally flows from a rock onto the land surface Streams and Groundwater There are 3 types of stream relationships with groundwater: 1. Gaining Streams Occurs in rainy regions Receives water from the saturated zone The surface of a gaining stream = water table 2. Losing Streams Occurs in dry regions A stream that loses water to the saturated zone The surface of a losing stream is above the water table 3. Dry Stream Beds Occur in dry regions A losing stream that dries up because water from surface has moved downward toward water table Local "bump" in water table due to addition of water from stream Effects of Groundwater Action Limestone can be dissolved easily by groundwater H2O + CO2 + CaCO3 Ca2+(aq) + 2HCO3 In other words, water with carbon dioxide in it dissolves limestone! Where extensive dissolution has occurred, caverns, caves, and fractures may occur in limestone. Sinkholes = closed depresssions that form from the collapse of a cave's roof or by solution as running water enlarges a void in limestone Disappearing Streams = places where surface water breaches the top of the bedrock water flows down into to ground instead of on top of the ground 6 Areas where significant dissolution of carbonate rocks has occurred are called areas of KARST. Karst topography describes an area with many sinkholes and disappearing streams that result from extensive cavern and fracture systems in limestone Groundwater Pollution and Remediation Old disposal methods were commonly detrimental to groundwater quality Dry wells Pour oil on roads to keep dust down In addition, many underground storage tanks (UST's) have leaked their contents into the subsurface Petroleum Products Solvents Various chemicals Behavior of Liquid Pollutants in Groundwater Different types of pollutants behave differently in groundwater due to their inherant densities LNAPL's = Light Non-Aqueous Phase Liquids Float on water table due to density < water Gasoline Behavior of Liquid Pollutants in Groundwater (cont.) DNAPL's = Dense Non-Aqueous Phase Liquids Sink in groundwater due to density > water Includes many organic solvents like TCE trichloroethylene (metal degreaser) PERC perchloroethylene (dry cleaning fluid) When contaminants enter groundwater, they commonly create a "plume" (a body of contaminated groundwater) 7 Remediation Remediation is the process of cleaning up contaminated soil and/or water pollution There are four types of remediation which are commonly used: 1. Natural Attenuation: Water dilutes pollution to safe level Chemical reactions break down pollutants over time Inexpensive, effective for many contaminants 2. Biodegradation: Micro-organisms (aerobic bacteria) are injected into the ground and they process the pollution into safe by-products Relatively inexpensive Generally quite effective given a period of time Commonly used in collaboration with other techniques (e.g. air sparging) 3. Air Sparging: Pump air/oxygen into the ground to create better living conditions for aerobic bacteria, and to oxidize pollution into safe end products Effective but more expensive than attenuation Very effective when used with bioremediation for certain types of contamination 4. Pump and Treat: Pump contaminated groundwater out of the ground to treat it Processed water commonly disposed of in sanitary sewer, and undergoes further treatment by municipal waste water facilities Very expensive Diminishing returns over a period of time 8 ... View Full Document

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