Gl102 groundwater PDF

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Chapter 16: Groundwater Importance of underground water: - Groundwater: water in pores of soil and sediment, and narrow fractures in bedrock - Largest readily available freshwater reservoir - Groundwater slowly dissolves rock such as limestone, creating surface depressions: sinkholes Distribution of underground water: - The shape of the water table is usually a subdued replica of the surface topography - Soil moisture: - Molecular attraction on particles near surface’ - Zone of saturation: - Phreatic zone - Water percolates down: all open spaces filled with water: groundwater - Water table: - Upper limit of zone of saturation (pressure surface) - Capillary fringe: - Extends upward from water table by surface tension in pore spaces - Zone of aeration: - Vadose zone - Area above water table - Capillary fringe and soil moisture - Water cannot be pumped by wells The water table: - Depth to water table is highly variable; vary seasonally and year to year - Water levels of wells coincides with water table - Water tends to accumulate beneath high areas between stream valleys - During extended drought. Water table may drop enough to dry shallow wells - Uneven water tables may also result from variations in rainfall and permeability Factors that influence the storage and movement of groundwater - Porosity: - Fraction of rock or sediment that is pore space - Controls how much water can be stored - Pore space depends on: - Grain size and shape - Packing efficiency - Degree of sorting - Amount of cementing - (Uniform Size has a lot more pore space) - Permeability, Aquitards, and Aquifers: - Permeability: ability of a material (water) to transmit fluid - Smaller pore spaces → slower groundwater movement - Groundwater: 1. Specific yield: portion that will drain with gravity

2. Specific retention: portion retained as film Aquitard: an impermeable layer that hinders or prevents water movement (such as clay) - Aquifer: permeable rock strata or sediment that transmits groundwater freely (such as sands and gravels) - Remember that porosity is not always a reliable guide to the amount of groundwater How groundwater moves: - *(figure 16.4 why is this figure wrong)* → arrows drawn with no respect or regard to scale, where is that water coming from, where does water go, misleading rep of what happens to groundwater - Most groundwater movement is slow → a few centimeters per day, due to gravity - Darcy's law (flow through a porous medium) - If permeable remains uniform: velocity of groundwater increase as slope of water table increases - v=k/n x deltah/deltaL - Q=AK x deltah/deltaL - Hydraulic head (deltah) → Vertical difference between the recharge and discharge points - Hydraulic gradient (deltah/deltaL) → slope of water table: hydraulic head divided by length of flow - Hydraulic conductivity (K) → factor that takes into account permeability of aquifer and fluid viscosity - Porosity (n) → fraction that is pore space -

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Artesian wells: - Water in arestial wells rises above the aquifer on its own due to pressure - In an arterial system 1. Water must be confined to a aquifer that is inclined so one end can receive water 2. Aquitards above and below the aquifer recent water from escaping and th weight (pressure) of the water forces th water to rise in the well Artesian wells: - Types: - Non-flowing: pressure surface below ground surface - Flowing: pressure above ground surface - Asrtersian spring occur when water rises to surface through fractures in rock→ in deserts; oasis

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Artesian system occurs when an inclined aquifer is surrounded by impermeable beds (figure 16.4 is very misleading) Environmental problems Mining groundwater: - Water table height balanced between rate of infiltration and rate of discharge - Groundwater is considered a nonrenewable resource in some places - Ex where water available to recharge aquifer is much less than the amount being withdrawn Subsidence: - Ground sinks when water is pumped faster than recharge - More pronounced with thick sediments - Water is withdrawn → - Water pressure decreases→ - Weight of overlying material pushes down Sewage - Septic tanks, broken sewers, farm waste - If harmful pathogens in sewage: 1. Purified: chemical oxidation or assimilation 2. Unpurified: if aquifer does not have the correct composition to purify or if the water moves to quickly no purification occurs - Figure 16.20 Sweeteners in groundwater: Cyclamate, acesulfame Groundwater contamination: - Road salt (Nacl) - Fertilizers (NO3) - Pesticides - Chemical and industrial materials - Leachate from landfills The geologic work of groundwater: - Groundwater a weak carbonic acid, dissolves soluble rock, like limestone - Limestone underlies millions of km2 - Groundwater reacts with calcite in the rocks to form calcium bicarbonate - Caverns: - Dissolves soluble rock at or just below water table - Dissolution creates cavities and caverns - Dissolved material removed, lowering the water table, and exposing caves to air - Caverns features form in the zone of aeration - Vaern features are called dipstone: evaporation of calite (in kettle) - Speleothems: all depositional features - Stalactites: hang from the ceiling - Stalagmites: grow up from the floor - Column: stalactites and stalagmites joined - Karst topography:

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Peters 110

Landscapes that have been shaped by the dissolution by groundwater Features include: - Sinkholes, sinks - Disappearing streams - Very irregular surfaces Development of karst landscape figure 16.24...