ESCI101 26 Groundwater1

Slide1 : Groundwater Incorporates Lecture Slides prepared by Bill Dupré and Peter Copeland Sources include Understanding Earth by Press et al., and more. (includes material copyrighted by W.H. Freeman & Co.) Figure 13.6 Vasey’s Paradise Groundwater discharges from the wall of Marble Canyon to form a series of natural springs. (Grand Canyon, Arizona)


Slide2 : The Hydrologic Cycle Recall from last time ….


Slide3 : Groundwater is one reservoir of the hydrologic cycle and it represents about 22% of fresh water.


Also, recall from last time …. : Also, recall from last time …. So there is about 100 times as much fresh water in ground water than in lakes and rivers combined….


Slide5 : The amount, availability, and movement of groundwater depends largely on Porosity and Permeability. With the exception of caves, there are no large open spaces for pools or rivers of water underground. The only space available for water is the pore space between grains of sand and other particles that make up the soil and bedrock and the space in fractures.


Porosity : Porosity The percent void space in a rock or sediment. It is a measure of the potential volume of water that can be stored in a rock.


Slide7 : Figure 13.7 MORE POROUS LESS POROUS Noncemented Sand Cemented Sandstone Porosity Varies with % Cement


Slide8 : Figure 13.7 MORE POROUS LESS POROUS Well Sorted Sand Poorly Sorted Sand Porosity Varies with Sorting


Slide9 : Figure 13.7 MORE POROUS LESS POROUS Fractured Shale Unfractured Shale Porosity Varies with Fracturing


Permeability : Permeability The ability of a material to transmit a fluid. It is a measure of how fast the fluid can travel through the rock or sediment


Slide11 : Figure 13.7 CONNECTED FRACTURES LESS POROUS Fractured Shale Unfractured Shale Permeability depends on Pore Connectivity UNCONNECTED PORES


Aquifer : Aquifer A geologic unit capable of storing and transmitting water in sufficient quantities to supply wells. Aquiclude (Aquitard) A geologic unit that resists water flow (relative to an aquifer).


Slide13 : Relative Porosity and Permeability of Different Aquifer Types Table 13.2 Type of Aquifer Porosity Permeability Gravel Very High Very High Coarse- to fine sand High High Fine-grained sand & silt Moderate Mod - Low Sandstone, mod. cemented Mod - Low Low Fractured Shale Low Low Metamorphic Rocks Low Very Low Unfractured Shale Very Low Very Low Generally, permeability correlates with porosity, but not always.


Types of Aquifers : Types of Aquifers Unconfined Aquifer: the permeable layer extends to the surface. It consists of an unsaturated zone separated from the saturated zone by the groundwater table. Confined Aquifer : the permeable layer is overlain and underlain by a less permeable layer (aquiclude)


Slide15 : Figure 13.8 Unsaturated Zone: Pores include both air and water Saturated Zone: Pores filled with water Groundwater Table Unconfined Aquifer


Water Table : Water Table • the top of the saturated zone of groundwater • the level to which water will rise in a hole • the level to which water will rise in an unconfined aquifer


Water Table : Water Table Groundwater moves downward due to the force of gravity: some water in unsaturated zone is on way to saturated zone rest is held by surface tension.


Slide18 : Figure 13.9 Dynamics of an Unconfined Aquifer in a Temperate Climate Rainwater infiltrates porous soil & rocks …. … and flows underground towards lakes and streams.


Slide19 : Figure 13.9 During the Wet Season… During wet periods, the water is high Natural springs are flowing. Both deep & shallow wells can be pumped …. … and ground water is dis-charged as it moves to lakes & streams.


Slide20 : Figure 13.9 During the Dry Season… During dry periods, evaporation discharges ground water in soils …. … springs stop flowing, river dries up … … the water tables falls, shallow wells dry up …. … and water from streams & lakes infiltrates and recharges the surface soil & rock.


Types of Aquifers : Types of Aquifers Unconfined Aquifer: the permeable layer extends to the surface. It consists of an unsaturated zone separated from the saturated zone by the groundwater table. Confined Aquifer : the permeable layer is overlain and underlain by a less permeable layer (aquiclude)


Slide22 : Figure 13.10 Confined Aquifer Recharge Area: where the confined aquifer is recharged by infiltration


Slide23 : Figure 13.10 Confined Aquifer Recharge Area: where the confined aquifer is recharged by infiltration Pressure Surface: Height to which water will rise in the confined aquifer


Recharge Area of a confined aquifer:- the area where rainfall enters (recharges) the aquifer. : Recharge Area of a confined aquifer: - the area where rainfall enters (recharges) the aquifer. Pressure Surface: - the level to which water will rise in wells in confined aquifers.


Note the Difference Between: : Note the Difference Between: Groundwater Table: - level to which water will rise in unconfined aquifer Pressure Surface: - level to which water will rise in confined aquifer* * If the pressure surface is above the ground surface, the water will flow freely out of the ground. Such wells are called flowing artesian wells.


Slide26 : Figure 13.10 Confined Aquifer Recharge Area: where the confined aquifer is recharged by infiltration Pressure Surface: Height to which water will rise in the confined aquifer Flowing artesian well


Slide27 : Pressure Surface Ground Surface Flowing artesian well Figure 13.10


Slide28 : Figure 13.11 Perched Water Table


Slide29 : The ground water table is not stable - Modifications of the ground water table may result in lowering of the water table, saltwater incursion, subsidence, and contamination.


Slide30 : Figure 13.12 Cone of Depression due to Withdrawal


Slide31 : Figure 13.13 Fissures and depression caused by ground subsidence due to over-pumping of groundwater (Edwards Air Force Base) Withdrawal of ground water changes the properties of the aquifer, e.g., causing: subsidence ground collapse


Slide32 : Figure 13.14 Dynamics of the Saltwater - Fresh Water Interface


Slide33 : Figure 13.14 The thickness of the fresh groundwater floating on top of the salty groundwater is affected by the balance between groundwater recharge and discharge


Slide34 : Figure 13.14 If the rate of discharge increases too much (by over-pumping), the saltwater will rise, causing saltwater intrusion in the well Inverted Cone of depression


Slide35 : Can we predict and prevent such changes? We need to understand how water flows through the subsurface: What drives flow: gravity, pressure What resists flow: low permeability, lack of recharge (no pressure)


Darcy’s Law : Darcy’s Law An equation by which the discharge (rate of flow) of groundwater can be calculated. Q = A ( K x S) Q = Discharge A = Cross-sectional area of flow K = Permeability (hydraulic conductivity) S = Slope of water table = h/l h = vertical drop l = flow distance


Slide37 : Figure 13.15 Darcy’s Law Rate of flow is directly proportional to drop in water table: - the greater the drop, the higher the flow. Rate of flow is indirectly proportional to flow distance: - the greater the distance, the lower the flow. Hydraulic Gradient: h/l Groundwater flow also affected by permeability, k Q = A ( K x h/l)


Slide38 : Figure 13.15 Darcy’s Law


Slide39 : To Maintain the Water Table, Discharge MUST be Balanced by Recharge If Imbalance Exists: Decrease discharge, i.e., water usage Increase recharge, i.e., pump recycled water Increase permeability??


How much water do we use per capita in the U.S. per day? : How much water do we use per capita in the U.S. per day? Choose 1: 2 liters? 10 liters? 300 liters? 6000 liters?


How much water do we use per capita in the U.S. per day? : How much water do we use per capita in the U.S. per day? 6000 liters…. A human can survive on just 2 liters a day. How do we use so much?


How do we use so much? : How do we use so much? Personal use per capita per day without conservation: Shower/bath: 60 liters Toilet: 80 liters Faucet: 45 liters Dish/clothes washer 75 liters Other domestic 40 liters TOTAL 300 liters Personal use per capita per day with conservation: Shower/bath: 50 liters Toilet: 40 liters Faucet: 45 liters Dish/clothes washer 50 liters Other domestic 25 liters TOTAL 210 liters Still just a few per cent of 6000 liters…


Slide43 : Main uses of water Agriculture: 43% (mainly irrigation) Industry: 38 % (includes manufacture of paper, packaging, cars, other appliances)