Lecture 11 - Water Resources - Groundwater 2024 PDF

CIVL 270 Introduction to Environmental Engineering Lecture 11 : Water Resources - Groundwater Dr. Mohamed Abdelsamie 2024 1 Reminder 2 Groundwater Aquifers Aquifers are underground regions of...

CIVL 270 Introduction to Environmental Engineering Lecture 11 : Water Resources - Groundwater Dr. Mohamed Abdelsamie 2024 1 Reminder 2 Groundwater Aquifers Aquifers are underground regions of soil or porous rock that are saturated with water. If the aquifer is physically separated from the groundwater, it is called a confined aquifer. Regions where the water can infiltrate the soil and reach the aquifer are called recharge zones. 3 Aquifer Systems Confined aquifer: is an aquifer that is confined between two confining layers (water kept under pressure) Unconfined aquifer: is an aquifer in which the water table forms the upper boundary; it is also called water-table aquifer 4 4 Unconfined aquifer - Groundwater Groundwater is located in a region of soil called the zone of saturation, where all of the spaces between soil particles are filled with water. The top of this region is called the water table. 5 Groundwater Flow Basics Factors that influence (i.e., govern) groundwater flow: Soil properties (e.g., porosity, hydraulic conductivity, homogeneity, isotropy, etc.) Hydraulic gradient (the driving force for flow) 6 Porosity Impacts: Flow of groundwater The amount of water that can be stored in a saturated aquifer depends on the Porosity of the soil and rock that makes up the aquifer. 7 7  Types of Porosity Primary –groundwater water in pores between grains. Secondary -groundwater in fractures, voids Effective porosity: porosity available for fluid flow, “connected pores”. Disconnected or “dead-end” pores. 8 8 Porosity 9 9 Specific Yield, Sy Storage term for unconfined aquifers Ratio of the volume of water that can be drained by gravity from an unconfined aquifer compared to the total volume 10 10 Example 11 11 Example 12 12 Groundwater Flow Basics Factors that influence (i.e., govern) groundwater flow: Soil properties (e.g., porosity, hydraulic conductivity, homogeneity, isotropy, etc.) Hydraulic gradient (the driving force for flow) 13 Hydraulic Conductivity, K Determines how fast a fluid (like water) can move through a porous media (like soil) 14 https://www.slideserve.com/ona/darcy-s-law 14 Hydraulic Conductivity, K Type of Soil K Value Range Well-Sorted Gravel 10-4 – 10-2 m/s Well-Sorted Sand 10-5 – 10-3 m/s Silt 10-8 – 10-6 m/s Clay 10-11 – 10-8 m/s Rock 10-13 – 10-10 m/s Fractured and/or Weathered Rock 10-8 – 10-2 m/s 15 15 Isotropy Isotropic = means that the porous media properties (specifically permeability) are the same in all directions 16  Homogeneity Homogeneous = the porous media has uniform properties throughout its entire volume Examples of heterogenous aquifers Changes in thickness Layering of geologic deposits Facies changes 17 Heterogeneity and Anisotropy of Hydraulic Conductivity The formation is Homogeneous: when K is independent of position K(x,y,z) = constant Isotropic: when K is independent of direction Kx=Ky=Kz=constant 18 18 Groundwater Flow Basics Factors that influence (i.e., govern) groundwater flow: Soil properties (e.g., porosity, hydraulic conductivity, homogeneity, isotropy, etc.) Hydraulic gradient (the driving force for flow) 19 How groundwater moves – Hydraulic Gradient 20 20 Hydraulic Gradient 21 21 Hydraulic Gradient 22 22 Example Problem You are working for a construction company and are building a school. In digging the foundation, you find water at 7 m bgs (below ground surface). One hundred meters away, you find water at 7.5 m bgs. Choose the datum as the confining layer that is 25 m bgs. (a) What is the piezometric surface at each point, the direction of groundwater flow, and the hydraulic gradient? (b) If the cross-sectional area of the aquifer through which water flows is 925 m2 and you are dealing with a coarse, sandy soil (K = 5.2x10-4 m/s), what is the Darcy velocity of groundwater in this aquifer? (c) What is the discharge? 23 Overconsumption Excessive water withdrawal can cause a lowering of the water table, called a cone of depression. This may cause nearby, shallower wells to run dry. 25 25 Overconsumption Wells located near the ocean can experience saltwater intrusion as the ocean water mixes with the groundwater. 26 26 Overconsumption Excessive groundwater consumption can also lead to subsidence, a compression and sinking of the zone of saturation. The San Joaquin valley in California has experienced subsidence of up to 28 feet. 27 27 Sources of Groundwater Contamination 28 28