Chapter 7 - Groundwater Hydrology (Part I) PDF

CEE 335 Hydrology Groundwater Hydrology Part 1 1 GROUND WATER 2  WHAT IS GROUND WATER  groundwater….technically refers to all water found beneath the surface of the earth – occurs in “saturated” zones of soil or rock » po...

CEE 335 Hydrology Groundwater Hydrology Part 1 1 GROUND WATER 2  WHAT IS GROUND WATER  groundwater….technically refers to all water found beneath the surface of the earth – occurs in “saturated” zones of soil or rock » pores between soil or rock particles are totally filled with water – occurs in “unsaturated” zones of soil or rock » pores are only partially filled with water  aquifer…. a water-bearing layer of soil, sand, gravel, or rock that will yield usable quantities of water to a well. – note that many soil and rock materials contain some water, but only those that produce useable quantities of water are 3 considered to be “aquifers”  DEFINITIONS  Aquifer: “a geologic unit that can store and transmit water at rates fast enough for supply” – Confined (artesian) aquifer: potentiometric surface above surface elevation of aquifer – Flowing artesian aquifer: potentiometric surface above ground surface – Unconfined Aquifer (water-table aquifer) » free-surface within aquifer with no underlying unsaturated material – Perched aquifer... free-surface aquifer above unsaturated material  Confining Layers: – Aquitard: “a geologic unit that may store but cannot transmit water at rates fast enough for supply – Aquifuge: “a geologic unit that cannot store or transmit water” – Aquiclude: Geologic material that cannot transmit 4 significant quantities of water are impermeable to UNCONFINED AQUIFER SYSTEM  Unconfined aquifer: an aquifer where the water table exists under atmospheric pressure as defined by levels in shallow wells  Water table: the level to which water will rise in a well drilled into the saturated zone 5  CONFINED AQUIFER SYSTEM  Confined aquifer: an aquifer that is overlain by a relatively impermeable unit such that the aquifer is under pressure and the water level rises above the confined unit  Potentiometric surface: in a confined aquifer, the hydrostatic pressure level of water in the aquifer, defined by the water level that occurs in a lined penetrating well #6  AQUIFER SYSTEM Recharge area Water Piezometric surface Ground table Flowing well surface Deep well well Water Table Unconfined aquifer Impermeable strata Confining Stratum Confined aquifer 7  GROUNDWATER FLOW Groundwater is always flowing, and the direction of flow is determined by the location of higher groundwater elevation. Note, however, that groundwater does not flow downhill; rather, it flows from higher hydraulic heads (or higher water elevation) to lower hydraulic heads. The distribution of hydraulic heads in the saturated zone determines the direction in which the water will flow. The speed with which groundwater flows, also called the velocity or flux, is determined by the difference in hydraulic head and the permeability of the sediment or rock through which it flows. Permeability is a number which describes the ease with which a fluid (like water) will move through a porous medium (i.e. a rock, soil, or sediment which has enough pore space to allow water to move through it). Later in the book, it will be shown how the difference in head from one point to the next, and the permeability, can be used to calculate the velocity of the 8 groundwater.  DARCY’S LAW One of the fundamental equations that govern groundwater flow is called Darcy’s Law, Q = K A dh/dl where: Q = discharge [L3/T] A = cross sectional area [L2] K = hydraulic conductivity [L/t] dh/dl = hydraulic gradient or change in hydraulic head (h) per change in distance (l) [ · ] In plain English, this equation states: Discharge (i.e., volumetric flow) through a cross sectional area is directly and linearly proportional to the hydraulic gradient, and the constant of proportionality that relates discharge to the hydraulic gradient is a 9 quantity called the hydraulic conductivity  FLUID ENERGY In simple, everyday terms, we think of hydraulic head as an elevation. More specifically, hydraulic head is the elevation of water in a manometer in a pressurized water pipe (Figure 2-1), or in a piezometer (Figure 2-2). - A manometer is a vertical tube in a pressurized water pipe used to measure pressure in the pipe. - A piezometer is a vertical tube with an open or slotted interval (usually called the screened interval or just the screen) inserted into the ground and used to measure hydraulic head in an aquifer; it is basically a well constructed for the sole purpose of measuring groundwater levels. Figure 2-1. Pipe of flowing water with Figure 2-2. Cross section of aquifer manometers showing 10 showing the loss of head along the flow hydraulic heads in three wells  FLUID ENERGY What do we mean by “energy”? Everything in the universe has some amount of energy associated with it, and that energy is present in various forms. Some sort of energy drives every natural process, and the key to understanding physical processes is in understanding the distribution of energy in a system. Potential Energy: energy stored in a piece of matter or at a point in a system; generally associated with position or with thermodynamics of the system (elevation, pressure, chemical, thermal) Kinetic: energy associated with motion (velocity). At every point in an aquifer, the fluid possesses some total amount of energy that is the sum of all the potential and kinetic energies in the fluid. As we previously stated, the fluid energy at a point in an aquifer manifests itself as the water level in a piezometer. So we could also say that the water level, or hydraulic head, represents the total energy in the aquifer at a given point, and we can use the various energy components of the hydraulic head (elevation, 11 pressure, velocity, etc.) to understand the driving forces behind  THE BERNOULLI EQUATION Fluid energies (and, subsequently, water levels) vary from one point in an aquifer to the next. Let us recall the first and second Laws of Thermodynamics. The 1st Law of Thermodynamics states that energy is conserved in any system; i.e. in cannot be created or destroyed, and any changes in energy must be accounted for in any system: energy added – energy subtracted = change in total energy This is basically the same as the conservation of mass. We could also express the first law in terms of the difference in energy at two points in a dynamic system: total energy(at point 1) + energy added/lost(between point 1 and 2) = total energy(at point 2) The 2nd Law of Thermodynamics states that closed systems tend to move towards increasing entropy. In a dynamic system like an aquifer, water will move from a point of higher energy (i.e., lower entropy) to a point of lower energy (higher entropy); in other words, groundwater moves in the direction of decreasing hydraulic head. Please note that groundwater does NOT (necessarily) flow downhill – it flows in the direction of decreasing head. The Bernoulli equation describes the total energy of a fluid at all 12 positions along a flow path in a closed system and is basically an  THE BERNOULLI EQUATION Z1 + P1/ρw g + v12/2g + Ha = Z2 + P2/ρw g + v22/2g + HL + HE where… z = elevation [L] p = pressure [M/L·t2] ρw = fluid density [M/L3] g = gravitational acceleration [L/t2] v = velocity [L/t1] Ha = heat energy added [L] HL = mechanical energy lost [L] HE = heat energy extracted [L] 13  HYDRAULIC HEAD AND HYDRAULIC POTENTIAL If we go with the assumption that we can ignore velocity and internal energy components when dealing with groundwater, we can drop all that out of the equation and express the fluid energy as the sum of the elevation and pressure components. That sum is what we call hydraulic head; in physical terms it is the fluid energy per unit weight, and in mathematical terms it is: h = z + p/ρwg where: h = hydraulic head [L] z = elevation [L] p = pressure [M/L·t2] ρw = fluid density [M/L3] g = gravitational acceleration [L/t2] If we multiply both sides of the equation by the gravitational constant, g, we get a quantity called hydraulic potential (Φ), which is the fluid energy per unit mass, or The hydraulic potential is simply a way of expressing the same fluid energy so that it isΦindependent = gz + p/ρ w , so (in of gravity that Φ= case yough would want to compare an aquifer on earth 14 with one on Mars or something like that). PRESSURE HEAD AND ELEVATION HEAD (LAB CONDITION)  = pressure head z = elevation head h =  + z = total head 15 PRESSURE HEAD AND ELEVATION HEAD (FIELD CONDITION)  = pressure head z = elevation head h = total head 16  POROUS MEDIA Up to this point, we have discussed the nature of water and the distribution of fluid energy (i.e., hydraulic head) in a flow system. Now we will turn our attention to the material through which the water flows. This topic will deal with the various aspects and properties of porous media, including: - Porosity - Permeability and hydraulic conductivity of porous media - Variability of these parameters with respect to location and direction - Measurement of these parameters Porosity All geologic materials have some amount of pore space, or empty space, in them. The term porosity (n) refers to the fraction of the total volume of a rock or sediment that is pore space. More precisely, it is defined as the volume of the voids divided by the total volume, or n = Vvoids/V total 17 3  POROSITY solids pores All porous geologic materials consist of solids surrounded by pore spaces “Porosity” = pore volume / total volume (where total volume = pore vol.. + solid vol..) Porosity is normally expressed as a percentage of the total volume of the material Example: if total aquifer volume = 1 ft3 & pore volume = 0.3 ft3 then porosity = 0.3 ft3 / 1.0 ft3 = 0.30 or 30 %  IMPORTANCE OF POROSITY  The pore spaces within a geologic material are potential “storage space” for ground water  The more pore space a material has (as indicated by it’s “porosity” measurement), the greater its potential to hold ground water  Fine-grained materials, such as clay or fine sand, have smaller individual pore spaces than coarser-grained materials  BUT, fine-grained materials also have a much larger number of pores ….so both fine- and coarse-grained materials can have nearly the same overall porosity  Geologic materials consisting of a mixture of fine- and coarse-grained materials tend to have lower porosity than either the fine- or coarse-grained components in the mixture because the smaller particles partially fill pore #19 spaces between the larger particles  VOID RATIO We can also identify a quantity called the Void Ratio (e), which is defined as the volume of voids divided by the volume of solids, or e = Vvoids/V solids where: e n 1 n Vvoids = volume of the voids [L3] Vsolids = volume of the solids [L3] Geologic materials are never completely dry; there is always some volume of water in them. We can think about geologic materials as composed of multiple phases – a solid phase, a water phase, and a gas phase. In turn, the solid phase can be further divided into its individual mineral phases. 20  MOISTURE CONTENT Moisture content can be expressed as gravimetric moisture content (i.e., moisture content by weight) or volumetric moisture content (i.e., moisture content by volume). The gravimetric moisture content (ω) is the weight of water in the sample divided by the weight of the solids in the sample, or: ω = weightwater/weight solids Volumetric moisture content (ɵ) is the volume of water in the sample divided by the total volume of the sample, or: ɵ = volumewater/volume solids Note that volumetric moisture content is always some value less than the porosity of the rock. We can also express the moisture content as the degree of saturation (Sd), which is the percentage of the void volume that is filled with water, or: Sd = volumewater/volume voids 21 If we combine the equations for porosity, moisture content, and degree of