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Chapter 9Subsurface HydrologyThe subsurface is invisible and, hence, mysterious. Sto-ries abound about subterranean worlds, underground riversand lakes, and lost civilizations. Because access to the sub-surface is limited to caves, mines, and other small open-ings, few p eople have ever visited this Stygian r e alm.In this chapter we focus on how water flows throughthe subsurface. We start with a ge neral description of thephysical environments that exist in the subsurface, moveon to how water flows through these environments, andconclude with an example from the Savanna h River Sitenear Augusta, GA.9.1 Subsurface FeaturesWater flow is a function o f two factors:• The physical structure of the subsurface with respectto the size and arrange ment of voids, especially thepresence or absence of aquifers , fractures, and otherfeatures; and• The availability of water and the resulting fluid pres-sures that develop.The firs t factor is q uantified using a coefficient called thehydraulic conductivity, which is a measure of the per me-ability of the geologic unit. The se c ond factor is repre-sented using the hydraulic gradient, which is a measure ofthe force driving water through the geologic structure.Because water flows downward due to gravity, a zone ofsaturation - or phreatic zone - develops within the subsur-face. Fluid pressures are positive within the zone of sa t-uration, with the pressure increasing as the depth belowthe water surface increases. Water flow in the saturatedzone is normally close to horizontal in aquifers - zonesof higher permeability - and is normally close to verticalin aquitards, also called confining layers - zones of lowerpermeability.Above the zone of saturation lies the unsaturated - orvadose - zone, where water is bound by capillary forces tothe soil. These forces are the r e sult of adsorptive, or sur-face tension, forces that attract water to soil surfaces, andresult in negative pressures. The unsaturated zone nor-mally extends from the ground surface down to the watertable, which is the surface that separates the saturated andunsaturated zones.Water flow in the unsaturated zone is normally closeto vertical due to the downwar d force of gravity, switchingover to horizontal flow once it r e aches the water table.Zones of perched water may exist within the unsatu-rated zone. Perched water results from the accumulationof downward percolating water on top of layers of lowerpermeability. Water flow in perched layers is more hori-zontal than in the rest of the unsa tur ated zone.The surficial aquifer is the uppermost saturated unitbelow the water table. The surficial aquifer extends fromthe water table downward to a confining layer, below whichlies one or more confined aquifers. Changes in fluid pres-sure within surficia l aq uifer s results in changes in the up-per surface, i.e., the water table, and vice versa.Confined aquifers are sandwiched above and below byconfining laye rs, and acts much as a pipe does. Changes influid pressur e doe s not affect the position of the confininglayer, and thus water in confined aquifer acts more like anincompressible fluid.Fluid HeadWater moves from high head to low head. The forces ofgravity, pressure, and inertia can be combined to yield ageneral equation of the energy status of water, describedby Bernoulli’s equation:h = z +Pγ+v22 g(9.1)where h is the total head, z is the elevation, P is the fluidpressure, γ = ρ g is the fluid weight, ρ is the fluid density,g is the gravitational constant, and v is the fluid velocity.The total head is the variable used to predict the direc-tion and magnitude of fluid flow, in both surface water andgroundwater systems. The velocity (inertial) componentcan generally be neglected if the flow is slow, e speciallytrue in groundwater systems, v ≪ 0. When these inertialeffects in ground-water systems can be neglected, then thetotal head, h (m), can be written using:h = z +Pγ= z + p (9.2)1CHAPTER 9. SUBSURFACE HYDROLOGY 2Drawdown - Water level decline due to pumping fromwell, relative to the baseline (initial) water level be-fore pumping.Hydraulic Gradient - Change in total head per unitdistanceFlux, or Darcian Velocity - The volume o f waterflowing per unit area per unit timeDarcy’s Law - A relationship between water flux andthe product of the hydraulic gradient and the hy-draulic conductivityTotal Head - Sum of elevation, pressure, velo c ity, o s-motic and other potentialsTotal Flow - The cumulative flow over an area, calcu-lated using the product of the flux with the cross-sectional area of flow.Fluid Velocity - The rate at which fluid particles aremoving, equa l to the darcian velocity divided by theeffective porosity.where p = P/γ (m) is the fluid pressure head. Pressurechanges with depth and time can result from barometric(atmospheric pressure) influences, tidal effects, fluid den-sity (sediment, salinity) changes, and vertical flow (non-hydrostatic conditions) within the water column.The pressure term can also be neglected by measuringthe water surface elevation, h = z, which is where the fluidpressure equals zero, p = 0. This simplification requireshydrostatic c onditions, within the monitoring device (e.g.,borehole, piezometer) i.e.:∆p = γ ∆z (9.3)which implies that a decrease in elevation, ∆z, is accom-panied by a corresponding increase in pressure , ∆p, at arate specified by the fluid weight, γ.Thus, the water surface elevation is only an appropri-ate measure of the total head of the system, h ≈ z, giventhat the velocity of the water is sufficiently small, pressurechanges with depth and time can be neglected, and freewater is present.Capillary and Osmotic Forces. Capillary and osmoticforces can also induce fluid movement and thereby affectthe total head. Failure to account for these forces mayresult in incorrect predictions of water flow and transport.Capillary forces arise due to the tendency of soil ma te-rials to abso rb and bind water to soil surfaces. The waterbound to soil sur fa c es resists the downward force of grav-ity, and does not readily drain from the soil. The totalhead must consider the negative fluid pressures that arisedue to these absorbtive forces (also call matric tension).Matric tensions are commonly measured using tensiome-ters.Water can move upward above the regional water tabledue to capillary forces. Finer grained media have greatercapillary forces, which result in higher capillary fringes.The height of the sa tur ated zone


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