GROUND WATER :GROUND WATER


GROUND WATERScience of the Occult :GROUND WATERScience of the Occult Ground Water or Groundwater Loosely, it is all water below the Earth’s surface; thus, also known as subsurface water Too general a definition for technical use More specifically, ground water is subsurface water within the zone of saturation; a more narrow definition to distinguish ground water from all other subsurface water Zone of Saturation or Saturated Zone Subsurface zone in which all the void spaces within the rock or sediment are filled with water A.K.A., known as the phreatic zone Unsaturated Zone Subsurface zone in which all void spaces in the rock or sediment are not filled with water A.K.A., known as the vadose zone or zone of aeration


More Definitions :More Definitions Water Table The top of the zone of saturation in an unconfined aquifer; therefore, the boundary between the zones of saturation and unsaturation that is at atmospheric pressure Distinct within a well; however really a transition between the zone of saturation and unsaturation delimited by the capillary fringe above the zone of saturation Typically a subdued representation of the ground surface With enough data points, configuration can be mapped liked topography Aquifer A formation, group of formations, or part of a formation that contains sufficient saturated, permeable material (rock, sediment, or soil) to yield significant quantities of water to wells or springs -- significant = economic Formation Geologic term that designates a specific assemblage of sediments or rock Typically mappable


EVEN MORE DEFINITIONS :EVEN MORE DEFINITIONS Permeability The property or capacity of a porous rock, sediment, or soil to transmit fluid Technically, permeability is an intrinsic property of the porous medium through which fluid moves and is independent of the properties of the fluid Hydraulic Conductivity The volume of water that will move in unit time under a unit hydraulic gradient through a unit area of a porous medium, which is at right angles to the direction of flow Unlike permeability, hydraulic conductivity is a function of not only the porous medium through which the fluid moves, but of the fluid itself Not uncommonly, in the fields of ground water hydrology and hydrogeology, the two terms are used interchangeably Porosity The volume of pore (void) space in a rock, sediment, or soil, commonly expressed as a percentage of that volume as compared to the total volume of the porous medium -- could be intergranular, fracture, or solution porosity Pore spaces are also known as “interstices”


POROSITY :POROSITY Rock, sediment, or soil may have pore space, but if not interconnected, lacks permeability


SATURATED ZONE, UNSATURATED ZONE, AND WATER TABLE :SATURATED ZONE, UNSATURATED ZONE, AND WATER TABLE


CONFINED AND UNCONFINED AQUIFERS :CONFINED AND UNCONFINED AQUIFERS Unconfined Aquifer An aquifer having a water table; i.e., the zone of saturation does not occupy the full thickness of the aquifer Upper surface of the zone of saturation is at atmospheric pressure, as expressed by the water level in a well A.K.A. known as a “water table aquifer” Confined Aquifer An aquifer bounded above and below by confining beds A confined aquifer does not have a water table The pressure at the top of the aquifer is greater than atmospheric pressure The water level in a well drilled into a confined aquifer will rise above the top of the aquifer until reaching atmospheric pressure or until flowing onto the ground With enough data points can map the configuration of the pressure surface A.K.A. known as an “artesian aquifer”


CONFINED AND UNCONFINED AQUIFERS ILLUSTRATED :CONFINED AND UNCONFINED AQUIFERS ILLUSTRATED


DARCY’S LAWHenry Darcy, 1856 :DARCY’S LAWHenry Darcy, 1856 Movement of Ground Water Discharge (Q) proportional to the cross-sectional area (A) and to the hydraulic gradient (i); Q = volume/unit time Hydraulic gradient (i) is the change in height over the change in length; it is, thus, dimensionless Given the definition of hydraulic conductivity, it (K) becomes a coefficient of proportionality with dimensional units necessary Thus: Q=KiA


DARCY’S LAW (Continued) :DARCY’S LAW (Continued) Discharge Volume Q= Volume/unit time, e.g., feet3/day A = Feet2 i = Feet/Feet, so is dimensionless Therefore, K = feet/day Discharge Velocity Velocity (V) = feet/day = Q/A V = Ki = Darcy Velocity To get true velocity of flow through a porous medium, divide by effective porosity “n” because some of the flow in A is blocked by solids Ground Water Velocity K for a clean sand is on the order of 103 to 104 But i on the order of 10-3 to 10-4 So, ground water velocities tend to be low -- a fraction of feet/day or a few feet/year


CONTINUITY EQUATIONConservation of Mass :CONTINUITY EQUATIONConservation of Mass Q(in) = Q(out) ± Change in Storage Two Types of Storage Storage within the porous medium Artesian storage Storage Within Porous Medium Storativity is the volume of ground water an aquifer takes in or releases from storage per unit area per unit change in water level Approximately equal to porosity A.K.A. Specific Yield Storage within a Confined Aquifer; i.e., artesian storage Same definition, but artesian storage is related to the slight compressibility and expandability of water and the aquifer Neither the aquifer or the water is highly compressible or expandable Thus, very little water is released from or taken into artesian storage by a unit change in water level Said another way, it takes a large change in water level in a confined aquifer to yield as much water as a small change in water level in a water table aquifer, but a decrease in artesian storage still leaves the aquifer full of water


CONTINUITY EQUATION (Cont.) :CONTINUITY EQUATION (Cont.) Q(in) = Recharge and Leakage from Sub- and Superjacent Aquifers Recharge is from precipitation falling on the Earth’s surface and leakage from streams and other water bodies; typically a few percent of average annual rainfall Q(out) = Ground Water Withdrawals and Leakage Withdrawals through wells and leakage to streams and surface water bodies, and leakage to sub- and superjacent aquifers


GROUND WATER DISCHARGE TO WELLS AND SURFACE WATER BODIES :GROUND WATER DISCHARGE TO WELLS AND SURFACE WATER BODIES


WELLSDischarge or Withdrawal of Ground Water :WELLSDischarge or Withdrawal of Ground Water Must Depress Water Table or Artesian Pressure Surface (Piezometric Surface) to withdrawal ground Water From Wells Inverted cone called cone of depression


IF Q(out) >> Q(in)Amount of Ground Water in Storage Must Decrease :IF Q(out) >> Q(in)Amount of Ground Water in Storage Must Decrease


COMMON HAZARDS :COMMON HAZARDS Leakage from underground storage tanks Poor waste disposal practices Accidental releases of chemicals Wrecks Pipeline breaks Use of agricultural chemicals Salt water intrusion Salt water disposal Land surface subsidence


LESS WELL KNOWN HAZARDSSOCIETAL QUESTIONS :LESS WELL KNOWN HAZARDSSOCIETAL QUESTIONS • Should ground water be treated as a depletable resource as are mineral resources? • Should ground water be privately owned as are mineral resources? • Should withdrawal of ground water for sale (to highest bidder?) be treated as a private property right? • Should water supply needs in a thriving urban area be justification for taking ground water from a rural area? • Should current needs now in one area be justification for taking ground water resources from areas with a surplus of ground water thereby limiting ground water availability for future generations and economic growth in those areas?