Subsurface Hydrodynamics

The economic and environmental suitability of any proposed well will be determined from surveying, testing, and the design of the well. When surveying and testing to determine site suitability, the confinement conditions, receptor zone and subsurface hydrodynamics should be studied with respect to the concentrate to be injected (Shammas et al. 2009). 

Successful operation of an injection well relies on the prediction of the direction and rate of concentrate flow, the displacement of pre-existing water, and the aquifer pressure change over time can be estimated. A knowledge of the subsurface hydrodynamics is therefore required to assist in these predictions (Shammas et al. 2009). 

Three primary problem areas exist in dating groundwater. These are (1) Formulation of realistic geochemical-hydrodynamic models needed to interpret data which are generated by field and laboratory measurements, (2) development of sensitive and accurate analytical methods needed to measure trace amounts of various stable and unstable nuclides, and (3) theoretical and field oriented studies to determine with greater accuracy the extent and distribution of the subsurface production of radionuclides which are commonly assumed to originate only in the atmosphere. 

The physical transport of dissolved organic compounds through the subsurface occurs by three processes advection, hydrodynamic dispersion, and molecular diffusion. Together, these three cause the spread of dissolved chemicals into the familiar plume distribution. Advection is the most important dissolved chemical migration process active in the subsurface, and reflects the migration of dissolved chemicals... 

Hydrodynamic dispersion refers to the tendency of a solute or chemical dissolved in the fluid, to spread out over time (i.e., to become dispersed in the subsurface). The mechanical component of dispersion results from the differential flow of the fluid through pore spaces that are not the same size or shape, and from different flow velocities and the fluid near the walls of the pore where the drag is greatest vs. the fluid in the center of the pore (Figure 5.4). 

GROUND-WATER RESTORATION Subsurface Effects of Contaminant Mobility Physical Containment Techniques Hydrodynamic Controls... 

Figure 6. Mass fraction of theoretical contaminant mass remaining vs. time for various values of the hydrodynamic dispersivity, a. Atmospheric pressure changes at land surface cause advective subsurface air flow, increasing dispersivity which significantly affects the rate at which volatile contaminants escape from a layer. Reprinted from Auer et al. (1996), Copyright 1996, pg. 157, with permission from Elsevier Science.

Subsurface solute transport is affected by hydrodynamic dispersion and by chemical reactions with soil and rocks. The effects of hydrodynamic dispersion have been extensively studied 2y 3, ). Chemical reactions involving the solid phase affect subsurface solute transport in a way that depends on the reaction rates relative to the water flux. If the reaction rate is fast and the flow rate slow, then the local equilibrium assumption may be applicable. If the reaction rate is slow and the flux relatively high, then reaction kinetics controls the chemistry and one cannot assume local equilibrium. Theoretical treatments for transport of many kinds of reactive solutes are available for both situations (5-10). 

Effective rates of sorption, especially in subsurface systems, are frequently controlled by rates of solute transport rather than by intrinsic sorption reactions perse. In general, mass transport and transfer processes operative in subsurface environments may be categorized as either macroscopic or microscopic. Macroscopic transport refers to movement of solute controlled by movement of bulk solvent, either by advection or hydrodynamic (mechanical) dispersion. For distinction, microscopic mass transfer refers to movement of solute under the influence of its own molecular or mass distribution (Weber et al., 1991). 

The net force acting on a vmit mass of oil or gas immersed in groundwater under hydrodynamic conditions, assuming that the subsurface is isothermal and isochemical, can also be described by Equation 4.11. 

As the magnitude and direction of g are fixed, and the magnitude and direction of - grad Pj/phc) re dependent on the geological subsurface conditions and the physico-chemical characteristics of the hydrocarbon-water system, the only variable that is changed under hydrodynamic circumstances in comparison with hydrostatic circumstances is - grad P PhcJ- From Equations 4.3 and 4.5 it follows that. 

Equations [3-6], [3-7a], and [3-7b] assume that the saturated aquifer thickness b is constant (as in a confined aquifer). In unconfined (phreatic) aquifers, which are somewhat more susceptible to subsurface contamination, the saturated thickness varies as the hydraulic head changes thus b is not, strictly speaking, constant. Unless otherwise stated, however, it is assumed in the following discussions that changes in the water table height of a phreatic aquifer are relatively small compared with the saturated thickness. When this is not the case, more complex expressions are needed to describe the hydrodynamics, and the reader is referred to Bear (1979). 

This book is a comprehensive mforonce on subsurface transport and fate processes. Topics covered include soli and contaminant properties affecting transport and fate, hydrodynamic processes, abiotic processes, biotic processes, physical modeis of contaminant transport, empirical modeis and vulnerability mapping, mathematical modeling of contaminant transport, and applications. 

The main transporter for these migration forms is subsurface water. That is why their mobility is determined by physical and chemical properties of water, gradient of the hydrostatic head, rock and deposit permeability. Aquaphiles migration direction, rate and distance is determined by hydrodynamics and noticeably depend on their depth. With depth, migration rates of both water and its components decline. 

Figure 23.1.1. Breakthrough curve in one dimension showing plug flow with continuous source resulting from advection only and from the combined processes of advection and hydrodynamic dispersion. [From T.H. Wiedemeier, H. S. Rifai, C. J. Newell and J.T. Wilson, Natural Attenuation of Fuels and Chlorinated Solvents in the Subsurface. Copyright 1999 John Wiley Sons. Reprinted by permission of John Wiley Sons.]...

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