Groundwater potential

Milhome MAE, de Sousa DDB, Lima PDF et al (2009) Assessment of surface and groundwater potential contamination by agricultural pesticides applied in the region of Baixo Jaguaribe, CE, Brazil. Engenharia Sanitaria E Ambiental 14 363-372... 

Contaminant volatilization from subsurface solid and aqueous phases may lead, on the one hand, to pollution of the atmosphere and, on the other hand, to contamination (by vapor transport) of the vadose zone and groundwater. Potential volatihty of a contaminant is related to its inherent vapor pressure, but actual vaporization rates depend on the environmental conditions and other factors that control behavior of chemicals at the solid-gas-water interface. For surface deposits, the actual rate of loss, or the pro-portionahty constant relating vapor pressure to volatilization rates, depends on external conditions (such as turbulence, surface roughness, and wind speed) that affect movement away from the evaporating surface. Close to the evaporating surface, there is relatively little movement of air and the vaporized substance is transported from the surface through the stagnant air layer only by molecular diffusion. The rate of contaminant volatilization from the subsurface is a function of the equilibrium distribution between the gas, water, and solid phases, as related to vapor pressure solubility and adsorption, as well as of the rate of contaminant movement to the soil surface. 

Figure 9 Hydrologic properties of a sandy silicate aquifer in northern Wisconsin (a) hydrologic conductivities (log K, tn s ), (b) groundwater potentials (tn), and (c) groundwater residence times (yr) (reproduced by permission of American Geophysical Union from Water Resources Research, 1992, 28, 579-589).

The driving forces for the flow of groundwater are groundwater potential gradients, temperature gradients, electrical gradients and chemical gradients (De Marsily, 1986 Freeze and Cherry, 1979). 

The net driving force for groundwater flow that results from the groundwater potential gradient only, can thus be expressed as... 

By introducing the appropriate expression for the hydraulic head, i.e. Equation 1.3 or 1.4, into Darcy s equation 1.8, it describes the flow for compressible and incompressible groundwater, respectively. The flow of compressible groundwater can thus be described by a more general form of Darcy s equation, in which the gradient of the groundwater potential is given by Equation 1.5... 

A shallow subsystem, characterized by cross-formational vertical upward flow of groundwater. The groundwater potential increases slightly with depth. The pressure-depth gradient is near hydrostatic to slightly superhydrostatic (Figure 2.10). 

The largest influence of hydrodynamic conditions in the intermediate subsystem on the entrapment conditions can be expected near parts of the basin with the largest vertical and lateral groundwater potential gradients, i.e. near the basin s depocentre. 

Each subsystem of burial-induced groundwater flow is characterized by specific vertical and horizontal changes of the groundwater potential (Section 2.1.3) ... 

A regional picture of changes in groundwater pressure with depth and groundwater potential with depth in combination with potentiometric surfaces constructed for different hydrogeological units permit the delineation of the vertical and lateral extent of the shallow, intermediate and deep subsystems of burial-induced flow in the studied area. 

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