Contaminants can migrate directly into ground water from below-ground sources (e.g. storage tanks, pipelines) that lie within the saturated zone. Additionaly contaminants can enter the ground-water system from the surface by vertical leakage through the seals around well casings, through wells abondoned without proper procedures, or as a result of contaminant disposal of improperly constructed wells (Boulding & Ginn 2004).
Generally three processes can be distinguished which govern the transport of contaminants in groundwater: advection, dispersion and retardation. Dispersion and density/viscosity differences may accelerate contaminant movement, while retardation processes can slow the rate of movement. Some contamination problems involve two or more fluids. Examples include air, water and organic liquids in the unsaturated zone, or organic liquids and water in an aquifer. Tracers are useful for characterizing water flow in the saturated and unsaturated zone.
The term advection refers to the movement caused by the flow of groundwater. Ground-water flow or advection is calculated based on Darcy's law. Particle tracking can be used to calculate advective transport paths (Walter & Masterson 2003). Particle tracking is a numerical method by placing a particle into the flow field and numerically integrating the flow path.
Dispersive spreading within and transverse to the main flow direction causes a gradual dilution of the contaminant plume (Figure 1). The dispersive spreading of a contaminant plume is due to aquifer heterogeneities (Figure 2). Dispersion on the macroscopic scale is caused by variations in hydraulic conductivity and porosity. Solute transport can be influenced by preferential flow-paths, arising from variations of hydraulic conductivity, at a decimeter scale (Zheng & Gorelick 2003).
Figure 1: Transport processes of contaminants in groundwater
Figure 2: Diagram illustrating the velocity variation within an individual pore (after Zheng & Bennett 1995)
Two major mechanisms that retard contaminant movement are sorption and biodegradation (see Microbiology).
If the sorptive process is rapid compared with the flow velocity, the solute will reach an equilibrium condition with the sorbed phase and the process can be described by an equlibrium sorption isotherm. The linear sorption isotherm can be described by the equation:
C* = Kd C
where C* = mass of solute sorbed per dry unit weight of solid (mg/kg)
C = concentration of solute in solution in equilibrium with the mass of solute sorbed onto the solid (mg/l)
Kd = distribution coefficient (L/kg)
5. None aqueaous phase liquids (NAPL)
Organic liquids that have densities greater than water are referred to as DNAPL (dense nonaqueous phase liquids). Nonaqueous phase liquids that have densities less than water are called LNAPLs (light nonaquesous phase liquids). Contamination by LNAPL typically involve spills of fuels like gasoline or jet fuel (Figure 3).
Figure 3: Migration Patterns for NAPL
5. Richards Equation
In unsaturated flow the pore water is under a negative pressure caused by surface tension, which is known as matric potential (ψ). The matric potential is a function of the water content (), temperature, and bulk density of the soil (Fetter 1999). The first to recognize the basic laws for the flow of water in soil was Buckingham (1907).
The Buckingham flux law is:
q = -K(ψ) ()
where q = the soil moisture flux (L3L-2T-1)
K(ψ) = the unsaturated hydraulic conductivity (LT-1) at a given ψ
() = the gradient of the total soil water potential, = ψ + Z (LL-1)
The Richards Equation combines the Buckingham Flux law with the continuity equation for soil moisture:
EPA “Modeling Subsurface Transport of Petroleum Hydrocarbons”
Journal of Contaminant Hydrology
Fetter, C.W. (1999): Contaminant Hydrogeology
Boulding, J.R., Ginn, J.S. (2004): Practical handbook of soil, vadose zone and ground water contamination: assessment, prevention and remediation, CRC Press.
Walter, D.A., Masterson, J.P. (2003): Simulation of Advective Flow under Steady-State and Transient Recharge Conditions, Camp Edwards, Massachusetts Military Reservation, Cape Cod, Massachusetts, Water-Resources Investigations Report 03-4053, USGS
EPA (1994) Symposium on Natural Attenuation of Ground-Water, EPA/ 600/R-94/162
EPA (2004): How to Evaluate Alternative Cleanup Technologies
Storage Tank Sites: A Guide for Corrective Action Plan Reviewers, (EPA 510-B-94-003; EPA 510-B-95-007; and EPA 510-R-04-002)