Transport hydraulic conductivity

This section discusses soil liners and their use in hazardous waste landfills. The section focuses primarily on hydraulic conductivity testing, both in the laboratory and in the field. It also covers materials used to construct soil liners, mechanisms of contaminant transport through soil liners, and the effects of chemicals and waste leachates on compacted soil liners. 

If effective porosity and other parameters are known, the time of travel (TOT) for a molecule of waste transported by flowing water through the soil liner can be calculated. TOT equals the length of the particular flow path times the effective porosity, divided by the hydraulic conductivity times the hydraulic gradient (Figure 26.10). 

Calculations show that after 10-30 years, molecular diffusion begins to transport the first molecules of waste 3 ft downward through a compacted soil liner. Accordingly, even with a perfectly impermeable liner with 0 hydraulic conductivity, in 1-3 decades contaminants will begin to pass through the soil liner due to molecular diffusion. 

NAPL will migrate from the liquid phase into the vapor phase until the vapor pressure is reached for that liquid. NAPL will move from the liquid phase into the water phase until the solubility is reached. Also, NAPL will move from the gas phase into any water that is not saturated with respect to that NAPL. Because hydraulic conductivities can be so low under highly unsaturated conditions, the gas phase may move much more rapidly than either of the liquid phases, and NAPLs can be transported to wetter zones where the NAPL can then move from the gas phase to a previously uncontaminated water phase. To understand and model these multiphase systems, the characteristic behavior and the diffusion coefficients for each phase must be known for each sediment or type of porous media, leading to an incredible amount of information, much of which is at present lacking. 

Transport-related non-equilibrium behavior (e. g., physical non-equilibrium) is excluded, which plays an important role in non-ideal solute transport in the field and in some experimental column systems. Physical non-equilibrium is due to slow exchange of solute between mobile and less mobile water, such as may exist between particles or between zones of different hydraulic conductivities in the subsurface soil column, and occurs for sorbing and non-sorbing molecules alike. 

Overland Runoff The fraction of rainfall or irrigation water that flows over a land surface from higher to lower elevations, known as overland runoff, is an additional pathway for contaminant transport. Runoff occurs when the amount of rain or irrigation water is greater than the soil infiltration capacity. The formation of a crust on the soil surface is a major contributor to runoff formation in arid and semiarid zones, because it decreases the infiltration capacity. The soil crust is a thin layer (0-3 mm) with a high density, fine porosity, and low hydraulic conductivity compared to the underlying soil. This skin forms as a result of falling raindrops or sodification of soil clays. 

Research is currently focusing on ways to lower the hydraulic conductivity of engineered biofihns to make them an effective barrier to contaminant transport this includes the introduction of ultramicrobacteria (UMB) to penetrate the soil matrix. 

This expression describes the fastest and most important mode of transport in groundwater. In fact, an important task of the hydrologist is to develop models to predict the effective velocity u (or the specific flow rate q). Like the Darcy-Weis-bach equation for rivers (Eq. 24-4), for this purpose there is an important equation for groundwater flow, Darcy s Law. In its original version, formulated by Darcy in 1856, the equation describes the one-dimensional flow through a vertical filter column. The characteristic properties of the column (i.e., of the aquifer) are described by the so-called hydraulic conductivity, Kq (units m s"1). Based on Darcy s Law, Dupuit derived an approximate equation for quasi-horizontal flow ... 

The physical factors include mechanical stresses and temperature. As discussed above, IFP is uniformly elevated in solid tumors. It is likely that solid stresses are also increased due to rapid proliferation of tumor cells (Griffon-Etienne et al., 1999 Helmlinger et al., 1997 Yuan, 1997). The increase in IFP reduces convective transport, which is critical for delivery of macromolecules. The temperature effects on the interstitial transport of therapeutic agents are mediated by the viscosity of interstitial fluid, which directly affects the diffusion coefficient of solutes and the hydraulic conductivity of tumor tissues. The temperature in tumor tissues is stable and close to the body temperature under normal conditions, but it can be manipulated through either hypo- or hyper-thermia treatments, which are routine procedures in the clinic for cancer treatment. 

Yeung, A.T. (1994) Effects of electro-kinetic coupling on the measurement of hydraulic conductivity. In Hydraulic Conductivity and Waste Contaminant Transport in Soils, ASTM STP 1142 (D.E. Daniel and S.J. Trautwein, eds.). Am. Soc. for Testing Materials, Philadelphia... 

Quinodoz HA, Valocchi AJ (1993) Stochastic analysis of the transport of kinetically sorbing solutes in aquifers with randomly heterogeneous hydraulic conductivity. Water Resour Res 29 3227-3240... 

Sudicky EA (1986) A natural gradient experiment on solute transport in a sand aquifer spatial variability of hydraulic conductivity and its role in the disperion process. Water Resour Res 22 2069-2082... 

Contaminants in the soil compartment are associated with the soil, water, air, and biota phases present. Transport of the contaminant, therefore, can occur within the water and air phases by advection, diffusion, or dispersion, as previously described. In addition to these processes, chemicals dissolved in soil water are transported by wicking and percolation in the unsaturated zone.26 Chemicals can be transported in soil air by a process known as barometric pumping that is caused by sporadic changes in atmospheric pressure and soil-water displacement. Relevant physical properties of the soil matrix that are useful in modeling transport of a chemical include its hydraulic conductivity and tortuosity. The dif-fusivities of the chemicals in air and water are also used for this purpose. 

The success of biorestoration depends on the hydrogeology of the site. If the hydrogeology is complex, success is problematic. Adequate procedures to characterize many sites are currently available. Moreover, the subsurface environment must be sufficiently permeable to allow the transport of the added N, P, and O2 to the microorganisms situated at the various subsurface sites containing the contaminants. This water movement — referred to as hydraulic conductivity—is critical for a positive outcome. 

Results of experiments utilizing crushed rock and equilibration with waste solutions to determine sorption behavior cannot be extrapolated to actual aquifer conditions, even if B.E.T. surface area is known. This technique has been commonly used in the past to assess waste-storage-site safety. As the primary hydraulic conductivity decreases and secondary conductivity becomes more prominent, this methodology becomes less and less viable for input to modeling of waste transport. The method presented in this report should result in more realistic waste-transport modeling. 

Simulation of He transport in the basin coupled with a groundwater flow model allowed the estimation of hydraulic conductivities for the different aquifers and, consequently, the estimation of groundwater residence times. Average turnover times for different aquifers are highly variable, ranging from 8,700 yr for the shallowest aquifer (Ypresian) to 30 Myr for the deepest... 

The flow of groundwater in a sedimentary basin results from the combined influence of the different driving forces for groundwater flow (mechanical, thermal, chemical and electrical driving forces) and the hydraulic conductivity of the subsurface. The transport of grovmdwater, heat and electricity, the mass transport of chemical components and the deformation of the solid part of the subsurface are coupled processes. 

Analysis of vadose zone transport is more complex than analysis of saturated zone transport, in part because changes in soil water content have a strong inbuence on both hydraulic conductivity and pore pressure. How does a decrease in soil water content affect ... 

The sensitivity of the hydraulic conductivity and other transport properties of discontinua (fractured media) to normal stress is typically substantially greater than that of continua (unfractured media). The stress-sensitivity has been demonstrated in numerous studies of fracture flow (e.g.. Gale, 1982). Natural fractures are a suspected cause of anisotropic water-flooding with a maximum rate of flood front advance approximately in the direction of the maximum horizontal stress (Heffer and Dowokpor, 1990). Natural fractures were recognised as significantly contributing to Clair well productivity (Coney et al., 1993). These fractures are aligned with the present day direction of the maximum horizontal stress in at least one of the wells in the Clair Field. 

---