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210 GANIC CONTAMINANTS IN SUBSURFACE ENVIRONMENTSCD B i � h -� ity of an area surrounding a wVl(Murdoch et al., 1990) This metho v s involves pumping fluid into a well until pressures exceed a critical valu4 LO beyond which fracturing begids. Sand is then pumped into the fracture LD ` to hold them open and provide a high permeability channel Th + 10 method, developed for enhanced oil recovery many years ago, is espe cially useful in regions of low permeability, and may help to improv a ' 10113 remediation methods that depend on mobilizing contaminants(grounc io r i3 fill^-y td water pumping, soil venting, etc.). F _ The main component of advection is the average linear groundwatc m ; .d ` I /' w velocity, which depends on the hydraulic conductivity and gradient o n t , t t the media,The average linear groundwater velocity is greatest in home w t C 1 geneous sods with high porosity, and with more soluble consdlue—, The conductivity can be increased for the purpose of remediation 22 is 4y flushing, extraction, etc.) by increasing the gradient. , The grain size distribution,easily measured in a laboratory from a so , sample, should be used to find the following parameters s sym 1-s. Appioiihnate Temperature of groundwater In the owterminaus United ' States at depths of 10 to 25 metals(degrees CeWue) Source 1`180, Dw — grain size diameter larger than 604►% (by weight) of the soil's 1993. grants t' , Dv - median grain size � D� = grain size diameter larger than 10% (by weight) of the soiPs ` large as it is at 25°C. Groundwater temperature within any region is grains , relatively stable, although it does vary across larger areas(Figure 8-5). The uniformity coefficient is equal to D�/D,�. Once these valuta ar� Advection. The average linear groundwater velocity (N), defined is known, they can be used with Figure 8-6 (Robson, 1978) to rind lla-n equation 8 3, is the main component of the advective term in equation hydraulic conductivity The range of values for hydraulic conductivity i 8.1, The hydraulic conductivity (K) and hydraulic gradient (dh/dl) very large(Table 8-1), from JO0 to 10-" cm/s Hydraulic conductivit• strongly influence the average linear groundwater velocity values can be measured in the field through a variety of aquifer les The saturated hydraulic conductivity is often measured rather than methods(e.g.,slug test or pump test).For neutral organic compoune r the intrinsic permeability of a media. water, typical groundwater velocities may range from tens to hundeW , of meters/year Naturally-occurrmg hydraulic gradients are typically of K r Los (see equation 8.3) the order of o.Ol (dh/dl), is Retardation. Contaminants dissolved in groundwater can become re ,where K - saturated hydraubc conductivity(crNsec) tarded by sorption onto mineral grains in soils or rocks in the saturatec k = intrinsic permeability(cm) zone.Sorption retards the movement of a contaminant compared to the P - density of fluid (g/cm') groundwater movement (See locus no. 4 and no 9) When partitionint g - sravitational constant(cm/sect � = dynamic viscosity tg/crn-sac) of a contaminant between the liquid and solid phases is eompleteh-u reversible, the retardation of the contamnant relative to the bulk mas- , H draulic conductivity is related to the grain stye of a porous media of the groundwater can be described by equation 14. t y y a in a mixture of reactive contaminants that are dissolved it N (see Figure 8-6). A simple empirical relationship to estimate the sato- S pea cn rated hydraulic conductivity Is based on the grain size distribution. groundwater will travel at different rates depending on their retardatior Hydraulic fracturing may also be useful for increasing the conductiv- factors (see Eq 8.4) Thus, the contaminant plume will exhibit a chro