Alternatives for Ground Water Cleanup (1994) / Chapter Skim
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2 Complexity of the Contaminated Subsurface
2 Complexity of the Contaminated Subsurface
Pages 35-79
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From page 35... ...
The complexities of the subsurface environment and of contaminant distribution, which are documented in this chapter, significantly complicate the cleanup task (see Box 2-1~. Although this chapter focuses on factors that complicate ground water cleanup, the arguments should not be interpreted to mean that restoration of ground water is impossible on theoretical grounds.
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From page 36... ...
Consolidated aquifers consist of more or less solid rock; they store water primarily in solution channels, fractures, and joints (although in material such as sandstone, some water may also be stored in interstitial pore spaces)
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From page 37... ...
As the flow lines indicate, the direction of ground water flow in such aquifers depends on the locations of the fractures and thus is often tortuous and difficult to predict. SOURCE: From Heath (1980)
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From page 38... ...
The hydraulic conductivity of a sandy aquifer is approximately two orders of magnitude greater than the hydraulic conductivity of a silty sand aquifer (average hydraulic conductivities for these types of materials are 30 meters per day and 0.3 meters per day, respectively)
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From page 39... ...
Flow paths in fractured rock are often difficult to ascertain because the fractures are not uniformly distributed and may not be interconnected. The fundamental law describing ground water flow is known as Darcy's Law, which can be expressed as follows:
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From page 40... ...
, which quantifies the pressure and gravity forces that drive flow and which is influenced by aquifer recharge, elevation, and pumping. Darcy's Law states that the rate of ground water flow is determined by the magnitude of the hydraulic gradient and the magnitude of the hydraulic conductivity of the aquifer material.
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From page 41... ...
In the top diagram, the wells are placed in a line perpendicular to the direction of ground water flow, and little recharge water reaches the discharge well. In the bottom diagram, the line from the recharge well to the discharge well is parallel to the direction of ground water flow, and nearly all of the water from the recharge well reaches the discharge well.
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From page 42... ...
Contaminants and products from microbial activity in the subsurface can also cause important changes in the water chemistry. As shown in Figure 2-4, as water moves through the unsaturated zone the concentrations of solutes increase as soil gases and minerals (which may contain metals such as iron and manganese)
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From page 43... ...
~ co N .,~ .
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From page 44... ...
Two broad classes of bacteria play important roles in the subsurface and in the development of new ground water cleanup technologies: aerobic and anaerobic. Aerobic organisms require oxygen to degrade or _, , ,~ , .
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From page 45... ...
45 On o ._ Cat V)
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From page 46... ...
Uncontaminated ground water in shallow aquifers is nearly saturated with dissolved oxygen. However, when the oxygen supply is depleted due to microbial consumption of organic matter, it is replenished slowly by oxygen dissolved in water that recharges the aquifer and to a lesser extent by diffusion from the unsaturated zone.
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From page 47... ...
In addition, leaking storage tanks that held leaded gasoline may be sources of lead in ground water. Detection of inorganic compounds at low concentrations at hazardous waste sites can also result from natural weathering processes or sampling procedures rather than from industrial wastes.
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From page 48... ...
The characteristics of the "carrier" fluid can greatly influence the migration pathways and persistence of a specific compound. NAPLs, in particular, have been the focus of substantial research and regulatory concern because NAPLs are extremely common and have complicated ground water cleanup at many hazardous waste sites.
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From page 49... ...
(Mercer and Cohen, 1990~. Chlorinated solvents are more dense than water and are thus called dense NAPLs, or DNAPLs.
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From page 53... ...
After its formation, this lens will migrate primarily in the direction of natural ground water flow. If the water table changes due to seasonal fluctuations or pumping, the lens can also spread vertically within the formation, enhancing the area of LNAPL contamination.
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From page 55... ...
cause the contaminant to spread as it threads its way through the soil pores. In Figure 2-7b, the effect of this spreading is shown at a larger scale, where the point source is observed to spread in the directions parallel and perpendicular to the average ground water flow direction.
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From page 56... ...
. At times t~ and t2, dispersion has caused the contamination to spread in directions perpendicular and parallel to the average direction of ground water flow.
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From page 57... ...
Vapor-Phase Transport Contaminants that volatilize into the air contained in soil pores can spread laterally and vertically through the unsaturated zone. Subsequent dissolution into flowing ground water or infiltrating recharge water may then enlarge the zone of ground water contamination (Sleep and Sykes, 1992~.
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From page 58... ...
LNAPLs tend to spread laterally at the top of the water table, whereas DNAPLs tend to sink vertically. Table 2-4 shows representative NAPL specific gravities.
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From page 59... ...
It also makes a spill's location quite difficult to detect because of the small lateral dimensions of the finger. Interfacial tension gives rise to the capillary forces that control the extent of lateral spreading of a NAPL as it migrates downward.
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From page 60... ...
Interfacial tensions can be affected by pH and dissolved constituents such as natural humic substances that behave as surface-active agents, reducing capillary forces. Overall, the pathways of NAPL migration are intricate, and the resulting contaminant distribution is highly nonuniform and complex.
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From page 61... ...
The total quantity of a contaminant in a unit volume of aquifer relative to that dissolved in the ground water is termed the "retardation factor." Retardation factors depend on contaminant properties (such as KoW) and soil properties (such as organic carbon content)
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From page 62... ...
30 40 FIGURE 2-9 Influence of sorption on contaminant transport. A contaminant is released at a concentration of 20 mg/liter over a short time period (i.e., in a pulse release)
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From page 63... ...
tend to entrap more contaminants. Due to the strength of the capillary forces holding it in place, the entrapped NAPL cannot be mobilized by ground water flow under typical operating conditions for pump-and-treat systems (Darcy velocities in the range of 0.01 to 50 meters per day)
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From page 64... ...
for five organic contaminants frequently found at hazardous waste sites. For most of these compounds, degradation by aerobic bacteria is faster than degradation by the other types of bacteria.
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From page 66... ...
Transformation rates generally proceed faster by microbial than by abiotic processes, but some abiotic reactions can be fast enough to influence the fate of contaminants in aquifers. One of the most commonly observed ground water contaminants, 1,1-dichloroethene (see Table 1-1)
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From page 67... ...
Because water moves more slowly through silt and clay than through sand and gravel, silt and clay layers can significantly change ground water and contaminant movement from what would occur in a sand and gravel aquifer without silt and clay deposits. Heterogeneities can cause wide variability in hydraulic properties such as hydraulic conductivity.
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From page 68... ...
68 ALTERNATIVES FOR GROUND WATER CLEANUP Laboratory model of the transport of a DNAPL contaminant through an aquifer with varying permeability; note the DNAPL pools above the low-permeability zones (the horizontal discs)
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From page 69... ...
- r -- -- I c7_ Chemical Heterogeneity Variable chemical composition within a formation may also affect contaminant transport over a range of scales. At the pore scale, a vari
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From page 70... ...
70 s o 1 1 1 1 c~ ct ~o 1 1 1 1 ~ z Lo ~co SH313W Nl '13A31 V3S 340aV BO(lililV o U C)
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From page 71... ...
Relatively little is known, however, about the degree to which sorption characteristics vary and are correlated with variations in hydraulic conductivity (Sudicky and Huyakorn, 1991~. Variable mineral composition and organic carbon content may also affect NAPL entrapment mechanisms, possibly creating zones of entrapped NAPL that are less accessible to flowing ground water.
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From page 72... ...
Advances in knowledge about subsurface processes are essential for improving ground water contamination assessments and cleanup technologies. These advances are needed in three broad areas: subsurface characteristics, contaminant transport and distribution, and reaction pathways and rates.
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From page 73... ...
· How can the costs of sampling methods be reduced to allow more extensive sampling at sites? · What new ideas and techniques can contribute to development of a reliable three-dimensional map of subsurface geology and ground water flow patterns at a site?
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From page 74... ...
· What critical parameters and properties govern transport of contaminants in various types of complex subsurface systems? Trapped globules of NAPL in a laboratory column containing a porous medium (glass beads)
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From page 75... ...
The fundamental study of such reaction systems is relatively new, and if pursued may open new vistas for in situ ground water cleanup. Research is needed to address the following questions: · What are the critical chemical and biological reactions affecting contaminants in various types of subsurface environments?
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From page 76... ...
· Subsurface environments have complexities and heterogeneities that make them inherently difficult to decontaminate. The complexity of the subsurface and the difficulty of characterizing it contribute in large measure to the problems experienced in ground water cleanup documented in the following chapters.
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From page 77... ...
1979. Modeling of ground water flow systems as they relate to the movement of contaminants.
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From page 78... ...
1986. A natural gradient experiment on solute transport in sand aquifers 1: approach and overview of plume movement.
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From page 79... ...
1986. A natural gradient experiment on solute transport in a sand aquifer: spatial variability of hydraulic conductivity and its role in the dispersion process.
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