This section discusses how field studies can be used in geochemical fate assessment and includes six cases of deep-well-injection facilities, documenting the geochemistry of the injected hazardous and other industrial wastes. Each case study is organized in the same format, with section headings as follows:
1. Injection Facility Overview describes the type of facility, its current status, and the characteristics of the injected wastes, and presents a brief history of injection and monitoring activities, including the distance traveled by the waste.
2. Injection/Confining-Zone Lithology and Chemistry provides information on the geology and chemistry of the injection zone formation fluids.
3. Chemical Processes Observed briefly describes the types of interactions and major physical effects that have been observed at the site and evaluates their significance.
Table 20.19 summarizes information about each study, including the location of the well, the lithology of the injection zone, waste characteristics, and the major geochemical processes observed. Current commercial-hazardous-waste, deep-well-injection facilities can be found on the Environment, Health and Safety Online (EHSO) web site.163
Field studies are an important complement to geochemical modeling and to laboratory studies. The following are two ways to investigate the interactions between injected wastes and reservoir material:
1. Direct observation of the injection zone and overlying aquifers using monitoring wells
In both instances, samples of the fluids in the zone are collected at intervals to characterize the nature of geochemical reactions and to track changes over time.
Monitoring wells drilled into the injection zone at selected distances and directions from the injection well allow direct observation of formation water characteristics and the interactions that occur when the waste front reaches the monitoring well. When placed near the injection well in the aquifer above the confining layer, monitoring wells can detect the upward migration of wastes caused by casing or confining-layer failure. Foster and Goolsby164 describe detailed methods for constructing monitoring wells.
Monitoring wells have several advantages, in that time-series sampling of the formation over extended periods is easy and the passage of the waste front can be observed precisely. Disadvantages include cost and the potential for upward migration of wastes if monitoring well casings fail. A monitoring well at the Monsanto plant had to be plugged when unneutralized waste reached it because of fears that the casing would corrode. The three Florida case studies and the North Carolina case study illustrate the usefulness of monitoring wells.
Backflushing of injected wastes can also be a good way to observe waste/reservoir geochemical interactions. Injected wastes are allowed to backflow (if formation pressure is above the elevation of the wellhead) or are pumped to the surface. Backflowed wastes are sampled periodically (and rein-jected when the test is completed); the last sample taken will have had the longest residence time in the injection zone. Keely165 and Keely and Wolf166 describe this technique for characterizing
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