Plumes, contaminant

Groundwater monitoring is a necessary component in any investigation of subsurface contamination. A wide variety of information can be gleaned from the data including groundwater velocity and direction, and contaminant identification and concentration. These data can be combined with other observations to infer various characteristics of the contamination. Examples are source and timing of the release, and future location of the contaminant plume. 

Pour typical weU patterns for contaminant plume containment are described in Ref. 16. The first is a pair of injection-production weUs. The second is a line of downgradient pumping weUs. The third is a pattern of injection-production weUs around the boundary of a plume. The fourth, the double-cell system, uses an inner ceU and outer recirculation ceU, with four ceUs along a line bisecting the plume in the direction of flow. Two other methods of plume containment are bio filters and a fuimel-and-gate system, which are described in the in bioremediation section. 

Determine exposure via groundwater define contaminant plume for evaluation of interception methods... 

Electromagnetic (EM) Conductivity Measures the electrical conductivity of materials in microohms over a range of depths determined by the spacing and orientation of the transmitter and receiver coils, and the nature of the earth materials. Delineates areas of soil and groundwater contamination and the depth to bedrock or buried objects. Surveys to depths of SO to 100 ft are possible. Power lines, underground cables, transformers and other electrical sources severely distort the measurements. Low resistivities of surficial materials makes interpretation difficult. The top layers act as a shunt to the introduction of energy info lower layers. Capabilities for defining the variation of resistivity with depth are limited. In cases where the desired result is to map a contaminated plume in a sand layer beneath a surficial clayey soil in an area of cultural interference, or where chemicals have been spilled on the surface, or where clay soils are present it is probably not worth the effort to conduct the survey. 

When groundwater contaminant plumes are suspected of having significant depth as well as lateral distribution, a three-dimensional array of monitoring points is needed to identify and characterize such plumes. Thus, groundwater data must be obtained from a number of different locations and from a number of different depths at each location. As a result, either a large number of drillholes are required, each with separate instrumentation installed, or instruments must be combined and installed at multiple levels in each of a smaller number of drillholes. 

Nadeau, R., J. Lafornara, G. Klinger and T. Stone. Measuring Soil Vapors for Defining Subsurface Contaminated Plumes. Management of Uncontrolled Hazardous Waste Sites Proceedings, Hazardous Materials Control Research Institute, Washington, D.C., 1985. 

FIGURE 10.48 The air shower principle creates a zone of clean air around the worker and pushes the contaminant plume away from the breathing zone. 

Groundwater contaminant plumes from accidental gasoline releases often contain methyl-tert-butyl ether. Experiments with certain soil microorganisms showed that a culture able to degrade methyl-tert-butyl ether did not degrade benzene and toluene. Further interactions were observed [468]. 

The nonaction alternative (1) provides no control of exposure to the contaminated soil and no reduction in the risk to human health posed through the groundwater. It also allows for possible continued migration of the contaminated plume and further degradation of the groundwater. 

FIGURE 18.4 Schematic of contaminant plumes showing methods by which groundwater can be contaminated. 

Groundwater models and other analytic techniques are available to assist in proper pump siting, choosing pump capacities, and calculating the movement of the contaminant plume. The characteristics of the aquifer, the flow of groundwater, and the size of the plume should be known. 

May cause a lateral spread of dissolved or separate phase contaminant plume Contamination may be transferred from groundwater to die vadose zone Has limited applicability at sites with confined aquifers Low soil permeability or other heterogeneous conditions may reduce effectiveness... 

May affect natural groundwater flow gradients at a site, potentially resulting in lateral or vertical migration of the contaminant plume... 

Has the potential to cause a lateral spread of dissolved or separate phase contaminant plumes. 

Electron donors—In direct biodegradation pathways, the contaminant acts as the electron donor or substrate. However, during cometabolic degradation, a different electron donor is metabolized, resulting in the consequential oxidation of the contaminant. In some contaminated plumes, other electron donors, such as other constituents of gasoline, may also be present. In cases where they are not, and cometabolic degradation pathways are desired, electron donors may be added. 

Important issues in groundwater model validation are the estimation of the aquifer physical properties, the estimation of the pollutant diffusion and decay coefficient. The aquifer properties are obtained via flow model calibration (i.e., parameter estimation see Bear, 20), and by employing various mathematical techniques such as kriging. The other parameters are obtained by comparing model output (i.e., predicted concentrations) to field measurements a quite difficult task, because clear contaminant plume shapes do not always exist in real life. 

Surface-water samples are usually collected manually in precleaned polyethylene bottles (from a rubber or plastic boat) from the sea, lakes, and rivers. Sample collection is performed in the front of the bow of boats, against the wind. In the sea, or in larger inland lakes, sufficient distance (about 500 m) in an appropriate wind direction has to be kept between the boat and the research vessel to avoid contamination. The collection of surface water samples from the vessel itself is impossible, considering the heavy metal contamination plume surrounding each ship. Surface water samples are usually taken at 0.3-1 m depth, in order to be representive and to avoid interference by the air/water interfacial layer in which organics and consequently bound heavy metals accumulate. Usually, sample volumes between 0.5 and 21 are collected. Substantially larger volumes could not be handled in a sufficiently contamination-free manner in subsequent sample pretreatment steps. 

The method or methods employed to remediate an aquifer vary, depending on the type, degree, and extent of contamination. Where pollution is shallow and dispersed over a small area, the sediments can sometimes be dug up and transported to a landfill designed especially to isolate the contaminants. Permeable reaction barriers can be installed to intercept a contaminant plume and strip pollutants from it, if the plume is shallow and narrowly focused. 

Radiation from a dirty bomb can emanate from a blast site in a contaminated plume of smoke or in contaminated debris. Radiation cannot be detected without special instruments, and radiation exposures can occur even without direct contact. Therefore, leaving a damaged building does not eliminate the risk of exposure. An effective tool to minimize or eliminate the potential for hazardous substance exposure is to move away from the site of the attack and into a building that provides protection from airborne contaminants. 

Large regional-scale dissolved contaminant plumes have been reported in many parts of the country. In California, many of the alluvial basin aquifers (i.e., major groundwater basins) were found to be impacted by chlorinated solvents during the period between 1979 to 1981. Beneath one valley a dissolved TCE plume incorporated the upper 50 ft of the saturated zone of a water table aquifer, was over 3 miles wide, and extended hydraulically downgradient over 14 miles. 

In all these cases, several characteristics of dissolved contaminant plumes and NAPL pools are assumed ... 

The use of degradation rates to determine the age of a contaminant plume assumes a contaminant from a single, slug-type release, which degrades to a more recalcitrant chemical compound (i.e., trichloroethene degrading to 1,2-dichloroethene), will enter the water table or aquifer at a point in time when none of the daughter product is present in the aquifer ... 

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