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Ground Water Quantity and Quality: Site Assessments
Investigations to determine ground water quantity and quality are undertaken at various sites in Maine to define potential ground water supplies, to disclose the areal extent and severity of existing contamination, and to predict changes in ground water quantity and quality that might come about as a result of some proposed development. Exploration techniques for ground water supplies were discussed previously under "Wells: Exploration." Site assessments more specifically aimed at defining existing or possible ground water contamination often utilize the same basic investigative techniques as used in water supply studies, but with different emphasis on details. The most often used technical approaches for these site assessments are discussed below.
Site Reconnaissance and Aerial-Photograph Interpretation
After study of existing data, aerial-photograph interpretation and on-site hydrogeologic reconnaissance are typically undertaken to identify major geologic features such as glacial deposits and bedrock fracture structures which may affect ground water availability and movement. Cultural features, such as homes and existing wells, as well as many other items of importance to a site assessment can be identified and plotted on maps.
Geophysical investigations use some physical property of porous media and the fluids in them to identify subsurface characteristics such as the depth to the water table, presence of contaminants, and the depth to bedrock. Commonly used methods are seismic refraction profiling, electrical resistivity soundings and profiles, and terrain conductivity profiles, all of which are run across the ground surface of the site. Seismic refraction uses the acoustical properties of geologic strata to generally define their thickness, depth, texture, and saturation. Electrical resistivity uses the electrical properties of these formations and the fluids in them (ground water and contaminants) to determine similar properties, but more often to determine the presence and areal extent of certain contaminants. Terrain conductivity is similar to electrical resistivity, but best used to map plumes of ground water contaminants. Geophysical investigations are often a prelude and guide to subsequent detailed test boring investigations.
Test Borings and Monitoring Wells
Test borings are made using a variety of techniques including auger, drive casing, and air percussion, to sample and explore glacial overburden and underlying bedrock. Samples of overburden are often collected by a split spoon device that is driven into the material, while bedrock is typically sampled by cutting a continuous core with a diamond bit. These methods provide a log of the composition, thickness, and texture of subsurface materials. Visual logs are sometimes augmented by subjecting the collected samples to laboratory analysis.
Hydraulic conductivity of subsurface materials is measured in test borings at various depths as they are drilled. A typical technique is to drive steel casing to a particular depth, wash out the casing, and fill the casing with water and observe the change in water level with time. The "falling head" of water in the casing will be more rapid in permeable sediments such as sand and gravel, than in poorly permeable sediments such as silt and clay. There are, in addition, other field techniques used to measure hydraulic conductivity in boreholes and in monitoring wells.
Once the drilling, sampling, and permeability measuring in a test boring are completed, a monitoring well is usually installed to observe ground water levels over a period of time and from which to collect ground water samples for chemical analysis. There are numerous materials used for construction of monitoring wells depending on the particular application. Many monitoring wells are built of PVC pipe (polyvinyl chloride plastic), using a slotted section for the well screen and solid pipe for the riser from the top of the screen to above ground surface. Also used for their non-reactive chemical nature are teflon, stainless steel, and polypropylene pipe.
Monitoring wells may be built to measure a water table or to measure ground water flow potential at one or more depths below the water table. Often a "cluster" of closely-spaced wells is constructed with one at the water table depth, a second at some intermediate depth, and the third just above or below the bedrock surface as illustrated by Figure 46. In some cases, two or more wells of a cluster may be in the bedrock aquifer. Water levels measured in these multiple depth wells are used to define the vertical components (vertical hydraulic gradient and direction of flow) of ground water flow. For example, in a recharge area, a well cluster would show a downward flow gradient, while in a discharge area an upward flow gradient would be shown. Similar minitoring points across a study area are used to map the horizontal components (horizontal hydraulic gradient and direction of flow) of the water table and of deeper (intermediate and regional) flow systems if they occur beneath the site. In all cases, it is prudent to take several sets of water level measurements spaced by several days or weeks in order to be sure the monitoring wells have equilibrated with the natural ground water systems. Furthermore, where detailed water level information is required, elevation of the measuring point at each well (usually the top of the casing) is determined by land survey.
Site assessment data, usually derived from completion of test borings and monitoring wells, include horizontal flow nets (Figure 47) and vertical flow nets (Figure 48). Figure 47 shows contour lines that represent the flow potential of shallow ground water (in this instance these equipotential lines describe the water table), and shows representative flow lines that indicate directions of shallow ground water movement. Figure 48 is a geologic cross section along line A-A' that illustrates the vertical characteristics of the shallow ground water flow system shown in Figure 47. These two flow nets were drawn from test boring and monitoring well data to graphically define the hydrogeologic conditions that controlled the movement of gasoline that leaked from a buried gasoline tank at the point marked in Figure 47.
Computer study of hydrogeologic problems is used to analyze and assess future conditions or, rarely, to reconstruct some past event such as a contaminant spill. There are a great variety of computer programs developed for different hydrogeologic settings and for different needs. Some programs deal with changes in flow potential and direction, while others can handle both ground water flow and migration of dissolved contaminants. There are three-dimensional programs available, as well as the more commonly used two-dimensional programs. Whatever the particular program, all require input of hydraulic data that are peculiar to the site being assessed. For general analysis, assumed hydraulic values are sometimes uses, but for detailed site analysis, field measured values of such things as hydraulic conductivity, hydraulic gradient, and saturated thickness must be used.
The computer program is first constructed to mimic the natural ground water flow conditions that are either assumed or are field measured. Once this "history matching" is completed within an acceptable limit of error, the program is run to evaluate future situations. In water supply work, for example, the interference among several proposed and existing municipal wells might be studied, while in contaminant assessment work, the future dimensions and extent of a plume might be investigated. In this way a great variety of pumping rates, waste-disposal methods, contaminant clean-up plans, and many other hydraulic situations are evaluated so that future conditions can be anticipated and the best possible alternatives can be selected for implementation.
In utilizing computer models, it must be remembered that the output is only as good as the field data used in the model. There is always a temptation to use reconnaissance-level data to predict the exact outcome of a land-use decision. Computer analysis is a valuable tool when used in conjunction with a well-designed field investigation program.
Last updated on March 25, 2009
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