Groundwater wells

To characterize flow in the subsurface environment, groundwater wells often are installed. Wells can be used to measure hydraulic head gradients, and thus to determine the direction of groundwater flow to conduct aquifer tests to 

When water is pumped from a production or remediation well, water from the surrounding aquifer enters the well in response to the head gradient created by the water removal. This leads to a lowering of the hydraulic head around the well, forming a cone of depression (Fig. 3-10). It takes some time for this cone of depression to fully develop while it is developing, the flow of water must be analyzed by transient techniques that account for flow changes over time (see Section 3.2.4). Ultimately, the water removed by a well must be replaced—in an unconhned aquifer, this is usually by rainwater percolation or by inflow from a river—or else the well will go dry. Wells often are 

FIGURE 3-9 Cross section of a typical groundwater monitoring well. Groundwater production wells are similar but may be much larger in diameter. 

At steady state, the principal of conservation of mass dictates that the rate at which water crosses an imaginary cylindrical boundary at a radius r from the well must be the same as the rate at which water is pumped from the well. The area of the cylindrical boundary is equal to its circumference (27rr) multiplied by its height (b), which is approximated by the saturated thickness 

Note that the slope of the water table in the radial direction, given by the gradient ds/dr, is proportional to the rate at which the well is pumped the slope is inversely proportional to hydraulic conductivity, distance from the well, and aquifer thickness. 

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Groundwater well injection—Oxidants may be introduced to the treatment zone through existing or new groundwater monitoring wells as a liquid, gas, or solid. This method relies on the natural migration of oxidants from the well into the formation. Injection wells need to be adequately spaced to allow for oxidant delivery to the entire treatment area. 

Lance injection, jetting, and fracturing—Use of a high-pressure lance can create microfractures in soils that increase soil permeability and allow for direct injection of oxidants and amendments into a desired treatment area without the need for an existing or new groundwater well. 

Regarding POCs, eight compounds (see Table 1) were selected as indicators from the qualitative analysis of leachate due to their frequent detection, to their abundance, and to their absence in groundwater wells monitored upstream the landfill. The selected compounds include plasticizers or their degradation products, insect repellent and natural compounds resulting from anthropogenic pollution. 

Wilson, J. E., Brown, S., Schreier, H., Scocill, D., Zubel, M. 2008a. Arsenic in Groundwater Wells in Quaternary Deposits in the Lower Fraser Valley of British Columbia. Canadian Water Resources Journal, 33, 397-412. 

Fuel oil no. 2 was detected along with gasoline in groundwater wells in Tiverton, Rhode Island. Over a 19-month period, total hydrocarbon concentrations in the water from one well decreased from 2,350 to 1,580 g/L during which time the proportion of hydrocarbons associated with fuel oil increased from 42% (987 g/L) to 78% (1,232 g/L), probably as a result of the more rapid degradation of the gasoline (Zheng and Quinn 1988). 

Aluminum levels in groundwater wells at neutral pH generally fall below 0.1 mg/L (100 ppb) (Brusewitz 1984). In areas receiving acid precipitation, aluminum levels in groundwater may be more than 10 times the levels found in areas with neutral pH levels in the water (Brusewitz 1984), possibly due to precipitation of aluminum compounds in the more alkaline medium, or the reaction of aluminum with available silicates. In another study, Miller et al. (1984) reported that the median concentration of aluminum in finished water obtained from groundwater was 0.031 mg/L (ppm) (range,... 

Photooxidation Diffusion Chemical changes due to photochemical reactions Introduction of contaminants from man-made materials, such as solvents from polyvinyl chloride (PVC) materials and PVC cement, plasticizers, and phthalates from polyethylene and polypropylene materials Protection from exposure to light, use of amber glass bottles Use of inert materials (PTFE, fiberglass-reinforced epoxy materials) steam-cleaning of groundwater well components prior to installation... 

We use water quality indicators, such as pH, temperature, conductivity (specific conductance), dissolved oxygen, oxidation-reduction potential (ORP), and turbidity, as groundwater well stabilization parameters. Stable values of three consecutive measurements of these parameters are considered an indication of a stabilized well. 

The measurement of pH is one of the most important measurements in water chemistry. The value of pH defines the types and the rates of chemical reactions in water, and the fate and bio availability of the living organisms. Together with temperature measurements, pH values serve as groundwater well stabilization indicators. 

Turbidity is a drinking water quality parameter and a groundwater well stabilization indicator. The clarity of water defines a physical property of turbidity. Suspended matter, such as clay and silt particles, organic matter, microscopic organisms, and colloids, causes natural waters to be turbid. Turbidity is measured optically as a lightscattering property of water. 

A map showing sampling points (groundwater well field a grid from which samples were collected a drawing of an excavation with measurements etc.)... 

Figure 4.19. Chromatograms obtained during continuous monitoring of a contaminated groundwater well. Sample Injections were made every 18 minutes (II, injection 1 and so on). (Reproduced from Ref. 65, with permission from Wiley-VCH.)...

Discussion. Looking at the water table elevations (Fig. 16.18) and the analytical results (Table 16.4), which wells may be regarded as representing uncontaminated groundwater Well 401 is upstream (Fig. 16.18), and on this basis alone should be uncontaminated. This is confirmed by the observation that the water in this well is lowest in Cl, NH4, COD, and TDS. Wells 404, 405, 406, 415, 416, and 417 are closest to the landfill and in the groundwater flow direction (Fig. 16.18). Therefore, they should to be suspected of contamination. The data reveal that these wells, and additional wells that are located downflow, are indeed contaminated. 

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