Extraction wells

Pump and treat technology is inherently slow because it depends on ground water for transport of the contaminant to the extraction well. This characteristic is particularly troublesome when the contaminant is only slightly water soluble, adheres to the soil, or collects ia pools within the aquifer. 

The extractive and photometric procedure of 2,4-D determination in aqueous solutions with crystal violet (CV) is developed. Determination method is based on interaction dye cation with formation of hydrophobic and ionic associate, which is extracted well by toluene. The colour intensity of toluene layer proportional to concentration of coloured cations and... 

This case serves to point out the importance of maintaining a constant hydraulic gradient on the inside and outside of a slurry wall. This can also become a factor if slurry walls are used with groundwater pumping systems (extraction wells). Then water must be reinjected to maintain a... 

An active perimeter gas control system can have any of the same configurations as a passive perimeter system with the addition of any combination of gas extraction wells, gas collection headers, vacuum blowers or compressors. Their ultimate purpose is to direct the gas to a treatment or utilization system. 

Corrective Action Application An acidic groundwater at a Florida site (pH 2.5-3) required treatment. The groundwater was collected by extraction wells, pumped to an above-ground reactor, and neutralized with lime. In the course of neutralizing the waste stream, precipitates were formed which were removed by clarification and filtration prior to discharge. Sludges produced from the clarification and filtration steps were dewatered by a filter press. 

The solution was heated on a steam bath for 6 hours. After cooling, the whole was poured into water and the oil extracted well with ether, the ether solution was discarded and the aqueous solution neutralized with 1 N sulfuric acid. A heavy oil separated which was taken into ether, washed with water, bicarbonate solution and again with water. After drying, the ether was removed to give a thick oil which solidified on standing (34.6 grams). After recrystallization from an ether-petroleum ether mixture it formed needles, MP 108°-112°C. 

To a cooled (—110X) solution of the silylvinyl lithium (from the stannylethene (37 mmol) and n-BuLi (41 mmol)) in THF (100 ml) was added precooled crushed C02 (ca. 150 ml). The mixture was stirred at — 110 X for 0.5h, and then allowed to warm to ambient temperatiire overnight. The mixture was poured into ether (200 ml) and extracted thoroughly with aqueous NaOH (1m). The basic extracts were acidified with dilute HC1 at 0°C and extracted well with ether. The ethereal extracts were dried, concentrated, and the residue was distilled to give the title acid (32 mmol, 86%), b.p. 68-74 °C/0.5mmHg. 

A. U. Sharipov, S. I. Dolganskaya, L. I. Ryabova, L. A. Chuma-chenko, and G. I. Petrukhin. Plugging solution for gas extraction wells—contains cement, pol3miethyl methacrylate modified with monoethanolamine, and water, and produces plugging stone of increased strength. Patent SU 1818464-A, 1993. 

Figure 14.1 presents the main components of a typical in situ SVE system.910 Vertical extraction wells are installed inside the contaminated zone at appropriate distances from one another. The SVE wells are typically constructed of PVC pipe, with a screened interval, which is placed within the contaminated zone. The wells are connected to blowers or vacuum pumps, which induce a continuous airflow through the pores of the unsaturated zone. The soil surface is sometimes covered with an impermeable seal, made from high density polyethylene (HDPE) or bentonite clay for example, to prevent the vertical influx of air from the surface, which might cause short-circuiting problems, and promote horizontal gas flow through the contaminated area. The airstream, which contains the contaminant vapors, passes initially through an air-water separation unit to remove the entrained moisture and is then directed to the gas treatment unit, where the contaminants are thermally destroyed or removed by adsorption. 

The movement of air in the subsurface during the application of SVE is caused by the pressure gradient that is applied in the extraction wells. The lower pressure inside the well, generated by a vacuum blower or pump, causes the soil air to move toward the well. Three basic equations are required to describe this airflow the mass balance of soil air, the flow equation due to the pressure gradient, and the Ideal Gas Law. 

Equation 14.11 introduces the notion of radius of influence, which is one of the important design parameters of S VE systems. Theoretically, the maximum radius of influence of a well is the distance at which the pressure becomes equal to the ambient atmospheric pressure, i.e., P = Patm. In practice, Pj is determined as the distance at which a sufficient level of vacuum still exists to induce airflow, e.g., 1% of the vacuum in the extraction well.912 The extraction wells are usually constructed using pipes with a standard radius, e.g., Pw = 5.1 cm (2 in.) or 10.2 cm (4 in.), and the vacuum applied in the wells typically ranges from 0.05 to 0.15 atm, i.e., Pw = 0.95-0.85 atm.9 12 If the vacuum required in the radius of influence is 1% of the vacuum in the extraction well, the... 

The airflow equations presented above are based on the assumption that the soil is a spatially homogeneous porous medium with constant intrinsic permeability. However, in most sites, the vadose zone is heterogeneous. For this reason, design calculations are rarely based on previous hydraulic conductivity measurements. One of the objectives of preliminary field testing is to collect data for the reliable estimation of permeability in the contaminated zone. The field tests include measurements of air flow rates at the extraction well, which are combined with the vacuum monitoring data at several distances to obtain a more accurate estimation of air permeability at the particular site. 

Any variety of configurations of injection wells, horizontal wells, trenches, infiltration galleries, aboveground sprayers or leach fields, and extraction wells, open ditches, or subsurface collection... 

Subsurface drains function like an infinite line of extraction wells, and can be used to contain and remove a plume or to lower the groundwater table (Figure 16.12). They are more cost-effective than pumping for shallow contamination problems at depths of less than 12 m (40 ft). Depths may be increased if the site is stable, if the soil has a low permeability, and if no rock excavations are encountered. 

FIGURE 16.10 Containment using extraction wells (a) cross-sectional view (b) plan view. 

The vacuum extraction process involves using vapor extraction wells alone or in combination with air injection wells. Vacuum blowers are used to create the movement of air through the soil. The air flow strips the VOCs from the soil and carries them to the surface. Figure 18.14 shows the flow diagram for such a process. During extraction, water may also be extracted along with vapor. The mixture should be sent to a liquid-vapor separator. The separation process results in both liquid and vapor residuals that require further treatment. Carbon adsorption is used to treat the vapor and water streams, leaving clean water and air for release, and spent GAC for reuse or disposal. Air emissions from the system are typically controlled by adsorption of the volatiles onto activated carbon, by thermal destruction, or by condensation. 

Biofouling of extraction wells can reduce system performance... 

During SVE, contaminated soil vapors are extracted by inducing a vacuum at one or more extraction points that are typically constructed as vertical vapor extraction wells. Horizontal extraction wells or trenches have also been employed as extraction points. In general, SVE is applied at depths... 

These factors affect the number of vapor extraction wells, vacuum level required, type of off-gas treatment, and length of time required for treatment.34... 

MPE provides several advantages when compared with the use of SVE or pump-and-treat alone. MPE provides for an increase in groundwater recovery rates, an increase in ROI in individual ground-water recovery wells, and recovery of shallow free product. By depressing the groundwater table in the vicinity of the extraction wells, MPE provides for remediation of the capillary fringe and smear zone, and remediation of volatile, residual contaminants located above and below the water table.46... 

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