Air Sparging in Gate Wells in Cutoff Walls and Trenches for Control of Plumes of Volatile Organic Compounds (VOCs)

Volatile organic compounds (VOCs) can be stripped from ground water by sparging air into water in wells or in trenches. This well/trench sparging (“WTS”) can remove VOCs from plumes of contaminated ground water as that water passes across the sparge zone. With sparging in wells, cutoff walls will be needed to force the contaminated water through the “gate” wells. With in situ sparging (“ISS”), air is sparged directly into a contaminated aquifer. ISS may be useful in treating local zones of high contamination, but WTS is better suited for treating large plumes of contaminated ground water. Interest in sparging methods is growing because: (1) they do not remove water from the subsurface, and so difficult disposal issues are avoided and an increasingly valuable water resource is not depleted; and (2) the Darcy velocity v in many systems is low, and so only a relatively small volume of water must be treated per unit time. The theoretical fractional efficiency of WTS is given by E = S/(l + S). The parameter S is named here as the “dimensionless sparge number,” with S = HR_g/(RTyzv) where H (atm‐m³/mol) = Henry's Law constant for the compound of interest; Rg (m3/s, at 1 atm pressure) = gas sparging rate; R = gas constant (= 8.2 X 10˜5 m3‐atm/mol‐deg); T = temperature (K); yz (m²) = cross‐sectional area producing the water which is passing into the sparging zone; and v = Darcy velocity (m/s). E increases as S increases. E increases as H increases because the volatility of a compound goes up as its H increases. Plots for E in WTS are given vs. S as well as vs. some of the variables making up S. Well/trench sparging (WTS) has the potential to become a useful treatment method for removing VOCs from contaminated ground‐water plumes. It is suited for use with most of the solvents and petroleum products which have caused extensive ground‐water contamination. The theory of the method is simple, and the theoretical removal efficiencies are predictable as well as adjustable.