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 spa rging (''WTS'') can remove VOCs from plumes of contaminated ground wat er 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 di rectly into a contaminated aquifer. ISS may be useful in treating loca l zones of high contamination, but WTS is better suited for treating l arge 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 valu able water resource is not depleted; and (2) the Darcy velocity v in m any systems is low, and so only a relatively small volume of water mus t be treated per unit time. The theoretical fractional efficiency of W TS is given by E = S/(1 + S). The parameter S is named here as the ''d imensionless sparge number,''with S = HR(g)/(RTyzv) where H (atm-m3/mo l) = Henry's Law constant for the compound of interest; R(g) (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 = D arcy velocity (m/s). E increases as S increases. E increases as H incr eases 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 variabl es 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 petro leum products which have caused extensive ground-water contamination. The theory of the method is simple, and the theoretical removal effici encies are predictable as well as adjustable.