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Wells which inject into lower strata are usually constructed and operated differently from <br />wells which inject into strata near the land surface. Such wells are drilled rather than dug or <br />bored, and emplace fluids into the subsurface by use of more sophisticated technology, materials, <br />and equipment. Wells of this sort require the use of casing and cementing.' Escape of injected <br />fluids into sources of drinking water is prevented by such casing, and by tubing and packer or <br />other methods. Injection is accomplished by either the force.of gravity or the application of <br />additional mechanical pressure to overcome the natural friction and hydrostatic,resistance of the <br />receiving formation.' <br />In addition, aquifers nearest the land surface most often supply water for domestic use.6 <br />Consequently, wells which inject into or above these aquifers increase the risk of human <br />exposure to the injected contaminants. <br />These considerations influenced the categorization of wells in Classes I and II separately <br />from those in Classes IV and V. Classes I and II encompass wells which normally inject into <br />strata below underground sources of drinking water. Class I wells are further limited to those <br />which inject beneath formations which contain an USDW within 1/4 mile of the well site; other <br />wells are included in Class V. The agency chose this approach since individual formations may <br />be identifiable for hundreds of miles and a formation which is suitable in one area as a source of <br />drinking water may not be in other sections. This limitation prevents a well from being subjected <br />to Class I requirements simply because it injects under an aquifer which, miles away, contains <br />drinking water. Class IV wells (which by definition inject into or above strata containing <br />underground sources of drinking water) will generally inject into or above the aquifers nearest <br />the land surface. Class V for the most part comprises wells which inject non-hazardous materials <br />into those same aquifers.' <br />Also influencing this proposed well class-ification was the nature of injected fluids. <br />Wells which handle hazardous materials war-rant close regulatory scrutiny. This consideration <br />influenced EPA to create a separate category (Class I) for wells which dispose of industrial and <br />municipal wastes. Such wastes commonly contain chemicals or other substances which can be <br />fairly characterized as noxious and, as appropriate, require separate performance criteria. <br />Nuclear waste practices are currently being reviewed by the Administration and therefore EPA is <br />' §§ 146.12(b); 146.22(b); 146.32(a) <br />5 See Generally Report to Congress, Section XI, XIII; Manual, Chapter IC: Preliminarv <br />Evaluation; Ground Water Pollution From Subsurface Excavations. U.S. Environmental Protection <br />Agency, 1973, Part 2, Section II, Ground Water Pollution." <br />6 Report to Congress, Sections III, IV; Manual, Chapter I, pp. I-10 -- 1-50. <br />' Report to Congress Section VIII, IX, XIII: Manual, Chapter I; Subpart F; Preliminary <br />Evaluation.. <br />Page 4 of 20