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Class II injection well monitoring provisions aze less stringent than those for Classes I <br />and 111.13 Continuous monitoring is not required for Class II; rather, depending on the actual <br />injection operation, monitoring frequency varies from daily to monthly. A stricter approach is <br />not essential for Class II wells because of the lesser toxicity and corrosivity of fluids which Class <br />II wells handle and because the total cost of imposing continuous monitoring on Class II wells <br />would have been inordinately burdensome in EPA's view" <br />Class III wells are also required to monitor, on a quarterly basis, water supply wells <br />adjacent to the injection site to detect any excursions from the injection site 55 This monitoring is <br />commonly practiced by operators of Class III wells 56 EPA is promulgating this requirement for <br />Class III wells (and not for Class I wells) because Class III wells are often designed to inject into <br />shallower strata, thereby increasing the possibility of contamination of aquifers nearest the land <br />surface. <br />This added risk has prompted the Agency to require monitoring wells at each project site, <br />located to maximize the probability of detecting any horizontal or vertical fluid excursion from <br />the injection zone. Weekly monitoring of the fluid levels in these monitoring wells and of <br />parameters appropriate to determine if any excursions of injected fluids are entering underground <br />sources of drinking water, is also required. This requirement, although involving additional <br />expense, was considered necessary to assure that any migration of these potentially harmful <br />injected fluids into underground sources of drinking water, which are often located quite close to <br />the injection zones, would be discovered and rectified promptly. Class III wells may be <br />monitored on a field or project basis rather than an individual well basis by manifold monitoring <br />e.g. using a common header with individual well points. This approach may be used with <br />facilities that consist of more than one injection well if the owner or operator can demonstrate <br />that manifold monitoring is comparable to individual well monitoring." <br />No monitoring requirements are proposed for Class V wells. These wells will be <br />assessed under the proposed regulatory scheme. The assessment should produce a substantial <br />amount of data upon which an entire regulatory approach, including monitoring, can be used. <br />53 §146.23(b). <br />59 Oil and Gas Wells, Chapter V-B, C. <br />55 § 146.33(b)(5). <br />56 Comments of Freeport Sulfur Co., Jan. 14, 1977; Statement byTexas Gulf Co., Oct. <br />13, 1976. <br />5' §146.33(b)(6). <br />Page 19 of 20