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protected zone.35 Using this mechanism, injection pressures can be restricted to provide <br />conservative protection even in the face of less than ideal geologic characteristics. For example, <br />if a confining stratus is known to be fractured or permeable, injection might be permissible if <br />done at predetermined pressure levels which under no circumstances could cause a rise of fluid <br />to the height at which it would enter a drinking water source. <br />PATHWAY 4 VERTICAL MIGRATION OF FLUIDS THROUGH IMPROPERLY <br />ABANDONED AND IMPROPERLAY COMPLETED WELLS <br />One of the common ways by which fluids can enter an underground source of drinking <br />water is by migration through improperly abandoned and improperly completed wells. This <br />would occur if fluids moving laterally within an injection zone encountered an improperly <br />abandoned or completed well, and, following the path of least resistance, flowed upward within <br />the well until entering an overlying underground source of drinking water or overflowing onto <br />the land surface. Because of the large number of wells drilled in the past, and because well <br />operation and abandonment have not always benefitted from close regulatory scrutiny, <br />contamination by this route can present a significant risk to public health. It is estimated that <br />there are about 17,000 improperly abandoned or improperly completed wells which could cause <br />this problem.36 <br />To prevent this contamination, the regulations require Directors to determine an "area of <br />review" for injection wells. This is the area around the injection well through which the <br />incremental pressure of injection can cause vertical migration. Operators of Class 1, III, and new <br />Class II wells (operators of existing and converted Class It wells are treated differently; see <br />below) must locate other wells within the "area of review" and correct any problems related to <br />improperly abandoned or improperly completed wells before beginning injection 37 Under this <br />approach, well injectors would have the affirmative responsibility to demonstrate that the <br />proposed injection operation would not cause contamination by this route. <br />Directors could choose either of two methods to determine the area of review. the first <br />method would be to require use of mathematical formulae to determine, on a case by case basis, <br />the lateral impact which an injection operation could cause. The formula would indicate the <br />distance outward from the well which this particular injection would or could affect. The <br />Regulations provide one formula which can be used for this purpose. It takes into account a <br />range of factors, including hydraulic conductivity, thickness of the injection zone, time of <br />injection, storage coefficient, injection rate, hydrostatic head and specific gravity. EPA is <br />proposing this particular formula because it is based on an equation which has been in common <br />use for years and, in that time has demonstrated satisfactory results; however, other suitable <br />3s § 146.06 <br />36 Oil and Gas Wells; Chapter VIII-D. <br />37 § 122.44(a). <br />Page 14 of 20