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In addition, the imposition of the "full" casing and cementing requirement on Class II <br />wells in existing injection fields would not yield significant environmental benefit. ff past <br />injection was performed in an unsafe way, nearby water resources will likely be too contaminated <br />for consumption as drinking water. Imposing casing and cementing in this instance would not be <br />helpful to the environment. On the other hand, if the injection has been performed historically in <br />a way which is protective of underground drinking water, it is reasonable to believe that the <br />injection method will continue to protect underground sources of drinking water. These facts are <br />particularly applicable to Class II wells because they are relatively older than wells in other <br />categories32 and are normally found in groups the members of which are similarly constructed." <br />Older wells, with longer histories of operation; are more likely to have contaminated drinking <br />water, if at all, by this time, than are newer wells. Moreover, the similar construction of wells in <br />specific fields increases the chances that, if contamination has occurred, it is already extensive. <br />Lastly, the need for the "full" casing and cementing of Class II wells is generally less <br />because-brine and other fluids associated with oil and natural gas production pose less threat to <br />human health than fluids which Class I and some Class III wells often inject. <br />PATHWAY 3 - MIGRATION OF FLUIDS FROM AN INJECTION ZONE THROUGH <br />THE CONFINING STRATA <br />The third way by which fluids can enter an underground source of drinking water is from an <br />injection zone through the confining strata. Upon entry into an injection zone, fluids injected <br />under pressure will normally travel away from the well laterally and through the receiving <br />formation. In most cases, this expected occurrence gives rise to no concern, but, if the confining <br />stratum which separates the injection zone from an overlying or underlying underground <br />source of drinking water is either fractured or permeable, the fluids can migrate out of the <br />receiving formation and into the protected region. <br />For obvious reasons, there are no well construction standards which can address this <br />problem of migration of fluids through this pathway. Consequently, the regulations propose two <br />provisions to assure that fluids do not travel this pathway into underground drinking water. First, <br />the regulations require that, prior to the issuance of a permit, the geologic characteristics of the <br />injection zone and confining strata be reviewed.34 Data already available to states can assist <br />Directors in making these reviews. A permit should only be issued upon the Director's finding <br />that the underground formations are sufficiently sound to contain fluids in the injection zone. <br />Second, the regulations require that well injection pressure be controlled to prevent <br />opening fractures in the confining strata or otherwise causing the rise of fluids into an overlying <br />3z Report to Congress , Section XI. <br />33 Id. <br />39 §§146.14(a)(1); 146.24(a)(1); 146.34(a)(1). <br />Page 13 of 20