42 USC 300j11 - Indian Tribes

(a) In general 
Subject to the provisions of subsection (b) of this section, the Administrator
(1) is authorized to treat Indian Tribes as States under this subchapter,
(2) may delegate to such Tribes primary enforcement responsibility for public water systems and for underground injection control, and
(3) may provide such Tribes grant and contract assistance to carry out functions provided by this subchapter.

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(b) EPA regulations 

(1) Specific provisions 
The Administrator shall, within 18 months after June 19, 1986, promulgate final regulations specifying those provisions of this subchapter for which it is appropriate to treat Indian Tribes as States. Such treatment shall be authorized only if:
(A) the Indian Tribe is recognized by the Secretary of the Interior and has a governing body carrying out substantial governmental duties and powers;
(B) the functions to be exercised by the Indian Tribe are within the area of the Tribal Governments jurisdiction; and
(C) the Indian Tribe is reasonably expected to be capable, in the Administrators judgment, of carrying out the functions to be exercised in a manner consistent with the terms and purposes of this subchapter and of all applicable regulations.
(2) Provisions where treatment as State inappropriate 
For any provision of this subchapter where treatment of Indian Tribes as identical to States is inappropriate, administratively infeasible or otherwise inconsistent with the purposes of this subchapter, the Administrator may include in the regulations promulgated under this section, other means for administering such provision in a manner that will achieve the purpose of the provision. Nothing in this section shall be construed to allow Indian Tribes to assume or maintain primary enforcement responsibility for public water systems or for underground injection control in a manner less protective of the health of persons than such responsibility may be assumed or maintained by a State. An Indian tribe[1] shall not be required to exercise criminal enforcement jurisdiction for purposes of complying with the preceding sentence.
[1] So in original. Probably should be capitalized.