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(1) The background ground water quality monitoring well(s) must be located so as to monitor the quality of ground water representative of the ground water passing the relevant point of compliance that has not been affected by leakage from the unit. At least 1 background water quality monitoring well is required at all facilities.   At least 2 background wells must be installed at facilities where statistics will be utilized for ground water quality data evaluation unless it can be demonstrated to the department's satisfaction that a single well will suffice for the statistical test method chosen for ARM 17.50.708.

(a) A determination of background quality may include sampling of wells that are not hydraulically upgradient of the waste management area where:

(i) Hydrogeologic conditions do not allow the owner or operator to determine what wells are hydraulically upgradient; or

(ii) Sampling at other wells will provide an indication of background ground water quality that is as representative or more representative than that provided by the upgradient wells and will represent the quality of ground water passing the relevant point of compliance.

(2) Downgradient ground water quality monitoring wells must be capable of detecting a migration of hazardous constituents from active and closed waste disposal areas. The number and location of downgradient monitoring wells must be approved in writing by the department. At least 2 downgradient monitoring wells are required, although the department may require more.

(a) The downgradient monitoring system must be installed at the relevant point of compliance specified by the department that ensures detection of ground water contamination in the uppermost aquifer.   When physical obstacles preclude installation of ground water monitoring wells at the relevant point of compliance at existing units, the down-gradient monitoring system may be installed at the closest practicable distance hydraulically down-gradient from the relevant point of compliance specified by the department.

(3) All wells shall be designed, installed, developed, sampled and documented in accordance with procedures outlined herein.

History: 75-10-204, MCA; IMP, 75-10-204, 75-10-207, MCA; NEW, 1991 MAR p. 1937, Eff. 10/18/91; AMD, 1993 MAR p. 1645, Eff. 10/9/93; TRANS, from DHES, 1995 MAR p. 2253.

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