|(a) A groundwater monitoring system must be installed that consists of a sufficient number of monitoring wells, installed at appropriate locations and depths, to yield representative groundwater samples from the uppermost aquifer as defined in §330.3 of this title (relating to Definitions). (1) Background monitoring wells shall be installed to allow determination of the quality of background groundwater that has not been affected by leakage from a unit. Background monitoring wells may be placed in locations that are not hydraulically upgradient of the waste management area if hydrogeologic conditions do not allow the owner or operator to determine which wells are hydraulically upgradient or if sampling at other wells will provide a better indication of background groundwater quality than is possible from upgradient wells. (2) The point of compliance monitoring system must include monitoring wells installed to allow determination of the quality of groundwater passing the point of compliance as defined in §330.3 of this title and to ensure the detection of groundwater contamination in the uppermost aquifer. Monitoring well spacing for a municipal solid waste landfill unit shall not exceed 600 feet without an applicable site-specific technical demonstration that may be supplemented with a multi-dimensional fate and transport numerical flow model as set forth in subsection (e) of this section. The owner or operator of a municipal solid waste landfill unit must install a groundwater monitoring system at the point of compliance, as required by 40 Code of Federal Regulations §258.51(a)(2). When physical obstacles preclude installation of the groundwater monitoring wells at existing units, the wells may be installed at the closest practicable distance to the point of compliance as defined in §330.3 of this title that will ensure detection of groundwater contamination of the uppermost aquifer. (b) The executive director may approve a multi-unit groundwater monitoring system instead of separate groundwater monitoring systems for each municipal solid waste management unit when the facility has several units, provided the multi-unit system meets the requirement of subsection (a) of this section and will be as protective of human health and the environment as individual monitoring systems for each unit, based on the following factors: (1) number, spacing, and orientation of the solid waste management units within an overall waste management area; (2) hydrogeologic setting; (3) site history; (4) engineering design of the units; and (5) type of waste accepted at the units. (c) The executive director may approve an alternative design for a groundwater monitoring system that uses other means in conjunction with monitoring wells to ensure detection of groundwater contamination in the uppermost aquifer from a solid waste management unit. The alternative design shall be at least as protective of human health and the environment as a monitoring-well system as specified in §330.403(a) of this title (relating to Groundwater Monitoring Systems). (d) All parts of a groundwater monitoring system shall be operated and maintained so that they perform at least to design specifications through the life of the groundwater monitoring program. (e) A groundwater monitoring system, including the number, spacing, and depths of monitoring wells or other sampling points, shall be designed and certified by a qualified groundwater scientist. Within 14 days of the certification, the owner or operator shall submit the certification to the executive director and place a copy of the certification in the operating record. The plan for the monitoring system and all supporting data must be submitted to the executive director for review and approval prior to construction. (1) The design of a monitoring system shall be based on site-specific technical information that must include a thorough characterization of: aquifer thickness; groundwater flow rate; groundwater flow direction, including seasonal and temporal fluctuations in flow; effect of site construction and operations on groundwater flow direction and rates; and thickness, stratigraphy, lithology, and hydraulic characteristics of saturated and unsaturated geologic units and fill materials overlying the uppermost aquifer, materials of the uppermost aquifer, and materials of the lower confining unit of the uppermost aquifer. A geologic unit is any distinct or definable native rock or soil stratum. (2) The owner or operator may use an applicable multi-dimensional fate and transport numerical flow model to supplement the determination of the spacing of monitoring wells or other sampling points and shall consider site-specific characteristics of groundwater flow as well as dispersion and diffusion of possible contaminants in the materials of the uppermost aquifer. Any model used shall: (A) have supporting documentation that establishes its ability to represent groundwater flow and contaminant transport, as needed; (B) have a sound set of equations based on accepted theory representing groundwater movement and contaminant transport; (C) have numerical solution methods that are based on sound mathematical principles and supported by verification and checking techniques; (D) be calibrated against site-specific field data; (E) have a sensitivity analysis to measure its response to changes in the values of major parameters, error tolerances, and other parameters; (F) show mass-balance calculations, where necessary; and (G) be based on actual field or laboratory measurements, or equivalent methods, that document the validity of chosen parameter values. (3) The owner or operator shall promptly notify the executive director, and any local pollution agency with jurisdiction that has requested to be notified, in writing of changes in facility construction or operation or changes in adjacent property that affect or are likely to affect the direction and rate of groundwater flow and the potential for detecting groundwater contamination from a solid waste management unit and that may require the installation of additional monitoring wells or sampling points. Such additional wells or sampling points require a modification of the site development plan.