a) The groundwater monitoring system must consist of a sufficient number of wells, installed at appropriate locations and depths to yield groundwater samples that:

1. 1) Represent the quality of background water that has not been affected by contamination from the facility or unit; and
2. 2) Represent the quality of groundwater at the compliance point or points.

c) Monitoring wells must meet the following requirements:

1. 1) Construction must be done in a manner that will enable the collection of groundwater samples;
2. 2) Casings and screens must be made from durable material that is resistant to expected chemical or physical degradation and that does not interfere with the quality of groundwater samples being collected; and
3. 3) The annular space opposite the screened section of the well (i.e., the space between the bore hole and well screen) must be filled with gravel or sand if necessary to collect groundwater samples. The annular space above and below the well screen must be sealed to prevent migration of water from overlying adjacent formations and the surface to the sampled depth.