(a) The nature and extent of any past, current and potential releases or migration of contaminants from the site must be defined to ensure that an adequate final closure plan is developed. For existing sites where information is known through previous monitoring of the facility during its operating life, some or all of the requirements of this section may be waived upon approval of the department. The minimum elements of a closure site investigation are as follows:

1. (1) a hydrogeologic investigation performed in accordance with section 363-4.4 of this Part that, at a minimum, must:

   1. (i) define the geologic and hydrogeologic conditions of the uppermost aquifer and, as required by the department, any other units in the critical stratigraphic section that may be impacted by the facility;
   2. (ii) establish a long-term monitoring well network in the uppermost aquifer and other units determined necessary by the department, to monitor potential environmental impacts from the landfill and the effects of closure or remediation; and
   3. (iii) analyze the initial round of samples or existing sampling data for each monitoring well point for baseline parameters. If contamination is detected, the department may require additional sampling and analysis as specified in section 363-4.6 of this Part;

2. (2) an explosive gas investigation performed to determine whether the site meets the requirements of section 363-7.1(e) of this Part, which must:

   1. (i) include at least three rounds of subsurface explosive gas monitoring performed along the perimeter outside the waste mass but within the property boundary. Monitoring must be performed at 100-foot maximum intervals where temporary sampling locations are used, or at 400-foot maximum intervals where permanent gas monitoring wells are constructed. Initial monitoring must be performed when atmospheric pressure and wind velocity are low and when the ground surface has been wet or frozen for several days. Monitoring must be done below the wet or frozen zone if present;
   2. (ii) identify the presence and concentration of explosive gases at or near the facility, including at the property line, in all on-site structures, and in potentially impacted off site structures;
   3. (iii) determine the extent of actual or potential gas migration off site; and
   4. (iv) describe the soil stratigraphy beneath and around the facility in terms of its potential to allow gas migration;

3. (3) a surface leachate investigation. This investigation must be performed when groundwater levels are at seasonal high elevations or at other times as specified by the department and must:

   1. (i) identify the presence of leachate seeps or other uncontrolled leachate at, or emanating from, the facility;
   2. (ii) document any instances where fugitive leachate from the facility is discharging into surface waters; and
   3. (iii) characterize the chemical constituents of surface leachate for baseline parameters;
4. (4) a vector investigation to identify the presence of any vectors at the facility. If a vector problem is identified an appropriate remediation plan must be implemented prior to cessation of waste acceptance at the facility.