Unless the context requires other-wise, in this part the following definitions apply:

(1) "Abandoned well" means a well whose use has been permanently discontinued from service and properly plugged with low permeability material to prevent the borehole from acting as a conduit for contaminants to enter the subsurface and to prevent inter-aquifer flow. Well abandonment must be in accordance with ARM 36.21.670 through 36.21.673 and 36.21.810.

(2) "Active life" means the period of operation beginning with the initial receipt of solid waste and ending at completion of closure activities.

(3) "Active portion" means any solid waste disposal unit, trench, cell or area that is receiving solid waste or has not been closed in accordance with department rules. Also known as "active unit" or "existing unit".

(4) "Aquifer" means any geologic formation, group of formations, or part of a formation capable of yielding significant quantities of ground water to wells or springs.

(5) "Aquitard" means a geologic formation, group of formations, or part of a formation, exhibiting low permeability and having little or no intrinsic permeability, that is stratigraphically adjacent to 1 or more aquifers and through which virtually no water moves. The hydraulic conductivity within an aquitard is much lower than in adjacent aquifers and is not sufficient to allow the completion of water supply wells within it. An aquitard is also known as a "confining bed", "confining layer", or "confining unit".

(6) "Assessment monitoring" means the monitoring that is required whenever a preventive action level has been detected in the ground water for 1 or more of the constituents in Table 1 [ARM 17.50.708].

(7) "Bentonite seal" is an impervious layer of hydrated bentonite (i.e., sodium montmorillonite) (e.g., granular or powdered) placed between the well casing and the borehole wall, 1 to 2 feet above the filter/gravel pack adjacent to the screened interval for the purpose of sealing and restricting ground water movement to the particular aquifer/zone designated for monitoring in each well.   The minimum sealing thickness must be 1½ inches around the outside of the casing on all sides, except for driven wells in accordance with ARM 36.21.806(2) .

(8) "Borehole" means an open or uncased subsurface hole created by drilling.

(9) "Casing pipe" means finished in sections with either threaded connections or beveled edges to be field welded, which is installed temporarily or permanently to counteract caving, to advance the borehole, or to provide structural protection to the well and restrict unauthorized access to the well.

(10) "Closed unit" means any solid waste disposal unit, trench, cell or area that no longer receives solid waste and has been closed in accordance with department rules.

(11) "Closure" means the process by which an owner or operator of a facility closes all or part of a facility in accordance with a department approved closure plan and all applicable closure requirements.

(12) "Confined aquifer" means an aquifer confined by an overlying stratigraphic unit that has a significantly lower hydraulic conductivity than the aquifer hosting the confined ground water.   Pore water pressure at the top of the confined aquifer is greater than atmospheric pressure.

(13) "Contamination" means the degradation of natural, background water quality by an undesirable substance not normally present or an unusually high concentration of a naturally occurring substance. There is no indication of specific limits, since the degree of permissible contamination depends upon the intended use, or uses, of the water.

(14) "Detection limit" means the lowest concentration for an analytical test method and sample matrix at which the presence of a substance can be identified in an analytical sample, with a stated degree of confidence, regardless of whether the concentration of the substance in the sample can be quantified.

(15) "Detection monitoring" means the ground water monitoring required by this subchapter, including sampling procedures specified in ARM 17.50.708(4) .

(16) "Drilling fluid" means a liquid or gas which may be used in the drilling operation to remove cuttings from the borehole, to clean and cool the bit, to reduce friction between the drill stem and the borehole wall, and to seal the borehole to prevent loss of drilling fluids.

(17) "Enforcement standard" means a numerical value expressing the concentration of a substance in ground water triggering assessment monitoring or corrective action, as specified in this subchapter.

(18) "Existing unit" means any solid waste disposal unit that is receiving solid waste as of October 9, 1993, and received it immediately prior to that date as well.   Waste placement in existing units must be consistent with past operating practices or modified practices to ensure good management.

(19) "Facility" means all contiguous land and structures, other appurtenances, and improvements on the land (licensed or unlicensed) used for landfilling or otherwise disposing of municipal solid wastes or any licensed class II landfill.

(20) "Filter pack/gravel pack" means a clean silica sand or sand and gravel mixture that is installed in the annular space between the borehole wall and the well screen or developed in-situ, extending an appropriate distance above and below the screen or monitoring device/point, for the purpose of retaining and stabilizing the particles from the adjacent strata.

(21) "Ground water" means all water below the land surface.

(22) "Ground water class" means a ground water quality classification established in ARM 17.30.1002.

(23) "Ground water quality standards" means the standards for ground water quality set forth in ARM 17.30.1003.

(24) "Grout" means an impervious or low permeability material placed in the annulus between the well casing or the riser pipe and the borehole wall to maintain the alignment of the casing and to act as a seal to prevent movement of ground water or surface water within the annular space.   It also includes the material placed in a borehole to plug the borehole during abandonment.

(25) "Grouting" means the operation by which grout material is placed in the borehole.

(26) "Hazardous substance" means a substance that because of its quantity, concentration, or physical, chemical, or infectious characteristics may pose an imminent or substantial threat to public health, safety, or welfare or the environment or is a substance that is defined as a hazardous or deleterious substance in 75-10-701(6) , MCA.

(27) "Hydraulic conductivity" means an indication of a formation's ability to transmit ground water (the more capable the formation, the higher the hydraulic conductivity) .   It is a function of the porous medium as well as the fluid, and is equal to the ratio of flow velocity to hydraulic gradient of a given formation.   Strictly defined it is the product of the intrinsic permeability of the earth material, the density of the fluid, the acceleration due to gravity, and the inverse of the dynamic viscosity of the water.

(28) "Hydraulic gradient" means the total change in hydraulic head with distance in a given direction.

(29) "Land application unit" means an area where wastes are applied onto or incorporated into the soil surface (excluding manure spreading operations) for agricultural purposes or for treatment and disposal.

(30) "Landfill" means an area of land or an excavation in which wastes are placed for permanent disposal, and that is not a land application unit, surface impoundment, injection well, or waste pile.

(31) "Lateral expansion" means a horizontal expansion of the waste boundaries of an existing disposal unit.

(32) "Leachate" means a liquid that has contacted, passed through or emerged from solid waste and contains soluble, suspended, or miscible materials removed from such waste.

(33) "Monitoring well" means a well that is used for ground water sample recovery, ground water quality monitoring, or ground water level measurement, but whose primary purpose is not withdrawal or injection of water. Monitoring well does not include water supply wells, geotechnical borings, pumping test wells, or corrective action or remedial action wells.

(34) "Municipal solid waste landfill unit (or MSWLF unit) " means a discrete area of land or an excavation that receives household waste, and that is not a land application unit, surface impoundment, injection well, or waste pile.   A MSWLF unit also may receive other types of RCRA subtitle D wastes, such as commercial solid waste, nonhazardous sludge, conditionally exempt small quantity generator waste and industrial solid waste.   Such a landfill may be publicly or privately owned.   A MSWLF unit may be a new MSWLF unit, an existing MSWLF unit or a lateral expansion.

(35) "New unit" means any solid waste disposal unit that has not received waste prior to October 9, 1993.

(36) "No migration petition" means a request from an owner or operator of a facility for a variance from these ground water monitoring rules as provided for in 75-10-207(2) , MCA.

(37) "Operator" means the person responsible for the overall operation of a facility.

(38) "Owner" means the person who owns a facility or part of a facility.

(39) "Permeability" means a measure of relative ease with which a porous medium can transmit a liquid under a potential energy gradient.   It is a property of the medium that is dependent upon the shape, size, and degree of interconnection of the pores.

(40) "Practical quantitation limit" means the lowest concentration for an analytical test method and sample matrix at which the quantity of a particular substance can be measured with a stated degree of confidence.

(41) "Preventive action limit (PAL) " means a numerical value expressing the maximum allowable concentration of a substance in ground water, triggering assessment monitoring.   The PAL is equal to the enforcement standard, or a statistically significant increase above background concentration, or a statistically significant decrease in pH.

(42) "Protective casing" means a section of pipe or tubing that is placed over the well casing at the surface to provide structural protection to the well and restrict unauthorized entrance into the well. A protective casing will usually extend a distance of several feet above and below land surface.

(43) "Qualified ground water scientist" is a scientist or engineer who has received a baccalaureate or post-graduate degree in the natural sciences or engineering and has sufficient training and experience in ground water hydrology and related fields as may be demonstrated by state registration, professional certifications, or completion of accredited university programs that enable that individual to make sound professional judgments regarding ground water monitoring, contaminant fate and transport, and corrective action.

(44) "Soil boring" or "rock boring" or "boring" means a hole intended solely to determine the hydrogeological properties, composition, stability, density, movement, pressure, stratigraphy, or other physical properties of soil or rock.

(45) "Static water level" means the elevation above mean sea level or a local datum of the top of ground water as measured in a well casing.

(46) "Surface impoundment" means a facility or part of a facility that is a natural topographic depression, human-made excavation, or diked area formed primarily of earthen materials (although it may be lined with human-made materials) , is designed to hold an accumulation of liquid wastes or wastes containing free liquids and is not an injection well.   Examples of surface impoundments are holding, storage, settling, and aeration pits, ponds, and lagoons.

(47) "Transmissivity" means the rate at which water of prevailing kinematic viscosity is transmitted horizontally through a unit width of an aquifer under a unit hydraulic gradient. Strictly defined it is the product of the hydraulic conductivity and the thickness of the aquifer.

(48) "Unconfined aquifer" means an aquifer in which hydrostatic pressures at the water table are equal to atmospheric pressure.   In unconfined aquifers, the water table is exposed to the atmosphere through pores in the overlying materials.

(49) "Unit" means a discrete area of land or an excavation used for the landfilling or other disposal of solid waste.

(50) "Uppermost aquifer" means the geologic formation nearest the natural ground surface that is an aquifer, as well as lower aquifers that are hydraulically interconnected with this aquifer within the facility's property boundary.

(51) "Water table" means the water surface elevation in an unconfined aquifer not influenced by capillary forces. The water table is strictly defined as the surface on which the water pressure in the pores of a porous medium is exactly atmospheric.

(52) "Well screen" means a pipe or cylindrical tubing with factory cut slots of a uniform width, orientation, and spacing designed and placed in the well so as to provide contact with all or a portion of the zone of saturation. For monitoring wells, well screen does not mean perforated or hand saw cut casing, or other well-site manufactured screen.

(53) "Zone of saturation" means a hydrologic zone in which all the interstices between particles of geologic material or all of the joints, fractures, or solution channels in a consolidated rock unit are filled with water under pressure greater than that of the atmosphere.