Section 503.21 - Special definitions(a)Active sewage sludge unit is a sewage sludge unit that has not closed.(b)Aquifer is a geologic formation, group of geologic formations, or a portion of a geologic formation capable of yielding ground water to wells or springs.(c)Contaminate an aquifer means to introduce a substance that causes the maximum contaminant level for nitrate in 40 CFR 141.62(b) to be exceeded in the ground water or that causes the existing concentration of nitrate in ground water to increase when the existing concentration of nitrate in the ground water exceeds the maximum contaminant level for nitrate in 40 CFR 141.62(b) .(d)Cover is soil or other material used to cover sewage sludge placed on an active sewage sludge unit.(e)Displacement is the relative movement of any two sides of a fault measured in any direction.(f)Fault is a fracture or zone of fractures in any materials along which strata on one side are displaced with respect to strata on the other side.(g)Final cover is the last layer of soil or other material placed on a sewage sludge unit at closure.(h)Holocene time is the most recent epoch of the Quaternary period, extending from the end of the Pleistocene epoch to the present.(i)Leachate collection system is a system or device installed immediately above a liner that is designed, constructed, maintained, and operated to collect and remove leachate from a sewage sludge unit.(j)Liner is soil or synthetic material that has a hydraulic conductivity of 1 * 10-7 centimeters per second or less.(k)Lower explosive limit for methane gas is the lowest percentage of methane gas in air, by volume, that propagates a flame at 25 degrees Celsius and atmospheric pressure.(l)Qualified ground-water scientist is an individual with a baccalaureate or post-graduate degree in the natural sciences or engineering who has sufficient training and experience in ground-water hydrology and related fields, as may be demonstrated by State registration, professional certification, or completion of accredited university programs, to make sound professional judgments regarding ground-water monitoring, pollutant fate and transport, and corrective action.(m)Seismic impact zone is an area that has a 10 percent or greater probability that the horizontal ground level acceleration of the rock in the area exceeds 0.10 gravity once in 250 years.(n)Sewage sludge unit is land on which only sewage sludge is placed for final disposal. This does not include land on which sewage sludge is either stored or treated. Land does not include waters of the United States, as defined in 40 CFR 122.2 .(o)Sewage sludge unit boundary is the outermost perimeter of an active sewage sludge unit.(p)Surface disposal site is an area of land that contains one or more active sewage sludge units.(q)Unstable area is land subject to natural or human-induced forces that may damage the structural components of an active sewage sludge unit. This includes, but is not limited to, land on which the soils are subject to mass movement.58 FR 9387, Feb. 19, 1993, as amended at 64 FR 42570, Aug. 4, 1999