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Parameter: A variable, measurable property whose value is a determinant of the characteristics of a system; e.g. temperature, pressure, and density are parameters of the atmosphere.

Parts Per Billion (ppb)/Parts Per Million (ppm): Units commonly used to express contamination ratios, as in establishing the maximum permissible amount of a contaminant in water, land, or air.

Perched Water: Zone of unpressurized water held above the water table by impermeable rock or sediment.

Percolation: 1. The movement of water downward and radially through subsurface soil layers and pore spaces in soil or rock, usually continuing downward to ground water. Can also involve upward movement of water. 2. Slow seepage of water through a filter.

Permeability: The rate at which liquids pass through soil or other materials in a specified direction.

Personal Protective Equipment (PPE): Clothing and equipment worn by environmental professionals and hazmat emergency responders to reduce their exposure to potentially hazardous chemicals and other pollutants (e.g. respirators, chemical suits, etc).

Petroleum: Crude oil or any fraction thereof that is liquid under normal conditions of temperature and pressure. The term includes petroleum-based substances comprising a complex blend of hydrocarbons derived from crude oil through the process of separation, conversion, upgrading, and finishing, such as motor fuel, jet oil, lubricants, petroleum solvents, and used oil.

Petroleum Derivatives: Chemicals formed when gasoline breaks down in contact with ground water.

pH: An expression of the intensity of the basic or acid condition of a liquid; may range from 0 to 14, where 0 is the most acid and 7 is neutral. Natural waters usually have a pH between 6.5 and 8.5.

Piezometer: A non-pumping well, generally of small diameter, for measuring the elevation of a water table (i.e. static water level).

Plume: 1. An elongated body of fluid, usually mobile and varying in shape.  2. A visible or measurable discharge of a contaminant from a given point of origin. Can be visible or thermal in water, or visible in the air as, for example, a plume of smoke.

Point Source: A stationary location or fixed facility from which pollutants are discharged; any single identifiable source of pollution; e.g. a pipe, ditch, ship, ore pit, factory smokestack.

Pollutant: Generally, any substance introduced into the environment that adversely affects the usefulness of a resource or the health of humans, animals, or ecosystems..

Polychlorinated Biphenyls: A group of toxic, persistent chemicals used in electrical transformers and capacitors for insulating purposes, and in gas pipeline systems as lubricant. The sale and new use of these chemicals, also known as PCBs, were banned by law in 1979.

Polyvinyl Chloride (PVC): A tough, environmentally indestructible plastic that releases hydrochloric acid when burned.

Porosity: Degree to which soil, gravel, sediment, or rock is permeated with pores or cavities through which water or air can move; related to the open spaces or voids that occur between mineral grains or in fractures of bedrock.

Potable Water: Water that is safe for drinking and cooking.

Potentiometric Surface: The surface to which water in an aquifer can rise by hydrostatic pressure.

Protocol: A series of formal steps for conducting a test.

Pumping Test: A test conducted to determine aquifer or well characteristics, such as recharge rate.

Purging:  Removing stagnant air or water from sampling zone or equipment prior to sample collection.  Wells are purged prior to sampling to ensure that the sample collected is representative of the surrounding groundwater conditions.

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