A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Paludification
The natural process of vegetation growth in lakes leading to their eventual infilling with accumulations of plant growth and decaying organic material. Many former kettle hole lakes resulting from the last glaciation (approximately 10,000 years ago in North America) are undergoing paludification.

Palustrine
Fresh water wetland environments, other than those located along a river or lake, dominated by trees, shrubs, emergent vegetation, mosses or lichens.

Particle Size
The diameter of sediments. Different types of sediment are classified according to particle size. The particle sizes of sediments in aquifers is used in designing the size openings for well screens.
See also grain size.

Parts per Billion
The number of "parts" by weight of a substance per billion parts of water. Used to measure extremely small concentrations.

Parts per Million
The number of "parts" by weight of a substance per million parts of water. This unit is commonly used to represent pollutant concentrations.

Pathogen
A living organism that is a disease producing agent. Generally, any viruses, bacteria, or fungi that may cause disease.

PCB (Polychlorinated biphenyls)
A group of synthetic, toxic chemical compounds which are chemically inert and not biodegradable (in the past these were often used in making paint and electrical transformers),. PCBs are typically associated with industrial wastes and are very difficult to remove once an aquifer is contaminated.

Peak Flow
The maximum discharge of a stream or river at a given location. It usually occurs at or near the time of maximum stage.

Peat
An acidic, fibrous, spongy soil that develops from the accumulation of dead plant material (especially sphagnum moss), that decays slowly. Decay is slow due to low oxygen levels and the acidic, nutrient poor conditions characteristically found in peatlands.

Per capita use
The average amount of water used per person during a standard time period, generally per day.

Perched Ground Water
Ground water that occurs above the main body of ground water, and is separated from it by unsaturated, impermeable sediments or rocks. Perched aquifers usually occur where there are discontinuous impermeable layers such as caliche.

Percolation
There are several interpretations of this word:
(1.) The movement of water through the openings in rock or soil.
(2.) The movement of a portion of riverflow or lake storage to ground water.
(3.) The process of downward movement of water in the unsaturated zone.
A similar word, infiltration, refers specifically to the movement of water from the atmosphere into the ground.
See also infiltration.

Perennial Stream
A stream that flows throughout the year in a well-defined channel.

Permafrost
Ground which is permanently frozen. Within permafrost areas they may be seasonal melting and areas of discontinuous permafrost. In areas of permafrost ground water may be the only available fluid water source. Wells and well equipment in permafrost areas are designed for sub-zero conditions.

Permeability
The property of sediments and rocks that allows the movement of water through them. Permeability is related to the size of openings and fissures in solid rock; the nature of the particles in unconsolidated organic and inorganic sediments, and the extent to which the void spaces are interconnected. High yielding aquifers usually have some rock layers or fractures with high permeability. Permeable materials, such as gravel and sand, allow water to move quickly through them, whereas impermeable material, such as clay, don't allow water to flow freely.

Pesticide
Any chemical used for control of plant or animal pests. Pesticides include insecticides, herbicides, fungicides, nematocides, and rodenticides.

pH
A measure of the relative acidity or alkalinity of water. Water with a pH of 7 is neutral; lower pH levels indicate increasing acidity, while pH levels higher than 7 indicate increasingly basic solutions. The range is 1 - 14.

Phreatic
The zone of sub-surface saturation.

Phreatophytes
Plants that are specifically adapted with deep rooting systems to draw moisture from the water table or capillary fringe.

Pitless Adaptor
A device installed in a vertical well casing to allow water to be piped horizontally below the frost line to its use point (usually a home).

Plume
A discrete occurrence of aquifer pollution extending down gradient from a defined source (such as a landfill or leaky storage tank) along the groundwater flow path. Plumes usually have maximum concentration at the their source with dispersion and diffusion of the contaminant down-gradient.

Point Source Pollution
Water pollution coming from a single point, such as a sewage outflow pipe.
See also non-point source pollution.

Pollutant
Substances such as solid waste, sewage, garbage, sewage sludge, radioactive materials, industrial, municipal, and agricultural waste that can detrimentally affect the quality of water.

Porosity
Describes the amount of water that can be stored in a rock formation. Porosity is the ratio of solid rock material and void space per unit volume of rock. Porous rock units may contain considerable amounts of water but may be poor aquifers because of low permeability (i.e. water movement is restricted). With respect to water movement, it is not just the total magnitude of porosity that is important, but the size of the voids and the extent to which they are interconnected. For example, clay may have a very high porosity with respect to water content, but it is not useful as an aquifer because the pores are usually so small and will not readily release water.

Potable water
Water of a quality suitable for drinking.

Potentiometric Surface
Theoretical (imaginary) surface of the static head of ground water in an aquifer.

Precipitation
The broad range of meteorological phenomena that contribute water to the hydrologic system, including rain, snow, hail, sleet, fog, mist and dew.

Pressure Tank
A tank installed as part of a water system to minimize the on-off cycles of the well pump. Pressure tanks typically store a few gallons of water and obtain their pressure from the well pump.

Primary Wastewater Treatment
The first stage of the wastewater treatment process where mechanical methods, such as filters are used to remove pollutants.

Prior Appropriation Doctrine
The system for allocating water used in most Western states. The doctrine of Prior Appropriation was in common use throughout the arid West as early settlers and miners began to develop the land. The prior appropriation doctrine is based on the concept of "First in Time, First in Right." The first person to take a quantity of water and put it to beneficial use has a higher priority of right than a subsequent user. The rights can be lost through nonuse; they can also be sold or transferred apart from the land.
See also riparian water rights.

Public Water Supply
Water withdrawn by agencies, such as a municipality or county water department, and by private companies that is then delivered to users. Most people's household water in the US is delivered by a public water supplier. The systems have at least 15 service connections (such as households, businesses, or schools) or regularly serve at least 25 individuals daily for at least 60 days out of the year.

Public Water Use
Water supplied from a public water supply and typically sold to towns for use in firefighting, street washing, municipal parks and swimming pools.

Pumping Level
The level of water in a well when pumping is in progress.

Pumping Test
Controlled pumping with associated measurements of water level changes that are used to determine aquifer characteristics and the hydraulic properties of wells.