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• Acre-Foot: A volume of water that could cover a one-acre area to a depth of one foot; approximately 326,000 gallons; about the amount of water required by two families for one year.
• Aeration Zone: The underground zone of soil and rock containing pore spaces of air and water above the saturated zone.
• Alluvium: Unconsolidated beds of sand, gravel, silt and clay deposited by flowing water.
• Aquifer:  An area that holds a lot of water, which can be pumped up with a well.
• Unconfined Aquifer: An aquifer that is not confined by an aquitard and is easily recharged.
• Aquiclude:  A layer of essentially impermeable material.
• Aquitard:  A formation that contains water but cannot transmit it rapidly enough to furnish a significant supply to a well; a confining bed that retards but does not prevent the flow of water from an adjacent aquifer.
• Artesian Well: A well penetrating a confined aquifer where the water is under pressure.
• Artificial Recharge: The addition of water to the groundwater system by some method of construction i.e.: streambeds, spreading grounds, pits, or direct injection.
• Confined Aquifer: A layer of water-bearing material sandwiched between two layers of much less pervious materials, such as between two clay layers.
• Contamination:  The presence of undesirable compounds above appropriate health or water quality standards.
• Discharge Area: A lake, well, spring, river or ocean where groundwater emerges.
• Groundwater: Water that is found beneath the earth’s surface that fills all pore spaces of the alluvium, soil or rock formation in which it is situated.
• Hardness:  The amount of carbonates and bicarbonates of calcium and magnesium dissolved in water.
• Hydrologic Cycle: The water circulatory system for all the earth’s fresh water.
• Inorganic Material: Non-carbon compounds such as minerals, metals, rocks, salts, and some gases.
• Land Subsidence: The lowering of the natural land surface in response to consolidation of soils during groundwater extraction. 
• Organic Material: Carbon-based compounds derived from living or once-living things.
• Overdraft:  The withdrawal of groundwater at a rate greater than its replenishment.
• pH: A measure of acid or alkalinity.  Neutral water has a pH of 7.
• Pollutant:  Any substance that degrades water quality.
• Recharge Area: The zone where water enters the aquifer, e.g., locally, stretches of the Arroyo Mocho and Arroyo del Valle recharge the groundwater basin.
• Salinity:  The concentration of salts in water, typically expressed as total dissolved solids or TDS.  Seawater has a TDS of about 35,000 mg/L.
• Saturation Zone: The underground zone at the water table and below where pore spaces are entirely filled with water.
• Seawater Intrusion: The migration of salty sea or bay water into fresh groundwater aquifers.
• Turbidity:  The measure of suspended matter in water.
• Unconfined Aquifer: An aquifer that has no clay or other restricting material at the top of the groundwater so that the groundwater levels are free to rise or fall.  The top of an unconfined aquifer is the water table.
• Water Table: The surface of the saturated zone where water pressure is equal to atmospheric pressure; the top of the saturated zone in an unconfined aquifer
• Withdrawal: Removing groundwater by drainage or by pumping from a well.