Acid mine drainage. The formation and movement of highly acidic water laden with heavy metals, generated as a result of the reaction of water with sulfur-bearing rocks, creating sulfuric acid that leaches heavy metals from rocks.

Acid rain/ acid deposition. Any form of precipitation (e.g., rain, fog, dust, or snow) that has a pH lower than typical rainfall (around pH 5.6). Acid deposition usually has a pH between 4.2 and 4.4.

Advection. The transport of a substance (i.e., a pollutant) dissolved in or carried by a fluid such as groundwater through the bulk motion of that fluid.

Agricultural Best Management Practices. A range of tools that can be implemented by farmers on their lands to improve water quality while maintaining agricultural productivity. 

Agricultural drought. Drought that affects crop productivity due to changes in temperature and precipitation.

Alluvial fan. A fan-shaped deposit of gravel, sand, and other sediments, typically located where the mouth of a river or stream flowing through a steep valley in a mountainous area opens onto a flatter plane.

Aquifer. A formation below the ground surface capable of storing large quantities of water. Water in an aquifer can be stored in fractures in rock formations or within the pores of unconsolidated materials like gravel. Aquifers that are open to the ground surface are known as unconfined aquifers. Aquifers that are contained between two layers are known as confined aquifers.

Atmospheric deposition. The process whereby precipitation, particulates, aerosols, and gases move.

Backwater effect. An increase in the upstream water level as a result of an obstruction to flow, such as a bridge opening or culvert. Backwater is measured as an increase above the normal surface water elevation.

Biogeochemical degradation. The process of breaking down a pollutant (often to a nontoxic substance) through interactions with microbes or minerals in the soil.

Blue baby syndrome. Common term for methemoglobinemia, a condition in infants where the child’s skin turns blue as the blood is unable to carry oxygen around the body. Methemoglobinemia is caused by ingesting water contaminated by nitrates. 

Brine. A hypersaline solution produced as a byproduct of the reverse osmosis desalination process.

Catalysis. The process of increasing the rate of a chemical reaction through the addition of a chemical known as a catalyst. For many biochemical reactions, this catalyst is an enzyme.

Chemical oxygen demand (COD) and Biological oxygen demand (BOD). The amount of oxygen consumed in the chemical/biological breakdown of organic matter in a water sample by a strong chemical oxidizing agent (COD) or bacteria present in the water (BOD). COD/BOD is used as an indicator of the amount of organic matter in water.

Closed basin. An internally draining watershed whose waters do not flow to the sea.

Concentrated Animal Feeding Operation (CAFO). An agricultural operation where animals are kept and raised in confined situations for the purposes of producing meat, milk, or eggs. An operation must contain at least 1,000 head of beef cattle, 700 dairy cows, 2,500 swine weighing more than 55 pounds, 125,000 broiler chickens, or 82,000 laying hens or pullets confined on site for more than forty-five days during the year to be defined as a CAFO.

Confining layer. A layer of a substance that water cannot pass through easily, like clay or shale, that acts as a bounding layer to an aquifer. Also known as an aquitard.

Consumption. The volume of water removed and not returned to the original water source, often as a result of evaporation.

Darcy’s Law. The equation governing the speed at which groundwater flows. Groundwater speed is proportional to the hydraulic gradient of the groundwater and the hydraulic conductivity of the aquifer.

De facto reuse. Unplanned reuse that occurs when a user discharges runoff or effluent back into the same stream or river that is the water source for another downstream user.

Deep-injection wells. Wells used to place water into porous geologic structures. Commonly used as a part of managed aquifer recharge.

Diffusion. Also known as molecular diffusion. The spreading out of a pollutant in time and space due to the thermal motion of individual pollutant molecules. In situations where the natural flow of groundwater is very slow (e.g., a layer of clayey sediment), diffusion might be the primary process defining how a pollutant moves.

Disinfection byproducts (DBPs). Toxic organic and inorganic compounds that can form as a result between naturally occurring organic compounds in water and a chemical disinfectant such as chlorine. Trihalomethanes are a family of common DBPs.

Dispersion. Also known as mechanical dispersion. The spreading out of a dissolved pollutant along the groundwater flow path due to variations in the velocity at which the pollutant is traveling relative to the average groundwater flow velocity. These changes in velocity are a result of slight variations in flow path length (some pollutant molecules will have further to travel than others) and variations in pore size in the aquifer medium (water tends to travel faster through larger pores than smaller ones). The combined effects of mechanical dispersion and molecular diffusion are known as hydrodynamic dispersion.

Dissolved Oxygen (DO). The amount of oxygen dissolved into a sample of water; the amount of oxygen available for aquatic organisms such as fish.

Energy-favorable reaction. A reaction that takes place spontaneously, without the required addition of further energy.

Erosion. The wearing away of surface materials (particularly topsoil) by the action of water and wind. Erosion of topsoil is a significant challenge for water quality and flooding.

Eutrophication. The enrichment of an aquatic environment with nutrients like nitrogen and phosphorus, leading to excessive blooms of algae and phytoplankton.

Flash drought. A rapid and sudden intensification of drought conditions.

Flash flood. Flooding that begins less than six hours (and often less than three hours) after heavy rainfall.

Flow duration curve (FDC). A graphical tool used by water resource managers to predict and evaluate changes in streamflow over time based on a historical record of probable streamflow.

Green infrastructure. The use of natural processes such as infiltration and evapotranspiration to capture, retain, and treat rainwater where it falls.

Gray infrastructure. Human-engineered systems for conveying, treating, and storing stormwater (including stormwater pipes, ditches, and retention ponds). Compare with green infrastructure.

Groundwater amendment. Manmade alterations to groundwater, often used to control the spread and cleanup of pollutants. Amendments are typically added to groundwater via injection wells and can include oxygen, reactive iron, or a food source for microbes in the soil that can aid with pollutant degradation. They can also be additions such as an acid, base, or buffer to control the groundwater pH and promote more favorable reaction conditions.

Groundwater. Water contained below the ground’s surface, either within the pores of saturated soil or within fractures in rock formations.

Hard engineering. The use of engineered hydraulic structures to protect coastlines from erosion and flooding. Compare with soft engineering.

Harmful algal blooms (HABs). Overgrowths of algae in waterbodies such as lakes, rivers, or marine bodies. Cyanobacteria (bluegreen algae) blooms can release toxic chemicals called microcystins into water bodies, while phytoplankton blooms in marine bodies can produce toxic “red tides.”

Heavy metals. Naturally occurring metals with a high atomic weight and a density at least five times that of water. Heavy metals include lead, cadmium, chromium, mercury, and arsenic and are highly toxic to both humans and wildlife.

Hydraulic conductivity. A physical property of the aquifer material that defines how fast water can move through pores or fractures under a specified hydraulic gradient. Often related to material properties such as grain size. Measured in the lab using a permeameter test.

Hydraulic fracturing (fracking). A process that artificially creates fractures in beds of shale and other similar geologic features to stimulate the production of oil and natural gas.

Hydraulic gradient. The “slope” of the groundwater.

Hydraulic head. A measurement of the energy in groundwater from the combined elevation of the water and the fluid pressure of the water in the soil pores. Given as a measure of height (in meters or feet). The hydraulic head is also the height of the water table in an unconfined aquifer, or the height of the water that would be observed in an observation well (also known as a piezometer) drilled into a confined aquifer.

Hydrograph. A graph of flow versus time past a given point of a river or stream. The size and shape of a hydrograph varies based on the shape of the watershed, how developed the land surface is, and surface water-groundwater interactions.

Hydrologic drought. Drought characterized by the effect of changes in precipitation on surface and subsurface water supply.

Hypoxia. Conditions characterized by dissolved oxygen levels lower than 2–3 milligrams per liter that can cause aquatic “dead zones.” Mainly a concern for estuaries and shallow coastal waters.

Ice jams. An accumulation of floating river ice that blocks the flow of water downstream. Ice jam breaches are a common cause of flash floods in colder climates.

Indirect potable reuse. Wastewater that is treated and pumped via deep-injection wells into an aquifer that is then used as a drinking water source for a municipality.

Initial abstraction. A parameter that accounts for all losses prior to runoff in flood modeling.

Interception. Capture and retention of rainfall, by the canopy of plants and trees, that prevents it from reaching the ground.

Interflow. The lateral flow of water through the unsaturated zone that emerges as surface water before infiltrating to deeper groundwater.

Lacustrine flooding. Flooding caused by short- or long-term fluctuations in the water levels of a lake.

Longshore drift. The movement of material along the coast by waves approaching at an angle to the shore but receding directly perpendicular to it.

Managed aquifer recharge (MAR). A suite of strategies used to replenish depleted aquifers through surface or subsurface recharge.

Meteorological drought. Drought characterized by a degree of dryness from a specified precipitation baseline over a given duration of time.

Microcystin. A family of toxic compounds produced as a result of the degradation of cyanobacteria blooms (see HABs). Microcystins are potent liver toxins.

Nitrogen. One of the three macronutrients needed for plant growth alongside potassium and phosphorus. Nitrogen can be found in many different forms in the soil, air and water, including nitrogen gas (N₂), nitrous oxides, nitrites (NO₂^-), ammonia (NH₃), ammonium (NH₄⁺), nitrates (NO₃^-), and organic nitrogen. 

Non-point source. A pollutant that is discharged over a wide area rather than a single, well-defined location. Examples include contaminated runoff from farm fields and urban areas.

Organic chemicals. Molecules that contain one or more carbon atoms chemically bonded to an atom of hydrogen, oxygen, or nitrogen. These molecules have the unique ability to form long chains.

Overtopping. Water elevations that exceed the crest of a dam through a combination of reservoir water level elevation and wind and wave action.

Phosphorus. One of the three macronutrients needed for plant growth alongside nitrogen and potassium. Phosphorus can be found in multiple forms in soil and water, primarily organic, inorganic soluble, and inorganic insoluble phosphorus.

Piping. Internal erosion of an earth dam that occurs when seepage carries soil particles away from the dam or its foundations or abutments. As this happens, an elongated cavity can be eroded backwards (i.e., upstream) towards the reservoir. When such a cavity reaches the reservoir, catastrophic failure of the dam can occur.

Point Source. A pollutant that enters the environment from an easily identifiable, defined, and confined location. Groundwater examples include underground tanks, defined spills, or injection wells. Note that this definition is slightly different from the statutory definition in the CWA: “any discernible, confined and discrete conveyance, including but not limited to any pipe, ditch, channel, tunnel, conduit, well, discrete fissure, container, rolling stock, concentrated animal feeding operation, or vessel or other floating craft.” Importantly, the CWA definition does not include agricultural discharges and return flows from irrigated agriculture.

Pollutant Retardation. The process of slowing down—but not eliminating—the transport of a pollutant dissolved in groundwater through the reversible process of soil sorption.

Porosity. The ratio of void volume to total soil volume.

Precipitation (chemical). A chemical reaction where a dissolved substance is transformed into a nonsoluble form and deposited (precipitated) out of the solution.

Priority pollutants. A set of 126 toxic pollutants regulated by USEPA, for which the agency has developed analytic testing protocols pursuant to the requirements of Section 307 of the CWA. These pollutants include heavy metals and several named organic chemicals, such as the pesticide atrazine.

Produced water. A byproduct of oil and natural gas extraction (especially via fracking) that contains a mixture of water, hydrocarbons, chemical additives, salts, and radionuclides. Produced water may be disposed of through injection into underground wells or treated and discharged to surface waters.

Reduction/oxidation. Reduction is a chemical reaction where a metal ion gains electrons (or loses oxygen). Oxidation is a chemical reaction where a metal ion loses electrons (or gains oxygen). Since the two chemical reactions occur simultaneously, they are often called redox reactions.

Redundancy. Provision of multiple layers of protection or backups to reduce the probability of failure of a component or systems.

Relative Mean Sea Level. The average sea elevation as measured with respect to a reference point on land.

Reliability. The probability that a component or system will not fail. Failure can be defined in multiple ways depending on the goals for the system in question.

Reservoir. A mode of long- or short-term storage of water in the natural environment. Includes lakes, groundwater, and water stored in ice.

Resilience. The ability of a system to “bounce back” from a shock or stressor.

Return flow. The amount of water returned to a waterbody following use and treatment.

Reverse osmosis (RO). A purification technique that uses a membrane to remove salts and other ionic substances from water. Large amounts of energy are used to power pumps to overcome natural osmotic pressure (i.e., the tendency of a solution to diffuse).

Riverine flooding. Flooding caused by a river or stream filling up and overflowing its naturally defined channel. This type of flooding is also known as overbank flooding.

Saturated zone. The section within the earth’s surface where the pores in the soil are completely filled with water.

Sediment starvation. The depletion of a sediment budget for a coastal estuary (often due to the creation of upstream dams), leading to coastal erosion and a decrease in overall land surface elevation. A particular challenge for estuaries on sediment-rich rivers such as the Mississippi.

Sedimentation. The process of depositing new sediment through suspended particles falling out of suspension due to reduced flow velocity. The dual processes of erosion and sedimentation are responsible for the natural reshaping of river valleys, floodplains, and deltas over time.

Seepage. The movement of water through a dam or its foundations or abutments.

Sheet flow. Runoff that flows over the surface of the ground without concentration in a channel. After around 100 feet, sheet flow deepens into shallow concentrated flow of between 0.1–0.5 feet.

Sink. The removal of water from a groundwater system through loss of surface water or pumping.

Soft engineering. Term used in coastal engineering to refer to the use of ecological principles to restore coastal areas and prevent erosion (also known as living shoreline). Compare with hard engineering.

Sorption. The physical and chemical processes by which one substance becomes attached to another. In the context of groundwater, sorption often refers to the process by which pollutant particles become attached to particles of soil. The process of sorption is reversible (desorption).

Source. The addition of water to a groundwater system through percolation from the ground surface, seepage from a surface waterbody such as a stream or lake, or an injection well.

Spillway. A structure designed to provide the gradual release of water from a dam or levee downstream.

Spreading grounds. An engineered water conservation facility that retains surface water long enough for it to percolate through underlying permeable soils into a hydraulically connected aquifer.

Stationarity. A time series without any definable trend, a constant variation, and defined auto-correlation structure (i.e., the relationship between one section of the time series and another, lagged section remains constant).

Stochastic. Relating to evaluation based on probabilities rather than a certain outcome.

Storm surge. Abnormal rise in water elevation generated by a storm above normal tide.

Subsidence. The gradual sinking or sudden settling of the land surface. Land subsidence can be caused by tectonic movements or by groundwater extraction.

Surface runoff. The flow of water across the ground surface when rainwater or meltwater can no longer infiltrate into the ground.

Tertiary (wastewater) treatment. An additional treatment stage beyond what is typically implemented for biologically treated wastewater effluent. The type of tertiary treatment depends on the designated end-use of the water. Also known as wastewater polishing.

Thermal stratification. The natural tendency of stagnant bodies of water (such as lakes) to form distinct layers separated by temperature.

Tidal flooding (nuisance flooding). The temporary inundation of low-lying areas caused by exceptionally high tides.

Total Kjehdahl Nitrogen (TKN). The sum of ammonia and organic nitrogen species in water. A common measurement used to evaluate nutrient concentrations in water.

Total Suspended Solids. The dry weight of solids that are not dissolved in water. A physical water quality indicator.

Transpiration. The loss of water vapor through a plant’s leaves.

Turbidity. A measure of the relative clarity of a liquid. A common physical water quality indicator.

Urban flooding (localized flooding). The inundation of land in the built environment resulting from a storm overwhelming existing drainage or sewer systems.

Vadose zone. The soil layer above the saturated zone that contains a combination of both air and water. Plant roots typically extract water from this layer.

Water scarcity. A condition that occurs when the demand for water outstrips supply.

Water table. The top of the saturated zone. Also the top of an unconfined aquifer.

Watershed. The land area that contributes runoff from precipitation or snowmelt to a stream or river at a given outlet point. Watershed definition is an important first step in flood modeling and forecasting, as well as pollutant modeling.

Withdrawal. The total volume of water abstracted from a surface or groundwater body for a given use.