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Contaminated and Hazardous Waste Site Management

Glossary D


Data Quality Objectives (DQAs): Statements developed by data users to define the quality of data needed to meet the objectives of a site assessment activity.

Darcy's Law: An empirically derived equation for the flow of fluids through porous media. It is based on the assumptions that flow is laminar and inertia can be neglected, and it states that the specific discharge, q, is directly proportional to the hydraulic conductivity, K, and the hydraulic gradient, I.

Decafluorotriphenylphosphine (DFTPP): Compound chosen to establish mass spectral instrument performance for semivolatile analysis.

Decay: The gradual reduction in the magnitude of a quantity.

Deconvolution: A type of inverse filtering; the process of recovering information that has been removed by natural and/or instrumental processes (eg. the Earth, recording equipment, etc.) from the true signal. Generally improves resolution, sharpens images.

Degassing: Process of removing dissolved gases from a liquid.

Degradation: The process by which a chemical is reduced to a less complex form.

Degradation Potential: The degree to which a substance is likely to be reduced to a simpler form by bacterial activity.

Deionized Water: Water from which all free ions have been removed.

De minimis: This legal phrase pertains to settlements with parties who contributed small amounts of hazardous waste to a site. This process allows the EPA to settle with small or de minimis contributors, as a single group rather than as individuals, saving time, money, and effort.

Denitrification: Bacterial reduction of nitrite to gaseous nitrogen under anaerobic conditions.

Dense Non-Aqueous Phase Liquid (DNAPL): A non-aqueous phase liquid (NAPL) with a specific gravity greater than 1.0. Because the specific gravity of water is equal to 1.0, DNAPLs sink through the water column until they encounter a confining layer. DNAPLs flow along the surface of the confining layer and can migrate in directions contrary to the hydraulic gradient. Because DNAPLs are found at the bottom of aquifers (rather than floating on the water table) typical monitoring wells will not indicate whether DNAPLs are present. DNAPL normally refers only to the immiscible phase liquid, however, it sometimes is taken generally to refer to chemicals such as chlorinated solvents, creosote and PCB oil and to include both the immiscible phase liquid and the mass of dissolved-phase emitted from the immiscible phase. To avoid possibilities for confusion, it is preferable to refer specifically to the immiscible phase of dissolved phase when wishing to distinguish the phase.

Density: The amount of mass per unit volume.

Desorption: Process of removing a sorbed substance by reversing adsorption or absorption.

Detection Limit: The lowest concentration of a chemical which can be reliably reported to be different from zero.

Dewatering: a) Removing water from a solid material by solid-liquid separation techniques (e.g. titration, centrifugation) b) Removing or draining water from an enclosure (e.g. a river bed) by pumping or evaporation.

Dielectric Constant: A measure of the ability of a material to store charge when an electric field is applied.

Dielectric Permittivity: Describes the charge separation or polarization in a medium.

Diffusion: The process by which molecules in a single phase equilibrate to a zero concentration gradient by random molecular motion (Brownian motion). The flux of molecules is from regions of high concentration to low concentration and is governed by Fick's Second Law.

Diffusion Coefficient: A parameter that measures how rapidly a constituent will diffuse in water.

Digestion Log: An official record of the sample preparation (digestion).

Dimensionality: The number (1,2,3) and orientation (vertical, areal, radial) of dimensions chosen when modelling aspects of the subsurface.

Dipole: A pair of equal charges or poles of opposite signs.

Discharge: The rate of flow at a given time, measured as volume per unit time.

Discharge Area/Zone: An area in which there is upward groundwater flow in the subsurface. Groundwater flows toward the surface in a discharge area and may escape as a spring, seep, or baseflow, or by evaporation and transpiration.

Dispersion: The process by which a substance or chemical spreads and dilutes in flowing groundwater or soil gas.

Dispersion Coefficient: A measure of the spreading of a flowing substance as a result of the nature of the porous medium.

Dispersivity: The property of a porous medium which determines its dispersion characteristics by relating the pore velocity components to the dispersion coefficient.

Displacement Currents: The movement of charge within a material by polarization, as opposed to the flow of free ions or electrons. Related to the applied electric field by the electric permittivity (dielectric constant).

Dissolution, Dissolve: Process of dissolving a solid or NAPL, to form ions or molecules uniformly distributed in water or another solvent.

Domain: In modelling, the segment of the subsurface being considered. It is defined by its boundaries and interior geometry (based on its hydrostratigraphy), and its material properties (porosity, hydraulic conductivity, etc.).

Downgradient: A downward hydrologic slope that causes groundwater to move toward lower elevations. Therefore, wells downgradient of a contaminated groundwater source are prone to receiving pollutants.

Drainage Pattern: The arrangement of natural stream courses in an area, in plan view. It depends on local geologic/geomorphologic features and history.

Drawdown: A lowering of the water table of an unconfined aquifer or the potentiometric surface
of a confined aquifer caused by pumping of groundwater from wells. The vertical distance
between the original water level and the new water level.

Drift (glacial): Glacial drift includes all rock material in transport by glacier ice, the deposits made by glacier ice and all materials mainly of glacial origin deposited in the sea or in glacial melt water bodies including materials rafted in by ice bergs or transported indirectly in the water itself. Glacial drift therefore includes till, rock fragments and stratified drift.

Drill Bit: The device that actual cuts through the subsurface materials. Usually advanced downward at the end of a drill string.

Drill Cuttings: The rock fragments, sand grains, etc. produced at the drill bit by the drilling action and circulated to the surface in the drilling fluid where they can be sampled.

Drilling Fluid: A water- or air-based fluid used during well drilling to remove cuttings, clean and cool the bit, hold the borehole open and reduce friction between the drill string and the borehole sides. Also called mud or drilling mud.

Drill Rod: Hollow jointed or coupled rods, rotated in the borehole through their connection at the bottom, to the drill bit and at the top, to the driving mechanism of the rig.

Drill String: The pipe string which extends down from the driving mechanism and serves to rotate the bit.

Drive Shoe: A forged steel collar on the bottom of a driven casing, with a cutting edge to shear off irregularities in the hole as it advances.

Driving Force: A force that produces movement of water, gas, NAPL, or of a chemical itself.

Drumlin: Irish term for a small hill. Consists of glacial drift shaped by ice action into a "hog- back", which is oval in plan and has its long axis pointing in the direction of ice movement. Drumlins often occur in groups.

Dry Weight: The weight of a sample based on percent solids. The weight after drying in an oven.

Dual-Phase Extraction: The active withdrawal of both liquid and gas phases from a well usually involving the use of a vacuum pump.

Dual Porosity: Describes subsurface material in which groundwater flow occurs both through fractures and through pore space. An example is a fractured till, where groundwater can flow through the fractures and also through the pore space among the till particles.

Duplicate: A second aliquot of a sample that is treated the same as the original sample in order to determine the precision of the method.

Dynamic Viscosity: A measure of a fluid's resistance to tangential or shear stress.



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