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Alluvial aquifer: The alluvial aquifer is composed of unconsolidated sediments (silt, sand, gravel, cobbles) deposited by stream flow.

Alternate concentration limit: Concentration of a constituent that may exceed the maximum concentration limit; or, a limit for a constituent without a maximum concentration limit. If DOE demonstrates, and the U.S. Nuclear Regulatory Commission concurs, that human health and the environment would not be adversely affected, DOE may meet an alternate concentration limit.

Ambient contamination: Naturally occurring constituents in groundwater (i.e., constituents that are not due to ore processing) that are present in concentrations sufficiently high to render the water undesirable or unfit for domestic use.

Aquifer: A body of rock or sediment that is saturated and sufficiently permeable to conduct groundwater in economically significant quantities to wells and springs.

Background level: 1. The concentration of a substance in an environmental medium (air, water, or soil) that occurs naturally or is not the result of human activities. 2. In exposure assessment the concentration of a substance in a defined control area, during a fixed period of time before, during, or after a data-gathering operation.

Baseline Risk Assessment: A baseline risk assessment describes the source of contamination, how the contamination reaches people and the environment, the amount of contamination to which people or the ecological environment may be exposed, and the health or ecological effects that could result from exposure.

Compliance strategy: The method used to meet U.S. Environmental Protection Agency (EPA) groundwater standards at a site regulated under the Uranium Mill Tailings Radiation Control Act (UMTRCA).

Contaminant: An undesirable substance from uranium-ore processing activities that may affect human health and the environment.

Contamination: Introduction into water, air, and soil of microorganisms, chemicals, toxic substances, wastes, or waste water in a concentration that makes the medium unfit for its next intended use. Also applies to surfaces of objects, buildings, and various household and agricultural use products.

Downgradient: Groundwater located in the same direction as groundwater flow from a specified location.

Environmental Assessment: A document that includes a brief discussion of the environmental impacts of the proposed action and alternatives and provides sufficient evidence and analysis for determining whether to prepare an Environmental Impact Statement or a Finding of No Significant Impact.

Environmental Impact Statement: A document that describes and evaluates the potentially significant impactsof major federal actions on human health and the environment from the proposed action and alternatives to the proposed action.

Exposure pathway: The course a contaminant takes from its source to the exposed individual. A complete exposure pathway generally requires four elements: (1) a source and mechanism of release, (2) a retention or transport medium, (3) a point of potential human contact with the contaminant (referred to as the exposure point), and (4) an exposure route (e.g., external gamma irradiation, ingestion, inhalation, dermal absorption). If any of these elements is missing, the exposure pathway is considered incomplete.

Exposure scenario: A series of assumptions based on factors such as land use and human activities at a site. The major assumptions inherent to a scenario aside from land use, are the frequency and duration of exposure to a contaminant. These exposure assumptions are assigned numerical values along with assumptions regarding exposure pathways and estimate of exposure point concentrations to calculate potential intakes of a contaminant by a receptor. Typical exposure scenarios are residential, commercial/industrial, and recreational.

Groundwater plume: A defined area of groundwater contamination. In this project, the term "groundwater plume" means the contaminated groundwater beneath a millsite and surrounding area that DOE determines to contain soluble radioactive or nonradioactive hazardous constituents that are present as a result of the uranium milling process.

Groundwater remediation: Treatment of grounwater to decrease the amount and mobility of contaminants.

Institutional controls: Controls such as deed restrictions, use restrictions, and permitting requirements that prohibit or limit activities that may result in exposure to contamination. Effective institutional controls must remain in effect for the duration of the hazard, survive a change in property ownership, and be enforceable. Institutional controls also include those which preserve knowledge and facilitate public education regarding hazards at a site in order to enhance protectiveness into the future.

Leachate: Water that collects contaminants as it percolates through wastes.

Limited-use groundwater: Groundwater that is defined in 40 U.S. Code of Federal Regulations 192.11(e) as "not a current or potential source of drinking water because...widespread, ambient contamination not due to activities involving residual radioactive materials from a designated processing site exists that cannot be cleaned up using treatment methods reasonably employed in public water systems...."

Maximum concentration limit: The EPA's maximum allowable concentration of certain constituents for ground-water protection. Constituents with maximum concentration limits that may be present in contaminated groundwater at UMTRCA-regulated sites include arsenic, barium, cadmium, chromium, lead, mercury, molybdenum, nitrate, radium, selenium, silver, and uranium.

Natural flushing: Allowing natural ground water movement and geochemical process to decrease contaminant concentrations.

Protectiveness: Maintaining risks to human health and the environment to within approved limits.

Radioactivity: The spontaneous emission of radiation, generally alpha or beta particles, often accompanied by gamma rays, from the nucleus of an unstable atom.

Radon: A colorless, naturally occurring, radioactive, inert gas formed by radioactive decay of radium atoms in soil and rocks.

Radon/infiltration barrier: A layer of compacted low-permeability clayey soil in the disposal cell cover that slows the movement of radon enough for the radon to decay before it escapes, and prevents precipitation water from entering the disposal cell.

Remediation: Removal of contamination at a site to levels that do not exceed pre-established goals, such as federal or state standards or alternate concentration limits that are protective of human health and the environment.

Residual radioactive material: Uranium mill tailings that have resulted from the processing of uranium ore and other waste at a processing site that DOE determines to be radioactive and that relates to such processing. The U.S. Environmental Protection Agency has interpreted this to include sludges and captured contaminated water from processing sites.

Risk: A measure of the probability that damage to life, health, property, and/or the environment will occur as a result of a given hazard.

Risk assessment: A process for organizing and analyzing information to determine if an environmental chemical might cause harm to exposed persons or the ecosystem. For human health, the EPA risk assessment process analyzes the possibility of cancer and noncancer effects caused by site contamination.

Site Observational Work Plan: A document that presents a summary of site hydrogeologic data and presents a site conceptual model. It presents an analysis of site environmental and health risk, identifies data gaps in the conceptual model, and identifies appropriate site-specific groundwater compliance strategies.

Stakeholders: Any organization, government entity, or individual who has a stake in or may be affected by an approach to environmental regulation, pollution prevention, energy conservation, and other activities.

Surveillance and maintenance: All activities necessary to ensure protection of human health and the environment following completion of cleanup, disposal, or stabilization at a site or portion of a site. Long-term surveillance and maintenance includes all engineered and institutional controls designed to contain or prevent exposure to residual contamination and waste, such as surveillance activities, record-keeping activities, inspections, groundwater monitoring, ongoing pump and treat activities, cap repair, maintenance of entombed buildings or facilities, maintenance of other barriers and contained structures, access control, and posting signs.

Supplemental standards: Regulatory standards that are protective of human health and the environment that may be applied when the concentration of certain constituents exceeds the standards.

Tiering: "Tiering" refers to the coverage of general matters in broader Environmental Impact Statements (such as national program or policy statements); subsequent narrower statements or environmental analyses (such as regional or ultimately site-specific statements) are "tiered" to the broader, general statements and incorporate them by reference. The narrower statements concentrate solely on the issues specific to the site.

Transient drainage: Gravity drainage of water expelled from the pore spaces of soil encapsulated in the disposal cell. Water is introduced into the cell during construction as water added for compaction and dust control, as moisture in waste materials, and from precipitation. The weight of overlying material squeezes water from pore space, which drains out the bottom of the waste material. "Transient" refers to the fact that the cell cover prevents recharge of water into the cell, and drainage is an artifact of construction that will eventually reach zero flow.

Uranium mill tailings: The sand-like material remaining after uranium ore has been crushed, ground, and leached with acids and solvents to extract the uranium and vanadium.

Vicinity properties: Properties outside a processing site boundary that have been contaminated by residual radioactive materials. These materials could have been dispersed by wind or water erosion, or removed by people.