Glossary of Terms, T – Z

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In this context, detailed goals that are often expressed as numeric goals. (e.g. to reduce contaminant X in this aquifer by 10% by 2009).

Threat Assessment – Tier 1

Preliminary examination of a drinking water threat based on readily accessible information.

Threat Assessment – Tier 2

Advanced examination of a drinking water threat through accessing more detailed information, interviews and perhaps when warranted, additional monitoring, modeling or studies.

Tier 1, 2, and 3 Water Budgets

Numerical analysis at the watershed/subwatershed (Tier1 and 2) or local area (Tier 3) level considering existing and anticipated amounts or water use within the watershed, as well as quantitative flow between the groundwater and surface water systems.

Time of Travel (TOT)

An estimate of the time required for a particle of water to move in the saturated zone from a specific point in an aquifer into the well intake.

Tolerance of a Water Supply System

A measure of the ability to sustain required pumping levels even during exposure events.

Uncertainty Analysis

Uncertainty analysis investigates the effects of lack of knowledge and other potential sources of error.

Uncertainty Score

Uncertainty addresses known gaps in data/information about, or deficiencies in methods of assessment for, threats and/or vulnerability. It reflects the degree of confidence in the semi-quantitative data used to calculate risk.

Unconfined Aquifer

An aquifer whose upper boundary is the water table.

Valuation of the Supply

An evaluation of the importance of a particular municipal well or intake to the whole municipal drinking water supply. For example, where there are multiple supplies, value may be smaller, versus a single supply where value may be greater.

Vulnerable Area

An area referring to a groundwater recharge area, a highly vulnerable aquifer, and a surface water intake protection zone or wellhead protection area.

Water Intake Reliability

The probability that a wellhead or surface water intake can meet demand.

Water Reserve

A proportion of surface water flow that must be sustained to support anthropogenic or ecological requirements.

Water Source

An aquifer or surface water body being used to supply drinking water.

Water Source Supply

The total amount of water flowing through a surface water or groundwater system.

Water Supply System

The group of surface water intakes and/or groundwater wells that pump water to supply a municipal water distribution system.

Water Quantity Exposure

The extent to which a threat or group of threats affects the availability of water at an intake or wellhead.

Water Quantity Receptor

A competing water demand or requirement in danger of incurring a potential impact. This includes other anthropogenic or ecological water uses within the watershed, particularly those that are required to be maintained by provincial or federal law (e.g. permitted wastewater assimilation flows, other Permits To Take Water, or fish habitat protected by Department of Fisheries and Oceans legislation).

Water Quantity Risk

The likelihood that the threats to water quantity may render an existing or planned drinking water source impaired, unusable or unsustainable.

Water Quantity Targets / Water Reserve Targets

These are detailed physical goals that are often expressed as numeric goals (e.g. to maintain streamflow above X).


An area that is drained by a river and its tributaries.

Wellhead Protection Area

The surface and subsurface area surrounding a water well or well field that supplies a municipal residential system or other designated system through which contaminants are reasonably likely to move so as to eventually reach the water well or well.


Land such as a swamp, marsh, bog or fen (not including land that is being used for agricultural purposes and no longer exhibits wetland characteristics) that, (a) is seasonally or permanently covered by shallow water or has the water table close to or at the surface, (b) has hydric soils and vegetation dominated by hydrophytic or water-tolerant plants, and (c) has been further identified, by the Ministry of Natural Resources or by any other person, according to evaluation procedures established by the Ministry of Natural Resources, as amended from time to time.

Source: Langbein, W.B and Iseri, K.T (1995) Manual of Hydrology: Part 1. General Surface-Water Techniques. Geological Survey Water-supply Paper 1541-A. Methods andpractices of the Geological Survey. Online:

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