This dataset comprises sand and gravel aquifers. Sand and gravel aquifers may be locally important (Lg) or regionally important (Rg).

Rg - A sand/gravel aquifer is classed as regionally important if it can supply regionally important abstractions (e.g. large public water supplies with ‘excellent’ yields >400 m3/d). It is highly permeable, more than 10 m thick or has a saturated thickness of at least 5 m, and should extend over at least 5 km2, and usually over 10 km2.

Lg - Locally Important Sand/Gravel Aquifer: Similar to a Regionally Important Sand/Gravel Aquifer (Rg), but with a smaller continuous area (c.1-10 km2) and/or less consistent permeability. Although the aquifer may supply ‘excellent’ yields, the smaller size limits the amount of recharge available to meet abstractions.

Sand/gravel deposits have a dual role in groundwater development and supply. Firstly, in some cases they can supply significant quantities of water for supply and are therefore classed as aquifers, and secondly, they provide storage for underlying bedrock aquifers. A sand/gravel deposit is classed as an aquifer if the deposit is highly permeable, more than 10 m thick and greater than one square kilometre in aerial extent. The thickness of the deposit is often used rather than the more relevant saturated zone thickness as the information on the latter is rarely available. In many instances it may be assumed that a deposit with a thickness of 10 m will have a saturated zone of at least 5 m. This is not the case where deposits have a high relief (for example eskers or deposits in high topographic areas) as these gravels are often dry.