ABSTRACT
Groundwater recharge in arid zones is characterised by low fluxes and high spatial and temporal variability. Environmental tracers are well suited for estimating recharge in these environments because they are more sensitive than most other methods to very low rates of recharge, and because they provide inherent spatial and temporal averaging. Groundwater recharge rates can be estimated from environmental tracer measurements on unsaturated zone soil profiles, or from water samples from piezometers completed at discrete intervals within confined and unconfined aquifers. Chloride is one of the most widely used tracers within the unsaturated zone. Under steady state conditions and in the absence of significant surface runoff, a mass balance of chloride can be used to estimate rates of groundwater recharge. Where rates of recharge have changed, either from a change in land use or due to changes in climate, chloride profiles can also be interpreted to reconstruct changes in infiltration over time. Where unsaturated zones are deep, however, there may be a timelag of many years or decades before changes in infiltration lead to changes in aquifer recharge. The magnitude of this timelag can also be estimated from water content and matric potential profiles.
Recharge rates can also be estimated from estimates of groundwater age, and the most widely used technique in arid zones involves measurements of radioactive decay of carbon-14. Carbon-14 has been used to estimate rates of diffuse recharge to unconfined aquifers and rates of recharge from ephemeral streams. In areas with deep unsaturated zones, however, measurements of the initial carbon-14 activity of groundwater recharge are required for accurate groundwater age estimates.
