Managed aquifer recharge (MAR) is a widely accepted method for augmenting water supplies for potable and non-potable use. The success of the MAR system is often defined by a substantial removal of chemical and biological contaminants during subsurface passage. To determine removal rates and to differentiate between removal and overall attenuation due to dilution, estimation of mixing proportions is a key element of tracer applications. This report provides an overview of tracers suitable for MAR and discusses advantages and disadvantages for each tracer. The ideal tracer may be defined by: a natural or anthropogenic origin, a clear uneven distribution in the studied system (e.g. sharp contrast between source and native groundwater), non-toxicity (human and environmental), easy and cost-effective measurement, and a conservative (neither sorbed nor (bio-)chemical reactive) or at least predictable chemical or physical behavior. A huge number of tracers exist, each with advantages and disadvantage. Tracers can be dissolved (e.g. chloride, bromide), stable or radioactive isotopes (e.g. ¹⁸O, ³H), gaseous (e.g. SF₆) or a physical properties (e.g. temperature). The use of heat as a tracer has several advantages over hydrochemical tracers. Temperature is inexpensive, easy and a robust parameter to measure. In contrast to chemical tracers, no laboratory analysis is required and the data is available immediately. Finally, a multi tracer approach (≥ 2 tracers) is always recommended, because the ideal tracer is rarely found. A reasonable combination is at least one conservative tracer (e.g. stable isotopes of water) with a retarded tracer (e.g. temperature) to evaluate short travel times from the point of recharge (e.g. riverbed or pond) to the recovery well.

An example of hydraulic characterisation based on temperature measurements can be found here.