Hydraulic characterisation of managed aquifer recharge sites by tracer techniques
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.