Inferring fault hydrology using groundwater age tracers

Faults can act as barriers to cross-fault flow, conduits to along-fault flow or both. Groundwater age inferred from hydrochemical data is one of the few tools that can estimate both barrier and conduit behaviour of the fault and associated fault damage zone. Conduit behaviour has been previously inferred from the existence of young water in deep aquifers or old water in shallow aquifers. While the ability of groundwater age to estimate the barrier behaviour of faults has been illustrated theoretically, it has not yet been observed in the field. Here, conceptual models of hydraulic head and groundwater age perpendicular to and across faults are discussed in terms of the resistance to flow across the fault (fault width divided by cross-fault transmissivity), the resistance to flow around the fault (fault length divided by aquifer transmissivity) and the recharge regime. To examine these conceptual models, field measurements of hydraulic head, ¹⁴C, chloride and electrical conductivity in groundwater are presented along transects across three contrasting fault systems. Large cross-fault head and age gradients are apparent at two of the three sites. At one of these sites, where regional flow is mostly parallel to the fault, ¹⁴C age and chloride data are used to estimate the rate of cross-fault flow. At the third site, there may not be a barrier to cross-fault flow, as there is no clear increase in either head or age gradient. The studies illustrate the importance of high data density to characterise fault zones, particularly for groundwater age which will vary both with depth and horizontal location along a transect. Although our data does not indicate conduit behaviour, such behaviour has been suggested based on groundwater chemistry at other locations along the fault strike at two of the sites.