The challenge of interpreting environmental tracer concentrations in fractured rock and carbonate aquifers

Abstract

Atmospheric environmental tracers are constituents and dissolved gases (e.g., chlorofluorocarbons, isotopes of hydrogen, helium, carbon, and oxygen) that are entrained in precipitation and recharged to groundwater. Often the time-varying atmospheric concentrations or processes affecting their evolution in groundwater are used with simplified conceptual models of groundwater flow to estimate residence times, which can then be used to infer velocity, recharge, and properties affecting chemical transport. For example, translating concentrations to residence time (measured in years) is often conducted using concepts of plug flow, binary mixtures of groundwater, and simple models of recharge and flow (Cook and Herczeg 2000). Successful interpretations of environmental tracer concentrations have been widely reported for unconsolidated, porous-media aquifers, and residence times have been introduced as calibration targets using particle-tracking methods in regional flow models (Sanford et al. 2004). To a lesser extent, interpretations of environmentaltracer concentrations have been applied to fractured-rock aquifers and carbonate formations that have been subject to karstification (Cook and Simmons 2000; Long and Putnam 2006). Such aquifers offer a degree of geologic complexity that exceeds that of porous-media aquifers. Consequently, methods of interpreting environmentaltracer concentrations commonly applied in porous media may not be applicable to fractured rock and carbonate aquifers (herein collectively referred to as fractured rock). The purpose of this article is to highlight those processes that affect environmental tracers in fractured rock, identify deficiencies in applying interpretive methods commonly used in porous media, and suggest alternative approaches for interpreting environmental-tracer concentrations to better understand the regional hydrogeology of fracturedrock aquifers.