CARBON MONOXIDE CONCENTRATIONS AS A TRACER OF RECHARGE EVENTS TO FRACTURED-ROCK AQUIFERS

Dissolved carbon monoxide (CO) is present in ground water produced from a variety of aquifer systems at concentrations ranging from 0.2 to 20 nanomoles per liter (0.0056 to 0.56 micrograms per liter). In two shallow aquifers, one a fractured-bedrock aquifer in West Trenton NJ and the other an unconsolidated coastal plain aquifer in Kings Bay, GA, long-term monitoring showed that CO concentrations varied over time by as much as a factor of ten. Field and laboratory evidence suggests that the delivery of dissolved oxygen to the soil zone and underlying aquifers by periodic recharge events stimulates oxic metabolism and produces transiently high CO concentrations. In between recharge events, the aquifers become anoxic and more substrate-limited, CO is consumed as a carbon source, and CO concentrations decrease. According to this hypothesis, CO concentrations provide a record of transient oxic metabolism that is detectable in ground-water systems after dissolved oxygen has been fully consumed. As such, CO concentrations may be useful for tracing the delivery and movement of recharge waters to the complex subsurface architecture characteristic of fractured-rock aquifers.