PHYSICAL AND HYDROCHEMICAL BEHAVIOR OF A BROMIDE TRACER IN SHALLOW GROUND WATER ADJACENT TO AN EPHEMERAL CHANNEL: INFLUENCE OF SEASONALITY AND PULSE RECHARGE IN THE SEMI-ARID SOUTHWEST

Shallow alluvial ground water in the semi-arid southwest is linked to episodic precipitation, runoff, and recharge events that create transient hydraulic and hydrochemical gradients. Solute (contaminant) exchange between ground and surface water reservoirs takes place in the aquifer near ephemeral channels during recharge events. To investigate the behavior of contaminant migration under transient hydrological conditions, we are conducting a KBr tracer test in a shallow alluvial aquifer in DP Canyon at Los Alamos National Laboratory. The DP Canyon system is made up of several reaches containing discrete alluvial aquifers and a head-cut spring where ground water “daylights” and re-enters the subsurface. Historical contaminants of concern that have accumulated in the aquifer since the 1950s include Sr-90, Cs-137, H3, and perchlorate.

A tracer mass of 47 kg of KBr dissolved in 100 gallons of formation water was injected into a well screened over the saturated thickness of the aquifer (approximately 5 ft). Br is currently being monitored in situ with ion selective Temphion probes and from samples collected manually at systematic intervals. Cation samples, including Sr-90, have been collected and analyzed to determine the extent of ion exchange as the plume migrates through the aquifer. Our observations indicate that runoff from the Los Alamos town site provides linear recharge along an ephemeral channel during snowmelt and precipitation events. The response of the water table is immediate with measured heads greater near, and diminishing away from, the axis of the channel. During the receding limb of the hydrograph, water table gradients relax allowing ground water, solute, and contaminants to flow back into the channel. As a result, surface/ground water interaction episodically flushes the aquifer adjacent to the channel and transports solute both in surface and ground water. Preliminary tracer data corroborate this model. Data show that although the centroid of the bromide plume remains in the upper to mid-part of the reach, Br has traveled extensively in the aquifer proximal to the channel during and shortly after runoff events. Ground/surface water exchange in the hyporheic zone appears to facilitate tracer migration close to the channel, with diminishing effects in the aquifer away from the channel.