With the increasing pressure to use desert basins as disposal sites for low level nuclear waste, estimating the vulnerability of groundwater to leachate potentially produced at these sites or other contaminant sources is becoming increasingly important. Chloride Mass Balance is one of many techniques that can be used to estimate infiltration in arid regions. This infiltration estimation technique has several assumptions including the assumption of piston flow (little to no preferential flow). The purpose of this thesis is to test some of the assumptions piston flow. Cores were collected from the Borrego Desert and an intense artificial infiltration event was conducted on them. Water drained from the bottom of the cores was collected and analyzed for chloride content. The cores were then dissected. Some of the soil samples were analyzed for chloride content by dilution and some were sampled for water by centrifuging at increasingly higher revolutions per minute. The smaller pores, emptied by higher RPMs, might be expected to have higher chloride concentrations since they would have a low hydraulic conductivity relative to the larger pores. The small pores may retain more chloride because during heavy recharge events the majority of water would preferentially flow through the larger pores leaving a higher chloride concentration in the smaller pores. The results of the experiments show that piston flow is the dominant transport mechanism within the 5.1 cm diameter cores of the soils sampled. The data from the centrifuged samples show that there is likely some preferential flow occurring on a pore scale.
