MODELING BROMIDE TRANSPORT THROUGH AN UNSATURATED FRACTURED BASALT

We present results and preliminary analysis from a bromide tracer test associated with a low-head weir in Los Alamos Canyon. The weir was built after the Cerro Grande fire to mitigate off-site movement of potentially contaminated sediment, which was expected to increase dramatically due to watershed damage further up the drainage. After construction, concerns about downward transport through fractured basalt from temporarily ponded water behind the weir prompted the Laboratory to install three monitoring boreholes. In conjuction with the monitoring plan, a bromide tracer test was designed to study transport. Prior to the summer rainy season of 2002, a solution of potassium bromide was sprayed onto the ponding area behind the weir. Bromide sampling was initiated in May 2002 and is ongoing. An 86 m vertical borehole with water sampling ports at 4 screened intervals (from 27 m to 82 m) provides high resolution temporal data of bromide concentration within the unsaturated zone beneath the weir. Data from the 4 sampling ports show that the initial transport to depth is quite rapid (60m in a few days), and that three observed ponding events have had a large impact on subsurface transport. Additionally, the data show that the bromide concentrations at all depths peaked within 1/2 year and are now decreasing. Simulations of bromide breakthrough are used to constrain the effective basalt porosity (0.001 - 0.01) and permeability (1e-12 - 1e-11 m2).