Following the May 2000 Cerro Grande fire at Los Alamos, NM, surface water control structures were constructed near Los Alamos to mitigate the transport of contaminant-bearing sediment toward the Rio Grande river due to increased runoff caused by the removal of vegetation by the fire. A low-head weir was constructed in Los Alamos Canyon, 5 km to the east of Los Alamos, to capture contaminant-bearing sediments and to allow runoff to pass downstream without significant ponding behind the weir. During construction of the weir, channel alluvium was removed and the underlying fractured basalt was exposed. To monitor any downward transport of contaminants into fractured basalt, and potentially downward to the regional groundwater, three boreholes (one vertical, and two angled) were installed for environmental monitoring. An innovative monitoring system was installed using FLUTe (Santa Fe, NM) liners for both vadose zone and perched groundwater zones. The vertical borehole intersects several perched water zones, and groundwater can be sampled from four ports. One angled borehole has an inflatable liner with sensors to measure relative water content. The second angled borehole was abandoned. Tracer tests were initiated in April 2002 and June 2003 with the application of solutions of potassium bromide and potassium iodide, respectively, onto the basin floor above the weir. The hydrogeologic characterization from drilling the boreholes, in conjunction with groundwater elevation and vadose zone moisture monitoring, and results from the tracer tests show that the subsurface hydrogeology is complex, and surface water and perched groundwater systems are in apparent close communication. Infiltration is rapid, and movement from the surface to the deepest perched zone (at a depth of 78 m) occurs within 8 to 14 d. Downward flow occurs predominantly via fracture flow.

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