Conceptual model of the geometry and physics of water flow a fractured basalt vadose zone

conceptual model of the geometry and physics of water flow in a fractured b asalt vadose zone was developed based on the results of lithological studie s and a series of ponded infiltration tests conducted at the Box Canyon sit e near the Idaho National Engineering and Environmental Laboratory. The inf iltration tests included one 2-week test in 1996, three 2-day tests in 1997 , and one 4-day test in 1997. For the various tests, initial infiltration r ates ranged from 4.1 cm/d (4.75 x 10(-7) m/s) to 17.7 cm/d (2.05 x 10(-7) m /s) and then decreased with time, presumably because of mechanical or micro biological clogging of fractures and vesicular basalt in the near-surface z one, as well as the effect of entrapped air. The subsurface moisture redist ribution was monitored with tensiometers, neutron logging, time domain refl ectrometry, and ground-penetrating radar. A conservative tracer, potassium bromide, was added to the pond water at a concentration of 3 g/L to monitor water flow with electrical resistivity probes and water sampling. Analysis of the data shows evidence of preferential flow rather than the propagatio n of a uniform wetting front. We propose a conceptual model describing the saturation-desaturation behavior of the basalt, in which rapid preferential flow occurs through the largest vertical fractures, followed by a gradual wetting of other fractures and the basalt matrix. Fractures that are satura ted early in the tests may become desaturated thereafter, which we attribut e to the redistribution of water between fractures and matrix. Lateral move ment of water takes place within horizontal fracture and rubble zones, enab ling development of perched water bodies.