Relation of hydraulic conductivity to depth, alteration, and rock type in the volcanic rocks of Pahute Mesa, Nevada, USA

A large data set of hydraulic conductivity ( K ) with depth was used to examine and isolate the influences of rock type, volcanic-rock alteration, and depth on K heterogeneity in the volcanic rocks of Pahute Mesa, Nevada, USA. K varied laterally up to seven orders of magnitude, even when controlling for alteration, rock type, and depth. Mean K values, however, differed by no more than two orders of magnitude between different alteration, rock type, or depth groupings. Shallow fractured lava flows (LF) and welded tuff (WT) in the upper 500 m of the saturated zone are hydraulically similar and have the highest mean K values of the volcanic rocks studied. Largely because of these high- K rocks, 90% of the transmissivity occurs in the shallow part of the flow system. Matrix-dominated nonwelded and bedded tuffs and deep LFs and WTs have low mean K values because of secondary alteration and limited open fracture networks. The relations of K with overburden pressure have weak correlations because of large heterogeneity, even in hydraulically similar data sets. Overburden pressure likely causes a one-to-three orders of magnitude decrease in K in the saturated zone within 2,000 m of the land surface, whereas lateral heterogeneity varies at least five orders of magnitude at any given depth. Field-scale, lateral and vertical K distributions from this work can be applied to conceptual and numerical studies of groundwater flow in complex volcanic settings.