ARCHITECTURE AND DIRECTIONAL SCALES OF HETEROGENEITY IN ALLUVIAL-FAN AQUIFERS

A wide range of structural, volcanic, and depositional processes produ ce heterogeneity in alluvial-fan aquifers. This heterogeneity is commo nly abrupt, and more importantly, it is directional. Prevalent concept s of scales of heterogeneity and stochastic hydrology emphasize nested or hierarchical aquifer structures that can be represented by station ary models. The argument for stationarity is that heterogeneity become s homogeneous, or spatially periodic, at some scale. Stationarity may exist in some braided and meandering aquifers, but stationarity is gen erally not valid for alluvial-fan aquifers, because hierarchical struc ture is generally present only at the microscopic and partially at the mesoscopic scale. Alluvial fans are directional landforms that extend downdip from a point source. Grain size and bed thickness generally d ecrease down fan. Abrupt to gradational facies relationships change do wn fan and may be substantially different in adjacent fans. Hydrogeolo gic properties, which can vary over 14 orders of magnitude, may parall el the down-fan fining trend or may show a 'humped'' pattern in a down -fan direction. As fans subside, prograde, and retrograde, a direction al aquifer is created whose complex heterogeneities require a directio nal model. The directional alluvial-fan aquifer is best characterized by three directional scales of heterogeneity (from smallest to largest ): (1) within-fan (microscopic scale to facies relationships), (2) bet ween-fan (parallel to depositional and structural strike), and (3) cro ss-fan (perpendicular to depositional and structural strike). A direct ional view of heterogeneities in alluvial-fan aquifers is essential fo r correct site characterization and for design of well networks, aquif er tests, and flow models.