Abstract: Anisotropic Groundwater Migration in Structurally Deformed Sedimentary Bedrock Terrains

JANES, STEPHEN D., The Janes Network, Santa Cruz, CA

Development of adequate conceptual models for groundwater movement in bedrock hydrogeologic settings is necessary because many new industrial sites and their associated groundwater problems overlie bedrock terrains. The intuitive recognition that low permeability zones might act to change flow direction has been empirically demonstrated through tracer study and the mapping of groundwater impacts at several sites overlying structurally deformed sedimentary bedrock. Despite locally strong hydraulic gradients, subtle differences in relative hydraulic conductivity between sedimentary strata control saturated zone flow direction.

The anisotropic character of dip parallel flow in the vadose zone and strike parallel flow in the saturated zone is the primary feature of groundwater movement in structurally deformed sedimentary bedrock hydrogeologic settings. While fluid infiltration through the vadose zone typically starts along a vertical pathway, it will deviate from this direction in dipping sedimentary bedrock settings when a relatively low permeability stratum is encountered. Vadose zone migration than follows a pathway parallel to the dip of bedding. Groundwater flow in the saturated zone is strongly influenced by stratigraphic geometry. Even in the presence of some fracturing, permeability contrasts between strata exert the primary influence on flow direction and effectively constrain it to a strike parallel pathway. Significant head differences in sandstone water bearing zones separated by intervening shale strata do not produce significant cross bed flow.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California