HYDRAULIC BARRIERS IN PLEISTOCENE BURIED-VALLEY AQUIFERS IN NORTH DAKOTA - TYPES, DETECTION, AND ORIGIN

Hydraulic barriers are relatively commonly occurring features of Pleistocene buried-valley aquifers. Two types of hydraulic barriers have been distinguished in North Dakota based on orientation with respect to the principal axes of buried valleys. Longitudinal hydraulic barriers are relatively narrow, linearly discontinuous ridges comprised of shale bedrock or till that are roughly parallel to the long axes of buried valleys. Transverse hydraulic barriers are narrow, continuous barriers comprised of silt and clay that cut across the long axes of buried valleys.

The principle directions of ground-water flow in buried-valley aquifers commonly are parallel to longitudinal hydraulic barriers and roughly perpendicular to transverse hydraulic barriers. As a result, longitudinal hydraulic barriers are more difficult to detect because they are manifested by small potentiometric surface discontinuities. Aquifer test analysis using areally extensive monitoring well networks is useful for detecting and inferring the location of longitudinal hydraulic barriers. Transverse hydraulic barriers are easier to detect because they are manifested by relatively large potentiometric surface discontinuities. Hydraulic barriers can be an important constraint on the long-term yield capability of buried valley aquifers at the local scale.

Pleistocene drainageways explain the occurrence and formation of hydraulic barriers in buried-valley aquifers. Longitudinal hydraulic barriers are erosional features that appear to be formed by stream-valley bifurcation in melt-water channels or glacial lake spillways. Transverse hydraulic barriers appear to be formed by catastrophic flooding along spillways from the rapid drainage of glacial lakes and subsequent deposition of silt and clay deposits by sluggish, underfit streams.