For purposes of simplicity we will consider 3 different groundwater provinces, each with differing geologic and aquifer characteristics:

• Eastern Nebraska
• Sand Hills region and the underlying Ogallala Group
• Tertiary terrestrial clastics of western Nebraska
• More complete description of groundwater in Nebraska.

Eastern Nebraska: below are units listed from youngest (most recent) to oldest:

• alluvium: e.g. along Platte, Missouri, Elkhorn rivers. Shallow, high quality aquifers, with quick recharge, but greater contamination potential.
• loess: wind blown silt deposits, fine-grained so relatively impereable, has fracture porosity.
• glacial outwash: well sorted sands and gravels, discontinuous in distribution, usually thin so not a good aquifer, potentially important as contaminant path.
• tills: poorly sorted deposits and hence impermeable, thicker to N and E, discontinuous in Omaha area,
• Dakota sandstones: good high porosity sandstones, thin to the E and locally absent in Omaha area, high TDS.
• Pennsylvanian age limestones and shales: generally impermeable, except where have fracture porosity, widespread, generally subsurface.

Sand Hills and the underlying Ogallala

• geologic nature of Sand Hills
• lakes, marshes and subirrigated meadows, groundwater fed rivers
• major recharge mechanism for underlying Ogallala paleo-channel deposits (old alluvium, unrelated to present river systems).
• Ogallala Group and aquifer, High Plains Aquifer

Tertiary strata in western Nebraska

• decrease in volcanic ash component with stratigraphic ascent, and corresponding decrease in permeability.
• basal White River Group Chadron conglomerate.
• local sandstone channels as exceptions.
• fracture aquifers in the Brule Fm..

Links for advanced exploration of the topic:

• Groundwater Foundation: a home brewed ground water protection organization.
• Description of pesticides and contamination concerns in Nebraska
• EPA description of Nebraska's groundwater concerns with a focus on nitrates.
• Description of USGS activities in Nebraska with a focus on water issues.