... are Groundwater flow systems?

Groundwater flow is an effective agent for the transport of contamination in the subsurface. Groundwater flow systems can transport dissolved pollutants for short distances of 100 meters or less, or they can transport contaminations for distances exceeding several kilometers or 10s of kilometers.

Groundwater flows through pores in unconsolidated deposits (sand, silt, clay) and through joints and fractures in consolidated bedrock (shale, sandstone). The pores or fractures may be more or less connected and thereby will be more or less permeable for water.

The topography of the groundwater surface is the determining boundary condition for groundwater flow. Groundwater flows always from highlands to lowlands, usually along curved flow lines such that water movement is downwards deeper into the groundwater body in highlands (recharge areas) and upwards towards the groundwater table under lowlands (discharge areas).

In general:

• groundwater does not flow in the direction of the pressure gradient,
  
• groundwater does not flow parallel to the groundwater table,
  
• groundwater flows in significant amounts through low permeability layers, clay and mud stone (shale) layers,
  
• groundwater flow does not necessarily follow the path of least resistance (a well-worn engineering statement), and
  
• one piezometer (groundwater table) upstream and two piezometers (groundwater table) downstream do not necessarily catch a plume of contaminants.

The understanding of groundwater flow systems is a necessary condition for any field investigation of subsurface flow and migrating dissolved contaminants.