An aquifer forms where groundwater accumulates in the subsurface to a volume that can be pumped by a well, ranging from small, local features to over 100,000 square miles. Confined aquifers usually sit within bedrock, whereas unconfined aquifers travel relatively freely through unconsolidated earth. Groundwater flow provides the transport mechanism; understanding it facilitates the modeling and predicting of the migration of contaminated groundwater.
To minimize offsite impacts, states typically regulate the location of the landfill -- vertically and horizontally -- relative to features such as the seasonal high groundwater level, the distance to drinking water supply wells and/or distance to 100-year floodplains. The nature of the groundwater itself also plays a role in the transportation of contaminants. The interaction between chemistry of the groundwater, influenced by the material through which it flows, and that of the potential contaminants helps determine the eventual fate and transport of released contaminants.
The subsurface geology near a landfill -- whether consolidated rock or unconsolidated soil and whether bedding planes or fractures affect the direction and speed of groundwater flow -- controls the migration of contaminated groundwater. Nearly all natural, subsurface materials transmit groundwater to some extent. In a sandy soil, for instance, water may flow at a rate of feet per day. In some hard rock, it may be fractions of an inch per decade.
Flow rate can increase, however, in places where materials with different properties are adjacent or where geologic processes result in fissures, cracks or fractures. These preferential pathways can be widespread or occur in discrete zones. More water will flow where resistance is lower, so zones located near a potential source of contamination may create a higher potential for contaminant migration.
Surface water bodies may include rivers, streams or lakes and can be classified as those that gain water from a nearby aquifer or lose water to one. Gaining bodies have increased risk for contamination as they can accumulate toxins, passing them on to wildlife or transporting them downstream.
A site conceptual model reflects the story compiled from the various hydrogeologic data collected during the field investigation. Regulations determine the tests conducted, as well as the density of test locations at a site. Subsurface data, such as soil and rock samples from borings and groundwater samples from wells; surface data collected using various geophysical techniques; and ground mapping to identify surface features that may be of interest are examined collectively for the creation of a coherent view of the system. No amount of sampling will completely describe the subsurface at any given location, however.