In its simplest form, the subsurface at Lowry AFB consists of alluvial sediment overlying claystone bedrock. The alluvial sediment can generally be found to about 30 feet below ground surface (bgs) and consists of fluvial (river) deposits of silty, sandy clay with interconnected lenses of silt, silty sand, sand, and gravelly sand. There are generally two types of units within the alluvium: intervals of relatively fast groundwater flow (generally sandy and gravelly sand paleochannels) and intervals of relatively slow groundwater flow (silts and clays). The unsaturated sand and silt units above the groundwater are referred to as the vadose zone and contain soil vapor and soil moisture.
The bedrock water-bearing zone consists of discontinuous sandstone lenses, and fractured and weathered claystone and siltstone of the upper Denver Formation. The upper portion of the bedrock contains water-bearing fractures that are directly connected to the overlying alluvium but are not highly connected in a lateral fashion, resulting in very slow or no groundwater flow in the actual bedrock matrix.
The bedrock surface topography influences the direction and magnitude of groundwater flow. Regionally, water moves from southeast to northwest, beneath Lowry. However, in areas with deep paleochannels, the direction of groundwater flow generally conforms to the trend of the paleochannel. At Lowry AFB, paleochannels can influence the direction of the groundwater flow and therefore, the pathways of contaminant movement in groundwater.