Groundwater Hydrogeology

Groundwater occurs in the subsurface, beneath the water table, where all pores in sediments and rocks are saturated. The rate of groundwater flow depends on both the drop in water-level elevation, or hydraulic head, along the flow path (hydraulic gradient), and the ability of different sediments or rocks to transmit water (hydraulic conductivity). High values of hydraulic conductivity are associated with aquifers, which yield water to wells or springs, whereas low values are associated with confining units that separate unconfined (water-table) aquifers from deeper confined aquifers. When wells are pumped, water is released from storage by deformation of the solid matrix, by expansion of water, and in unconfined aquifers by dewatering of pore space. Extensive pumping can cause compaction of sediments and land subsidence. Values of aquifer properties such as storativity and hydraulic conductivity can be determined by pumping tests. At the regional scale, groundwater flows from recharge zones in upland areas, where rainfall or snowmelt infiltrates to the water table, to discharge zones in lowland areas, where water emerges at springs and seeps. Local-scale topographic relief and spatial variability in hydraulic conductivity (heterogeneity) can affect the positions of groundwater flow systems. Solute transport is driven by groundwater flow and to a lesser extent by diffusion, or mass movement following concentration gradients. Consequently, heterogeneity affects the spreading of solutes in groundwater.