All fresh groundwater contains dissolved constituents and all fresh groundwater is in motion conveying its dissolved load through the flow system towards discharge areas. This process has been going on throughout geologic time. The type and magnitude of these dissolved constituents render the groundwater either useful or not useful for drinking, agriculture and industrial processes. The main features of Earth’s surface, as it exists today, were formed by geologic events in the past, millions to billions of years ago when forces deep in Earth’s crust formed mountains while erosion and deposition of sediments created valleys and plains. In the north and far south, glaciers gouged the landscape and smeared mixtures of geological material over the land to form today’s shallow geologic layers. The hydrologic cycle was established long ago in synchrony with the climate and landscape, evolving together over geologic time.
Water flowing on Earth’s surface is the main agent controlling the evolution of the landscape through erosion and deposition. Leopold et al. (1964) wrote that “Rivers are the gutters down which flow the ruins of continents”, in reference to the mass of sediments transported by the riverine systems to the ocean. This pithy statement could be adapted to “Groundwater is the enigmatic web through which flows the decimation of rocks”, in reference to the mass of solute discharged to the ocean by groundwater.
Flowing groundwater is the foundation that provides the ongoing supply of water to the surface water system through both wet and dry periods and occasionally alters the landscape through development of salt flats in arid areas and the formation of caves with associated sink holes in karst areas. Approximately half of the water flowing in rivers is from long‑term flow through the persistently saturated portion of the groundwater system discharging to the river, and half is from storm runoff. Some storm runoff occurs over the ground surface, though most flows through the shallow subsurface, often initiated in the vadose zone through the capillary fringe or via temporary saturated zones perched on low permeability layers.