In summary, with some exceptions, groundwater evolves from a nearly pristine state (low concentrations of dissolved species) where precipitation falls on the earth and the concentrations of dissolved species increase as water travels through the vadose zone. In the vadose zone, carbonic acid is generated by dissolution of CO2 gas from roots, decaying organic matter and microbes. The carbonic acid dissolves minerals, which increases the total dissolved solids (TDS) in vadose zone water. Evapotranspiration further increases the TDS because water molecules are released into the atmosphere while all salts are left behind in the water feeding the underlying groundwater flow system. While moving along the flow paths in the groundwater zone, the water may undergo little or no increase in TDS before it discharges to the surface, as is often the case in local groundwater flow systems, or it may acquire much higher TDS as is common in regional flow systems. The groundwater in motion provides conveyor belts for the dissolved constituents that carry the dissolved load from recharge areas to discharge areas.
There are two categories of conveyor belts (Figure 60). First, there are those operating in dry climates, where all of the groundwater discharge evaporates or evapotranspires while the salts are left behind to accumulate in saline soils, salt flats, or salt lakes such as the Great Salt Lake in North America, Lake Eyre in Australia, Lake Titicaca in South America, and Lake Chad in Africa. The chemical nature of the groundwater discharge zone in the dry climates is influential, perhaps the dominant influence, on the ecology of surface vegetation and aquatic systems. In contrast, the second, wet climate conveyor belts discharge their chemical loads into surface water bodies such as springs, streams, rivers, marshes and swamps and much of this water eventually discharges into the oceans.
When considering flow rates and the distribution of hydraulic head, groundwater systems are generally in balance with the present‑day hydrologic conditions on today’s landscape. In other words, the distribution of water pressures in the groundwater zone are in a near‑steady condition established by the average conditions of today’s climate, except where there is excessive groundwater pumping. However, this is not the case for the chemical composition of groundwater, which, nearly everywhere, is not in a steady condition and generally not in geochemical equilibrium with the minerals the groundwater encounters along the conveyor belts. In locations that have not reached geochemical equilibrium, geochemical processes combine with groundwater flow to continually extract chemical mass from the geology for discharge at the surface. In contrast, in parts of the world that have been geologically stable for tens of millions of years, the geochemical factory and conveyor belts have extracted essentially all of the available soluble materials. In those places, the groundwater is fresh, even as deep as one or two kilometers. This fresh groundwater has exceptionally low TDS, influenced primarily by the chemical load contributed from the atmosphere.