Origin of Dissolved Constituents in Groundwater
The dissolved chemical load in groundwater comes not only from interaction with the geologic materials it flows through, but also from the constituents in the atmosphere that the recharge water was exposed to before infiltrating. Precipitation that infiltrates through the vadose zone to form groundwater recharge carries wet and dry aerosols that the atmosphere transports over the continents. These aerosols are from ocean spray, smoke, volcanoes, continental dust, and lightning, as well as chemicals from human activities such as burning of fossil fuels and application of agricultural herbicides and pesticides. Owing to their small sub-micron size and the acidic nature of rain due to the presence of carbon dioxide (CO2) in the atmosphere, dry aerosols are dissolved in the precipitation water. This dissolved material includes many of the elements in the periodic table in very low concentrations. The concentration of each element in the atmosphere varies with distance from its source. For example, the distribution of chloride precipitated on the United States as shown in Figure 52 indicates higher amounts of chloride near the coasts and the Great Salt Lake as a result of aerosols in spray from those water bodies.
Although the chemistry of natural groundwater starts off with the contribution of dissolved constituents from the atmosphere, nearly all groundwater takes on additional dissolved mass of constituents as the water infiltrates through the vadose zone, as represented schematically in Figure 53. The chemistry of infiltrating water is altered in the vadose zone in two ways: by amplification related to evapotranspiration and by geochemical reactions (Figure 53). Even if the vadose zone were chemically and microbially inert so as to contribute no chemical mass to the recharge water, the chemical concentrations in the infiltrating water would increase substantially (increasing the TDS) because of the evapotranspiration amplification effect.
Evapotranspiration transfers much of the vadose zone water to the atmosphere (the global average is about 70%), but substances dissolved in that water remain behind in the soil water, so the concentrations of dissolved substances in the water that is left behind increase compared to those in the precipitation. In arid and semi arid areas most of the solutes are derived from precipitation. However, the vadose zone is not inert; it is chemically reactive because oxygen enters from the atmosphere and carbon dioxide (CO2) is produced within the vadose zone from microbial decay of organic matter in the soil and from respiration of plant roots. The combination of these reactions and the evapotranspiration amplification effect uses the CO2 content of gas in the vadose zone to be 10 to 100 times higher than that of the Earth’s atmosphere in drier climate regions, and 2 to 5 times higher in wetter climates. This CO2 dissolves into the water, forming carbonic acid, and the acidic soil water dissolves minerals. In many regions, the vadose zone has minerals containing sulfur, such as iron bearing minerals like pyrite, and these minerals are oxidized in the presence of oxygen gas to produce sulfuric acid, further increasing acidity of the soil water and its ability to dissolve minerals. This process of minerals dissolving in the vadose zone is an important part of landscape weathering (the breaking down of rock). The presence of oxygen and carbon dioxide in the vadose zone make the vadose zone especially reactive geochemically.
Almost without exception, the TDS of groundwater increases as groundwater moves along the flow path from the recharge to discharge areas (Figure 54). This increase occurs largely from the release of relic salinity from low permeability zones within which groundwater is nearly stagnant. Relic salinity includes solutes that remain from past geologic eras when sea water was present in the geologic formations. Some minor amounts of solutes may be acquired from additional weathering, but most are from relic salinity and underlying brines.
Older geologic landscapes become less geochemically reactive over geologic time as the soluble minerals are flushed out by groundwater flow. In humid areas, groundwater in these exceptional landscapes has relatively low TDS even after traversing long flow paths with large groundwater residence time because sources of soluble constituents have been diminished by millions of years of groundwater flushing. In contrast, geologically young landscapes typically have higher TDS groundwater because there has not been sufficient time to flush the soluble components from the geologic materials. For example, areas glaciated during the past few hundred thousand years such as in Canada, parts of the northern United States, northern Europe and much of Russia, are geologically young landscapes formed by glaciers. Generally, these geologic materials are comprised of minerals and rock fragments that have been exposed to groundwater for only 10,000 to 15,000 years after the most recent glaciers receded from the mid latitudes. As a result, substantial amounts of readily soluble minerals such as calcite, dolomite and pyrite, among others, are available for dissolution by infiltrating water and in some of those areas even groundwater with short residence time has elevated TDS, causing them to be brackish. Groundwater moves more slowly in deep zones, so there has been less flushing, and thus, in general, deeper water is saltier. In arid areas, groundwater tends to contain more dissolved solutes owing to concentration by evaporation.
Generally, flow paths that are short and well flushed have bicarbonate (HCO3–) as the dominant anion and usually calcium (Ca2+) as the dominant cation (Figure 55). This composition is primarily the result of processes that occur in the vadose zone as shown in Figure 53. As discussed above, the vadose zone is especially geochemically active because root respiration and microbial decay produces abundant CO2 that dissolves to form carbonic acid (H2CO3), which enhances mineral dissolution thus producing bicarbonate (HCO3–). Even if there are no reactive minerals in the vadose zone, some HCO3– is produced in this zone. The groundwater that makes up nearly all the baseflow to streams and rivers is from local groundwater systems in which the dominant anion is HCO3–. On longer flow paths more typical of intermediate flow systems, sulphate (SO42-) often exceeds HCO3– as the dominant anion (Figure 55), but there are many exceptions. In some groundwater systems, SO42- declines due to natural microbiological processes (sulphate reduction) that consume sulphate and produce CO2 that causes a rise in HCO3– and a decrease in pH. The regional flow systems (Figure 55) generally have chloride (Cl–) as the dominant anion and sodium (Na+) as the dominant cation because these longer flow paths have more opportunity to be influenced by releases of halite (sodium chloride) from low permeability zones and underlying formations. Also, common chloride minerals have the highest solubility of all the major minerals. The lower solubilities of major minerals that produce HCO3– and SO42- keep those ions at low to moderate concentrations.
Chloride minerals are so soluble that over geologic time chloride is removed from the higher permeability zones that have been flushed by large volumes of fresh recharge water. But chloride commonly lingers as “relic chloride” in low permeability zones and in deep flow systems (Figure 56). In general, the longer the flow path, the more likely zones of slow flowing groundwater will release relic chloride into zones of active flow.