Natural rainfall contains dissolved air and a very small amount of dissolved solids. During the travel of rainfall through soil, sediment, and rock to reach an aquifer, the water dissolves additional solids. Most of the soluble solids picked up by the water are inorganic because inorganic minerals are the main component of soils, sediments, and rocks, and natural organic substances tend to be less soluble than common mineral salts. Water flowing deeper into the subsurface may encounter very soluble minerals such as rock salt and produce saltwater after dissolving the salt. Other deep subsurface waters can be highly saline because they were originally trapped seawater that has been further altered through geologic time.
The total dissolved solids content (often abbreviated TDS) is a general measure of the salinity of a water. The most common substances contributing to the dissolved solids of most ground waters are the inorganic constituents calcium, magnesium, sodium (positively charged cations) and bicarbonate, chloride, and sulfate (negatively charged anions). Bicarbonate may also be represented as alkalinity in some water analyses. Inorganic constituents commonly contributing minor amounts to ground-water TDS are silica (uncharged), potassium (a cation), nitrate and fluoride (anions). There are a large number of other individual and combined elements dissolved in water, including gases and metals. Table 2 lists the names, properties, and chemical symbols or representation for common major and minor dissolved inorganic substances as well as several dissolved metals in Dakota aquifer waters. Concentrations of these dissolved substances are often reported as milligrams per liter (abbreviated mg/L). This concentration unit is essentially the same as a part per million (ppm) in freshwater, because the weight of a liter of water with dilute concentrations of dissolved constituents at at ground-water temperatures is very close to 1,000 grams (one million milligrams). In saltwater, a mg/L is a few percent different from a ppm because the density of the solution is greater than for freshwater. Concentrations of trace amounts of dissolved constituents are often listed as micrograms per liter (mg/L) which is very close to parts per billion (ppb) in freshwater.
Freshwater is often defined as water containing less than 1,000 mg/L TDS. Freshwaters in the outcrop and subcrop portions of the Dakota aquifer are usually calcium-bicarbonate or calcium, magnesium-bicarbonate type waters. Most soils and near surface rocks in Kansas, including the Dakota aquifer, contain at least small amounts of calcium carbonate present as calcite (CaCO3). The calcite contains small amounts of magnesium and the mineral dolomite (CaMg(CO3)2) can also be present. During infiltration of rainfall, the carbonate minerals dissolve and add calcium, magnesium, and bicarbonate to the water. Small amounts of other inorganic constituents are also dissolved from soils and near surface rocks. These substances are present in the main carbonate minerals and in the small amounts of other soluble minerals, are adsorbed on clays, or have been concentrated as salts in soils during dry periods. Typical ranges of major constituents and fluoride concentrations in the most common chemical types of Dakota waters are listed in Table 2.
Fine-grained sediments in the Dakota aquifer and overlying rocks often contain the mineral pyrite (FeS2). The pyrite weathers to produce dissolved iron and sulfate. The iron can then oxidize and precipitate as iron oxide and oxyhydroxide (hydrated oxide) which produces the red to orange coloration commonly occurring in Dakota strata. The solution from pyrite weathering is acidic and dissolves additional calcite in a natural neutralization process. These processes increase the calcium and sulfate concentrations dissolved in Dakota waters. Rocks overlying the Dakota aquifer such as the Graneros Shale often include gypsum (CaSO4 . 2H2O), a very soluble mineral. Water infiltrating through these rocks can have relatively high concentrations of calcium and sulfate from dissolving gypsum. Recharge passing through rocks with gypsum and entering Dakota strata can substantially increase the calcium and sulfate content of waters in the upper aquifer. In some cases calcium-sulfate waters may result, although this water type is not as common as other chemical types in ground water from the Dakota (Table 3).
Large areas of the Dakota aquifer contain saltwater (primarily dissolved sodium and chloride). Concentrations of TDS can be in the ten's of thousands of mg/L (Table 2). Geochemical tests have identified the main source of this saltwater as dissolution of rock salt (NaCl) in Permian rocks underlying Dakota strata. Although most of the Dakota sediments probably contained seawater either during their deposition (the marine shales and sandstones) or after deposition when the sea covered these units, nearly all of the seawater has been flushed out by surface recharge. However, saltwater from the underlying Permian rocks has been slowly intruding into Dakota strata for millions of years. The salt-dissolution brine replaced the seawater source of salinity long ago. During more recent geologic time, freshwater recharge has been slowly flushing saltwater from the Dakota aquifer.
The past occurrence of saline water in Dakota aquifer strata resulted in the adsorption of large amounts of sodium on the clays in the shales, siltstones, and sandstones. As freshwater of calcium-bicarbonate type slowly flushed the saline water from the aquifer, the process of natural softening of the water occurred as dissolved calcium and magnesium adsorbed on the clays and released sodium to solution. The decrease in calcium concentration allowed some calcite to dissolve where present in aquifer strata, thereby supplying additional calcium and bicarbonate to the water. The added calcium was then available for more cation exchange with sodium. Some additional bicarbonate may have been generated from slow oxidation of organic matter trapped in the aquifer framework. The combined effect of these processes increased dissolved sodium and bicarbonate concentrations while decreasing dissolved calcium, magnesium, and chloride concentrations in confined parts of the Dakota aquifer where the water is now fresh to slightly saline. The water types range from sodium-bicarbonate to sodium-chloride, bicarbonate to sodium-chloride with excess sodium in the direction of increasing salinity. These waters are typically soft because the calcium and magnesium concentrations are relatively low. Typical ranges of major dissolved constituents in sodium-bicarbonate waters in the Dakota aquifer are listed in Table 2.
The pH (a measure of how acidic or alkaline a water is) of the sodium-bicarbonate waters in the confined Dakota aquifer is alkaline and usually in the 7.5 to 8.5 range. (The pH of a water indicates how acidic or alkaline is a water; a value of 7.0 units represents a neutral solution at room temperature.) Elevated concentrations of dissolved fluoride are also usually associated with the sodium-bicarbonate waters. Dissolved fluoride contents can be from over 1 mg/L up to several or more mg/L in comparison with less than 1 mg/L for calcium-bicarbonate type waters. The high fluoride derived from calcium minerals containing fluoride (probably mainly apatites). The low calcium concentration resulting from the cation exchange that produced the sodium-bicarbonate water allowed the calcium minerals to dissolve. Some fluoride adsorbed or weakly attached to clays was released in the higher pH waters by exchange with hydroxyl ion (OH-).
Other naturally occurring constituents of interest in Dakota aquifer waters are iron and manganese. Dissolved concentrations of iron range from less than a few mg/L to over 10 mg/L and manganese range from less than a mg/L to nearly a mg/L. The greater concentrations occur in two types of environments. One occurrence is in the outcrop or subcrop area of the Dakota aquifer where recharge with dissolved oxygen reaches strata containing pyrite. Oxidation of pyrite was referred to earlier as a source of sulfate as well as dissolved iron in ground waters. Such waters can have a pH between 6 and 7 (very slightly acidic). The other occurrence exists where reactions with dissolved constituents and sediments have essentially completely consumed dissolved oxygen and produced a chemically reducing environment. This commonly occurs in the confined portion of the Dakota aquifer because the age of the water is old and no recent recharge with significant oxygen can enter. The reducing environment allows iron, manganese, and some other heavy metals to dissolve from the sediments. These waters can sometimes have a high enough hydrogen sulfide (H2S) content to give a "rotten egg" odor. Ammonium ion (NH4+) levels can be over a mg/L in the reducing environment (Table 1).
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