Chloride, Salinity, and Dissolved Solids
Excess dissolved solids is a common water problem
Groundwater age, climate, geology, and human actions all play a role
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All natural waters contain some dissolved solids (salinity) from contact with soils, rocks, and other natural materials. Too much, though, and dissolved solids can impair water use. Unpleasant taste, high water-treatment costs, mineral accumulation in plumbing, staining, corrosion, and restricted use for irrigation are among the problems associated with elevated concentrations of dissolved solids.
The dissolved solids concentration in water is the sum of all the substances, organic and inorganic, dissolved in water. This also is referred to as “total dissolved solids”, or TDS. Calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, nitrate, and silica typically make up most of the dissolved solids in water. Combinations of these ions—sodium and chloride, for example—form salts, and salinity is another term commonly used to describe the dissolved solids content of water.
Total dissolved solids can be monitored in real time in surface water and groundwater by measuring its surrogate, specific conductance. Specific conductance measures the ability of water to transmit an electrical current. That ability increases with the amount of dissolved ions in the water.
Concentrations of dissolved solids in water can be so high that the water is unsuitable for drinking, irrigation, or other uses. Concentrations greater than the recommended value for drinking water of 500 mg/L give water an unpleasantly salty taste. Elevated concentrations of dissolved solids in water distribution systems can contribute to corrosion of plumbing fixtures and reduce the lifespan of equipment.
When used for irrigation, water with high dissolved solids can reduce crop yield because the dissolved salts make it more difficult for plants to extract water from the soil. Dissolved solids in irrigation water can cause salts to build up in soils and aquifers and can eventually make the land unsuitable for growing crops.
What causes dissolved solids to be high?
High concentrations of dissolved solids are more likely to be a problem in groundwater than in surface water. That’s because when groundwater moves through the rocks and sediments that make up an aquifer, some of the minerals in those rocks and sediment dissolve, a process called “weathering”. Groundwater that has been in an aquifer a long time has had more time to react with and weather aquifer materials than groundwater that has recharged recently.
Groundwater age is just one of the factors that can affect the concentration of dissolved solids. Other factors include climate, geology, and human actions.
Climate affects concentrations of dissolved solids in groundwater through precipitation, evaporation, and groundwater recharge. In arid regions, where precipitation is low and evaporation rates are high, there is less water to dilute the products of rock weathering. Evaporation of shallow groundwater, where the water table is near the land surface, also concentrates dissolved solids in groundwater in arid regions. Climatic differences extend across the wide spatial scales and result in broad regional patterns in dissolved-solids concentrations. Consequently, elevated concentrations of dissolved solids tend to be higher in groundwater in the arid western part of the U.S. than in the humid east. In a study of groundwater in Principal Aquifers in nine regions of the U.S., the highest concentrations of dissolved solids in groundwater were measured in the Denver Basin, High Plains, and Southwestern Basin-Fill Principal Aquifers.
Geology affects dissolved solids concentrations because some types of rocks weather more readily than others. Some sedimentary rocks, such as shales, carbonate rocks, and evaporites, are more soluble and easily weathered than quartz-rich sandstones or crystalline rocks such as granites. Geology can vary at regional and local scales, and can even vary with depth in an aquifer.
Human activities can affect concentrations of dissolved solids in groundwater. Groundwater pumping can pull deep saline water upward to shallow depths, or in from the coast into freshwater aquifers. Irrigation can increase concentrations of dissolved solids in groundwater in several ways, particularly in arid regions. When irrigation water evaporates or is taken up by plants, it leaves the dissolved salts it contained in the soil. Excess irrigation water can flush minerals that have accumulated over thousands of years in soils and the unsaturated zone down to the water table. Irrigation can raise the water table close to the land surface, so that direct evapotranspiration of shallow groundwater can further concentrate dissolved solids. Human activities can add dissolved solids to recharging groundwater. Detergents, water softeners, fertilizers, road salt, urban runoff, and animal and human waste all contain elevated concentrations of dissolved solids that are delivered to groundwater by wastewater disposal, septic systems, or direct application to the land surface. As a result, dissolved solids concentrations are more likely to be high in shallow, recently recharged groundwater near the water table beneath urban, suburban, or agricultural areas than in shallow groundwater beneath undeveloped areas or in deeper groundwater.
► Learn more about dissolved solids and the processes that cause them to be elevated in groundwater in Principal Aquifers.
Urbanization and chloride—a concern for streams and groundwater
Chloride is a major component of dissolved solids. The use of road salt—sodium chloride, the same chemical as table salt—for deicing is a major manmade source of chloride to surface water and groundwater. Application of road salt in the United States has tripled since the 1970s, while other uses of salt have remained stable or decreased.
Concentrations of chloride have been increasing in U.S. streams, especially in urban areas affected by snow. Elevated concentrations of chloride in streams can be toxic to some aquatic life. Additionally, the presence of chloride increases the potential corrosivity of the water. Corrosion in water distribution systems affects infrastructure and drinking water quality.
► Learn more about corrosivity.
Chloride also is a concern in groundwater, and concentrations are increasing in many aquifers across the U.S.
In the glacial aquifer system, which extends across the northern United States, chloride concentrations were highest in shallow groundwater beneath urban areas, in some cases exceeding the guideline for drinking water of 250 mg/L for taste and odor. During low-flow conditions, when groundwater is the dominant source of water to streams, high concentrations of chloride in groundwater in this aquifer system can cause chloride in streams to exceed the chronic aquatic criterion developed to protect fish and other aquatic life.
High concentrations of dissolved solids can indicate other problems
High concentrations of dissolved solids sometimes are accompanied by other nuisance constituents. For example, water with high total dissolved solids usually is hard, because calcium and magnesium—the two elements that define hardness in water—are two of the major components of dissolved solids in groundwater. Hard water reacts poorly with soap and sometimes leave scale deposits in pipes and water heaters. Although high dissolved solids is not in itself a health concern, it can sometimes signal the presence of elevated concentrations of arsenic, uranium, radium, or other trace elements in the groundwater as well. The occurrence of high dissolved solids in drinking water therefore can indicate that testing for a broader range of constituents might be warranted to assess possible risks and to determine options for reducing those risks.
► Learn more about nuisance constituents that can cause taste and odor problems in drinking water.