Groundwater Quality NT
By Darrel Dunn, Ph.D., PG, Consulting hydrogeologist. (Professional Synopsis)
The purpose of this web page is to present a brief, non-technical overview of factors that affect groundwater quality. The quality of groundwater is generally expressed in terms of chemical constituents, index parameters, and dissolved gas. Natural inorganic chemical constituents have been classified as major, secondary, minor, and trace constituents. Concentrations of natural dissolved organic chemical constituents are generally low in groundwater compared to the inorganic constituents. Water quality index parameters are measurable attributes that are affected by more than one constituent (bulk parameters).
Major Inorganic Chemical Constituents
Major inorganic chemical constituents are sodium, calcium, and magnesium, which are positively charged ions; and bicarbonate, sulfate, and chloride, which are negatively charged ions. The major constituents are common in groundwater, and the individual constituents are generally found in concentrations ranging from 1 to 1000 parts per million (ppm) in potable water. Non-ionized silica is also common at concentrations greater than 1 ppm. None of the major natural groundwater constituents are subject to U.S. Environmental Protection Agency (EPA) primary drinking water standards. These are legally enforceable standards that apply to public water supplies. They are deemed to protect public health. Calcium is a major cause of water hardness (discussed below). Magnesium also contributes to hardness. Chloride can impart a salty taste when the concentration is greater than 250 ppm and the water contains sufficient sodium. People unaccustomed to drinking water high in sulfate can experience diarrhea (Epsom Salt is magnesium sulfate). Sulfate greater than 250 ppm can give water a bitter taste.
Secondary Inorganic Chemical Constituents
Secondary constituents include boron, carbonate, fluoride, iron, nitrate, potassium, and strontium. Secondary constituents are generally found in concentrations less than 10 ppm in potable water. Boron, carbonate, iron, potassium, and strontium are not subject to EPA primary drinking water standards. The EPA primary water quality limit for fluoride is 4 ppm. Potential sources of fluoride in groundwater include dissolution of fluorine-bearing minerals, agricultural fertilizers, and pesticides. Commercial lawn fertilizers do not contain fluoride. The natural concentration of fluoride may be limited by its tendency to precipitate as calcium fluoride (the mineral fluorite), and it has been observed that groundwater with a high concentration of calcium does not contain more than about 1 ppm fluoride. The EPA primary drinking water standard for nitrate is 10 ppm nitrogen in nitrate. Most nitrate in groundwater originates as water percolates through soil. Sources in soil include decomposition of organic matter, fertilizers, and wastewater from septic tanks. Nitrate compounds are highly soluble, so nitrate tends to move with the groundwater without precipitating. However, nitrate can be taken out of groundwater by organisms. Where the groundwater contains virtually no oxygen, bacterial action can cause denitrification of nitrate to gaseous elemental nitrogen. Iron in groundwater can be a nuisance. Concentrations greater than 0.3 ppm cause staining (clothing plumbing fixtures, objects wetted by lawn sprinkling). Concentrations greater than 0.5 ppm may impart taste.
Minor Inorganic Chemical Constituents
Many minor constituents occur in groundwater at concentrations less than 0.1 ppm. Those subject to EPA primary drinking water standards include antimony, arsenic, barium, cadmium, chromium, copper, lead, and mercury. Antimony and arsenic are uncommon in natural minerals and rocks, so their concentrations in natural groundwater should be very small. However, about 10 percent of groundwater in the the United States exceeds 0.01 ppm. Antimony may be present in industrial wastes and spills. Elevated concentrations of arsenic in groundwater may be caused by pesticide contamination. Low concentrations of barium, chromium, copper, and lead may occur naturally in groundwater as a result of dissolution of limestone, which tends to contain small amounts of these elements. Lead concentrations in groundwater tend to be low due to the low solubility of lead carbonate (the mineral cerussite). Mercury concentrations in groundwater tend to be low because it is readily adsorbed on solid surfaces. However, it can form chemical complexes with humic substances, and some groundwater contains humic substances if it has passed through organic-rich soils. All of the aforementioned minor constituents have industrial, agricultural, or household uses; so they may occur in groundwater as contaminants.
Minor constituents not subject to EPA primary standards are aluminum, manganese, nickel, phosphate, and zinc. Aluminum is an abundant constituent in natural minerals, but it is relatively immobile in groundwater systems unless the water is acidic or organic rich. The chemical behavior of manganese in groundwater is similar to iron, but it is less abundant. The EPA has issued a health advisory for manganese due to potential neurological effects. The Lifetime Health Advisory is 0.3 mg/L (ppm). The EPA Secondary Drinking Water Maximum Contaminant Level (a non-enforceable guideline) is 0.05 mg/L (ppm) based on staining and taste considerations. Nickel and zinc are important industrial metals, but their natural concentrations in groundwater are low. Some people can taste zinc in water at concentrations above 5 ppm. Phosphorus generally occurs in groundwater as phosphate. Phosphorus tends to be precipitated in groundwater so its concentration is usually low. The usual sources are fertilizer and sewage. It is used by organisms in the groundwater as a nutrient.
Trace Inorganic Chemical Constituents
Most natural dissolved organic matter is derived from decaying organic material in soil that the water has passed through. It consists of components that remain after incomplete decay facilitated by microbes. Natural organic matter may impart color to groundwater. A plethora of synthetic organic compounds produced by human activity may enter the groundwater system. Fortunately, there are many mechanisms that tend to protect the groundwater. These include precipitation, chemical degradation to simpler compounds, ingestion by organisms present in the subsurface, and adsorption onto mineral surfaces. Nevertheless, some toxic organic compounds or products of their degradation are mobile in groundwater systems and may exceed drinking water standards near contamination sources.
Groundwater Quality Index Parameters
Commonly used index parameters used to characterize groundwater quality include total dissolved solids (TDS), hardness, pH, color, and total organic carbon (TOC). TDS is the solid residual of a filtered water sample after evaporation. It is mostly inorganic but may contain a small amount of organic matter. Water for most domestic and industrial uses should be less than 1000 ppm TDS, and less than 500 ppm is recommended for drinking water. Hardness of water is a measure of its tendency to precipitate soap and reduce its cleansing action. It is caused primarily by calcium and magnesium, but is also affected by iron. It is normally expressed in terms of equivalence to ppm of calcium carbonate. A value less than 60 ppm indicates soft water and a value greater than 180 ppm indicates very hard water. Water from limestone aquifers is likely to be hard to very hard. The value of pH is a measure of acidity of water. A value of 7 standard units is neutral, neither acidic nor alkaline. Values less than 7 are acidic, and values greater than 7 are alkaline. Most groundwater pH is between 5.5 and 8. Acidic water tends to attack metals. Excessively alkaline water may also attack metals. Color is true color after turbidity has been removed, and is measured on an intensity scale. Color above 15 or 20 units is considered objectionable. TOC represents organically-linked carbon. It does not include inorganically-linked carbon, such as carbonates and bicarbonates.
Gases in Groundwater
The solubility of gases in water is directly proportional to pressure. Analyses of gases in groundwater are rarely made. Dissolved gas probably occurs in concentrations of 1 to 100 ppm in most groundwater. Hydrogen sulfide and methane are common dissolved gases. Hydrogen sulfide may be produced from sulfate by bacteria. Low concentrations of hydrogen sulfide may impart an unpleasant odor. Methane may be produced by bacterial fermentation of organic matter in the groundwater system. Methane released to the atmosphere in a confined space (like a well house) may explode if ignited. At room temperature and pressure, methane is a colorless, odorless, nontoxic gas. If it displaces too much oxygen it can cause asphyxiation. The concentration at which asphyxiation becomes significant is higher than the concentration of an explosive mixture with air. Other gases that are common in groundwater are nitrogen, oxygen, carbon dioxide, radon, and nitrous oxide (laughing gas). Radon is a health hazard due to its radioactivity.
Chemical Evolution of Groundwater
The chemical composition of groundwater is determined by the original composition of the water before it entered the subsurface and the evolution of the water chemistry as it moves through the subsurface materials. The original composition may be that of rainwater, stream water, lake water, wastewater, irrigation water, ocean water or water from any other source. Water in the unsaturated part of the soil evolves by dissolution of minerals (including fertilizer), dissolution of soluble organic chemicals (including pesticides), precipitation of minerals, plant and animal metabolism and respiration, decomposition and fermentation of dead plant and animal material, decomposition of organic chemicals, adsorption of dissolved constituents onto solid surfaces, equilibrium with gases in the soil, and concentration by evaporation.
Water moving through the saturated subsurface materials below the water table is still affected by most of these processes. Exchange of gases with the soil atmosphere is less. No direct evaporation occurs, and metabolic processes are primarily bacterial. Dissolved oxygen is depleted by bacteria and reaction with oxidizable organic and inorganic material. The loss of oxygen affects chemical equilibria, as does the change in pH caused by various interactions as the water moves through the subsurface. The loss of oxygen also affects the type of bacteria that exist in the pores of the subsurface material, which affects the nature of the chemical reactions mediated by bacteria. As the water moves deeper into the grounder system its chemical composition moves toward equilibrium with the minerals in the aquifer and constituents that are readily adsorbed move slower than the water or not at all.
To see an example of a report on the groundwater quality in a limestone aquifer click here.
Posted March 7, 2016. Last revised August 21, 2018.