Hardness Information Sheet

Background

Hardness is one of the most commonly tested parameters of drinking water. Because water is a good solvent (able to dissolve substances) and can pick up impurities easily, rarely is our water pure. It usually contains ions and compounds picked up from underground, land surfaces, pollution, and other human impacts. Hardness is one test that helps us determine the amount of certain dissolved minerals.

Hardness is defined as the sum of the polyvalent cations (ions with a charge greater than +1) present in the water. The minerals of calcium (Ca+2) and magnesium (Mg+2) are usually the predominant cations responsible for hardness levels. Other ions, such as iron (Fe+2), manganese (Mn+2), aluminum (Al+3), may contribute to hardness, but in natural waters these other ions are usually found in insignificant amounts. Just like total dissolved solids (TDS), hardness is a parameter that somewhat summarizes the amount of various substances that may be in the water. Though methodology for hardness tests can vary to account for different ions, most simple tests focus on just calcium and magnesium. Hardness test kits often express results in parts per million (ppm) of CaCO3 (calcium carbonate), which then can be converted directly to calcium and magnesium concentrations. Hardness can also be recorded in grains per gallon (gpg), degrees hardness (dH), or the molar concentration of CaCO3. 1 grain per gallon (gpg) = 17.1 mg/L. Parts per million (ppm) is the same amount as mg/L.

Waters with high hardness values are referred to as "hard", while those with low hardness are "soft". Hard water simply has excessive amounts of the polyvalent cations in them. There are various verbal categories / levels to classify the numerical results of hardness; they vary depending on the resource or agency. A chart of examples of the classifications follows:

Calcium and magnesium may be added to a natural water system as it passes through soil and rock containing large amounts of these elements in mineral deposits. Hard water is usually derived from the drainage through calcareous (calcite-rich) sediments and rock, such as limestone, sandstone, siltstones. Dolomites are rich in magnesium. These rocks are found in Western Pennsylvania, thus affecting the hardness levels in our water. In this area, if water has had the opportunity to interact with bedrock, rock, and soils for a long time (such as groundwater), it will be hard. If the cations responsible for making water hard are not calcium and magnesium, but are iron, sulfate, chloride, manganese, or aluminum, etc. instead, this is considered to be "non-carbonate hardness".

Water that has entered waterways directly without soaking into the ground, will be significantly softer. Collected rainwater is usually soft because it has not interacted with any geological sources of the cations. Soft water is also derived from the drainage of igneous rocks, because these rocks don't weather very easily, don't release many cations, and don't always contain calcium and magnesium.

Hardness in water can have some biological impacts on waterways. Calcium is an important component of aquatic plant cell walls, and the shells and bones of many aquatic organisms. Magnesium is an essential nutrient for plants and is a component of the chlorophyll molecule. If there is very little calcium in a waterway (less than 10 mg/L), only sparse plant and animal life can be supported because this waterway does not usually contain enough organic matter and nutrients. Hardness is also helpful in limiting metal toxicity for fish because calcium and magnesium keep fish from absorbing metals such as lead, arsenic, and cadmium (which are other polyvalent cations) into their bloodstream through their gills. The greater the hardness, the harder it is for toxic metals to be absorbed through gills. In addition, hard water is usually also high in alkalinity, which can help maintain pH levels that aquatic life need in order to survive (see Alkalinity and pH Information sheets for more information). Hard water can also affect fish osmoregulation, the process that controls the concentration of internal body fluids. There is a continuous movement of surrounding water into and out of a fish's body. The greater the difference in body fluid concentration and the surrounding water - the greater the osmotic effect. Hard water with more ion concentration is closer to body fluid levels, making the job of osmoregulation a little easier for fish. Soft water or very hard water will disrupt this balance and fish have to adapt their osmoregulation process.

Human Impact

Hardness is a characteristic of water that is not considered a pollutant and in most cases not considered a major health-related concern. Although beneficial because calcium and magnesium are essential minerals to a healthy human diet, hard water is generally considered a nuisance rather than health benefit or threat. Hardness increases the amount of soap that is needed to produce foam or lather because the calcium and magnesium ions form complexes with soap keeping it from sudsing. The calcium and magnesium deposits from water can cause a film on sinks, bathtubs, hair, skin, fabrics, ice cube trays, glassware, and dishes. This calcium buildup (solid precipitate) from water can also coat the interior of water pipes, boilers, heat exchange equipment, household appliances, and some industrial equipment. Deposits (often called scale) can become so thick that they clog pipes or cause equipment malfunctions, thus becoming an economic burden. These hardness deposits can have a white or greenish color to them. In addition to all of these annoyances, some people do not like the taste of hard water.

Even though hardness can cause plumbing nightmares, hardness is also desirable because it reduces corrosion rates in our pipes. This reduces the amount of lead (from lead solder), copper, zinc, and other metals from plumbing that may enter our drinking water. Unlike hard water, soft water with few positive ions is more reactive to picking up cations such as metals from pipes or the surrounding environment. The calcium coating on the inside of the pipes can also help reduce corrosion.

Some homeowners actively try to soften their water using a number of different systems. An ion exchange filter/ system exchanges the positive sodium ions from salt (sodium chloride) for calcium and magnesium. The sodium enters the drinking water instead, which can also affect taste in large quantities. Sometimes this system is used only for hot water heaters so extra sodium is not drunk, but expensive pipes and equipment are protected. Another softening method is a lime-soda process, involving the reaction of lime and soda ash reacting with hard water producing a insoluble precipitate with calcium and magnesium ions. These ions no longer interfere with cleaning soaps, but the precipitate can still leave films and be drunk.

Humans can increase the amount of hardness in waterways in a few ways. Drainage from operating or abandoned mine sites can add calcium, magnesium, iron, manganese, and other cations from the newly exposed rocks or overburden (soil and crushed rock removed from mining operations). Some industrial discharges can be high in calcium and metals. Wastewater from homes is often high in cations from household cleaning agents, food residue, human waste, and from rinsing out the trapped calcium and magnesium from an ion exchange filter system when changing the salt. All of these cations cannot always be removed by water treatment facilities before discharging to a stream.

Water Quality Criteria

Because hardness is a characteristic of water and not a pollutant, there are no published standards for overall hardness, calcium, or magnesium from the Environmental Protection Agency (EPA). Government agencies do hope that a level of hardness does exist so that pipes are less corrosive, limiting the release of metals that do need to be regulated.