Arsenic in Groundwater
What is arsenic?
Arsenic is a chemical element with the symbol “As.” It is found naturally in rocks in the earth’s crust. Arsenic is recognized as a poison and cancer causing substance (carcinogen). It occurs within organic compounds (combined with hydrogen and carbon), and within inorganic compounds (combined within sulphur, chlorine or oxygen).
In water, arsenic has no smell or taste and can only be detected through a chemical test. The ambient concentration of arsenic in surface and ground waters in Canada is very low, usually ranging from 0.001 to 0.002 milligrams per litre (mg/L). Concentrations in groundwater are often higher than those measured in surface waters. Some creeks that are recharged by groundwater with high levels of arsenic may also have high levels. The Canadian drinking water guideline for arsenic sets a Maximum Acceptable Concentration of 0.010 mg/L.
What are the known sources of arsenic?
Localized high concentrations of arsenic have been found in well water from several regions in British Columbia, almost always associated with arsenic-containing bedrock formations. The most common sources of elevated arsenic levels in groundwater are:
• Weathering of arsenic bearing minerals and ores
• Infiltration or runoff from locations of past mining activities.
What are the environmental health concerns?
Water that contains arsenic is only a concern if it is being used for drinking or food preparation. Exposure through breathing and skin contact is not considered significant or harmful. Ingested arsenic is transmitted through the blood stream and may concentrate within the internal organs, skin, hair and nails. It is eliminated from the body mainly in urine.
Exposure to high levels of arsenic can cause short term or acute symptoms, as well as long-term or chronic health effects. Symptoms of exposure to high levels of arsenic may include stomach pain, vomiting, diarrhea and impaired nerve function that may result in “pins and needles” sensation in hands and feet. Arsenic can also produce a pattern of changes in your skin which includes darkening of wart-like growths – most frequently found on the palms or soles. Because children tend to drink more water per unit of body weight than do adults they may have a greater exposure to arsenic from drinking water and as a result be at increased risk of adverse effects when elevated concentrations of arsenic are present.
Long-term (years to decades) exposure to even relatively low concentrations of arsenic in drinking water can increase your risk of developing certain cancers including skin, lung, kidney and bladder cancer. Cancer is the critical health effect used in setting the Canadian guideline for arsenic in drinking water.
Arsenic Removal From Groundwater
The presence of arsenic in groundwater is both naturally occurring and/or the result of contamination associated with agricultural and industrial practices. Severe human health effects have been observed in populations that are exposed to persistent arsenic in groundwaters destined for potable, drinking water applications. Industrial and manufacturing processes utilizing groundwaters containing arsenic need to address treatment as it relates to the influent, recycle and reuse requirements.
Arsenic can be found in groundwater from areas that are in contact with arsenic rich rocks or other industrial pollution. In the US, this naturally occurring contamination is generally focused in areas of the Southwest, Northeast and Midwest, however, many small areas of contamination exist outside these regions and are linked to industrial activities, pesticides and wood preservatives.
The most significant needs for treatment of arsenic in groundwater sources relates to protection of human health. Industrial and manufacturing processes utilizing groundwaters containing arsenic need to address treatment as it relates to the influent, recycle and reuse requirements.
Groundwater used for non-potable applications, such as feed to an industrial and/or manufacturing process, also requires attention and possible removal of arsenic. In addition to the concerns already stated for human health and the environment, arsenic removal occurs in industrial applications in order to meet influent water quality requirement needed by the manufacturing process and equipment.
With reference to human consumption, the Environmental Protection Agency (EPA) drinking water standards require arsenic levels to be less than 10 ppb. Exposure to arsenic above this maximum contaminant level (MCL) has been linked to skin damage or problems with circulatory systems and may cause an increased risk of cancers of the bladder, lungs, skin, kidney, nasal passages, liver and prostate.
The US Environmental Protection Agency has designated the following treatment methods as Best Available Technology (BAT) for removing arsenic from groundwater.
Precipitation technology uses chemicals to transform dissolved contaminants into an insoluble solid The solid is then removed from the liquid phase by clarification or filtration. Precipitation/coprecipitation is more often used to remove contaminants other than arsenic from the water, such as hardness or suspended solids. However, the precipitation/coprecipitation processes applied to groundwaters usually also removes arsenic.
Ion exchange is also frequently used as a treatment technology for arsenic removal. As contaminated water is passed through the resin, contaminant ions are exchanged for other ions in the resin. Ion exchange is often preceded by treatments such as filtration to remove organics, suspended solids, and other contaminants that can foul the resins and reduce their effectiveness. Ion exchange resins must be periodically regenerated to remove the adsorbed contaminants and replenish the exchanged ions. Regeneration water and spent resin containing high levels of arsenic would require additional treatment prior to disposal or reuse. Alternatively, single-use, non-regenerable ion exchange resins may be used.
