An Insidious Threat

Arsenic contamination in drinking water wells is a common concern. A variety of technologies are effective in removing arsenic from well water.
An Insidious Threat
Chemical storage tanks near the campground where customers complained of symptoms that are common in cases of arsenic ingestion.

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A small campground that maintains a public water system with 26 full-time customers came to us concerned that its clients were complaining of digestive tract irritation. A number also had skin abnormalities, such as dark and light spots on their skin and small corns on their palms, soles, and the trunks of their bodies.

We noted that the water system had a drinking water well close to an industrial cattle operation. During the course of the day, the cattle were dipped in an arsenic bath to eradicate ticks. (Cattle-dipping vats were built in the state to eradicate the tick responsible for transmitting a disease called southern cattle fever.)

The lesions and other symptoms the campground customers had were characteristic symptoms of chronic arsenic consumption.

Known toxicity

Oral exposure to inorganic arsenic is known to cause digestive tract pain, nausea, vomiting, diarrhea, decreased production of red and white blood cells, abnormal heart function, blood vessel damage, liver or kidney damage, and impaired nerve function, as well as skin abnormalities. In addition, arsenic ingestion is reported to increase the risk of cancer, especially in the liver, bladder, kidney and lung.

Virtually everyone knows arsenic is toxic. It has been featured in many murder mysteries, such as Arsenic and Old Lace. One lesser-known fact of arsenic toxicology is that the lethal dosage varies with its speciation. In general, methylated and organo-arsenic compounds are less toxic than inorganic arsenic.

At the cattle-dipping site, the presence of uncoated sand allowed the arsenic to be mobile, and an arsenic plume found its way into the campground's drinking water well, a quarter-mile away. The arsenic level found in the well was 49 parts per billion (ppb).

Common worldwide

There have been more than 40 major incidents worldwide involving groundwater arsenic contamination. An estimated 60 million people are drinking groundwater with arsenic concentrations above the World Health Organization standard of 10 ppb.

In the U.S., arsenic is most commonly found in the groundwater of the southwest. Parts of New England, Michigan, Wisconsin, Minnesota and the Dakotas are also known to have significant arsenic concentrations in groundwater. According to the U.S. EPA, millions of private wells have unknown arsenic levels, and in some areas, more than 20 percent of wells contain levels that exceed the established arsenic limit.

Effective treatments

We explained to the campground owner that there are several ways to alleviate an arsenic problem. We outlined the drinking water treatment processes that remove arsenic effectively. Co-precipitation with either iron or aluminum oxides has been shown to remove arsenic from water. The use of iron as a coagulant has been found to remove arsenic with efficiencies exceeding 90 percent.

An inexpensive method used to remove arsenic from contaminated well water is to sink wells 500 feet or deeper. A 2011 study funded by the National Institute of Environmental Health Science demonstrated that deep sediments can remove arsenic through a process called adsorption: Arsenic sticks to the surfaces of deep sediment particles, naturally removing it from well water.

Also, magnetic separation of arsenic at very low magnetic field gradients has been demonstrated in point-of-use water purification utensils using high-surface-area nanocrystals of iron oxide (Fe3O4). Because of the high specific surface area of Fe3O4, the waste mass associated with arsenic removal is dramatically reduced with this method.

The most common valence states of arsenic are arsenic (V), or arsenate, and arsenic (III), arsenite. Arsenic (V) is prevalent in aerobic surface waters, and arsenic (III) occurs in anaerobic groundwater. In the pH range of 4 to 10, the arsenic (III) compound is neutral in charge, while the arsenic (V) species is negatively charged.

Removal efficiencies for arsenic (III) are poor compared to arsenic (V). Arsenic (III) can be converted to arsenic (V) using chlorine, ferric chloride or potassium permanganate.

Exploring technologies

There are several common technologies for removing arsenic (V):

Coagulation and filtration. The type of coagulant and dosage used affect the efficiency of the process. Performance is slightly lower with alum than with ferric sulfate. The disposal of the arsenic-contaminated coagulation sludge can be of concern.

Lime softening. Operation at the optimum pH range of 10.5 provides a high percentage of arsenic removal. However, it may be difficult to reduce concentrations consistently to 1 ppb by lime softening alone.

Activated alumina. This method is effective in treating water with high total dissolved solids (TDS). However, selenium, fluoride, chloride and sulfate, if present in high concentrations, compete for adsorption sites. Activated alumina is highly selective toward arsenic (V).

Ion exchange. This process is effective, although sulfate, TDS, selenium, fluoride and nitrate compete with arsenic, and the competition can affect the run times of the unit. Suspended solids and precipitated iron can cause clogging of the ion exchange bed so that systems containing high levels of these constituents may need pretreatment.

Reverse osmosis (RO). Removal efficiencies of greater than 95 percent are possible when operating at the ideal pressure. If RO is used by small systems in the western U.S., water recovery needs to be optimized because of the scarcity of water resources and because the water recovery rate is 60 percent.

Electrodialysis reversal. This technology achieves removal efficiencies of 80 percent. One study demonstrated arsenic removal to 3 ppb.

Nanofiltration. Arsenic removal of greater than 90 percent is achievable.

Help for small systems

We explained to the campground owner that the 1996 Safe Drinking Water Act amendments specifically identify point-of-use and point-of-entry devices as options that can be used for compliance with National Primary Drinking Water Regulations. These devices can be effective and affordable compliance options for small systems in meeting the arsenic Maximum Concentration Limit.

One case study performed by the EPA with the Village of San Ysidro in New Mexico determined that point-of-use RO units can satisfactorily function in place of central treatment facilities to remove arsenic from the drinking water supplies of small rural communities. The RO units removed 86 percent of the total arsenic.


John Rowe, Ph.D., is a professor of Water Resources at Florida Gateway College in Lake City, Fla.


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