In January 2006, the U.S. Environmental Protection Agency (USEPA) reduced the maximum allowable concentration of arsenic in drinking water to 10 micrograms per liter from the existing 50 micrograms per liter. This move put the United States arsenic drinking water standard in line with the recommendations from the World Health Organization (WHO). It also brought into focus the fact that over 10 million homes in the U.S. with private wells have arsenic levels in excess of the new maximum contaminant level (MCL).
Long-term exposure of consumers to even low levels of arsenic via drinking water consumption has been linked to an increased risk of skin, prostate, lung and bladder cancers, as well as diabetes, circulatory disorders and tremors. Recent studies at Dartmouth Medical School on New Hampshire residents confirmed the association of low to moderate levels of arsenic in drinking water with an increased risk of bladder cancer, especially if the consumer was also a smoker. This increased awareness of the health effects of even low-level arsenic concentrations has caused some states to impose an MCL for arsenic in drinking water that is even more stringent than the U.S. EPA MCL. The New Jersey Department of Environ-mental Protection has adopted an MCL for arsenic of only 5 micrograms per liter.
Arsenic is tasteless, odorless and colorless and its presence is only detected by testing. USGS maps delineating arsenic occurrence in ground water in the United States can be found here: http://water.usgs.gov/nawqa/trace/arsenic/.
Although public water utilities are now required to meet the new arsenic limit of 10 micrograms per liter, households relying on domestic wells are not regulated. However in several areas of the United States, compliance with the arsenic MCL is mandated for real estate transactions. This means that many millions of consumers are being exposed to arsenic levels that could have a negative impact on health.
For instance, in New Hampshire, approximately 40 percent of the population rely on domestic wells and it has been estimated approximately 10 percent of these wells have arsenic concentrations between 10 and 50 micrograms per liter. Municipalities are required to regularly test their finished water to ensure compliance with the new arsenic limit and other standards.
Increasing public awareness of the health effects of arsenic in drinking water is translating into an increased demand for home water treatment systems designed specifically for arsenic removal. Whole-house point-of-entry (POE) units are preferred over smaller point-of-use (POU) devices designed for individual faucet treatment since this guarantees arsenic-free water for the household. Recent studies in New Jersey have shown that showering in arsenic-tainted water leads to an increase in urine arsenic levels, demonstrating that arsenic can enter the body through the skin.
Arsenic and Water Chemistry
Arsenic exists in water in two major chemical forms: arsenate (or As(V)) and arsenite (or As(III)). The form in which the arsenic is present is highly dependent upon the water chemistry, with arsenate predominating in oxygen-rich waters and arsenite predominating under oxygen-deficient conditions. However, it is not uncommon to find both species existing simultaneously in a specific water. Of the two species, arsenite is the more toxic and also the most difficult to treat because it is
usually present as an uncharged species and cannot be removed by all arsenic-removal technologies. Fixed-bed adsorption systems are probably the most suitable treatment system since most adsorption media will remove both arsenite and arsenate.
Arsenic and Health Issues
One of the best-documented incidences of arsenic poisoning due to the consumption of arsenic-tainted drinking water is Bangladesh. Here, millions of tube wells were drilled in the 1970s to provide the local population with potable groundwater to replace the surface-water supplies, which were tainted by water-borne pathogens resulting in a host of serious diseases, including cholera, dysentery and typhoid. The incidence of disease dropped rapidly, but was replaced by a host of ailments directly related to arsenic in the well water. Arsenic concentrations in Bangladesh routinely run into the hundreds of micrograms per liter, which is considerably higher than levels typically found in U.S. groundwater. Symptoms of arsenic poisoning in Bangladesh include skin lesions, hyperkeratosis and peripheral vascular disorders. Very high concentrations of arsenic in well water are not limited to Bangladesh and do occur in the United States. Our laboratory routinely analyzes domestic well-water samples as part of our POE arsenic treatment program. We have had several instances of water from private wells with arsenic concentrations between two and three hundred micrograms per liter. The highest concentration of arsenic measured to date in the laboratory originated from a well in Arizona, where arsenic levels in excess of two milligrams per liter (two thousand micrograms) were recorded.
Arsenic Removal Technologies
There are a number of arsenic-removal systems available that are designed to treat the water for an entire household. The most popular methods use a fixed bed of an arsenic-selective adsorbent to treat the water coming into the house. These systems operate in a similar manner to a water-softening system. Water passes through the sorbent where arsenic is bound to the solid media to produce a finished product that is essentially arsenic-free drinking water. Unlike a water softener, common ions such as sulfate, bicarbonate and chloride are not removed so the taste of the cleaned water is unaffected. Many of the arsenic-selective media will last several years in a typical household before needing to be replaced, unlike water softeners, which need to be regenerated with brine on a regular basis.
Ideally, the arsenic-removal system is operated in a lead-lag configuration as shown schematically in Figure 1 (below). Water passes sequentially through a
filter and then through two columns containing the sorbent before being distributed throughout the household.
This configuration, coupled with regular testing of the water, gives added assurance that the finished water is always free of arsenic. The arsenic concentration in the
water after the first column is monitored and once the level at this system mid-point exceeds the MCL, the lead bed is replaced. The lag bed, which has been exposed to very little arsenic, is then placed in the lead position and a new column of media placed in the lag position. The presence of the lag bed guarantees that any arsenic passing through the lead bed is caught by the second column, ensuring no arsenic in the finished water.
Without the added assurance of the lag bed, there is the real danger of the media becoming exhausted between testing periods and arsenic occurring in the treated water. Arsenic breakthrough can be rapid in many instances and levels in the treated water can increase from non-detectable levels to concentrations above 10 micrograms per liter over a period of a few weeks. Depending upon the sampling schedule, this means untreated water could be consumed for weeks or months before it is known that the media has expired. Regular testing of the treated water is essential to ensure the system is operating as designed.
Water Testing for Arsenic Removal
Water chemistry has a significant effect on the adsorbent column. High pH and high silica levels will greatly reduce the capacity of all media and any phosphate present will compete with arsenic for available adsorption sites. As a consequence, when deciding on an arsenic-removal system and a water-testing frequency, some knowledge of the well’s basic water chemistry is highly desirable. This can be obtained by sending a sample of water to a certified analytical laboratory for a complete water analysis.
The detection of very low levels of arsenic in drinking water requires highly specialized instrumentation to
accurately quantify the amount present. Test kits are available that rely on a chemical reaction between reagents and arsenic to produce a color on a test strip, similar to pH paper. This color is then compared to a chart, which matches arsenic concentration with a specific shade. However, the subtle difference in the shades between low levels of arsenic is not always easy to accurately gauge. To be confident of arsenic levels in the water, it is probably best to send a sample to an analytical laboratory where it can be tested by professionals. Test strips are best employed to qualitatively determine whether arsenic is present in the water rather than to accurately assess concentrations.
Arsenic is a serious health hazard that is found in many private wells in the United States. Public water systems are mandated to test and treat arsenic to reduce concentrations to 10 and even 5 micrograms per liter. Private wells in known arsenic regions should be tested and treated for arsenic. Water chemistry needs to be evaluated to help determine the best treatment alternative and testing regime. Though many arsenic-removal technologies are available, sorbent media-based, redundant (lead/lag) systems are recommended as a reliable and cost-effective solution. As testing necessitates the detection of 10 or less micrograms per liter, laboratory analysis is recommended to determine base-water chemistry, and for system testing to ensure arsenic concentrations are being reduced to acceptable levels.