Health Stream Literature Summary - Issue 55 - September 2009

Arsenic in groundwater in New England - point-of-entry and point-of-use treatment of private wells.
Moller, T., Sylvester, P., Shepard, D. and Morassi, E. (2009) Desalination, 243(1-3); 293-304.

Arsenic concentrations in groundwaters in the US are mostly low, but levels greater than 10 ppb (the current US Environmental Protection Agency maximum contaminant level (MCL) for arsenic) are prevalent in some regions including New England and several southwestern states. There are currently no federal requirements to monitor arsenic levels in private wells and owners are responsible for the quality and safety of their well water. Point-of-entry (POE) and point-of-use (POU) arsenic removal systems may be an effective treatment option for individual households and small communities using groundwater in these areas. Devices based on fixed bed arsenic adsorption technology on hybrid hydrous iron oxide/polymer media (ArsenXnp®) have advantages over other technologies such as ion exchange (needs regular regeneration) or reverse osmosis (reject water may be wasted). The functional service life of hybrid hydrous iron oxide/polymer media systems depends on the concentration of different arsenic species in the water (arsenate and arsenite), the cumulative volume of water filtered, the pH and the presence of other ions such as silica and phosphate. Using data from field studies with a variety of water conditions, it is possible to predict the service life of the units with reasonable accuracy for a particular water quality.

A study was conducted based on data collected from a water sampling and testing service provided to customers in eastern New England who had installed devices of the npXtra TM System (containing ArsenXnp® medium). These systems contain two media columns in series with a sampling point located between the columns so that partially treated water can be sampled in addition to untreated and treated water. Samples of untreated, partially treated and treated water were collected at the time of installation from 236 POE and 39 POU systems over an 18 month period from November 2005 until August 2007. After 6 months (for POU) or 9 months (for POE) customers were sent a sample kit and asked to submit water samples, with 62 supplying samples. Of the tested well waters (untreated), 77% had arsenic concentrations less than 50 ppb which was the USEPA MCL for arsenic until January 2006, and 22% had levels below 10 ppb. About 9% of the samples had levels greater than 100 ppb arsenic with only five sites exceeding 300 ppb. The highest arsenic levels were found in New Hampshire and Maine while on average New Jersey had the lowest concentrations. The largest number of tested private wells having arsenic levels above the 10 ppb MCL occurred in Maine. The collected data was generally consistent with the findings of US and state Geological Surveys of the region.

During the 1.5 years of operation, some of the first installed systems started to show arsenic breakthrough in partially treated water samples, indicating that the first media column was nearing saturation. This was consistent with predictions based in water quality characteristics. Generally it appears that when arsenic levels are below 50 ppb, the POU/POE units will last at least 1.5 years. However, when waters have pH values higher than 8.5 and arsenic levels above 100 ppb, the life of the POE and POU systems are shorter, often 10-12 months. Premature exhaustion of the ArsenXnp® media can also be due to elevated silica and phosphate levels in the water. An unexpected early breakthrough can also occur when arsenic exists in arsenite form which adsorbs more poorly to the medium than arsenate.

Periodic arsenic analysis of the customers waters have shown that the POE and POU systems operate as predicted and meet the requirements of the customers in terms of arsenic removal and ease of operation. No physical problems such as flow, back pressure build-up or blocking of the systems have been reported. Only 10 of the systems have needed to be replaced since becoming operational which corresponds to 4.2% of all the devices installed.

Comment The authors note that only 52% of the expected number of water samples have been returned by customers, but it is not known whether all customers received the sampling kits. The effectiveness of POU/POE devices for reducing arsenic exposure can only be assessed by a longer term study so that the willingness and/or ability of customers to maintain systems in good working order is evaluated. This is particularly important when failure/exhaustion of the water treatment device is not detectable by the consumer in terms of clogging or poor aesthetic water quality.

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