Arsenic is an inorganic toxin that occurs naturally in soils. It can enter water supplies from many sources, including: erosion of natural deposits; runoff from orchards; runoff from glass and electronics production wastes; or leaching from products treated with arsenic, such as wood. Synthetic organic arsenic is also used in fertilizer.

Arsenic toxicity is primarily associated with inorganic arsenic. Arsenic ingestion has been linked to cancerous health effects, including cancer of the bladder, lungs, skin, kidney, nasal passages, liver, and prostate. Arsenic ingestion has also been linked to noncancerous cardiovascular, pulmonary, immunological, and neurological, endocrine problems. According to EPA's Safe Drinking Water Act (SDWA) Arsenic Rule, inorganic arsenic can exert toxic effects after acute (short-term) or chronic (long-term) exposure. Toxicological data for acute exposure, which is typically given as a LD50 value (the dose that would be lethal to 50 percent of the test subjects in a given test), suggests that the LD50 of arsenic ranges from 1- 4 milligrams arsenic per kilogram (mg/kg) of body weight. This dose would correspond to a lethal dose range of 70 to 280 mg for 50 percent of adults weighing 70 kg. At nonlethal, but high, acute doses, inorganic arsenic can cause gastroenterological effects, shock, neuritis (continuous pain) and vascular effects in humans. EPA has set a maximum contaminant level goal of 0 for arsenic in drinking water; the current enforceable maximum contaminant level (MCL) is 0.050 mg/L. As of January 23, 2006, the enforceable MCL for arsenic will be 0.010 mg/L.

The SDWA requires arsenic monitoring for public water systems. The Arsenic Rule indicates that surface water systems must collect one sample annually; groundwater systems must collect one sample in each compliance period (once every three years). Samples are collected at entry points to the distribution system, and analysis is done in the laboratory using one of several EPA-approved methods, including Inductively Coupled Plasma Mass Spectroscopy (ICP-MS, EPA 200.8) and several atomic absorption (AA) methods. However, several different technologies, including colorimetric test kits and portable chemical sensors, are currently available for monitoring inorganic arsenic concentrations in the field. These technologies can provide a quick estimate of arsenic concentrations in a water sample. Thus, these technologies may be useful for spot-checking different parts of a drinking water system (for example, reservoirs, isolated areas of distribution systems) to ensure that the water is not contaminated with arsenic.

The two primary technologies for evaluating arsenic concentrations in the field are colorimetric test kits and portable chemical sensors. These two technologies are described in further detail below:

The field test kits detected by mixing the water sample with powdered reagents, which converts the arsenic to arsine gas. A colorimetric test strip is then immersed in the sample, removed, and compared to a reference table to determine the arsenic concentration in the sample. Several vendors (including Industrial Test Systems, Inc., and Peters Engineering) also offer a battery-operated tester, which measures the color change electronically and displays the results on the unit.

There are two portable sensor technologies currently on the market (Monitoring Technologies International's PDV 6000 and TraceDetect's Nano-Band™ Explorer). These analyze arsenic using anodic stripping voltammetry (ASV) technology and transmit results to a laptop (not included with the sensor product) loaded with specialized software to interpret, display, and store the results. The ASV technology works through a standard oxidation/reduction (redox) chemical reaction in the test solution. First, a "reducing potential" is applied at the working electrode. When the "reducing potential" exceeds the "ionization potential" of the arsenic ion in solution, the arsenic is "reduced" and collected on the electrode. After a pre-specified time, "oxidizing potential" is applied to the working electrode. This strips off the arsenic and creates an electric current, which is measured and compared to a reference standard to determine the sample concentration. The results are then displayed on the laptop.

The Nano-Band™ Explorer uses a unique electrode configuration to increase its response time. Its electrode is composed of 100 sub-electrodes, and the increase in mass that can be transported across these multiple electrodes allows measurement of the arsenic concentration in a much shorter time relative to conventional electrode technologies (usually within a few seconds).

The U.S. EPA Environmental Technology Verification (ETV) program has evaluated multiple products for measuring arsenic in water samples, including seven different test kits and two portable sensors. For each product, EPA evaluated accuracy, precision, linearity, method detection limit, matrix interference effects, inter-unit reproducibility, rate of false positives/false negatives, and other factors. In general, EPA evaluated solutions ranging from 0.001 to 0.1 mg/L arsenic. This translates to 10 percent to 1,000 percent of the new 0.010 mg/L standard for arsenic in drinking water. A summary of results of these evaluations are presented below. For a full discussion of the tests and results, see http://www.epa.gov/etv/verifications/vcenter1-21.html.

Accuracy is a measure of how close a measurement is to the true value. The measured difference between sample reading and the true value is referred to as "bias." Bias is reported as a percentage of the measure value relative to the true value, and can be positive (sample reading is above the true value) or negative (sample reading is below the true value). Bias can vary in magnitude between different analytical techniques or procedures. It should also be noted that small errors can result in a large bias when the actual concentration in the sample is low. For example, an error of 1 ppb in measuring concentrations of 10 ppb arsenic vs. 100 ppb arsenic would result in a positive bias of 10 percent for the first measurement, but only 1 percent for the second measurement.

EPA found that many of the arsenic test kits could have a large bias (up to almost 10,000 percent in some cases) in measuring a known arsenic concentration. Most of the biases were positive (i.e., the results were reported as higher than the actual concentration), although at least one kit (Industrial Test Systems [ITS] Quick™ Ultra Low II) produced results that had negative biases.

Results from the portable sensors also showed biases. While the Nano-Band™ Explorer showed only high bias in the EPA trials, the PDV 6000 showed both positive and negative bias.

Precision measures the repeatability of a measurement (e.g., the bias in measuring the same sample a number of times). EPA's ETV program expresses precision as the Relative Standard Deviation (RSD) of replicate analyses.

Precision for the test kits ranged from 0 to 139 percent of the original measurement.

Precision for the Nano-Band™ Explorer ranged from 3 to 91 percent. Precision for the PDV 6000 ranged from 3 to 16 percent.

In summary, it can be difficult to generalize or draw conclusions regarding the overall accuracy and precision of arsenic test kits or portable sensors because there is a wide variation between the different products. Therefore, the use of arsenic test kits or sensors must be tempered with the knowledge of their limitations. While they do provide a quick, inexpensive method to evaluate general concentrations of arsenic in water, they may not be reliable for providing a consistent, accurate result. However, since the use of arsenic testing for security purposes should not require a highly accurate result (i.e., decisions regarding the immediate safety of water would most likely be based on much higher arsenic concentrations than those used for regulatory monitoring, and thus more accurate testing may be done to confirm exact concentrations at a later time), these kits and sensors may be useful for water security applications.

A summary of several available products, their ranges, and the total test times is provided in Table 1 below:

Sodium chloride, iron, sulfate, and acidity can potentially interfere with arsenic measurements in water. However, in general, EPA found that the test kits and portable chemical sensors were not affected by the presence of sodium chloride, iron, sulfate, or acidity, and that measurements of arsenic were similar in samples that contained these potential interfering chemicals vs. samples that did not. EPA did find that high iron and/or hydrogen sulfide concentrations biased arsenic measurements by the PDV 6000 analyzer. Arsenic results in samples with high concentrations of iron and/or hydrogen sulfide were biased high.

Costs for arsenic detection systems can vary greatly depending on the level of system sophistication. Costs for test kits depend on the number of tests in the kit, and the range of the test. For example, Industrial Test System, Inc. kits can range from $16 for a two-test kit to $250 for an ultra low-range 25-test kit. The Peters Engineering test kit capable of analyzing 100 samples is $220; refill packs for 100 additional tests can be purchased for $60. The AS 75 tester is $330.

The Nano-Band™ Explorer Portable Water Analyzer costs $8,000. This includes the battery-powered, rechargeable instrument, software, one Nano-Band™ Explorer electrode, an auxiliary electrode, a reference electrode, a cleaning and reconditioning kit for the electrode, and a temperature sensor. The PDV 6000 portable analyzer for the detection of heavy metal ions has a list price of $7,900. This price includes the analyzer unit, software, batteries, charger, and carrying case. Neither system includes a laptop computer, which is necessary to run either technology in the field.

Several arsenic test kits and sensors have been evaluated by the EPA Environmental Technology Verification program. Information on these technologies can be found at http://www.epa.gov/etv/verifications/vcenter1-21.html.