Monitoring arsenic in the environment: a review of science and technologies with the potential for field measurements

Abstract

This review examines available field assays and other technologies with the potential to measure and monitor arsenic in the environment. The strengths and weaknesses of the various assays are discussed with respect to their sensitivity, ability to detect the chemical states of arsenic, performance in various media, potential interferences, and ease of operation. The state of the science and development efforts of selected technologies is presented.

Introduction

Arsenic is a relatively common, toxic element that is also a known carcinogen [1]. Arsenic is found in a wide variety of chemical forms throughout the environment and can be readily transformed by microbes, changes in geochemical conditions, and other environmental processes [2]. While arsenic occurs naturally, it also may be found as a result of a variety of industrial applications [3], including leather and wood treatments [4], and pesticides [5]. Anthropogenic arsenic contamination results from a variety of activities: manufacturing metals and alloys, refining petroleum, and burning fossil fuels and wastes. These activities have created a strong legacy of arsenic pollution throughout the world.

The combination of high toxicity and widespread occurrence has created a pressing need for effective monitoring and measurement of arsenic in soils and groundwater. Toxic concentrations of arsenic have been detected in water supply wells in the United States [6] and abroad [7], creating a health risk for a large fraction of the world's population [8]. Arsenic is second only to lead as the main inorganic contaminant in the original National Priority List (NPL) of Superfund sites [9]. It also is one of the toxic materials regulated under the Resource Conservation and Recovery Act (RCRA). Therefore, the need exists for arsenic monitoring at Superfund sites, RCRA landfills, facilities handling arsenic-containing wastes, and sites where arsenic is found at toxic concentrations in groundwater.

The current maximum contaminant level (MCL) for all forms of arsenic in groundwater is 50 μg/l (50 ppb), set by EPA in 1975 based on a Public Health Service standard originally established in 1942. On 22 January 2001, EPA adopted a new standard for arsenic in drinking water at 10 ppb, to be enforced by January 2006 [10]. Arsenic-contaminated waste is restricted under RCRA as a hazardous waste and must be treated to meet limits determined by a prescribed aqueous extraction protocol, the toxicity characteristic leaching procedure (TCLP), usually performed in a laboratory. However, recent studies have shown that the TCLP may not accurately measure the ability for arsenic to migrate from a landfill [11]. Arsenic-contaminated soil is often treated as a hazardous waste with the same limitations on treatment or disposal, and often, additional regulations [12], [13]. Specific limits requirements vary but soil arsenic requires measurements down to mg/kg (ppm) concentrations. The new groundwater limits may affect disposal procedures for waste-containing arsenic, increasing the pressure to directly monitor RCRA waste sites, as well as arsenic containing soils for their potential to leach arsenic into groundwater.

Unlike organic pollutants, arsenic cannot be transformed into a non-toxic material; it can only be transformed into a form that is less toxic to organisms in the environment. Because arsenic is a permanent part of the environment, there is a long-term need for regular monitoring at sites where arsenic-containing waste and at sites where it occurs naturally at elevated concentrations. A range of analytical field assays for pollutants such as arsenic provide valuable tools to support improved site characterization [14].

This review presents a brief overview of the scientific literature on existing technologies that are available for detecting arsenic in liquid and solid media that may find application for measuring arsenic in soil, waste, and groundwater, including research developments that may affect those technologies.. This review intends to provide a critical overview of existing methods and technologies under development and offers insights into the most plausible future developments for detecting arsenic in the field.

One of the main sources of information on current analytical technologies is EPA's publication SW-846, “Test Methods for Evaluating Solid Waste, Physical/Chemical Methods”, which is a compendium of analytical and sampling methods that have been evaluated and approved for use in complying with RCRA regulations [15]. The environmental analytical chemistry of arsenic has been reviewed, along with selenium, antimony, tellurium, and bismuth with a strong emphasis on laboratory methods [16]. Field methods for the analysis of arsenic in the environment have also been reviewed in a paper published by the EPA, which forms the basis of this review [17].