Arsenic in ground water

Most water supply programmes in Cambodia have focused on providing access to bacteriologically safe water, an approach which has led to an increasing reliance on ground water, especially in rural areas. However, there have been very few data collected on the chemical quality of the nation’s drinking water sources, and few water supply programmes have the capacity to assess chemical quality. The study was designed to address this data gap by conducting a low-cost, rapid assessment of drinking water sources nationwide to determine whether there were any chemicals of concern in Cambodian water supply sources. Results of the assessment confirm that there are several parameters of health and aesthetic concern; dissolved arsenic is the most significant. Elevated arsenic levels (some exceeding 500mg l21) were detected in aquifers of moderate depth in several highly populated areas, confirming that further investigation of the occurrence of arsenic contamination in Cambodia is warranted. Other chemicals of health concern include nitrate, nitrite, fluoride and manganese. Additionally, many ground water sources are negatively impacted by parameters of aesthetic concern, such as iron, manganese, hardness and total dissolved solids. Elevated levels of these parameters have caused consumers to reject newly installed water supplies, often in favour of surface water sources that are bacteriologically unsafe.

INTRODUCTION

The Kingdom of Cambodia, a nation of nearly 14 million, struggling to revitalize its economy and infrastructure after decades of war and civil conflict, is burdened by some of the highest levels of poverty and disease in the region. Cambodia’s under-five mortality rate of 124 per 1,000 live births is the second highest in Asia, and nearly 40% of the population lives on less thanone dollar per day (NIS 2003;UNICEF2004). As water-related diseases are among the leading causes of morbidity and mortality among Cambodian children, provision of safe water is a high priority for the Royal Government of Cambodia (MEF 2004; WHO 2004a). Efforts to improve drinking water supplies undertaken since the early 1980s have been moderately successful in urban areas, as the proportion of urban residents with access to safe water is estimated to exceed 70% nationally (NIS 2004) and may exceed 85% in Phnom Penh (NIS 2002). (In Cambodia the term ‘safe water’ typically means piped water, water obtained from drilled wells with handoperated or powered pumps, or water obtained from concrete-lined hand-dug wells. Though the bacteriological safety of any of these sources can reasonably be questioned, most agencies consider these sources ‘safe’ as a working definition.) However, among Cambodia’s rural population (who constitute 84% of the total population), only 34% are estimated to have access to safe water (NIS 2004).

Most rural residents access surface or shallow ground water sources that are bacteriologically contaminated. The government and many of its partner agencies have been implementing programmes to install ground water wells as a means of providing year-round water supplies that are bacteriologically safer than those currently available. The actual number of rural water supply wells in Cambodia is not known, although anecdotal evidence from the Ministry of Rural Development and other agencies suggests that the number of public and private wells has increased rapidly within the past ten years. Though wells installed by the government and its partners were intended to provide bacteriologically safe drinking water, the chemical quality of the water they provided had not been systematically characterized prior to this assessment, which was completed in January 2001. The assessment was conducted by the Ministry of Rural Development and the Ministry of Industry, Mines, and Energy, and was supported technically and financially by WHO. The assessment was designed to rapidly determine whether Cambodia faced any chemical water quality problems that were of significant public health concern. In addition, it was intended to provide a body of data to serve as a baseline for comparison with future water quality studies. The assessment focused exclusively on chemical water quality. Though significant to human health, microbiological parameters were not included in this study for several reasons: 1) sector professionals in Cambodia were already well aware of the threat of microbiological contamination; 2) many bacteriological tests had already been performed on Cambodian drinking water sources and local capacity for this existed; 3) performing bacteriological tests on the sampled water sources would have greatly increased the time, staff, equipment and budgetary requirements of the project. The scope of the assessment was therefore kept purposefully narrow so as to focus the limited available resources on an issue that had received little prior attention. The assessment was executed over a 12-month period and provided new information regarding Cambodia’s drinking water quality situation, including identifying arsenic as a potentially significant public health issue. Results of the assessment have led or contributed to a number of follow-up actions by the government, including the promulgation of national drinking water quality standards, further investigation of arsenic occurrence and mitigation techniques, and a reassessment of the technical and policy approaches being used to solve the nation’s urgent drinking water supply problems.

METHODS

Study area and sample locations The study area comprised 13 of Cambodia’s 20 provinces and contained approximately 80% of the total population. The study area did not include the municipality of Phnom Penh, as much of the capital is served by a large water treatment and distribution system. Budget and time constraints permitted collection and analysis of approximately 100 samples during the first and main round of sampling. Excluding duplicates and trip blanks, this resulted in the collection of 94 unique samples during the ‘main’ round of sampling. (A portion of the budget was reserved to conduct follow-up sampling should further clarification or confirmation of any findings be needed, which also reduced the size of the main round of sampling.) The limited number of samples meant that the sampling density was relatively thin, but given the fairly uniform geography of much of central Cambodia, it was decided it would be more worthwhile to screen a larger area rather than focusing in more detail on a smaller portion of the country. If the highly populous area of the country had been more geographically and geologically diverse, a greater number of samples would probably have been required to accomplish the same task. A total of seven or eight samples were collected from each province within the study area. Within each province, one sample was obtained from the public water supply system in the provincial capital, and the balance of samples was collected from rural drinking water sources. Samples were collected during the dry season, which permitted greater access in rural areas, as well as making it more likely that ground water sources used for drinking on a seasonal basis would be in regular use. A small number of samples were also collected from perennial surface water bodies that served large numbers of rural residents, even