Mapping of arsenic content and distribution in groundwater in the southeast pampa, Argentina.

Introduction

The relationship between groundwater and the chemical, physical, and kinetic processes affecting the various rock and sediment components could be the reason for the appearance of arsenic in some sources of water supply (Toth, 2000). The content in water depends more on speciation than on the amount of arsenic present in the environment (Bhumbla & Keefer, 1994). Upper, or phreatic groundwater, tends to be highly mineralized water containing considerable amounts of arsenic, fluoride, boron, vanadium, and other minerals.

According to the World Health Organization (WHO, 1998), the carcinogenic effect of ingesting water containing inorganic arsenic above the recommended maximum level of 0.01 milligrams per liter (mg/L) is well demonstrated and is reflected in an increased incidence of skin cancer in humans.

Morras, Blanco, and Paoloni (2000) reported excessive levels of arsenic in the groundwater of the Chaco-pampa region in Argentina. A study by Sastre, Rodriguez, Varillas, & Salim (1997), surveyed a population that had resided in the Salta Chaco (northwest Argentina) for over 10 years and found that 8.6 percent of those surveyed were suffering from chronic regional endemic hydro-arsenism (CREHA).

Volcanic ash in quaternary sediments in the pampa plains of north La Pampa Province, Argentina, show high concentrations of arsenic (7 to 12 mg/L), as well as other oligoelements (Nicolli, Smedley, & Tullio, 1997; Smedley, Nicolli, Macdonald, Barros, & Tullio, 2002).

A considerable number of studies have been undertaken in Argentina on high arsenic content in water and the consequences for human health, (Arribere et al., 1997; Ayerza, 1918; Biagini et al., 1995; Blanco, Paoloni, & Fiorentino, 2000; Bolzicco, Bettig, & Bojanich, 1997; Formigli, Revelant, Marinozzi, & Olguin, 1997; Salvador, 1987; Tello, 1951; Trelles, Larghi, & Paez, 1970).

Building on the results of a previous paper on arsenic in the groundwater of an extensive area (Paoloni, Fiorentino, Sequeira, & Echeverria, 2000), the study team decided to evaluate the water over a smaller area in order to increase sampling and better characterize the spatial distribution of arsenic. The study area has a large rural and suburban population for whom groundwater constitutes the only readily available source of water for human consumption and agricultural and livestock production.

Taking into account WHO's recommended tolerance level in water of 0.01 mg/L (1998), the study reported here set out to map the arsenic content in groundwater within the study area and highlight the areas of greatest toxicity. It is hoped that the dissemination of the results will lead to a wider public appreciation of the dangers involved, provide the basis for preventive action, and assist with the therapeutic treatment of affected populations. The results of the study should be made available not only to districts within the study area, but also to all regions in the country where a similar public health risk exists.

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Materials and Methods

The study area extends along the southeast border of the pampa plains between 38[degrees]18' and 38[degrees]58' S, and 60[degrees]43' and 61[degrees]43' W, covering an irregular surface of approximately 6,000 square kilometers, which corresponds to the Coronel Dorrego district in the south of the Province of Buenos Aires, Argentina. The cartography of the study is based on charts from the Instituto Geografico Militar (Military Geographic Institute), with scales of 1:50,000 and 1:100,000, and on Landsat satellite images.

Selective and specific sampling was carried out in 104 wells and perforations used for exploiting groundwater. The water is accessed by means of traditional piston pumps driven by aeolian power (windmills) and in some cases by electrically powered centrifugal pumps. Water depth and temperature were recorded in all cases.

The study team carried out hydrochemical analyses for the quantitative determination of arsenic in the laboratory by means of the electrometer method, using a selective electrode and the arsine test technique. This method involves the addition of two reagents, zinc powder and hydrochloric acid, to the solution, causing a reaction between the reagents. The reaction generates hydrogen gas ([H.sub.2-]). This gas reacts with any arsenic that is present within the test tube to form arsine gas (As[H.sub.3-]). The arsine gas turns the color of the reaction zone on a test strip. The strip is then compared to color standards to determine the arsenic concentration.

Using as reference the recommended maximum tolerance values for arsenic, the authors drew a map, highlighting the zones considered most at risk and those where the water was found to be free of arsenic, thus showing relative vulnerability to this specific natural contaminant throughout the study area (Hirata & Reboucas, 1999).

Results and Discussion

More detailed than the preliminary study of Paoloni and co-authors (2000), the work reported here makes it clear that the selected area contains sectors where not only the local population but also livestock and sensitive crops are exposed to a high level of risk.

The results are shown on a map of the area (Figure 1), with isolines of arsenic concentration in mg/L demarcating the affected zones and the variations found in arsenic content. The map also shows the location of the main town and of smaller towns with low population density, where many residents have no running-water supply for drinking purposes. The remainder of the population is rural, and each homestead relies on its own source of water for consumption and for agricultural and livestock production.

Because the distribution of waters with very high concentrations of arsenic bears no apparent relation to the direction of groundwater flow, a map of underground hydrodynamic behavior, or isohypse chart (meters above sea level [MASL]), was drawn with equipotential lines showing the morphology of the phreatic surface (Figure 2). The dominant flow is north-northeast to south-southeast, with a net tendency to discharge in the direction of the maritime coast. The configuration of the equipotential lines in the northern sector is irregular, with a relatively high hydraulic gradient. In addition, several watersheds are evident, with well-defined line of discharge. In the southern sector, the distribution of the lines is more regular and widely spaced, showing a marked parallelism suggestive of a more attenuated and gentle hydraulic gradient. Since the study area is flat and the circulation is slow, the vertical movement of water via processes such as filtration and evapo-transpiration is considered more important than the horizontal movement of the underground flow. This situation suggests that the arsenic in the groundwater could well derive from the washing and dragging of certain materials present in the quaternary loess sediments in the region (Morras et al., 2000).

The spatial variability and concentration of arsenic in groundwater could be the outcome of water-environment interactions of the type reported by J. Toth (2000), giving rise to "in situ" environmental effects, which can occur at any depth, since groundwater acts as a geological agent between the environment and spatial distribution. This interpretation is supported by the nonsignificance (0.155) of the correlation coefficient between the depth of the upper limit of phreatic water and the arsenic content (Figure 3).

It was found that a mere 16.5 percent of the water samples (representing approximately 21 percent of the total study area) were completely free of or showed only slight levels of arsenic. In 27.2 percent of the samples, arsenic content ranged from just under the recommended maximum level of 0.01 mg/L to 0.05 mg/L, and the remaining 56.3 percent of samples were severely affected, with arsenic concentrations of 0.06 to 0.50 mg/L. Over the course of its various field trips, the research team was able to establish that the local population of the area in question consumes and utilizes the groundwater on a daily basis. In most cases, members of the population are ignorant of whether the water they consume is contaminated by arsenic and of the consequences of ingesting contaminated water over prolonged periods.

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Conclusions

Areas have been detected where arsenic content in the groundwater exceeds to different extents the acceptable level. Groundwater used for human consumption and livestock and agricultural production is seriously affected by arsenic contamination in approximately 79 percent of the study area, reaching levels of up to 0.5 mg/L, and only 21 percent of the area is completely free of or has only low concentrations of arsenic.

The gravity of the situation calls for advisory services to be set up to counsel those already affected, implementation of a control strategy to contain the damage and minimize future risk, and overall dissemination of information on the carcinogenic effects of ingesting water contaminated with arsenic.

The ability to highlight particularly vulnerable areas through detailed study, as reported here, should pave the way for increased monitoring and control.

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Corresponding Author: Juan Dario Paoloni, Departamento de Agronomia, Universidad Nacional del Sur-Conicet, Complejo Palihue, Bahia Blanca, Argentina 8000. E-mail: [e-mail].