No.8 - Impacto de la geoquímica en la salud: una aproximación basada en estudios de casos de ecosistemas tropicales en la República Dominicana

A. J. Hernández, S. Alexis, C. Vizcayno, J. Pastor


Prompted by the high heavy metal levels frequently encountered in topsoil samples of the tropical forests of the Jaragua-Bahoruco-Enriquillo Biosphere Reserve, Dominican Republic, and considering the most common land uses of this area, we decided to address this pollution problem and its effects on health. The source of these pollutants is linked to geoedaphic processes more than to human impacts, in a region that comprises core, intensive agriculture and buffer areas of the reserve, harbouring mines (bauxite and limestone), crops and livestock. The hypothesis that heavy metals liberated by geochemical actions in some of these tropical ecosystems could be related both to productivity and to human and animal health, led us to assess metal bioavailability in the area’s main crops as the primary source of food or fodder.
This study presents an experimental design in which microcosms were set up using soils selected according to the known presence of several metals (Cd, Cr, Cu, Zn, Ni and Al) and seeds obtained from the original crops in the area: kidney beans (Phaseolus vulgaris), the legume "guandules" (pigeon pea Cajanus cajan), corn (Zea mays) and sorghum (Sorghum bicolor). These crops are consumed by humans, herbivores (goats mainly and cows) and other animals (e.g. hens and pigs); crop remains are even used as organic fertilizer. The complex trophic network that includes heavy metals generated by geochemical actions in these tropical ecosystems makes them real scenarios for studies on relations between environmental health and human health.
To establish the context of the heavy metal pollution, we characterized the geoedaphic features of the region. The predominant rocks are sedimentary limestones: with limestone colluvial deposits dominant in the tropical conifer forest and rain forest of the Sierra de Bahoruco; crystalline limestones in the tropical latifoliated forest; and Quaternary detritic rocks and reefs (carbonates overlying alterites) in the dry tropical forest. Across the territory, there is a marked predominance of soils that range from surface soils to shallow, poorly developed stony soils of low natural fertility. Most can be classified as entisols. Soils of recent alluvial origin lack pedogenetic horizons and are subjected to diverse humidity and temperature regimens. Slopes are pronounced and relief and altitudes vary. Their profiles include A-R horizons characterised by displaying an ochre epipedon over a fractured rock bed whose depth is shallow, and A-C horizons of a sandy to clayey soil and subsoil texture, whose colours range from dark brown to grey and depths from very shallow to deep. Soils occur from the mountains to landscapes including rivers or sandy coasts. The main types of clay are: hematite, kaolin, bohemite (the most abundant) gibbsite and calcite. Textures range from sandy-silty to clayey. Sand and clay fractions seem more abundant than silt ones. Soil pHs are generally in the basic range with infrequent acid soils. Organic matter and total nitrogen levels are not low, especially OM in the dry forest and N in the latifoliated forest. Available K contents are low in mountain forests and high in dry forests. Available P contents are generally low to very low.
The heavy metals in the soils are mainly of lithogenic origin and levels are generally higher than those of the reference soils mentioned in the bibliography (established polluted soils that could harm the plants they sustain). Specifically, Ni, Cu and especially Cd, were markedly high in crop soils.
The chemical health state of the area’s waters was also examined. In terrestrial ecosystems with the exception of one, pHs were basic. Highest electrical conductivity values were recorded in wells. Carbonates and bicarbonates showed scarce variation. In inland ecosystems, most marked anion contents were those of chlorides and sulphates detected in ponds, which also showed high levels of Mg and Na. One of these wells found at a bauxite mine is used as a drinking trough by livestock and wild animals and also contains elevated Al and Fe. In coastal ecosystems, waters exhibit extremely high sulphate and especially chloride concentrations. Finally, existing organic compounds are those frequently found and pose no harm.
Although this is a preliminary approach to the topic, we present our initial results in terms of the heavy metal contents of the above-ground plant mass (essentially the leaves and seeds as the main parts consumed). The results of our bioassays on the four main crops of the area indicate significant differences in the heavy metal contents of kidney beans depending on the soils. The Cu contents of the beans in all the soils were higher than reference values, while Zn and Mn levels surpassed these references in one of the soils. There was no detectable presence of harmful metals such as Cd and Cr, and Ni, concentrations were low. Leaf metal contents of the kidney bean also varied according to the soil they grew in with the exception of Al, which always appeared in high levels. Cd was absent in leaves; Mn and Cu concentrations were high while those of Zn, Cr and Ni were low. Appreciable levels of Cu, Ni, Cd, As and Cr were also found in different tissues of the leaves of pigeon peas. We also provide the results of an electron microscopy study designed to identify the plant organs capable of accumulating most heavy metals. Thus, the leaves of sorghum showed high proportions at the cell level of Cu, Ni and Cr along with the presence of Cd. In corn leaf cells growing in the soils of the bauxite mine, Cu levels were high followed by Ni, and Cd concentrations were sufficiently high to be taken into account. Moreover, small quantities of As should also be considered. Finally, we discuss our results from the perspective of the possible ecopathologies produced by heavy metals in the topsoil that could affect the health of agroecosystems, animals and humans.

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1309 (during 2009)


biosphere reserve, forest soils, lithogenic metals, water composition, ecopathologies

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