WUILLOUD Rodolfo German
congresos y reuniones científicas
Hydrogeologycal study of the prheatic aquifer in groundwater coupled arid ecosystems under a world-wide climate change context
Conferencia; International Conference on Arid and Semi-Arid Development through Water Augmentation; 2010
Institución organizadora:
In arid regions or drylands, which cover about 45 % of the Earth land surface, the groundwater plays an important role for human activities and biological production. The NE region of Mendoza (Argentina) is one of the most arid regions of the Monte desert, with long term (1957-2008) mean annual precipitation of 214 mm, mostly (73%) concentrated in the austral summer (from November to March), and with absolute temperatures that vary from -9º C in winter to 43 ºC in summer, with a mean of 16,2 ºC. Shallow groundwater in this region is used by local communities, their livestock and phreatophic vegetation, allowing the subsistence of human populations. Groundwater of the unconfined aquifer in these areas derives from snow precipitated in the Andes mountains, 200 Km to the West, that crosses irrigated oasis and reaches the area with a high salinity and arsenic content. Snow precipitation and water use for irrigation in agriculture and urban areas may be affected by future climate changes, impacting the characteristics and availability of groundwater in the desert. Large agricultural and urban areas are located seventy km upgradient and to the West, on the alluvial plains of the Central Andes.   The objective of this study is to characterize hydrogeologically the phreatic aquifer in the central Monte desert (Argentina), including the direction and velocity of the groundwater flow, its chemical composition, and suitability for human consumption and other economic uses. The study area includes 23,000 Km2, is located in an arid zone with local pastoralist communities including descendants of the indigenous Huarpes. Geomorphologically, the region has been shaped by aggradation processes influenced by the lifting of the Andes. The study area is clearly differentiated from the western Andes Mountains. The plain fillings are composed by continental Terciary and Quaternary deposits (aeolian and fluvial sediments) (Gonzáles Díaz y Fauqué, 1993), reworked by winds that developed dune systems oriented NNW-SSE hosting discontinuous valleys with elevation gradients of 10 to 30 meters.   Groundwater is used by domestic animals and in many areas is the only water source for human consumption. The aquifer is composed of fine sand sediments of aeolian and fluvial origin. Estimated values of hydraulic conductivity and porosity resulted in 5 m/day and 5%, respectively. The thickness of the unsaturated zone showed a variability between 4 m to 15 m, decreasing in wells located near footslopes of the dunes and near Desaguadero and Tunuyan rivers. The groundwater flow has a general W?E direction with estimated velocities from 0.1 to 0.25 m D-1.   The groundwater was mostly alkaline, with high salinity and arsenic content (up to 5800 mg  L-1. and 600 µg L-1, respectively), with nitrate concentrations that vary in all areas and depths. Better conditions for unconfined groundwater are found in paleochannel areas. The arsenic and salinity derive from the contact with the sediments. High nitrate concentrations (13 to 60 mg L-1) indicate organic contamination, probably from surface soils. Nitrate leached in cattle accumulation areas may reach the aquifer and domestic wells. The occurrence of local recharge in unvegetated sand dunes, as indicated by chloride mass balances (Jobbagy et al., 2010 in press), may increase groundwater velocities above those calculated from the regional groundwater flow, transporting nitrate from livestock accumulation areas to domestic wells in a relatively short time (a few years). Under the current climate and land-use conditions, nitrate contamination did not exceed current health limits in most of the sites analyzed, but increasing grazing pressures and deforestation may cause higher levels of groundwater contamination with nitrate. Low concentrations of nitrates  (0.4 to 1.6 mg  L-1) are found in wells located in paleochannels and in deep wells of the plain. This analysis suggests that agriculture and urban areas located upgradient do not have an important impact in the local groundwater under current climate and land-use conditions, although it may change in the future under different conditions.   Further studies include stable and radioactive isotope analysis to indicate the age and origin of groundwater, as well as local and remote recharge processes of the phreatic aquifer in the central Monte.