Beatriz N. Losinno

Electrical conductivity and depth of groundwater at the Pergamino zone (Buenos Aires Province, Argentina) through vertical electrical soundings and geostatistical analysis

Abstract In the humid Pampean region of Argentina, a rich agricultural zone, the periodic occurrence of droughts of different intensity is one of the most important factors in the variability of crop yield. Because complementary irrigation is a highly efficient resource to increase such yields, an understanding of groundwater resources is important. This knowledge is limited in topographically smooth zones by the absence of outcroppings and observation boreholes. Water conductivity is another limitation factor if the goal is to avoid soil degradation by irrigation and negative effects for animal and human consumption. The aquifers of the northeastern zone of the Buenos Aires province have been studied regionally, but information at the local scale is limited to sparse boreholes. In this work, a survey using vertical electrical soundings was carried out to determine the depth, thickness, and continuity of shallower aquifers. Both a mapping of the water table and the electrical conductivity distribution of free aquifers were achieved from well data and geophysical results using geostatistical techniques. Recharge areas of the aquifer were recognized as those areas with low conductivity and topographic highs. The discharge areas, mainly at the bed of the Pergamino River, have higher values of conductivity; two zones north and south of the city of Pergamino have conductivities greater than 2000 μS cm−1. Isolines of depth to the fresh-salty water interface showed different values over the Pergamino River, with some local maxima at the swamp zone and near Pergamino.

Modeling of Electromagnetic Soundings to Detect Groundwater Contamination by Intensive Livestock Farming

The feedlots, farms for feeding animals in corrals, are spreading at Buenos Aires Province, Argentina. It is important to evaluate the contamination by mineralization of organic matter. Geophysical exploratory methods give an important tool for monitoring groundwater and soil quality. The objective of this work was to perform electromagnetic soundings and to do a modeling of these data by an inversion procedure for different frequencies at a corral of a feedlot and the surroundings. The survey was repeated the following year. Bellow the corral, the unsaturated zone has lower resistivity (between 8 and 17 ohm m, more conductive) than the test zone. The second year, the test zone showed greater range of values for resistivity (up to 54 ohm m) but the layers bellow the corral showed an important decrease of resistivity. Groundwater samples were taken along four years. An increase of water conductivity was found at the lower well along the time. The EMI modelling allowed characterizing the distribution of resistivity bellow the corral and the background, reaching the groundwater, detecting an increase of salinity bellow the corral which may be mainly due to leaching of nitrates.

GEOFÍSICA APLICADA A LA EVALUACIÓN DE LA VULNERABILIDAD A LA CONTAMINACIÓN EN ZONAS RURALES

Agricultural and cattle activities are considered to be the source of high risk of contamination of groundwater. The contamination is the result of the interaction between the contaminant charge and the aquifer vulnerability. The aim of this work was to evaluate the vulnerability of the Pampeano aquifer at some rural zones of the highly productive area of Pergamino-Arrecifes river basin. The traditional method of indexation was used taking into account the principal factors which influence: the depth of free groundwater level, the degree of confinement of the aquifer and the lithology of non saturated zone. The first one was obtained through geoelectrical prospecting, due to the lack of well information at some places. The last factors were estimated based on hidrogeological features known from some wells. The influence of the edaphic zone (soil) was incorporated as a new index by means of texture properties of soils sampled at each site and soil maps. Even if vulnerability was moderate for all the studied sites, some places along both margins of the Pergamino river had higher indexes than the surroundings of Arrecifes town. The small urban centers had greater risk of contamination of groundwater due to their intrinsic vulnerability (higher indexes) and the potential contaminant charge that intensive agricultural and cattle activities, which are developed around these sites, may generate. Higher values of nitrate concentrations were found at these places with a sharp increasing tendency during the last years, indicating the need of stronger control over the effect of these activities on groundwater contamination.