Groundwater dynamics, land cover and salinization in the dry Chaco in Paraguay

Abstract

A two-year experimental study on saline groundwater dynamics in the Paraguayan dry Chaco is reported. This study was designed to better understand dryland salinity as a natural phenomenon and a potential consequence of land use changes, in order to avoid adverse economic and environmental impacts. A total of 28 wells at two study sites located nearby Loma Plata were monitored approximately every two weeks for groundwater conductivity and water table level, following transects from sown pasture to native forest and topographic gradients from bush or pasture to a nearby temporarily flooded lagoon. Results show a relation among topography, land cover, precipitation, groundwater salinity and surface salinization. Under pasture, groundwater was less saline in the average and the water table was slightly higher than under adjacent forest. Under forest, infiltrating rainwater never reached groundwater level, whereas under pasture, rainwater diluted the groundwater at several occasions during the observation period. Along the topographic gradients, there was a fringe prone to surface salinization along the borderline of the lagoon. At higher topographic levels, the saline ground water was low enough to not reach the surface by capillary ascension. On the other hand, high water table in the center of the lagoon fed by inflowing rainwaters (which had a slightly higher altitude level than the water table under adjacent land covers), displaced the saline groundwater vertically and horizontally. These findings provide information for land use planning in the region, which is prone to dryland salinity due to a generally high saline water table.

Introduction

Water is vital to satisfy both human needs, production and to maintain ecosystem functions. Despite this fundamental role, there is a high probability that water resources are among the most limited during this century, due, among other factors, to the continued growing World population and the increase in demand for fresh and potable water (Gottle and Sène, 1997; Heathcote, 2009; Jewitt, 2002; Oki and Kanae, 2006; Ostfeld et al., 2012). Water availability and quality are also threatened around the world, not only because of their scarcity in some regions, but also because of inadequate management, high levels of pollution and degradation of ecosystems (le Polain de Waroux et al., 2018; Ostfeld et al., 2012; Boer and Radersma, 2011; Peters and Meybeck, 2000; Rengasamy, 2006). Water pollution poses significant threat to marginal populations particularly in countries where water balance is negative or where dry conditions prevail (Marchesini et al., 2017; Nelson and Chomitz, 2006; Vorbsmarty et al., 2000).

Water quality not only depends on the management of its distribution and health in the cities, but also on the management measures taken at their sources of origin. In addition to measures which should accompany the development of cities at the urban level, it is necessary to consider that what happens at the rural level and in the catchments as well as the rainfall pattern and water infiltration do have a great influence on the availability and quality of this resource (Laino et al., 2016). From this perspective, the practices of adequate soil management and conservation are closely related to the improvement of the water supply, both in its quality and in its quantity, and justify the approach of the human activities being integrated with ecosystems and abiotic factors, particularly in the areas of water catchment, runoff, infiltration and groundwater recharge (Cotler-Ávalos 2004; Dourojeanni and Nelson, 1987; Heathcote, 2009). The hydrological cycle is the movement of water from the atmosphere to the earth and back again. Most salts are highly soluble, so water is key to the movement of salts in the landscape (Podmore, 2009). Even though soluble salts are inherent in all soils, there are many processes that can contribute to the build-up of salts in a given soil layer (Boer and Radersma, 2011). Soil salinization, defined as the accumulation above a certain level of water-soluble salts within soil layers, can lead to undesirable effects on the ground surface, limiting production and altering ecosystems functionality (Williams, 1999).

Soil salinity is described and characterized in terms of the concentration and composition of soluble salts. Evaporation, agricultural activities, historical depositions, fresh water extraction, soil mineral weathering and pollution can lead to salinization of soil and groundwater (Boer and Radersma, 2011). These factors combined with climatic, hydrological, geological, as well as groundwater depth and salinity, vegetation cover and human influences determine where salinization occurs (Williams, 1999). Salinization commonly causes damages when saline groundwater reaches the soil surface by capillary ascension (Podmore, 2009). Dryland salinity is strongly dependent on saline groundwater dynamics along landscape profiles (Williams and Chartres, 1991), and land use cover has shown to have a direct impact (although not necessarily the predominant one) on the depth of the water table (Peck and Williamso, 1987; Smerdon et al., 2009) thus land use change could eventually modify vulnerability to salinization. All soil types can be affected by salinization (Marchesini et al., 2017; Rengasamy, 2006).

Salinization is a phenomenon that can have an important impact on ecology and productivity (Williams, 1999). Ecohydrological transformation in the dry Chaco and the risk of dryland salinity has been studied and warned of by few studies (Marchesini et al., 2017; Magliano et al., 2016), however, these studies focus on the Argentinean dry Chaco and not the Paraguayan. While much attention has been paid to coastal areas (Cardona et al., 2004; Carol et al., 2009; Giménez and Morell, 1997) and to some of the world's most populated areas prone to salinization such as in some regions of Australia (Gordon et al., 2003; Ruprecht and Schofield, 1991) and China (Wang et al., 2008; Wu et al., 2014), the semi-arid Chaco, especially in its Paraguayan portion still remains understudied or studied but unpublished. Even though some studies have focused on the benefits of land use planning to maintain connectivity (Torrella et al., 2018) or others have analysed bird conservation (Macchi et al., 2016) in agricultural landscapes in the dry Chaco, few have focused on studying salinization as a land use planning tool. Understanding saline groundwater and salinization dynamics and the effects of land use and land cover is crucial to prevent damage and decide on proper agricultural and livestock activities allowing for sustainable production. The challenge to find grazing systems that are sustainable is shown to lie in recognizing that ecosystems must be studied in an integrated way. Not focusing only on short term animal productivity without considering all other ecosystem essential components (Williams and Chartres, 1991). This study aims to provide insights of the relation among water fluxes, soil cover and salinization in areas dedicated to livestock production in the Paraguayan dry Chaco.