Effects of woodland land use conversion to pastoralist settlements on hydrological processes and biogeochemistry in the Monte desert

VILLAGRA, PABLO EUGENIO; JOBBÁGY, ESTEBAN GABRIEL; ARANIBAR, JULIETA NELIDA; MEGLIOLI, PABLO ANDRÉS; MAGLIANO, PATRICIO NICOLÁS

San Luis

Workshop; I Regional Workshop on Isotope Ecohydrology; 2020

Grupo de Estudios Ambientales, IMASL (CONICET ? UNSL)

In the Central Monte Desert (central-west Argentina), Prosopis flexuosa woodlands offer the opportunity to study the interactions between phreatophyte vegetation, groundwater and local precipitation, considering different intensities of land use given by pastoralist settlements. In this arid ecosystem, we explore the consequences of pastoralist settlements in interdune lowlands on ecohydrological processes related to the recharge and discharge of the aquifer. We propose that vegetation changes associated with the pastoralist settlements allow a greater drainage of rainwater and, as a consequence, a higher local groundwater recharge and solutes leaching to the aquifer. The objectives were to compare the fluctuations of water table and the chemical and isotopic compositions of the aquifer, in disturbed (livestock stations) and undisturbed sites (control woodlands). In the Telteca Reserve, boreholes or piezometers were completed in interdune valleys, three in each of two land uses. Each borehole drilled, extending 1 m below the water table level, was used to measure water levels (manually and by pressure transducers) and to collect groundwater samples, every 3-4 months, from November 2011 to June 2017. In laboratory, we measured pH, electric conductivity, chlorides and nitrates, using conventional methods. Oxygen and hydrogen isotopic analyses were performed (GEA-IMASL, CONICET, San Luis) on groundwater samples collected during the 2013-2016 rainy seasons. Temporal variation in water table height showed seasonality in both land uses, being lower during summer and higher during autumn and winter. Intense rainfalls produce a relative recovery of groundwater levels, suggesting partial decoupling of phreatophytes with the aquifer during the summer. Isotopic composition of groundwater in both land uses was close to the signature of Mendoza river water and distinct from local rainfalls. However, groundwater in the livestock stations was slightly enriched in heavier isotopes (2H and 18O) compared with control woodlands. The removal of vegetation for the establishment of the livestock stations did not produce changes of hydrological relevance. Land use changes, however, had implications on the chemistry of groundwater, determining that the solutes and nutrients in the soil profile leaching into the aquifer, altering the quality of this water reservoir.