Coupled soil water balance and isotopic approach to determine spatial and temporal distribution of groundwater recharge

Lobina F;Da Pelo S.;Coppola A.;Vacca A.;Arras C.;Piscedda A. F. &;Porru M. C.

2023-01-01

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Abstract

The expansion of human activities such as agriculture has led to an over-exploitation and depletion of groundwater resources coupled with diffuse contamination. The spreading of pesticides and fertilizers has caused the release of varying amounts of chemical pollutants into the soil, which the leaching action of rainwater then carries into the water table. In these contexts, it is essential to quantify the aquifer’s active recharge and assess the timescale at which this recharge occurs. When applying groundwater recharge methods, evaluation of potential amounts that can reach the water table is performed, but how long the water takes to reach it is usually neglected. The Arborea Plain (Italy) has been designated as a Nitrate Vulnerable Zone since 2005, but despite the restrictive regulations imposed by the Nitrates Directive, the latest reports published by the agencies proposed for monitoring did not show any improvement in groundwater quality, but rather it was classified as being a low quantitative status. In this study, the Soil Water Balance (SWB) code and the isotope soil pore water profile method were employed to evaluate both the amount of groundwater recharge in the Arborea NVZ and the time lag between infiltration and water table attainment. The SWB code utilizes a modified version of the Thornthwaite-Mather soil-water balance approach at a daily time step and incorporates spatially distributed soil, meteorological, and land cover data (Westenbroek et al., 2010). Soil pore water stable isotope analysis is based on the peak-shift method, according to which on the assumption that seasonal effects on the isotopic composition of precipitation are traced through the soil vertical isotopic profile. An experimental approach combining hydrogeology, satellite remote sensing and soil analysis was used to estimate the data required for the application of the two methods by combining on-site, laboratory and remote data. In this study, the application of the SWB model and the isotope approach proved to be useful and simplified tools in estimating the net water infiltration. An accurate estimation of spatial and temporal distribution of groundwater recharges contributes to better land management in terms of optimizing agricultural practices with the purpose of performing specific interventions that can reduce groundwater pollution.