RESUMEN
In several countries, hail is considered as the most harmful climatic phenomenon from an agricultural perspective. The surroundings of the Gallocanta Lake (North-East Spain), is one of the areas where the storms affecting the Ebro Valley are formed. For this reason, silver iodide from hail suppression systems has been emitted to the atmosphere for half a century. Nowadays, there is an increasing social concern about the potential environmental consequences of this activity, which has promoted the analysis of the influence of hail suppression systems regarding the amount of silver concentration in the ecosystem. This study focuses on silver atmospheric deposition and its accumulation in soils. To this end, silver concentrations in rainfall (5 gauges, 16 samples per site, from April 2017 to March 2019) and soils (72 samples) distributed across the hail suppression network managed by the Anti-hail Consortium of Aragón, were analysed. The results show that the amount of silver is much higher in rainfall gauges and soils close to ground-based silver iodide generators (85 µg/m2·day and 10 mg/kg soil, respectively), but concentrations considerably decrease when samples are collected far from them (downing to 0.3 µg/m2·day and 0.1 mg/kg soil). Apart from the samples obtained nearby silver iodide generators, most of the other soil samples display silver concentrations below the legal threshold established for the most vulnerable activities (1 mg/kg soil in agricultural and forestry land uses). Nevertheless, silver content, both in precipitation and in soils, is higher in regions where hail suppression has been developed for decades when compared to nearby areas in which silver iodide emissions did not occur. Silver content observed in soils is not high, but their cumulative effect in sediments and biota should be analysed, which is the aim of the second part of the present study.
RESUMEN
The silver content in soils in the area in the surroundings of the Gallocanta Lake (Aragón, NE Spain) is lower than expected considering the estimated silver emission during the last 50 years by hail suppression systems. To understand the silver accumulation processes, selected water (4 surface, 3 groundwater), biota and sediment (6 surface sediments and 3 cores from Gallocanta and Campillo de Dueñas Lakes and Used Reservoir) samples have been analysed. An essay comparing the growth of wheat in pots with different silver iodide concentrations has been carried out. Finally, silver content in 7 tissues from two sheep that graze in areas both with and without hail suppression systems during 6-8 years were analysed. Our results show that after 50 years of silver iodide emissions to the atmosphere, silver accumulation in the waters and sediments of the lowlands, including some wetlands of high ecological value, has not been significant. Sediment cores did not show any peaks associated to silver emission. We propose that constant absorption of silver by plants could explain the low concentration in water and sediments. Crops and grass could accumulate the excess silver, as both wheat and sheep are able to absorb significant amounts of silver. These bio - accumulation processes could have helped to avoid a progressive environmental deterioration of the surroundings of the Gallocanta Lake. However, this hypothesis should be corroborated and quantified by further research on the analysis of natural and agricultural areas under the influence of hail suppression systems.
