Spatial content and variability of mercury in agricultural soils in the province of Valencia (Spain), with an emphasis on those dedicated to rice crop

Mercury (Hg) is a metal potentially dangerous that can accumulate in soils, move to plants and cause significant ecotoxicological risks. The province of Valencia is the third in Spain and has a great agricultural, industrial and tourist vocation; it has an area of 10,763 km², of which it devotes 272,978 ha to cultivation, most of which are irrigated soils. To the south of the city of Valencia, is the Albufera Natural Park (ZEPA area and Ramsar wetland) with 14,806 ha dedicated to rice cultivation. Pollution and burning of rice straw in rice paddies are serious problems. Therefore, the concentration of Hg in agricultural soils in the province of Valencia according to use, with an emphasis on rice paddy soils, and spatial distribution were determined; and the effects of rice straw burning on Hg accumulation on rice paddy soils was assessed. Systematic sampling was carried out throughout the agricultural area at an intensity of a grid of 8 x 8 km, in which samples composed of soil between 0 and 20 cm were collected in a total of 98 plots; and a simple random sampling in the case of rice paddies in 35 sites, distinguishing between plots where the incineration of rice straw was carried out and where it was not. The concentration of Hg was determined with a direct DMA-80 Milestone analyzer in the previously pulverized sample. The detection limit was 1.0 g kg^-1, the recovery was 95.1% to 101.0% ± 4.0%. The analyses were performed in triplicate. A basic descriptive statistic (means, medians, deviations, and ANOVA) was performed. Samples were grouped according to land use. For geostatistic analysis and in order to obtain the map of the spatial distribution of the concentration of Hg in soils, the classical geostatistic technique was used by ordinary kriging. The concentration of Hg in the soils of the province of Valencia showed great variability. The soils of the rice paddies together with those dedicated to the cultivation of citrus and horticultural of the coastal plain, showed the highest levels of Hg, in contrast to the soils of the interior areas dedicated to dry crops (vineyards, olive, almond and fodder). Spatial analysis reflected a concentration gradient from west to east, suggesting that the Hg in the soils of the interior has a geochemical origin, while in the coast soils it is of anthropic origin. On the other hand, it was observed that the burning of rice straw increased the Hg concentration in rice paddy soils. This research is the first information on the distribution of Hg in the soils of the province of Valencia and a contribution that can help weigh the effects of open burning of rice straw on Valencian rice paddies.