Geochemical modeling of systems affected by irrigation: The case of Lerma basin (Spain, 2004-2020)

Geologically "saline" zones with scarce pluviometry, which are already susceptible to the salinization of natural drainage, can experience the acceleration of the salinization of the receiving water systems with the implementation of irrigation. The main objective of this paper is to analyze the geochemical processes that control the variations of the hydrosaline balance due to the implementation of irrigation of the Lerma basin (Spain) from the beginning of its transformation into irrigation land (2004) until the consolidation of irrigation (2020). The results of this study evidence the dissolution of some mineral phases, such as halite, gypsum, and dolomite, and the precipitation of others, such as calcite. Additionally, the final composition of the irrigation return flows cannot be explained without consideration of the NaCa exchange. Part of the dissolved Ca²⁺ is deposited in the soil, which, in turn, contributes with Na⁺ to the solution. These natural processes are accelerated with irrigation but progressively slow down as the soil salts are washed with time. Although less evident, there is an additional negative agroenvironmental effect associated with the precipitation of calcite and the possible formation of petrocalcic horizons in the soil. The results obtained herein indicate that studies focusing on the salinity of irrigated zones should go a step further and include the geochemical processes in quantifying the global mass of exported salts.