Impact of the shift from groundwater to surface water irrigation on aquifer dynamics and hydrochemistry in a semi-arid region in the south of Portugal

In semi-arid regions, the shift from locally extracted groundwater to regionally supplied surface water for irrigation can have a large impact on the aquifer dynamics and hydrochemistry, as is shown for an area of intensive citrus culture in the south of Portugal. Studying the changes that occur is important, particularly in the light of European and national policies that seek to preserve the quality of groundwater and the involved ecosystems. In September 2005, the application of Nitrates Directive 91/676/CEE to the study area led to its designation as a vulnerable zone and the obligation of implementing measures that reduce the risk of nitrate leaching to groundwater. In addition, the area is bordered to the south by the internationally recognized ecosystem of the Ria Formosa lagoon, which requires a careful management of water quantity and quality. This article shows that the substitution of the irrigation source indirectly caused the reduction of the nitrogen load on soil and groundwater. The study is based on a large number of groundwater head and quality data gathered over the past decade. It is observed that irrigation with surface water triggered freshening of the upper aquifer, inverting a long-existing trend of increasing groundwater salinities caused by irrigation with local groundwater. The occurrence of both phenomena in the same aquifer is rather unique and is simulated with the hydrogeochemical software PHREEQC. In terms of aquifer dynamics, a sharp rise of the water table is observed, together with an almost complete attenuation of its interseasonal oscillations. A further rise, which is to be expected in years with average or higher rainfall, could cause the water table to enter the root zone of the citrus trees in some areas, with potentially negative consequences. Therefore, an integrated mixed-source irrigation system using both surface and groundwater is highly recommended.