Long-term water monitoring in two Mediterranean lagoons as an indicator of land-use changes and intense precipitation events (Adra, Southeastern Spain)

Abstract

During recent historical times the Adra river delta, a detrital coastal aquifer of nearly 32 km 2 located in a semi-arid, mountainous area of SE Spain, has undergone different changes caused by human activity. Within this context, both the river dynamics in the plain and the geomorphology of the coastline have at various times resulted in the formation of small lagoons. At present only two small (<0.5 km 2) lagoons exist, at the eastern edge of the aquifer, which, although closely surrounded by commercial market-garden greenhouses, are protected under international agreements. During the last 30 years of the twentieth century traditional agricultural irrigation techniques have undergone significant changes to improve their efficiency. Surface-water resources in the Adra river basin are regulated via the Beninar reservoir. In addition, the use of groundwater is increasing progressively. Both these factors affect the recharge of the coastal aquifer. To monitor these changes measurements of electrical conductivity and water level fluctuations have been recorded in these lagoons for the last 35 years (1975–2010). A comparison of the hydrochemical characteristics of the water in the lagoons and of the surrounding groundwater from 2003 to 2010 shows marked differences induced by the different hydrological dynamics in each lagoon, as well as by the hydrogeological impact of changes in land use in the delta. The increase in water demand is a consequence of the extension of irrigated areas from the fluvio-deltaic plain to its slopes, originally occupied by unirrigated crops. A reduction in irrigation return-flow is linked to the use of new irrigation techniques. These modifications affect both the recharge regime of the aquifer and its water quality. Moreover, extreme precipitation events, which are characteristic of Mediterranean semi-arid environments, can affect the lagoons’ hydrological dynamics to a considerable extent. One such example is the unusually rainy period from January to March 2010 (>600 mm). This event, along with other effects, has dramatically lowered the salinity of the water in both lagoons. This case study reveals the extreme vulnerability of deltaic environments and also how lagoons can reflect anthropogenic changes over the whole river basin.