Effect of wetland transformation on groundwater quality and soil salinization in neighbouring agricultural areas

Wetlands, and specially salt marshes, represent the final stage of the filling of depressions, embayments, levelling of marine delta plains and other depositional environments. Due to their origin, they are porous media, with high permeability, permanently flooded, or with very shallow oscillating groundwater table. They are rich in wildlife and among the most primary producers environments in the Globe. Hydrological connections exist between the wetlands and the groundwater, and hydrological connections exist between wetland and seawater, in coastal areas. The management of groundwater in coastal sedimentary areas is particularly difficult, because overexploitation of freshwater can lead to marine intrusion. Several activities have acted concurrently in a semiarid area of the West Mediterranean Region, in Spain, degrading the wetland, and promoting salinization of groundwater and soils. Peat mine activity has voided of soil a large coastal wetland extension, and promoted accelerated evaporation from free water surface, decreasing the pore space to be followed by intruding seawater. Over-pumping of groundwater for irrigation has decreased the hydraulic gradient between inland freshwater and seawater, which in turn intrudes the porous wetlands aquifer. This water is used for irrigation and, hence the risk of salinization propagates to neighbouring agricultural areas, and is already a fact in some parts of the study area. Also farmers have tried to cultivate in the marginal areas of the wetland, getting poor results. Subsidence, quick oxidation of organic matter and oxidation of sulphidic sediments, change the initial conditions to very adverse. The periodical analysis of water from wells, show a progressive worsening of the quality with time and the hydro-chemical indexes show clearly the phenomenon of blending of freshwater and seawater. Also, the oscillation of quality during natural recharge/artificial discharge indicates some complex connections between karstic and sedimentary aquifers. In this paper the general connections between the wetland and the agricultural system of this Spanish study case is presented, highlighting the soils and water degradation processes, their causes and extent.