Assessment of the environmental impact of an agricultural area near a coastal lagoon using geophysical and geochemical techniques

© 2019 by Nova Science Publishers, Inc.Unsustainable agricultural practices exert high pressure on soils and water, especially in those areas near to protected or vulnerable zones, causing environmental risks. The ¿Mar Menor¿ coastal lagoon is located in SE of Spain; is one of the biggest and most beautiful coastal lagoons in Europe, where tourism is one of the leading economic activities of this area. However, the intensive agriculture practices carried out during the last decades in the surroundings of the lagoon could be affecting the water and sediments quality, putting the environmental and economic values of this coastal lagoon at risk. Therefore, in order to assess the potential environmental impacts of the agricultural activity in the coastal lagoon, an agricultural area near to the coastline was selected; geochemical and tomography techniques were applied with the following objectives: i) determine the spatial and vertical distribution of nutrients and soluble salts in the agricultural area; ii) identify the different layers of soil and subsoil; iii) evaluate the relationships between the identified layers and leaching of salts and nutrients. Fifteen superficial soil samples and thirty-three subsoil samples up to 15 m deep from three mechanical probes were collected. pH, electrical conductivity, carbonates, organic carbon, total nitrogen, available phosphorus, iron, manganese, zinc and copper, texture, soluble sulphates, chlorides, nitrates, nitrites, calcium, magnesium, potassium, and sodium were analysed. In addition, six tomography profiles of 215 m long were applied. Electrical Resistivity Tomography profiles and borehole core samples revealed the presence of three well-defined layers; one formed by clay, another by gravel and sand, and the last one by sandstones and marls. The water table is relatively superficial, around 5 m deep. Nonetheless, a trait of marine water intrusion appeared as an ascending path of salt water with south to north direction. Spatial distribution analysis showed high values of pH, nitrates, nitrites, and available nutrients in the agricultural soils, whose concentrations were very heterogeneous through the agricultural area. Only concentrations of soluble calcium and potassium were higher in the centre and the southern area close to the lagoon. However, the concentrations of nitrates, sulphates, chlorides, and sodium decreased with depth, which suggested that together with the found correlation among subsoil layers and salts/nutrients concentrations, there is neither nutrients nor soluble salts leaching from the surface to the deep subsurface layers or lagoon.