Groundwater Quality Risk due to Conventional Irrigated Agriculture in the “Apulian Tavoliere”

High-value and intensively managed crops usually take advantage of large amounts of irrigation water and nitrogen fertilisers, therefore greatly contributing to the risk of groundwater contamination due to drainage water and nitrate leaching. An real accounting of this risk is a priority knowledge at the field as well as at the land scale, in order to start up effective agro-environmental measures to control or mitigate those impacts. The problem is much more difficult to manage in case of brackish irrigation water, due to saline groundwater and sea-water intrusion in coastal areas. The leaching technique is the only way to avoid soil salification but, at the same time, it produces impacting drainage with water of very critical quality. The overall conventional farming management is consequently pressed for an answer, in line with the fundamental criteria of (environmental and economical) sustainability. This study was carried out in order to assess the main composition and quality of the drainage water resulting from the current cultivation practice (irrigation and fertilization) performed by an ordinary farmer with respect to an area of the Apulian Tavoliere, close to the coast of the Manfredonia gulf (Adriatic sea). Particularly, salt load and nitrate were considered, periodically measuring the electrical conductivity (EC), the sodium adsorption ratio (S.A.R.) and the N-nitrate concentration of the drainage water intercepted from the cultivated field. A permanent experimental field-unit was established in autumn 2006; three plots of 100 m2 each (6.4 x 15.6 m) were delimited; at the center of each plot an artificial draining basin was arranged digging the soil out of a trench, 3.2 m wide, along the entire plot length; the bottom of each trench was covered with a plastic sheet in order to prevent water percolation; a set of drains (two groups per trench, three drains per group) were displaced over the plastic cover to collect the percolating water and conveying it into thanks placed at the edge of each plot (two thanks per plot). The trenches were then filled with the same soil obtained by the excavating procedure, trying to correctly reproduce the original soil stratification. Two subsequent crop cycles were considered, from spring to winter 2007: tomato and spinach, respectively. Tomato was transplanted on 20 April and harvested on 26 July; spinach was sowed on 18 October and harvested on 27 December. While tomato was actively irrigated (540 mm plus 117 mm of rain), autumn rainfalls (148 mm) completely satisfied the spinach water requirements and no irrigations were needed. A flow-weighted averaging of nitrate concentration over multiple drainage water samplings allowed to calculate an higher overall loss of nitrogen during spinach cultivation, as a consequence of an higher fertilization rate. An increasing nitrate concentration was registered in the drainage water from the start to the end of the experimental period. The drainage S.A.R. values were always significant lower than those of the irrigation water. This first investigation confirmed the very worrying and dangerous conditions of the local agricultural practices and the need of drastic and quick technical adjustments.