Salt and nitrate concentrations in the surface waters of the CR-V irrigation district (Bardenas I, Spain): diagnosis and prescriptions for reducing off-site contamination

The return flows from irrigated agriculture may increase the salt and nitrate concentrations of the receiving water bodies, limiting their agricultural, industrial, urban and ecological uses. The objectives of this study are (i) to analyze the sources and levels of salt and nitrate concentrations in the surface waters of the irrigation district n° V of Bardenas I (CR-V), and (ii) to prescribe management practices aimed at protecting the quality of water resources. The electrical conductivity (EC) and nitrate concentration ([NO3−]) were measured in water samples collected in 28 drainage ditches of CR-V and in eight points along the Riguel River (the main drainage outlet for CR-V) in 14 dates of the 1999–2000 hydrological year. Drainage waters were moderate in salts (year-average EC=0.87 dS/m) and high in nitrates (year-average [NO3−]=55 mg/l), and both of them increased during the non-irrigation season. The lowest EC and [NO3−] in the Riguel River were measured at the entrance of CR-V and during the April–September irrigation season (season-average=0.45 dS/m and 2 mg/l, respectively), and attained the highest values at the river outlet (end of CR-V) and during the October–March non-irrigation season (season-average=1.55 dS/m and 50 mg/l, respectively). Salt loadings at the river outlet were correlated with river flows (P<0.001), but nitrate loadings were independent of flows (P>0.05) due to the higher nitrate variability in drainage waters. The unitary annual upper limit load emissions from CR-V were 7.2 t total dissolved salts/ha and 59 kg NO3−-N/ha. The optimization of nitrogen fertilization, the improvement of irrigation efficiency and the internal reuse for irrigation of the low EC-high NO3− drainage waters are the key management strategies for decreasing salt and nitrogen load emissions from CR-V and protecting the quality of the receiving water bodies.