Environmental impact of salinity by diffuse contamination on the water quality of the Arba River

Degradation of water quality of surface waters in the Ebro river basin constitutes an important environmental concern because of the increasing salinity trends observed in this region. A significant contribution to the salt load transported by the rivers is supplied by non-point sources from irrigation return flows. The Bardenas I irrigated area occupies 30000 ha in the semiarid central part of the Ebro valley. Return flows from this area have been monitored during the last decade. Since 1990, a total of 16 sampling points on return irrigation canals and another 6 sites along the Arba river, the natural drainage system of this area, have been sampled on a weekly/fortnightly basis. A high spatial variability in salinity levels has been found in the area, thus electrical conductivity varies from 0.3 to 10 dSm-1 depending on the irrigation period and on the soil types drained. The electrical conductivity in the Arba river increases from 0.3 to 4.7 dSm-1 from the inlet to the outlet of the Bardenas area. Therefore, the salt load supplied from the irrigation return flows amounts more than 50% of the total load transported by the Arba river. This river presents the highest salinity levels in the Ebro basin. Poor water quality due to high salinity determines restricted use for several purposes. In order to reduce this environmental impact, it is necessary to establish adequate management practices involving design of more water efficient uses to avoid further water quality deterioration.