Temporal variation of arsenic and nitrate content in groundwater of the Duero River Basin (Spain)

• Temporal variation of arsenic and nitrate content in groundwater.
• Mechanism of As mobilization in groundwater.
• Influence of geochemical reactions on As removal in groundwater.
• Correlation between arsenic and bicarbonate concentration in groundwater.
• Bicarbonate ions can displace arsenic from surface of minerals and sediments.

High concentrations of arsenic (As) in groundwater of the Duero River Basin have created a public health concern in some provinces of Spain. However, the mechanism of As mobilization and influence of surface water–groundwater interaction, nutrients and different geochemical reactions on As removal have not yet been clearly reported. Therefore, this study investigated the possible mechanisms of As release, and temporal variations of As with respect to nitrate content in groundwater. Hydrogeochemical characteristics of groundwater sampled along three years in a region of central Spain showed high As contents exceeding EU guideline value of 10 μg/l. Significant positive correlations were found between As and bicarbonate concentrations in water samples. These results suggest a possible mechanism of As mobilization from sediments to groundwater as follows: bicarbonate ions can displace HAsO42- adsorbed on surface of aquifer Fe oxyhydroxide, other minerals and sediments. In addition, the high pH values of the groundwater might also favor the As desorption processes. The results showed that As concentrations in water samples decreased along with the increase in nitrate concentrations across the whole period of study (2001, 2003 and 2007). The negative and significant correlations between As and nitrate contents in water reflect the temporal variation of As concentration due to the effect of surface water–groundwater interaction and nutrient input into groundwater. The increased use of nitrogen fertilizers and pig manure in agriculture practices increased the nitrate content in groundwater during the period of this study, which could have favored the precipitation of Fe oxyhydroxides and As adsorption.