Groundwater Baseline Composition and Geochemical Controls in the Doñana Aquifer System, SW Spain

Abstract

The geochemical controls of groundwater baseline chemistry of the Doñana aquifer system (DAS -SW Spain) have been characterised using major and minor inorganic components and environmental isotopes. In unconfi ned areas, groundwater baseline is controlled mainly by rainwater composition, which is of Na-Cl type, equilibrium with silica and dissolution of soil CO 2 , Naand K-feldspars, and CaCO 3 where present. The resulting mineralisation ranges from very low to moderate. Groundwater baseline changes from the unconfi ned areas to the confi ned sector mostly by mixing with old marine water, Na/Ca-(Mg) cation exchange, sulphate reduction and calcite dissolution/ precipitation. The resulting salinity ranges from 1 mS cm -1 up to 80 mS cm -1 .In the shallower layers (<40 m) of the unconfi ned areas, groundwater baseline has been modifi ed by different human activities, as shown by the presence of agrochemicals (nutrients, metals, pesticides) and of industryderived airborne pollutants (mainly metals coming from the nearby Huelva industrial site and from open-pit mining sulphide exploitation). This is supported by tritium based ages, which show residence times greater than 40 years for fl ow lines deeper than 35-40 m.Groundwater composition in the confi ned areas is mostly naturally derived (baseline), as supported by 14 C ages ranging from 1 to more than 15 kyr. Agriculture-derived pollutants are present in groundwater below irrigation areas exploiting groundwater confi ned layers close to the northern boundary of the aquifer confi ned under the marshes, as well as the unconfi ned aquifer.

Monitoring and current research of groundwater resources and recharge in the Doñana Natural area (SE Spain)

“…Each piezometer has a 2 m screen in its base. Data from these new observation boreholes provided good understanding of vertical gradients, both in the recharge and in discharge zones of the aquifer, and also about the natural composition of the groundwater, the processes controlling it, the transit time of groundwater, the existence and extent of agricultural pollution and the relationship between the different wetland types and the aquifer (Iglesias 1999;Trick and Custodio, 2004;Lozano 2004;Lozano et al 2005;Manzano et al 2005Manzano et al , 2007Manzano et al , 2008Manzano et al , 2009Manzano et al , 2011Manzano et al , 2013Higueras 2014).…”

“…The composition of the Na-Cl waters is the result of slightly evaporated rainwater infiltrating and dissolving soil CO2 and Na/K feldspars, accompanied by cation exchange. The other chemical facies are the result of Na-Cl water type dissolving CaCO3 in the deepest layers of the Eolian Unit (Iglesias 1999;Lozano et al 2005;Manzano et al 2008).…”

“…Figure 15A shows that ammonia fertilizers seem to be a main source of NO3 in the shallow groundwater of the Mimbrales-La Vera sector in the Eolian Mantle. Ammonia fertilizers also contribute to the atmospheric source or recharge, represented by samples L2 and Monitoring and current research of groundwater resources and... Boletín Geológico y Minero, 130 (4): 661-690 Manzano et al (2005Manzano et al ( , 2008. Figura 12.…”

The origin of groundwater composition in the Pampeano Aquifer underlying the Del Azul Creek basin, Argentina

“…The study of the origin of the chemical composition of groundwater has many applications, but the most immediate is to use the knowledge gained to set sustainable groundwater management at local, regional and basin scales (Nieto et al, 2005;Manzano et al, 2003Manzano et al, , 2008Shand et al, 2007;Edmunds and Shand, 2008;Custodio and Manzano, 2008;Wendland et al, 2008).…”

“…The study of groundwater baseline composition has been performed in many aquifers around the world mostly due to national or international requirements derived from the regulations operating in each country (Edmunds and Shand, 2008;Manzano et al, 2003Manzano et al, , 2008Wilcox et al, 2005;Wendland et al, 2008;Daughney et al, 2009;Morgenstern and Daughney, 2012;Krishan et al, 2014). Although Argentina does not have a water law that requires the identification of groundwater baseline, some studies focused on groundwater hydrogeochemistry have been performed in the last decade.…”