Groundwater composition and pollution due to agricultural practices at Sete Cidades volcano (Azores, Portugal)

Groundwater is a strategic resource of the Azores archipelago given that agriculture is a major economic activity. A field study was undertaken at Sete Cidades volcano (Sao Miguel, Portugal) to characterize the composition of soil water at several depths in two sites: one without anthropogenic pressure (village site) and the other pasture land (Pa I site), and groundwater in the saturated zone of both sites. The composition of groundwater from springs discharging on the volcano flanks and inside the summit caldera is similar, composed principally of poorly mineralized Na-HCO3 and Na-HCO3-Cl water types, as suggested by median electrical conductivity values. Samples of groundwater collected in seven piezometers spread inside the Sete Cidades caldera are characterized by a conductivity between 95 and 232 mu S/cm and the dominant water type is Na-HCO3. Soil water at the Sete Cidades village site is of Na-Cl type and its compositional similiarity to rain water suggests control by evapotranspiration. The limited soil depth of the site, as well as the high precipitation and soil hydraulic conductivity, cause vertical homogenization of soil water composition. In contrast, soil water sampled at the pasture site (Pa1) shows greater mineralization when compared to the previous site, and waters are mainly HCO3 to HCO3-Cl types with a Mg-Ca trend for cations. A trend was indicated for the relative soil water composition at site Pa I, from the more superficial suction cup (G1) to the deeper G14-15. This is an evolution similar to perched-water bodies inside the Sete Cidades Caldera. The Mg-Ca-dominated composition at lower depth is explained by the application of fertilizers. Nevertheless, other processes are also influencing water chemistry evolution, and the observed relative decreases in Ca, Mg and K with depth may result from clay formation and the uptake by ion exchange of Ca2+ and Mg2+ for Na+. Although, the relative increase in Na could also result from silicate weathering. Besides contributing to the Na content, silicate dissolution may also explain the relative increase in HCO3, both being associated with silica in solution. The impact of the excessive application of fertilizers is reflected by the NO3 contents of soil water at Pa I and the village sites, as values are higher where agriculture is developed, with a similar trend being observed in the saturated zone. (c) 2012 Elsevier Ltd. All rights reserved.