Geochemical processes regulating F-, as and NO3- content in the groundwater of a sector of the Pampean Region, Argentina

Abstract

The presence of F- and As in groundwater is common in volcanic aquifers. Excessive concentrations of these ions affect the quality of drinking water and can be harmful to health. When there is an anthropogenic source in phreatic aquifers, NO3- is incorporated to the groundwater components, deteriorating its quality. The objective of this work is to assess the geochemical processes that regulate the contents of F-, As and NO3- of the groundwater in a sector of the Pampean Region in Argentina. This area is supplied with water by exploiting a multilayer aquifer, composed of a phreatic aquifer occurring in loess sediments and a fluvial semi-confined aquifer, separated by an aquitard. The results obtained show that the phreatic aquifer has a higher concentration of F-, As and NO3- than the semi-confined aquifer. Fluoride derives from the dissolution of volcanic glass at a slightly alkaline pH and from anion exchange; however, it may also be absorbed by the reprecipitating carbonates. The As is released by desorption, with the main source being the glass and lithic fragments of the loess. The NO3- originates from the decomposition of organic matter, mainly in the septic tanks of the peri-urban areas. Meanwhile, the As and F- content in the semi-confined aquifer is lower and its origin is the result of water inflow by vertical downward infiltration from the phreatic aquifer through the aquitard. The Pampean Region is one of the areas with the largest volume of agricultural exports in the world and at present it is undergoing a strong social and economic growth. Understanding the geochemical processes that regulate the quality of drinking water is of vital importance to generate water management guidelines aiming at minimizing the deterioration of drinking water sources.