Hydrodynamic and hydrogeochemical evaluation of groundwater and linkage with herbicide pollution: Central Argentina

Abstract

Surface water and unconfined and semiconfined (SAC B) aquifers of Marcos Juárez plain, in the central Pampa of Argentina, were studied to assess hydrogeological features and the presence and distribution of herbicides in water. The studied area, located near a regional hydrological discharge area shows streams and channels of high salinity fed by the groundwater with low values of herbicides as a consequence of hydrodynamic dispersion. The unconfined and semiconfined aquifers (SAC B) are impacted by the presence of herbicides although the percentage of positive samples is low. The unconfined aquifer, directly connected to the atmospheric phase, has a shallow water table being more likely to be reached by effective water infiltration. Despite their high Koc and low GUS index, glyphosate and AMPA were present in the highest concentrations in the unconfined aquifer. It is assumed that this situation is the result of its widespread use, its high solubility in water, and its high persistence. The chemical characteristics of metolachlor and imazethapyr, such as their relatively low Koc, moderate and high GUS index, and high persistence have allowed also their leaching into groundwater. Herbicide migration from the unconfined aquifer to the semiconfined aquifer layer in the loessical plain can occur due to the differences in their hydraulic potential. This situation facilitates the mixing of water, a process that is interpreted through fractures or cracks in the confining units (aquitards) or due to the presence of broken old pipes. The multivariate statistical analysis of the unconfined groundwater chemistry leads us to the conclusion that approximately 53% of the variance is explained by different levels of salinization resulting from natural geochemical processes (such as cation exchange, dissolution of salts, etc.), while the remaining 47% is attributed to contamination processes (arrival of herbicides and nitrates). The second and third positions of herbicides in the PCA analysis demonstrate the impact of agricultural activity, especially in the unconfined aquifer.