Dynamics of salt deposition and water contamination in a poorly drained region of the pampa plain at field-scale processes in urban and rural settings

Abstract

The study area is part of the South Aeolian Sandy Plain (Holocene-Pleistocene). It has a nearly flat to gently undulating relief with low topographic slopes and moderate to severely impeded drainage. The landscape consists of dissipated dune bodies and lagoons that significantly control groundwater flow dynamics and water table morphology. Elevated active dune bodies act as local recharge areas with descending flows, while low-lying areas function as intermediate and regional discharge sectors with ascending flows, showcasing hydro-halomorphic processes due to shallow water table depths and groundwater chemical composition. Groundwater baseline is dominantly of sodium chloride/sulfate type and high salinity, aligning with the regional hydrogeological framework. This lowland plain collects regional groundwater flows, where mineralization occursas a result of natural geochemical evolution during extensive and prolonged circulation. This process involves interaction with sediments and the incorporation of salts through various weathering processes, in addition to local evapo-concentration. Freshwater lenses of the sodium bicarbonate type are observed below active dune bodies. The temperate and sub-humid climate shows an annual precipitation of 869 mm, mainly concentrated in spring and summer, with cycles of dry periods leading to significant evaporation from lagoons and soils, promoting salt precipitation. Precipitated salts occur cyclically, forming calcite between 0–99% evaporation, gypsum crystals above 60% evaporation (visible to the naked eye), and halite at 99% evaporation, despite the urban or rural sectors. Urban activities significantly impact groundwater flow. In Canals City, the northern sector is shaped by recharge from on-site sanitation systems. In contrast, the southern sector shows a water table depression due to pumping for sewer system installation. Lower saline contents in the northern sector are linked to artificial recharge from an aqueduct (fresh water), whereas higher saline contents in the southern sector result from pumping and extracting more saline intermediate groundwater flows. In urban areas, organic matter from on-site sanitation systems consumes dissolved oxygen (DO) and generates bicarbonates, lowering pH, and creating a complex pattern of aerobic and anaerobic patches in the aquifer, affecting nitrate (NO3−) values and leading to a suspected denitrification. In rural areas, local contamination scenarios, such as from pens and dairies, contribute organic matter to the aquifer, leading to similar conditions as in urban areas.