What is the source of geogenic (natural or native) solutes in groundwater? The orthodox explanation suggests it is largely a function of water–rock interaction (weathering of the soil zone and aquifer mineral framework). It is proposed herein that atmospheric deposition (combination of wet and dry aerosols from ocean spray, smoke, volcanoes, continental dust, and lightning) is a significant source, and in many cases the dominant source, of the major and minor geogenic solutes in groundwater. Solute mass-balance analyses suggest that much of the mass of major and minor ions must be transported into the aquifer from an external source. Example case studies are presented: analysis of groundwater in a coastal marine aquifer located in an arid area (United Arab Emirates) suggests that over 50% of several major ions potentially originate from atmospheric deposition; in an alluvial fan in a semi-arid system (High Plains, USA), 100% of most solutes potentially originate from atmospheric deposition; and in a humid glacial aquifer system (Michigan, USA), 20–30% of many major ions are potentially from atmospheric deposition. These observations contrast with many hydrogeologic textbooks, which still propose the origin to be water–rock interaction—hence, the myth.

Arsenic occurrence in groundwater near the Cimino-Vico volcanoes (central Italy) was analysed considering the hydrostratigraphy and structural setting and the shallow and deep flows interacting within the Quaternary volcanics. Groundwater is the local source of drinking water. As documented in the past, arsenic in the groundwater has become a problem, and the European maximum allowable contaminant level was recently lowered to 10 μg/L. Chemical analyses of groundwater were conducted, sampled over an area of about 900 km², from 65 wells and springs representative of the volcanic aquifer and thermal waters. Considering the type of aquifer, the nature of the aquifer formation and its substratum, the hydrochemical data highlight that the arsenic content of the groundwater is mainly connected with the hydrothermal processes in the volcanic area. Thermal waters (54–60°C) fed from deep-rising fluids show higher arsenic concentrations (176–371 μg/L). Cold waters sampled from the volcanic aquifer are characterized by a wide variability in their arsenic concentration (1.6–195 μg/L), and about 62% exceed the limit of 10 μg/L. Where the shallow volcanic aquifer is open to deep-rising thermal fluids, relatively high arsenic concentrations (20–100 μg/L) are found. This occurs close to areas of the more recent volcano-tectonic structures.