Campi Flegrei is one of the most closely watched volcanoes in the world. Although its last eruption, which formed the Monte Nuovo cone, occurred in 1538, the caldera remains active. It experiences bradyseism, a slow alternating uplift and subsidence of the ground driven by injections of CO2-rich magmatic fluids into the base of the hydrothermal system, along with low- to medium-energy seismicity and intense degassing. The most dramatic recent crisis unfolded in 1982–1984, when the ground rose by as much as 1.8 meters and thousands of shallow earthquakes, none exceeding magnitude 4.0, forced the evacuation of roughly 40,000 people from Pozzuoli. After a period of subsidence that lasted until 2004–2005, a new uplift phase began, and from 2012–2013 onward the deformation rate, gas emissions, and seismicity accelerated. In response, the Italian Civil Protection raised the alert level from “background” to “yellow-attention,” and ground uplift rates reached about 3 centimeters per month in 2024.

Against this backdrop, the research team, led by Vincenzo Allocca and colleagues at the University of Naples Federico II together with scientists from the INGV Osservatorio Vesuviano, set out to accomplish three goals: reconstruct a comprehensive hydrogeological characterization of the caldera, correlate the physical and chemical properties of groundwater with the volcano-stratigraphic and volcano-tectonic framework, and evaluate how groundwater circulation changed over nearly three decades by comparing water table maps from 1985 to 2013. A key part of the study involved rescuing unpublished data. Much of the hydrogeological work carried out after the 1982–1984 bradyseismic crisis, including master’s and doctoral theses, had never been fully published and was essentially unknown to the international scientific community.

The team compiled piezometric measurements from a series of increasingly dense well networks. A 1985 survey measured 12 wells, a 1985–1986 campaign tracked 23 wells at monthly frequency, and a 1990 study examined 530 wells in a census, selecting 61 for piezometric measurements. For the most recent picture, the researchers measured 67 wells between 2012 and 2013, combining newly censused wells, wells revisited from the 1990 survey, and wells provided by the North-Western Campania Basin Authority and the MANITERM project. Well elevations were determined with differential GPS, and water levels were measured with probes up to 200 meters long. The team interpolated the measurements using kriging in a GIS environment to generate water table maps for 1985, 1986, 1991, and 2013, and then calculated differences between consecutive maps. A reliability assessment, accounting for data density and distance from observation points, gave confidence values ranging from 50 percent in sparsely monitored areas to 98 percent in well-covered sectors, and the associated uncertainty proved small relative to the observed variations.

To quantify the hydraulic properties of the aquifer, the researchers performed pumping tests on 36 wells distributed across the study area. They measured the discharge of each pumping well and the steady-state drawdown to compute specific capacity, which they converted to transmissivity using an empirical relationship developed for porous aquifers. The results spanned a remarkable range, from 8.90 × 10−1 to 3.33 × 103 square meters per day, with a mean of about 1.68 × 101 square meters per day. When classified by magnitude, the values fell into low, intermediate, high, and very high classes, with the low class, covering 39 percent of values, associated with compact tuffs and fine-grained ash deposits, and the intermediate and high classes associated with highly fractured tuffs and coarse loose pyroclastic material. According to the standard classification scheme the authors adopted, Campi Flegrei is an aquifer with very high transmissivity variability, reflecting considerable hydraulic heterogeneity inherited from its complex volcanic stratigraphy.

The time-lapse comparison of water table maps revealed a dome-shaped piezometric structure with radial flow toward the coastline, which acts as the main discharge area. In the 2013 map, the highest water table elevations, between 20.0 and 24.8 meters above sea level, correspond to the Quarto plain recharge area, and an elevation of about 24 meters was observed at the Gauro volcanic edifice. A striking anomaly emerged at Solfatara-Pisciarelli, the area of most intense fumarolic activity, where the water table rises to about 60 meters above sea level, forming a dome-shaped bulge. The authors attribute this bulge to the condensation of steam from a vertically extended “gas plume,” formed by complete vaporization of hydrothermal brines by magma-sourced volatiles, which feeds the local aquifer from above. This feature went undetected in the 1985, 1986, and 1991 maps simply because those surveys lacked well coverage around Solfatara.

The maps consistently showed a groundwater divide between Monterusciello and Cuma, approximately coinciding with the northern structural boundary of the Neapolitan Yellow Tuff caldera, where the piezometric surface peaks at 23.01 meters above sea level. This divide splits the caldera into two groundwater basins with contrasting behavior. The northern basin, which coincides with the recharge area, has a gentle mean piezometric gradient of 0.5 percent and normal temperatures. The southern basin shows steeper gradients, averaging 2.3 percent at Gauro and 7 percent at Solfatara-Pisciarelli, and hosts warmer, more mineralized water. Between 1985 and 2013, piezometric changes of several meters occurred in both directions: rises of up to 15 meters concentrated along the northern edge of the Gauro crater and in the central caldera, and lowerings of 2 to 8 meters in the northeastern, northwestern, and southeastern sectors, including part of the Agnano basin. The authors link these fluctuations to long-term rainfall variability and groundwater exploitation, while cautioning that hydrothermal contributions, such as increased pore water pressure from magma degassing or heat and vapor supply, may also play a role.

The hydrogeochemical survey added a chemical portrait of the same division. Temperature measurements on the 67 samples ranged from a minimum of 15.5 degrees Celsius at a Quarto well to a maximum of 85.8 degrees Celsius at a well in the Solfatara area, with a clear trend of increasing temperature from northwest to southeast. Twenty-eight samples were classified as cold or hypothermal, 23 as hypothermic, 7 as thermal, and 9 as hyperthermal. Electrical conductivity ranged from 97.9 to 37,100 microsiemens per centimeter, and pH from 1.5 to 8.9, with the most acidic values recorded at Solfatara, where redox potential also dropped to −381 millivolts. Piper diagram analysis showed that the dominant facies were sodium-chloride type, with 41 samples, and mixed calcium-sodium-bicarbonate type, with 31 samples, supplemented by smaller populations of calcium-bicarbonate and mixed calcium-magnesium-chloride waters.

The spatial distribution of these water types mirrors the hydrogeological structure. North of the divide, in the Licola and Quarto recharge zone, cool calcium-bicarbonate waters with very low salinity and positive redox values dominate. South of the divide, the sodium-chloride waters show temperatures from 15.9 to 85.8 degrees Celsius, electrical conductivity exceeding 20,000 microsiemens per centimeter near the coast, and the influence of ascending deep, CO2-rich mineralized fluids that rise through faults and fractures. Maps of chloride and magnesium concentrations, chosen as conservative tracers of seawater mixing, show high values along the southern coastline, confirming seawater intrusion at low piezometric levels, and low values in the north. The authors interpret this pattern as the result of increasing mineralization and longer water-rock interaction times along flow paths from the caldera interior toward the coastal discharge zone, with geological discontinuities locally mixing distinct water bodies into hybrid chemistries.

The findings have direct relevance to volcanic hazard assessment because the sectors of the aquifer with the highest temperatures and highest chloride and magnesium concentrations correspond closely to the areas affected by ongoing bradyseismic uplift and seismicity. High-precision locations of earthquakes recorded over the past decade show epicenters coherent with the fracture zones activated during the 1982–1984 crisis, but also reveal new sources involved in the current unrest. Proposed triggering mechanisms include the intrusion of small magma batches at shallow depth of 3 to 4 kilometers, or the injection of hot magmatic fluids from shallow intrusions into the hydrothermal system. In either case, variations in groundwater levels, temperature, and chemistry can reflect changes in magmatic degassing, heat flow, and caldera deformation, making groundwater a potentially invaluable tracker of escalating volcanic episodes.

The study is not without limitations. The monitoring networks grew in density and spatial resolution over the decades, from 12 wells in 1985 to 67 in 2013, and although the homogeneous distribution of wells in key sectors permitted comparison, the Solfatara anomaly could not be reliably compared with earlier periods. Disentangling the interplay between rainfall, groundwater extraction, and hydrothermal input in driving piezometric fluctuations remains challenging, and the chemical dataset represents a snapshot from 2013 rather than a continuous record. The authors also note that previous hydrogeochemical studies had focused on individual sectors such as the Bagnoli-Fuorigrotta plain, Agnano, Solfatara, and Cuma, and that a comprehensive groundwater chemistry survey since the resumption of unrest in 2005 had only recently appeared, underscoring how much of the caldera’s subsurface fluid dynamics was previously unresolved.

Looking forward, the researchers propose that the wells identified in this study form the basis of a dedicated hydrogeological and hydrogeochemical monitoring network, in which selected wells serve as long-term observation points for groundwater level, temperature, electrical conductivity, redox conditions, and major ion chemistry. Such a network, focused on the sectors with documented thermal anomalies, chemical enrichments, and localized piezometric rises, could detect early signs of changes in hydrothermal circulation that precede variations in volcanic activity. Combined with the existing multiparametric surveillance at Campi Flegrei, including seismicity, soil CO2 flux, fumarolic composition, and ground deformation, this hydrogeological perspective would strengthen the ability to track temporal trends and rapidly identify deviations from baseline conditions in one of the world’s most densely populated volcanic areas.