Ancient Rain Locked Beneath a Chinese Gabbro Pluton Rewrites the Story of Jinan’s Karst Waters

Beneath the bustling city of Jinan, famous for its gushing limestone springs, lies one of the most remarkable natural laboratories in hydrogeology. A new study published in Hydrogeology Journal has mapped, sample by sample, how cold spring water and deep geothermal karst water around the Jinan Pluton tell two radically different chemical stories — and how a single slab of Early Cretaceous gabbro acts as the geological wall separating them. The findings, drawn from 36 groundwater samples analyzed with ion chromatography, inductively coupled plasma optical emission spectrometry, and isotope dating, reveal that the water beneath one of China’s most iconic spring cities is far older, stranger, and more chemically evolved than anyone had fully appreciated.

The Jinan Pluton, composed predominantly of gabbro with two-pyroxene diorite, formed during the Early Cretaceous and intrudes into Cambrian–Ordovician dolostone and dolomite aquifers. To the south of this igneous barrier lies the cold-water karst system that feeds the city’s famous springs, including Baotu Spring, Heihu Spring, Bai Spring, and Baimai Spring. To the north lies the Jibei Geothermal Field, where deeply buried dolostone hosts thermal water reaching temperatures of approximately 70 degrees Celsius. The pluton, together with Carboniferous–Permian sandstone, mudstone, and shale on its flanks, forms an aquitard that forces karst groundwater to circulate slowly and deeply beneath it, giving rise to the geothermal system. This configuration makes the pluton’s periphery a natural observatory for studying how karst water evolves from shallow, fresh recharge to deep, chemically mature thermal water.

The research team collected 23 sets of geothermal karst samples, 12 sets of cold-water karst samples, and one cold-water sample from igneous fractured bedrock between 7 and 17 June 2022. Major anions were measured using a Thermo Fisher ICS-600 ion chromatograph, major cations with a PerkinElmer ICP-OES, and bicarbonate by hydrochloric acid titration. Stable isotopes of hydrogen and oxygen, along with radiocarbon dating, were analyzed at laboratories of the Chinese Academy of Geological Sciences and partner institutions. The team then employed PHREEQC geochemical modeling to calculate mineral saturation indices, alongside sodium–chloride coefficients, desulfurization coefficients, and chloro-alkaline indices, to quantify water–rock interaction intensity and the degree of hydrogeological isolation.

The results expose a striking south-to-north chemical gradient. Cold-water karst springs exhibit low total dissolved solids, typically below a few hundred milligrams per liter, with hydrochemical types dominated by Ca–HCO3 and Ca–HCO3·SO4 facies. As groundwater flows northward and descends beneath the pluton into the geothermal field, the hydrochemical type evolves progressively through HCO3·SO4, SO4·HCO3, SO4·Cl, and finally Cl·SO4, accompanied by a continuous rise in TDS. In the westernmost segment of the geothermal field, TDS values range from 655 to 881.5 milligrams per liter, while in the eastern segment they climb to as high as 7277 milligrams per liter. The dominant anion shifts from sulfate to chloride, and the dominant cation from calcium to sodium, reflecting increasingly intense and prolonged water–rock interaction in progressively more isolated aquifers.

Isotope data provide the temporal dimension of this evolution. Stable isotope values for cold-water karst range from −65 to −54.6 per mil for δ2H and −9.1 to −7.8 per mil for δ18O, clustering near the Global Meteoric Water Line and confirming meteoric precipitation as the ultimate recharge source. Geothermal karst waters show generally more negative values, from −80.67 to −55.5 per mil for δ2H and −10.5 to −6.9 per mil for δ18O, indicating recharge under colder climatic conditions and possibly at higher elevations than today. Radiocarbon dating reveals the full age spectrum: water from Bai Spring, at the interface between the cold and geothermal systems, is just 2475 years old, while geothermal waters in the Jibei Geothermal Field range from 12065 to 39640 years. The oldest waters occur in the northern and eastern parts of the field, farthest from the recharge zone, confirming that geothermal renewal capacity weakens with distance and depth.

Mineral saturation indices calculated with PHREEQC show that readily soluble constituents such as halite, gypsum, and anhydrite remain undersaturated in both cold and geothermal karst waters, meaning these minerals continue dissolving and contributing calcium, sodium, sulfate, and chloride to groundwater. In contrast, less soluble phases such as calcite, aragonite, dolomite, quartz, and talc are generally at or near saturation. Critically, saturation indices for halite, gypsum, and anhydrite are significantly higher in geothermal karst water than in cold-water karst, explaining the higher TDS and the shift toward sulfate and chloride dominance. In geothermal waters farthest from the dolostone belt, saturation indices for chalcedony, celestite, fluorite, and serpentine are also elevated, leading to higher concentrations of fluoride, strontium, and silicic acid — trace components that serve as geochemical fingerprints of deep, prolonged circulation.

The study also uncovers an unexpected anomaly in the cold-water system. The western segment of the cold-water karst discharge area, roughly between the Mashan Fault and the Qianfo Mountain Fault, exhibits TDS values significantly lower than the central and eastern segments — and even lower than the southern mountainous recharge area. The explanation lies with the Yellow River, which flows as a perched river across the region. In western Jinan, the Quaternary sediments between the riverbed and the underlying carbonate aquifer are less than 100 meters thick, and in places only several tens of meters, allowing Yellow River water to infiltrate and recharge the karst aquifer. Because Yellow River water in this reach carries average sodium and chloride concentrations of 138 and 129 milligrams per liter respectively, this recharge elevates the chloride proportion in groundwater. During infiltration, reverse cation exchange adsorbs sodium onto rock and soil particle surfaces while chloride, an inert ion, migrates freely into the aquifer, driving the sodium–chloride ratio below 1 and producing the anomalous low-TDS signature.

In eastern Jinan, by contrast, the total thickness of Quaternary and Neogene strata between the Yellow River bed and the carbonate aquifer roof exceeds 500 meters, with multiple laterally continuous clayey soil layers and mudstone interbeds preventing substantial river recharge. The eastern segment of the cold-water karst discharge area also hosts the lowest karst water level across the entire study region — lower than both the western discharge segment and the Baimai Spring Basin to the east. This regional hydraulic low makes the eastern segment a convergence center for karst groundwater flow, creating a relatively isolated hydrogeological environment with respect to the surface. The combination of negligible Yellow River recharge and strong confinement explains why both the cold-water karst and the geothermal karst in the eastern segments exhibit the highest TDS values and the most chemically evolved signatures in the study area.

Cation exchange processes, quantified through the cation exchange adsorption ratio and chloro-alkaline indices, further refine the picture. Linear fitting of the exchange parameters yields a regression slope of −0.8216, close to the theoretical value of −1, with a coefficient of determination of 0.7719, confirming that cation exchange is one of the primary processes controlling calcium, magnesium, and sodium concentrations in the groundwater. For most geothermal karst samples, forward cation exchange dominates, increasing sodium content while decreasing calcium and magnesium. However, in geothermal waters farthest from the dolostone belt in the eastern segment and in cold-water karst of the discharge area, reverse cation exchange prevails, adsorbing sodium from solution and releasing calcium and magnesium from rock surfaces — a process that reduces the sodium–chloride ratio and contributes to the distinctive chemistry of these zones.

The implications extend well beyond academic curiosity. Jinan’s karst aquifers supply drinking water to millions and feed the springs that define the city’s identity, while the Jibei Geothermal Field represents a substantial renewable thermal resource. Understanding where water is young and vulnerable versus old and confined is essential for protecting recharge zones, managing extraction sustainably, and preserving spring discharge. The study demonstrates that the Jinan Pluton is not merely a passive geological boundary but an active control on regional groundwater circulation, water temperature, and water quality. As climate variability and urbanization continue to pressure northern China’s karst water systems, the chemical and isotopic fingerprints documented here provide a baseline for monitoring change — and a reminder that some of the water beneath Jinan fell as rain before the last Ice Age ended.

Subject of Research: Hydrogeology

Article Title: Hydrochemical differences and origins of karst waters around the Jinan Pluton, North China

Article References: Hydrochemical differences and origins of karst waters around the Jinan Pluton, North China. (n.d.). https://doi.org/10.1007/s10040-026-03187-z

Image Credits: AI Generated

DOI: 10.1007/s10040-026-03187-z

Keywords: karst water, Jinan Pluton, hydrochemistry, geothermal, Yellow River, water-rock interaction, isotope hydrology, North China, Hydrochemical, differences, origins, karst