Arsenic is not some rare industrial pollutant that only shows up near factories. It is a naturally occurring element woven into the structure of common rocks and minerals around the world, and the slow, steady interaction between water and these rocks is the main reason tens of millions of people drink arsenic-contaminated groundwater today. The minerals that host arsenic, the chemical reactions that set it free, and the conditions that make contamination worse or better are all well studied, yet the problem persists because it is fundamentally geological. Understanding how arsenic lives inside rock, and how it escapes, is the starting point for understanding one of the largest mass poisonings in human history.
Where Arsenic Hides in Rock
Arsenic does not float around as a free element inside the Earth’s crust. It sits locked within the crystal structures of specific minerals, most commonly sulfide minerals. The most important of these is arsenopyrite, an iron-arsenic-sulfide mineral that is the single most abundant arsenic-bearing mineral on the planet. Pyrite, the familiar “fool’s gold,” is another major host. When pyrite contains more than about one percent arsenic by weight, it is called arsenian pyrite, and in some deposits the arsenic content can reach as high as 20 percent by weight when arsenopyrite is absent from the rock.1Ore Geology Reviews. Phase relations of arsenian pyrite and arsenopyrite In the deep subsurface soils beneath Tokyo, for example, framboidal pyrite clusters barely ten micrometers across have been identified as the primary sink holding arsenic in place within the sedimentary formation.2PubMed. Unveiling the potential mobility and geochemical speciation of geogenic arsenic in the deep subsurface soil of the Tokyo metropolitan area
Beyond pyrite and arsenopyrite, arsenic shows up in a surprising variety of mineral phases. Orpiment and realgar, both arsenic sulfides, are vivid yellow and orange-red minerals that have been known since antiquity. Less common arsenic sulfides include alacranite, bonazziite, and wakabayashilite.3SpringerLink / CrossRef. Pigments—Arsenic-based yellows and reds Arsenic also hitches a ride on iron oxyhydroxides, the rusty coatings on sediment grains that act like chemical sponges, adsorbing arsenic onto their surfaces. In volcanic tuff samples from central Italy, iron oxyhydroxides were found to hold up to 70 percent of the total arsenic in the rock, with smaller fractions bound to calcite and sulfides.4PubMed Central. Water-Rock Interaction Processes: A Local Scale Study on Arsenic Sources and Release Mechanisms from a Volcanic Rock Matrix
Volcanic Rocks as a Major Source
When people think of arsenic contamination, they often picture old mines or industrial waste. But some of the most widespread arsenic problems on Earth trace back to ordinary volcanic rocks that have never been near a mine. In the Altiplano-Puna plateau of the Central Andes, researchers identified volcanic rocks as the main regional source of naturally occurring arsenic in surface and groundwater. The average arsenic concentration in unaltered volcanic rocks there was about 9 milligrams per kilogram, roughly double the average for the upper continental crust. Arsenic levels climb with the silica content of the rock, meaning that silica-rich volcanic types like dacites and rhyolites carry the highest concentrations.5Chemical Geology. Silicic volcanic rocks, a main regional source of geogenic arsenic in waters: Insights from the Altiplano-Puna plateau, Central Andes
This pattern is not limited to the Andes. Across the arid and semi-arid regions of Latin America, arsenic and fluoride frequently show up together in drinking water, and the source is almost always the same: fine-grained volcanic particles mixed into the loess and alluvial sediments that make up the local aquifers. Alkaline groundwater conditions and limited recharge make the problem worse.6PubMed. Co-occurrence of arsenic and fluoride in groundwater of semi-arid regions in Latin America: genesis, mobility and remediation A similar volcanic connection has been documented in parts of Italy, Mexico, and East Africa. The takeaway is that volcanic terrain, especially terrain built from silica-rich eruptions, is inherently risky for arsenic in groundwater, even without any human disturbance.
How Arsenic Escapes From Rock Into Water
Arsenic locked inside a sulfide mineral is largely harmless. The danger begins when that mineral meets water and oxygen. Arsenopyrite weathering follows two main pathways, both of which produce scorodite, an iron-arsenic oxide mineral, as an intermediate product. Scorodite then dissolves unevenly to form arsenic-bearing iron hydroxides, which can themselves break down and release arsenic into solution.7Journal of Hazardous Materials. Arsenic release from arsenopyrite weathering: Insights from sequential extraction and microscopic studies At the molecular level, arsenopyrite first oxidizes to produce reduced iron, reduced arsenic, and elemental sulfur, which are then further oxidized to their higher-valence forms and sulfate.8PubMed. Arsenic release from arsenopyrite weathering in acid mine drainage: Kinetics, transformation, and effect of biochar
But sulfide oxidation is only one route. In many aquifer systems, arsenic is not stored in sulfides at all but is adsorbed onto iron oxide and iron hydroxide coatings on sediment grains. When conditions shift, that arsenic can desorb. Strongly alkaline groundwater is one trigger: laboratory tests on iron-rich mineral waste showed that arsenic leaching follows a V-shaped pattern with respect to pH, peaking under very acidic conditions and again under very alkaline conditions, with maximum arsenic release at pH 11.9PubMed. Arsenic release from iron rich mineral processing waste: Influence of pH and redox potential In irrigated agricultural areas, competitive desorption by other anions like phosphate and bicarbonate, combined with alkaline conditions, can strip arsenic off the surfaces of iron and manganese minerals and push it into groundwater.10PubMed. Occurrence and behavior of arsenic in groundwater-aquifer system of irrigated areas
In oxidizing, alkaline groundwater, arsenic typically exists in its oxidized form and is released from host iron oxide minerals at high pH values.11Applied Geochemistry. Arsenic mobilization in an oxidizing alkaline groundwater: Experimental studies, comparison and optimization of geochemical modeling parameters The practical consequence is that you do not need acid conditions to mobilize arsenic. Alkaline water percolating through volcanic ash, loess, or iron-coated sediment can be just as effective at stripping arsenic off mineral surfaces.
Microbes That Pull Arsenic Out of Minerals
Chemistry alone does not explain the worst arsenic crises. Microorganisms play a central role, particularly in the reducing conditions found in deep aquifers and waterlogged sediments. The dominant mechanism in the heavily contaminated alluvial and deltaic aquifers of South and Southeast Asia is thought to be microbial reduction of iron and arsenic: bacteria use organic carbon as fuel and, in the process, dissolve the iron minerals that had been holding arsenic in place.12Chemical Geology. The role of electron donors in arsenic-release by redox-transformation of iron oxide minerals – A review Certain bacteria can directly “breathe” arsenic the way we breathe oxygen. Dissimilatory arsenate-respiring prokaryotes convert the less mobile oxidized form of arsenic into a more mobile reduced form, effectively pulling it out of the mineral phase and into the water.13PubMed. Sulfate enhances the dissimilatory arsenate-respiring prokaryotes-mediated mobilization, reduction and release of insoluble arsenic and iron from the arsenic-rich sediments into groundwater
Research on deep sediments in the Jianghan Plain of central China isolated a novel bacterium, Aeromonas sp. JH155, that could completely convert arsenic from its oxidized to its reduced form in just 72 hours under laboratory conditions, and efficiently promote arsenic release from minerals into solution.14Science of The Total Environment. Microbial communities involved in arsenic mobilization and release from the deep sediments into groundwater in Jianghan plain, Central China A more recent discovery added a twist: methane dissolved in groundwater can serve as the fuel source for a completely different set of microbes, methanotrophs, which trigger the same kind of iron mineral dissolution and arsenic release. Since methane is common in arsenic-contaminated aquifers worldwide, this methane-driven pathway may be a significant and previously underappreciated contributor to the problem on a global scale.15Communications Earth & Environment. Arsenic mobilization by anaerobic iron-dependent methane oxidation
Seasonal Swings and Irrigation Effects
Arsenic concentrations in groundwater are not static. In the Jianghan Plain of the Yangtze River Basin, some wells show arsenic concentrations that swing by more than tenfold within a single year, from around 100 to 1,200 micrograms per liter. Groundwater pumping and surface-water recharge during the dry season push oxidizing water into the otherwise oxygen-free aquifer, temporarily knocking arsenic levels down for one to three months. Once the oxygen supply stops, the aquifer’s own buried organic carbon re-establishes reducing conditions, and arsenic concentrations climb back up.16PubMed. Aquifer Arsenic Cycling Induced by Seasonal Hydrologic Changes within the Yangtze River Basin
Irrigation adds another layer of complexity. In northern China’s Datong Basin, the infiltration of arsenic-laden irrigation water alters the chemistry of the unsaturated zone above the water table, introduces both arsenic and iron into shallow soils, and causes the water table to rise and fall in ways that repeatedly change whether conditions favor arsenic being locked up or being released.17Journal of Hydrology. Effects of irrigation-induced water table fluctuation on arsenic mobilization in the unsaturated zone of the Datong Basin, northern China The net effect is a slow, cumulative loading of arsenic into the shallow groundwater system, driven in part by the very agricultural practices meant to sustain local food production.
Mining Waste Rock and Acid Drainage
Natural weathering releases arsenic slowly over geological time. Mining can accelerate the process dramatically by excavating sulfide-bearing rock, crushing it, and leaving it exposed to air and rain in enormous waste dumps. Studies of coal mine waste rock have found arsenic concentrations in the range of 0.3 to 8 milligrams per kilogram, and leachates from the most acidic waste rock samples contained arsenic at levels ranging from micrograms to milligrams per liter, enough to degrade natural water quality.18Journal of Geochemical Exploration. Potential of coal mine waste rock for generating acid mine drainage Gold mines present an even sharper problem. At a gold mine in Thailand, massive sulfide waste from certain dump sites generated acidic drainage with elevated levels of arsenic, copper, lead, zinc, and other metals. The kinetic testing showed that some of these waste rock samples will continue to produce acid and leach metals for very long periods.19PubMed. Acid mine drainage potential of waste rocks in a gold mine (Thailand): application of a weathering cell test and multivariate statistical analysis
The irony with gold mining specifically is that arsenopyrite often physically encapsulates fine gold particles, making the ore “refractory,” meaning the gold cannot be dissolved by standard leaching methods until the arsenopyrite is broken down first. One approach uses sodium hydroxide at room temperature to attack the arsenopyrite and liberate the gold: while untreated arsenopyrite ore yielded only about 23 to 29 percent gold recovery, pretreatment pushed recovery to 81 percent, with gold extraction directly tied to how much arsenic was removed.20Hydrometallurgy. Pretreatment of a refractory arsenopyritic gold ore using hydroxyl ion Pressure oxidation at high temperature and oxygen pressure can push gold recovery even higher, to over 98 percent.21REM, Int. Eng. J. Recovery of gold from refractory ore employing pressure oxidation These industrial processes generate large volumes of arsenic-rich waste that must be managed carefully to avoid contaminating surrounding water.
The Bengal Basin and Other Contamination Hotspots
The most devastating example of arsenic-rock contamination anywhere on Earth is the Bengal Basin, spanning Bangladesh and the Indian state of West Bengal. The original source of the arsenic traces back to the weathering of rocks along the collision zone between the Indian and Eurasian tectonic plates, the Indus-Tsangpo suture zone in the Himalayas. From there, arsenic was carried by rivers and stored in the sediments of an ancient foreland basin over millions of years. During the last roughly 10,000 years, intense tectonic activity and heavy monsoon rainfall remobilized that arsenic and transported it toward the Bay of Bengal, where it became concentrated in the young alluvial sediments that millions of people now rely on for drinking water.22PubMed. Bengal arsenic, an archive of Himalaya orogeny and paleohydrology
In Bangladesh, the problem extends beyond drinking water. Irrigating a rice field with groundwater containing just 0.55 milligrams per liter of arsenic, using a typical water application of 1,000 millimeters, adds an estimated 5.5 kilograms of arsenic per hectare per year to the soil. In some affected areas, soil arsenic concentrations have reached 80 milligrams per kilogram, and certain vegetables accumulate striking amounts: arum, a commonly grown root vegetable, has been found to contain over 150 milligrams per kilogram of arsenic.23PubMed Central. Arsenic contamination in food-chain: transfer of arsenic into food materials through groundwater irrigation Rice, which grows in flooded paddies where reducing conditions favor arsenic uptake, is a particular concern for food-chain exposure.24PubMed Central. Arsenic in the water and agricultural crop production system: Bangladesh perspectives
What Chronic Arsenic Exposure Does to the Body
Inorganic arsenic, the form that dissolves out of rocks, is a confirmed human carcinogen that can cause cancers of the skin, lung, and bladder, with additional associations reported for liver, prostate, and kidney cancer.25PubMed Central. Health effects of chronic arsenic exposure But cancer is only part of the picture. Chronic exposure through drinking water causes a distinctive set of skin changes that are typically the earliest visible symptoms: darkened patches called melanosis, lightened patches called leucomelanosis, and thickened, calloused skin on the palms and soles called keratosis. Beyond the skin, long-term exposure has been linked to high blood pressure, diabetes, neurological effects, respiratory problems, and cardiovascular disease.26PubMed. Chronic exposure of arsenic via drinking water and its adverse health impacts on humans These effects are dose- and duration-dependent, and they can take years to become apparent, which is one reason contamination often goes unrecognized until a large population has already been exposed.
Drinking Water Standards and the Gap Between Rules and Reality
The World Health Organization first set a drinking water limit for arsenic in 1958 at 200 micrograms per liter. That limit was lowered to 50 micrograms per liter in 1963, and then to the current provisional guideline of 10 micrograms per liter in 1993 as evidence of harm at lower doses accumulated.27PLoS ONE. Arsenic in drinking water: An analysis of global drinking water regulations and recommendations for updates to protect public health Many countries have adopted the 10 microgram per liter limit, but adoption is closely tied to wealth. A global survey found that arsenic is one of the contaminants whose national standards are most tightly correlated with a country’s economic resources: wealthier countries are far more likely to have standards that meet the WHO guideline, while lower-income countries are more likely to either have a higher limit or no national standard at all.28PLOS ONE. A comprehensive survey and analysis of international drinking water regulations for inorganic chemicals with comparisons to the World Health Organization’s drinking-water guidelines
Having a standard on paper does not guarantee clean water. In Bangladesh, where groundwater arsenic commonly exceeds 50 micrograms per liter across large areas, enforcement and infrastructure remain enormous challenges. The gap between regulation and reality is where most of the ongoing exposure happens.
Cleaning Up Arsenic-Contaminated Water
Given how deeply geological the arsenic problem is, you cannot simply stop it at the source. Remediation focuses on intercepting arsenic after it has entered the water. One well-studied approach uses zero-valent iron, essentially metallic iron particles, which adsorb arsenic onto their surfaces. In laboratory tests, nanoscale zero-valent iron removed arsenic rapidly, with the reaction largely complete within minutes. Over 90 days, the iron gradually corroded into magnetite and maghemite, and about a quarter of the adsorbed arsenic was chemically reduced, but the arsenic remained bound to the solid surface rather than being released back into solution.29Environmental Science & Technology. Arsenic(V) Removal from Groundwater Using Nano Scale Zero-Valent Iron as a Colloidal Reactive Barrier Material
At a larger scale, permeable reactive barriers installed in the path of contaminated groundwater have shown promise. Column experiments using mixtures of municipal compost, limestone, and zero-valent iron successfully cleaned acid mine drainage of arsenic and metals. The arsenic was trapped by co-precipitation with and adsorption onto iron and aluminum hydroxides that formed as the pH rose. When zero-valent iron was included in the mixture, arsenic concentrations in the treated water consistently dropped below 10 micrograms per liter, meeting the WHO guideline.30PubMed. In situ removal of arsenic from groundwater by using permeable reactive barriers of organic matter/limestone/zero-valent iron mixtures These are relatively low-cost, passive systems that do not require electricity or chemical dosing, which makes them attractive for remote or low-income settings where the arsenic problem is worst.
Arsenic Minerals as Ancient Pigments
Arsenic-bearing minerals have been part of human culture for thousands of years, long before anyone understood their toxicity. Orpiment, the bright yellow arsenic sulfide, and realgar, its orange-red counterpart, were prized pigments in painting, manuscript illumination, and decorative arts across cultures from ancient Egypt to medieval Europe and East Asia. A comprehensive review of these materials catalogued at least a dozen distinct arsenic-bearing mineral phases that appear in cultural heritage objects, including less well-known species like pararealgar, anorpiment, and claudetite.31SpringerLink / CrossRef. Pigments—Arsenic-based yellows and reds One complication for conservators is that realgar degrades under light exposure, transforming into pararealgar with a shift from red to yellow. This means that the colors seen in ancient artworks today may not match what the original artist intended.
The trade in orpiment and realgar was extensive in the ancient and medieval world, and the minerals were also used in medicine, cosmetics, and pest control. Their toxicity was recognized at least partially by some classical writers, but widespread occupational poisoning of painters and miners was common. The modern study of these materials in artworks relies on the same analytical techniques, like X-ray fluorescence and X-ray absorption spectroscopy, that geochemists use to study arsenic in contaminated sediments. It is a reminder that the connection between arsenic and rock is not purely an environmental story but a deeply human one.
Why Natural Organic Matter Makes Things Worse
A detail that complicates remediation and prediction is the role of natural organic matter in aquifer sediments. Organic compounds interact with both arsenic and the iron minerals that hold it, and not in a helpful direction. On pure iron hydroxide surfaces, a fraction of reduced arsenic that adsorbs is naturally oxidized to its less mobile form. But when organic matter coats those same iron surfaces, that oxidation is suppressed: about 40 percent of reduced arsenic was oxidized on clean iron hydroxide, compared to only about 29 percent when organic matter was present.32PubMed. Arsenite and arsenate binding to ferrihydrite organo-mineral coprecipitate: Implications for arsenic mobility and fate in natural environments Organic matter also shifts the pH at which different arsenic forms are preferentially adsorbed, generally promoting conditions that make arsenic more mobile. In aquifers rich in peat, buried plant material, or other organic-rich layers, the combined effects of organic matter and microbial activity create a one-two punch for arsenic release.
The behavior of the two main arsenic forms on iron mineral surfaces also changes with pH. Below about pH 5 to 6, the oxidized form is more strongly held; above pH 7 to 8, the reduced form sticks better.33PubMed. Comparison of arsenic(V) and arsenic(III) sorption onto iron oxide minerals: implications for arsenic mobility This crossover means that the common assumption “reducing arsenic from its oxidized to its reduced form always increases mobility” is an oversimplification. At near-neutral pH and in the presence of iron minerals, reducing arsenic can actually decrease its mobility or have little effect. The real-world outcome depends on the specific combination of pH, mineral surface, organic matter content, and microbial activity in any given aquifer, which is part of why predicting arsenic contamination from geological maps alone remains frustratingly imprecise.