Two Kansas State University geologists are shedding new light on how tungsten metal is leached from the sediment surrounding aquifers into the groundwater — findings that may have implications for human health.
Tungsten is a naturally occurring metal primarily used nowadays for incandescent light bulb filaments, drill bits and as an alternative to lead in bullets. Although tungsten is generally regarded as nonhazardous to the environment and nontoxic to humans, it can be poisonous if ingested in large amounts, and in recent years has been tentatively linked to cases of childhood leukemia in the Western U.S.
“Very little is known about the biogeochemistry of tungsten in the environment,” says Dr. Saugata Datta, professor of geology at Kansas State University, in a KSU release. “We need to understand how this metal is leached from the soils into groundwater because humans can be exposed to tungsten through multiple pathways.” Dr. Datta teaches a number of courses in hydrogeology, low temperature geochemistry and water resources, and the occasional service course, his primary expertise being in studies on trace element and oxyanion migration and contamination in the environment, especially in groundwaters, urban air particulates, subway microenvironments, and unproductive soil environments using hydrological and geochemical tools including synchrotron spectroscopy. He has been recently active in research in geochemical implications of CO2 sequestration, and has been also working in delineation of areas of groundwater problems in Kansas and neighboring states.
Dr. Datta, along with Chad Hobson, master’s student in geology, Lavonia, Ga., and colleagues at Tulane University and the University of Texas, Arlington, found that the likelihood that tungsten will seep into an aquifer’s groundwater depends on the groundwater’s pH level, the amount of oxygen in the aquifer and the number of oxidized particles in the water and sediment. Analysis also showed that tungsten-VI is the most common form of tungsten in natural sediments.
These latest findings appear in the study “Controls on tungsten concentrations in groundwater flow systems: The role of adsorption, aquifer sediment Fe(III) oxide/oxyhydroxide content, and thiotungstate formation,” published in the journal Chemical Geology.
The study, Co-authored by Karen H. Johannesson of the Department of Earth and Environmental Sciences at Tulane University, Heeral B. Dave of the Department of Earth and Environmental Sciences, The University of Texas at Arlington, and T. Jade Mohajerina and Saugata Datta of the Department of Geology, Kansas State University, analyzed groundwater samples collected along flow paths within the Carrizo Sand aquifer (southeastern Texas) and the Aquia aquifer (coastal Maryland) for dissolved tungsten (W) concentrations, along with the major solutes, pH, and measures of in situ redox conditions [e.g., dissolved Fe(II), Fe(III), and S(-II) concentrations]. In addition, sediment samples were collected from both aquifers to evaluate the solid-phase speciation of W. Tungsten concentrations in the Carrizo Sand aquifer range from 3.64 to 1297 pmol kg 1 (mean ± SD = 248 ± 440 pmol kg 1), with the lowest concentrations reported from the recharge area. Tungsten concentrations progressively increase down gradient along the flow path within Carrizo Sand aquifer groundwaters, reaching the highest levels in sulfidic groundwater roughly 50–60 km from the recharge zone.
The Carrizo Sand aquifer is recharged where it outcrops in northern Atascosa County , and groundwater flows towards the southeast. The Texas Water development board describes the Carrizo-Wilcox Aquifer as a major aquifer extending from the Louisiana border to the border of Mexico in a wide band adjacent to and northwest of the Gulf Coast Aquifer. It consists of the Wilcox Group and the overlying Carrizo Formation of the Claiborne Group. The aquifer is primarily composed of sand locally interbedded with gravel, silt, clay, and lignite. Although the Carrizo-Wilcox Aquifer reaches 3,000 feet in thickness, the freshwater saturated thickness of the sands averages 670 feet.
The aquifer’s groundwater, although hard, is generally fresh and typically contains less than 500 milligrams per liter of total dissolved solids in the outcrop, whereas softer groundwater with total dissolved solids of more than 1,000 milligrams per liter occurs in the subsurface. High iron and manganese content in excess of secondary drinking water standards is characteristic of the deeper subsurface portions of the aquifer. Parts of the aquifer in the Winter Garden area are slightly to moderately saline, with total dissolved solids ranging from 1,000 to 7,000 milligrams per liter. Irrigation pumping accounts for slightly more than half the water pumped, and pumping for municipal supply accounts for another 40 percent. Water levels have declined in the Winter Garden area because of irrigation pumping and in the northeastern part of the aquifer because of municipal pumping.
The Edwards Aquifer Website notes that water in sand aquifers like Carrizo-Wilcox tends stay in place or move very slowly, with water injected into unconfined sand forming a stationary dome, and if there are confining layers then water spreads out horizontally. Either way, it is possible to store water in sand and come back years later and extract the exact same water. For more information about exploitation and development associated with Carrizo-Wilcox, and the greater El Paso and San Antonio areas see:
In addition to the publication, Datta and Hobson presented the findings at the International Conference on Biogeochemistry of Trace Elements. For the study, researchers looked at Fallon, Nev.; Sierra Vista, Ariz.; and at the Cheyenne Bottoms Refuge near Hoisington, Kan. The sites were chosen based on previous studies analyzing plants and dust collected on trees in the locations. Additionally, these areas have natural tungsten mineral deposits, nearby military bases, and mining and smelting operations in the area, Dr. Datta notes.
In 2002, the Centers for Disease Control investigated several clusters of acute lymphatic leukemia in both Nevada and Arizona. The investigation found that residents’ urine had tungsten levels above the 95th percentile. “This was important for us to know because the goal is to clarify valuable information about tungsten’s geochemistry,” says Dr. Datta. “So, we needed sites that had tungsten — and enough tungsten to measure easily. The benefit of this study is that tungsten’s geochemistry has been overlooked and until recently, largely unknown. This work will help fill the gaps in the knowledge of tungsten, which is possibly carcinogenic, and help determine its future use.”
Dr. Datta and Chad Hobson analyzed sediment samples lining the aquifers while researchers at Tulane University and the University of Texas, Arlington analyzed the groundwater samples. The National Synchrotron Light Source was used for spectroscopic analysis of the individual particles. This helped researchers understand the speciation of tungsten in natural sediments in the environment and helped them detect why tungsten forms organosulphur complexes that can be soluble in groundwater, Dr. Datta explains. Analysis also showed that tungsten-VI is the most common form of tungsten in natural sediments.
Analysis of the sediment and groundwater also showed that iron oxide and oxyhydroxide particles in both substances play a key role in regulating how much tungsten is in the groundwater. The fewer iron oxides or oxyhydroxide particles, the higher the amount of tungsten, Datta said. Similarly, the team found that the number of tungsten-regulating iron oxide particles is controlled by the pH in the groundwater. A higher pH results in more tungsten entering the water.
“Tungsten is specifically bound to these iron oxides and oxyhydroxides,” says Dr. Datta. “One of the major factors controlling tungsten’s mobility and bioavailability is pH. Ranging values of pH can affect how tungsten behaves or transforms between different tungsten species, which have different properties and factors controlling mobility.”
When tungsten is in the water it is surrounded by oxygen atoms and forms an anion, Dr. Datta observes. When in the presence of phosphates, this anion tends to bind with other transition metals, commonly iron, to form poloyoxometalates. In this form, tungsten can become more soluble in water. Researchers also found that aquifers with less dissolved oxygen had greater traces of tungsten in the groundwater than aquifers with high dissolved oxygen levels.
The process of tungsten being leached from the surrounding sediment into the groundwater can be reduced if the ironoxides are in the water and the water has a neutral pH level, according to Datta.
The study is part of a three-year, $515,000 National Science Foundation-funded project between Kansas State University and Karen Johannesson at Tulane University that is titled “Collaborative Research: Chemical Hydrogeologic Investigations of Tungsten: Field, Laboratory, and Modeling Studies of an Emerging Environmental Contaminant.” It focuses on biogeochemistry of tungsten’s reaction to the environment and how it is transported from sediments into groundwaters once it becomes geochemically mobilized.