Tracing Groundwater Uranium Cycling in the Northern Plains, United States

[NNadir]

The paper I'll discuss in this post is this one: Isoscapes as a Regional-Scale Tool for Tracing Groundwater Uranium Cycling in the Northern Plains, United States Arijeet Mitra, Randall Hughes, Tracy Zacher, Rae O’Leary, Bayley Sbardellati, Mason Stahl, Reno Red Cloud, Shams Azad, James Ross, Benjamin C. Bostick, Steven N. Chillrud, Alex N. Halliday, Ana Navas-Acien, Kathrin Schilling, and Anirban Basu Environmental Science & Technology 2025 59 (50), 27635-27647.

It may also be relevant to point to this "commentary" in the same issue of the journal in which the above citation appears: Safe Water Is Rapidly Becoming a Private Good in the United States Joe Brown Environmental Science & Technology 2025 59 (50), 26911-26913. This brief commentary is open to the public, whereas the former citation requires subscription or other access. I merely point out the "commentary" and will only discuss the paper initially cited.

The first article refers to Native American communities, largely located in South Dakota. Other Native American communities, notably those in the "Four Corners Region" Northeastern Arizona, Northwestern New Mexico, South Eastern Utah and Southwestern Colorado, have been affected by uranium in their regions. Famously the latter region is the site of former and operable uranium mines. As for the former, well, the text tells the story:

Safe drinking water is a critical concern in Native American communities due to limited infrastructure and widespread groundwater contamination. (1,2) Approximately 6.5% of Native American households lack access to safe and adequate water sources or waste disposal facilities, compared to less than 1% of households in the general U.S. population. (3,4) Additionally, groundwater contamination from uranium (U) and arsenic (As) poses serious health risks, especially in Native American communities that rely on private, unregulated wells. (2,5−9) Communities participating in the Strong Heart Water study (SHWS), located in the Northern Plains, have groundwater U concentrations as high as 60 ppb, nearly double the United States Environmental Protection Agency’s (U.S. EPA) maximum contaminant level (MCL) of 30 ppb. (10,11) Public health studies have shown that residents in these areas experience a 40% higher cancer mortality rate than the national average (12) and have the highest documented U and As exposure among all demographic groups in the U.S. (13,14)

While history of U mining in parts of South Dakota has raised concerns about mining-related contamination, (15−19,18,19) there are no legacy or active U mines within or near SHWS study sites. Instead, elevated groundwater U concentrations in this region are primarily attributed to geogenic sources, as U mineralized zones are dispersed throughout the subsurface. (19) The dissolution of U-bearing bedrock results in nonpoint source contamination in aquifers, particularly in areas where residents rely on untreated private wells for drinking, livestock water, and irrigation. Despite ongoing infrastructure improvements, many households in these communities remain dependent on private wells, which increases their exposure to naturally occurring contaminants. Understanding the processes that govern the release or removal of U in groundwater is essential for identifying high-risk areas and developing effective mitigation strategies.

While dissolved U concentrations provide a general indication of U contamination, they alone are insufficient to characterize the geochemical processes that control U mobility in groundwater. A key factor influencing U mobility in groundwater is its oxidation state. Under reducing conditions and near-neutral pH, the highly soluble U(VI) undergoes reduction to insoluble U(IV), leading to the formation of U-containing solids, such as uraninite, which are effectively immobilized. This reduction is primarily driven by microbially mediated terminal electron-accepting processes in aquifers, such as iron [Fe(III)] and/or sulfate (SO₄^2– reduction. (20−26) However, reduced U(IV) solids are susceptible to reoxidation and easily oxidized by dissolved oxygen (O₂) or nitrate (NO₃–) to form soluble U(VI). (27,28) In aqueous environments, with elevated Ca²⁺ concentrations, U predominantly forms neutral calcium–uranyl–carbonate complexes, including both neutral Ca₂UO₂(CO₃)₃⁰ and negatively charged CaUO₂(CO₃)₃^2– species. (29−36) The presence of complexing agents like bicarbonate and organic ligands stabilizes dissolved U(VI) in groundwater and enhances its mobility. (37,38) Therefore, local variations in aquifer geochemistry, mineral composition, organic matter content, and microbial activity create heterogeneous redox zones that further complicates the interpretation of the spatial distribution of dissolved U in groundwater. This challenge can be addressed by using indicators sensitive only to the U redox reactions. Fractionation of U isotopes (i.e., changes in ²³⁸U/²³⁵U) serves as an indicator of environments conducive to U(VI) reduction and mobilization (Table S1 of the Supporting Information). (39−42)

Uranium isotope fractionation occurs due to the nuclear volume effect (NVE), which results from differences in nuclear size rather than mass between isotopes. (43−45) Because ²³⁸U has a disproportionately larger nuclear volume than ²³⁵U and therefore a lower electron density at the nucleus, ²³⁸U is preferentially incorporated into reduced U(IV) phases during reduction, which results in a more thermodynamically stable configuration.

Uranium is a ubitiquous element, found widely distributed on Earth, and while it can be (and often is) mobilized by mining activities, it is also, in many places on Earth, especially those reliant on groundwater, present as "NORM" (Naturally Occurring Radioactive Materials). I have shown elsewhere in this space that natural uranium is almost always in secular equilibrium with its decay products, notably radium, but can be disturbed from that equilibrium by anthropogenic means, one of which is uranium mining, another being fracking for natural gas. (I have provided references in this space showing that in many cases fracking flowback water in Pennsylvania and Michigan is more radioactive than groundwater in underground nuclear weapons test sites in Nevada: 828 Underground Nuclear Tests, Plutonium Migration in Nevada, Dunning, Kruger, Strawmen, and Tunnels ) In the case presented here, the uranium is "NORM;" not the result from uranium mining.

Although displacement from secular equilibrium between decay products of uranium is most commonly achieved either by mining activities or enrichment activities, anthropogenically, they can also occur from natural processes.

To wit:

Variation in U activity ratios or (²³⁴U/²³⁸U) in groundwater may serve as an effective indicator for determining U source proximity (Figure S1). In sandstone-hosted U deposits, (²³⁴U/²³⁸U) approaches secular equilibrium over approximately one million years, as the ingrowth of ²³⁴U is balanced by the decay of ²³⁴U, which produces a U activity ratio of 1. (50) However, in natural systems, this equilibrium is often disrupted by water-rock interactions that preferentially mobilize ²³⁴U. Such disequilibrium arises from α-recoil, in which recoil energy from the decay of ²³⁸U displaces its daughter nuclide ²³⁴Th by approximately 20-30 nm in silicate minerals. As a result, some of these atoms are ejected from the mineral into groundwater or relocated toward grain boundaries or microfractures, where they are more susceptible to leaching. (51,52) The extent of ²³⁴U recoil loss is influenced by the grain size and geometry of U-bearing minerals and tends to increase as grain size decreases. (53−55) This process can lead to (²³⁴U/²³⁸U) values below 1 in U-bearing minerals. Importantly, U transport in groundwater directly influences (²³⁴U/²³⁸U). When dissolved U stays close to its source, groundwater (²³⁴U/²³⁸U) reflects the U activity ratio of the source and is maintained near secular equilibrium. (56−59) In contrast, when U is transported over longer distances without precipitation, continued α-decay in groundwater and sustained ²³⁴U release from minerals along the flow path led to progressively elevated (²³⁴U/²³⁸U) values in groundwater. (56−59) This process is largely unaffected by adsorption–desorption of U, in contrast to the pronounced isotopic effects of α-decay of ²³⁸U in water and preferential leaching of ²³⁴U from recoil-damaged lattice sites of U-bearing minerals. Dissolved and surface-bound U(VI) rapidly reach isotopic equilibrium, (60−63) and attain identical (²³⁴U/²³⁸U) values within 2-24 h (48,49) at ionic strengths typical of groundwater. Overall, U activity ratios in groundwater provide a robust tracer for distinguishing local versus distal U sources.

The authors thus claim that the departure from secular equilibrium, (²³⁴U/²³⁸U) ratio, can distinguish how far well water sources are from uranium deposits through which they percolate.

Some figures from the text:



The caption:

Figure 1. Spatial distribution of uranium (U) concentrations (μg/L) in groundwater samples (n = 140) across the study region. The heatmap is generated using IDW interpolation, with warm colors (red) indicating higher U concentrations and cool colors (blue) representing lower concentrations. Different symbols represent groundwater sampling locations (n = 140) based on their cluster assignments. The inset map shows the location of Pine Ridge Reservation in southwest South Dakota. A scale bar and north arrow are included for spatial reference. The black thin arrows indicate groundwater flow path, and the black thin continuous lines indicate potentiometric surface. Gray numerical labels show potentiometric surface elevations (contour interval: 15 m), referenced to the NAVD 88 vertical datum traced from published studies. (83)

The caption:

Figure 2. Spatial distribution of δ²³⁸U and (²³⁴U/²³⁸U) in groundwater samples (n = 140) across the study region. (Top) Groundwater δ²³⁸U isoscapes showing warm colors (red) indicate more positive values, and cool colors (blue) represent more negative values. (Bottom) Spatial variation of groundwater (²³⁴U/²³⁸U), with darker brown shades indicating lower ratios close to 1 and dark green shades indicates higher ratios.

One can hear nonsense all the time from antinukes that uranium is a depletable resource, usually based on their ignorance of breeder technologies and their poor scientific backgrounds and educations with respect to other topics like, but not limited to, geology. One can also hear this from some nuclear power advocates something along the same lines, coupled with the claim that thorium is a better fuel since it is more available from terrestrial ores. (Most of the world's radioactive thorium is currently dumped in the mine tailings from lanthanide, "rare earth," mining to service our useless - from a climate perspective - wind turbines and electric cars.) In neither case can this be considered true; uranium is inexhaustible whereas thorium is not. The reason is that uranium in the +6 oxidation state is marginally soluble at concentrations high enough to extract from aqueous sources, like the groundwater in South Dakota, as well, more importantly in the ocean. Although the concentrations are low, solid phase resins, and other systems, for example coral proteins (which concentrate uranium) can extract these at a manageably higher cost that mining. I note that these same resins (although not coral) can be used to remove uranium from drinking water, whereupon the resins become uranium ores that in theory, if not in practice, can be exploited for use in nuclear powerplants. As it is, in breeding settings (which in the CANDU case would involve thorium and plutonium along with depleted or natural uranium), the world's uranium and thorium already mined (and in the latter case dumped) could provide, in theory, all of humanities energy needs for many centuries, eliminating the need for all coal mining, all oil mining and all gas mining, while restoring riparian ecosystems destroyed by dams. This would not be as inexpensive and dumping the onus on future generations as is our current practice of trashing the environment and destroying the planetary atmosphere, but it would be wise, but wisdom is currently a very, very, very, very rare commodity in conversations about energy.

Have a nice weekend.

[biophile]

1. This is so important! Thank you!

Here in PA (as in so many states) the effects of fracking on groundwater reservoirs and wells is frightening.

[NNadir]

2. My pleasure. Thank you for reading.

[biophile]

3. I'm not going to pretend that I am in the same scientific league

I worked with isotopes but not a nuclear engineer. And I have forgotten much of my college work because I didn’t use it on the daily in my career. But I did read your piece even if some of it was not in my wheelhouse 😆.