publications

2025

1. Application of the DRASTIC Model to Assess the Vulnerability of Groundwater Contamination Near Zaporizhzhia Nuclear Power Plant, Ukraine

   E. W. Slessarev, A. Nezgoduk, Jon K. Golla, B. Faybishenko, D. Dwivedi, P. S. Nico, J. T. Birkholzer, D. O’Ryan, O. Alvarez, A. B. Kersting, and M. Zavarin

   ACS ES&T Water, Jan 2025

   Publisher: American Chemical Society

   Abs DOI

   Russia’s invasion of Ukraine continues to have a devastating effect on the well-being of Ukrainians and their environment. We evaluated a major environmental hazard caused by the war: the potential for groundwater contamination in proximity to the Zaporizhzhia Nuclear Power Plant (NPP). We quantified groundwater vulnerability with the DRASTIC index, which was originally developed by the United States Environmental Protection Agency and has been used at various locations worldwide to assess relative pollution potential. We found that there are two major gradients of groundwater vulnerability in the region: (1) broadly higher risk to the northeast of the NPP and lower risk to the southeast driven by a regional gradient in water availability and water table depth; and (2) higher risk in proximity to the channels and floodplains of the Dnipro River and tributaries, which host coarser-textured soils and sedimentary deposits. We also found that the DRASTIC vulnerability index can be used to identify and prioritize groundwater well-network monitoring. These and more detailed assessments will be necessary to prioritize monitoring and remediation strategies across Ukraine in the event of a nuclear accident, and more broadly demonstrate the utility of the DRASTIC approach for prognostic contamination risk assessment.

2. Deep Roots Supply Reactivity and Enhance Silicate Weathering in the Bedrock Vadose Zone

   Ivan D. Osorio-Leon, Daniella M. Rempe, Jon K. Golla, Julien Bouchez, and Jennifer L. Druhan

   AGU Advances, Jan 2025

   _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2025AV001692

   Abs DOI

   In upland environments, roots commonly extend deep below soil into partially saturated bedrock. This Bedrock Vadose Zone (BVZ) has been shown to store and circulate water, host organic carbon respiration and serve as a critical source of rock-derived nutrients. However, the extent to which deep roots influence chemical weathering rates remains poorly understood. Here, we report 4 years of depth-resolved major ion chemistry over a 16-m thick BVZ hosting a deep rhizosphere in a catchment subject to a Mediterranean climate. These data allow development and validation of a reactive transport model (RTM), revealing that the timescales of water storage and drainage in the BVZ are sufficient to facilitate substantial chemical weathering of the shale bedrock. However, observed solute concentrations are only reproduced by the RTM when we explicitly include measured rates of {\vphantom}}textCO_2(g)} production meters below soil driven by the deeply rooted forest. By combining direct observations and a process-based RTM we conclude that the carbon respiration promoted by deep roots significantly enhances chemical weathering rates in the BVZ, constituting 43% }pm } 3% of total solute flux from the base of the BVZ to the water table.

2024

1. Weathering Incongruence in Mountainous Mediterranean Climates Recorded by Stream Lithium Isotope Ratios

   Jon K. Golla, Julien Bouchez, Marie L. Kuessner, and Jennifer L. Druhan

   Journal of Geophysical Research: Earth Surface, Jan 2024

   _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2023JF007359

   Abs DOI

   Lithium isotope ratios (δ7Li) of rivers are increasingly serving as a diagnostic of the balance between chemical and physical weathering contributions to overall landscape denudation rates. Here, we show that intermediate weathering intensities and highly enriched stream δ7Li values typically associated with lowland floodplains can also describe small upland watersheds subject to cool, wet climates. This behavior is revealed by stream δ7Li between +22.4 and +23.5‰ within a Critical Zone observatory located in the Cévennes region of southern France, where dilute stream solute concentrations and significant atmospheric deposition otherwise mask evidence of incongruence. The water-rock reaction pathways underlying this behavior are quantified through a multicomponent, isotope-enabled reactive transport model. Using geochemical characterization of soil profiles, bedrock, and long-term stream samples as constraints, we evolve the simulation from an initially unweathered granite to a steady state weathering profile which reflects the balance between (a) fluid infiltration and drainage and (b) bedrock uplift and soil erosion. Enriched stream δ7Li occurs because Li is strongly incorporated into actively precipitating secondary clay phases beyond what prior laboratory experiments have suggested. Chemical weathering incongruence is maintained despite relatively slow reaction rates and moderate clay accumulation due to a combination of two factors. First, reactive primary mineral phases persist across the weathering profile and effectively “shield” the secondary clays from resolubilization due to their greater solubility. Second, the clays accumulating in the near-surface profile are relatively mature weathering byproducts. These factors promote characteristically low total dissolved solute export from the catchment despite significant input of exogenous dust.

2. Antecedent Hydrologic Conditions Reflected in Stream Lithium Isotope Ratios During Storms

   Jon K. Golla, Julien Bouchez, and Jennifer L. Druhan

   Geophysical Research Letters, Jan 2024

   _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2024GL109624

   Abs DOI

   Antecedent hydrological conditions are recorded through the evolution of dissolved lithium isotope signatures ( {\vphantom}}delta ^7}Li) by juxtaposing two storm events in an upland watershed subject to a Mediterranean climate. Discharge and {\vphantom}}delta ^7}Li are negatively correlated in both events, but mean {\vphantom}}delta ^7}Li ratios and associated ranges of variation are distinct between them. We apply a previously developed reactive transport model (RTM) for the site to these event-scale flow perturbations, but observed shifts in stream {\vphantom}}delta ^7}Li are not reproduced. To reconcile the stability of the subsurface solute weathering profile with our observations of dynamic stream {\vphantom}}delta ^7}Li signatures, we couple the RTM to a distribution of fluid transit times that evolve based on storm hydrographs. The approach guides appropriate flux-weighting of fluid from the RTM over a range of flow path lengths, or equivalently fluid residence times. This flux-weighted RTM approach accurately reproduces dynamic storm {\vphantom}}delta ^7}Li-discharge patterns distinguished by the antecedent conditions of the watershed.

2022

1. Subsurface weathering signatures in stream chemistry during an intense storm

   Jon K. Golla, Julien Bouchez, Marie L. Kuessner, Daniella M. Rempe, and Jennifer L. Druhan

   Earth and Planetary Science Letters, Oct 2022

   Abs DOI

   Long-term relationships between stream chemistry and discharge are regulated by watershed subsurface structure and biogeochemical functioning. The extent to which these mechanisms are expressed and may be explored in the geochemical response of streams during storm events remains an open question. Here, we monitor an intense storm as it infiltrated an upland hillslope draining into a small steep canyon stream that is typified by chemostatic concentration-discharge relationships in rock-derived solutes. Our approach couples a high-frequency record of stable lithium isotope ratios (δ7Li) in the stream with novel sampling of rock moisture within the hillslope. At peak discharge, lithium-sodium ratios (Li/Na) increased from 0.58 μM/mM to 0.82 μM/mM and δ7Li decreased from +28.9±0.1‰ to +26.4±0.4‰ in the stream. Hillslope hydrologic monitoring reveals that the rainwater infiltrated the subsurface, yet attenuated breakthrough of the heavily depleted δD signal of this storm (as low as −86‰) only reached the upper 3-4 meters of the vadose zone. These δD data show that the storm water mixed with previously stored rock moisture and displaced stored fluid to deeper depths, causing an observable rise in the water table. Groundwater 87Sr/86Sr and δ7Li demonstrate consistency in the fluid-rock interactions that occur below the water table prior to and during the storm. In total, these observations indicate that the transfer of fluid and generation of solutes through the interior of the hillslope produce the variability of Li/Na and δ7Li within the stream during the storm, and support application of a previously established 1-D reactive transport model framework developed for the evolution of lithium within the hillslope to this extreme hydrologic event. Based on the model, both Li/Na and δ7Li versus discharge relationships reflect an overall shorter transit time of fluid through the interior of the hillslope. These model results are consistent with our hydrologic observations and indicate that Li from further upslope (where the vadose zone becomes thicker) contributes to stream solute chemistry at the height of the storm. We conclude that in this system, stream lithium isotope signatures record the routing of water and generation of solutes within the hillslope even during intense storm events.

2021

1. The evolution of lithium isotope signatures in fluids draining actively weathering hillslopes

   Jon K. Golla, Marie L. Kuessner, Michael J. Henehan, Julien Bouchez, Daniella M. Rempe, and Jennifer L. Druhan

   Earth and Planetary Science Letters, Aug 2021

   Abs DOI

   The stable isotopes of lithium (Li) serve as a robust proxy of silicate weathering. The fate and transport of these isotopes in the dissolved load of major rivers have been characterized to infer changes in both contemporary weathering regimes and paleo-conditions. In this contribution, we deconvolve this integrated signal into the individual processes that fractionate Li at the inception of silicate weathering by directly measuring Li isotope ratios of waters (δ7Li) transiting through a rapidly eroding first-order hillslope. We use these data to develop a multicomponent reactive transport framework, which shows that net dissolution of weathered material generates light δ7Li signatures (as low as −9.2‰) in the shallow portion of the vadose zone. An increase in δ7Li deeper into the vadose zone (as much as +18‰) reflects an increasing contribution of secondary mineral formation. Below the water table, congruent weathering occurs and imparts elevated cation concentrations and bedrock δ7Li. Silicate weathering continues within the saturated zone as groundwater travels downslope (δ7Li = +13 to + 24‰) to the stream. The stream signatures (δ7Li = +28 to +29‰) reflect the terminus of this network of silicate weathering reactions and the relative magnitude of each contributing process (e.g., transitions in secondary mineral formation, dissolution of weathered material). We show that fluid progressing through the weathering profile of this first-order hillslope is distinguished by a sequence of characteristic Li isotope signatures, which can be reproduced in a forward, process-based model framework. This model development offers an improved quantitative basis for the use of metal(loid) stable isotopes in disentangling catchment-scale chemical weathering fluxes.

2019

1. Multivariate Statistical Method Validation Using Aqueous Geochemistry from Yellowstone National Park

   Cary R. Lindsey and Jon K. Golla

   In Geothermal Resources Council Transactions, Aug 2019

2018

1. Using Principal Component Analysis to Aid in Visualization and Interpretation of Geothermal Solute Chemistry: An Application to Yellowstone Thermal Waters

   Jon K. Golla

   In Geothermal Resources Council Transactions, Aug 2018

2017

1. Comparison and Controls of Thermal Spring Chemistry in Cascade Range and Olympic Mountains Geothermal Provinces, Washington

   Jon K. Golla and Jeffrey H. Tepper

   In Geothermal Resources Council Transactions, Aug 2017