Ryan T. Bailey

Assessing selenium contamination in the irrigated stream-aquifer system of the Arkansas River, Colorado.

Prudent interventions for reducing selenium (Se) in groundwater and streams within an irrigated river valley must be guided by a sound understanding of current field conditions. An emerging picture of the nature of Se contamination within the Lower Arkansas River Valley in Colorado is provided by data from a large number of groundwater and surface water sampling locations within two study regions along the river. Measurements show that dissolved Se concentrations in the river are about double the current Colorado Department of Public Health and Environment (CDPHE) chronic standard of 4.6 microg L(-1) for aquatic habitat in the upstream region and exceed the standard by a factor of 2 to 4 in the downstream region. Groundwater concentrations average about 57.7 microg L(-1) upstream and 33.0 microg L(-1) downstream, indicating a large subsurface source for irrigation-induced dissolution and mobilization of Se loads to the river and its tributaries. Inverse correlation was found between Se concentration and the distance to the closest identified shale in the direction upstream along the principal groundwater flow gradient. The data also exhibited, among other relationships, a moderate to strong correlation between dissolved Se and total dissolved solids in groundwater and surface water, a strong correlation with uranium in groundwater, and power relationships with nitrate in groundwater. The relationship to nitrate, derived primarily from N fertilizers, reveals the degree to which dissolved Se depends on oxidation and inhibited reduction due to denitrification and suggests that there are prospects for reducing dissolved Se through nitrate control. Current and future results from these ongoing studies will help provide a foundation for modeling and for the discovery of best management practices (BMPs) in irrigated agriculture that can diminish Se contamination.

Modeling variably saturated multispecies reactive groundwater solute transport with MODFLOW-UZF and RT3D

A numerical model was developed that is capable of simulating multispecies reactive solute transport in variably saturated porous media. This model consists of a modified version of the reactive transport model RT3D (Reactive Transport in 3 Dimensions) that is linked to the Unsaturated-Zone Flow (UZF1) package and MODFLOW. Referred to as UZF-RT3D, the model is tested against published analytical benchmarks as well as other published contaminant transport models, including HYDRUS-1D, VS2DT, and SUTRA, and the coupled flow and transport modeling system of CATHY and TRAN3D. Comparisons in one-dimensional, two-dimensional, and three-dimensional variably saturated systems are explored. While several test cases are included to verify the correct implementation of variably saturated transport in UZF-RT3D, other cases are included to demonstrate the usefulness of the code in terms of model run-time and handling the reaction kinetics of multiple interacting species in variably saturated subsurface systems. As UZF1 relies on a kinematic-wave approximation for unsaturated flow that neglects the diffusive terms in Richards equation, UZF-RT3D can be used for large-scale aquifer systems for which the UZF1 formulation is reasonable, that is, capillary-pressure gradients can be neglected and soil parameters can be treated as homogeneous. Decreased model run-time and the ability to include site-specific chemical species and chemical reactions make UZF-RT3D an attractive model for efficient simulation of multispecies reactive transport in variably saturated large-scale subsurface systems.

The influence of nitrate on selenium in irrigated agricultural groundwater systems.

Selenium (Se) contamination of groundwater is an environmental concern especially in areas where aquifer systems are underlain by Se-bearing geologic formations such as marine shale. This study examined the influence of nitrate (NO₃) on Se species in irrigated soil and groundwater systems and presents results from field and laboratory studies that further clarify this influence. Inhibition of selenate (SeO₄) reduction in the presence of NO₃ and the oxidation of reduced Se from shale by autotrophic denitrification were investigated. Groundwater sampling from piezometers near an alluvium-shale interface suggests that SeO₄ present in the groundwater was due in part to autotrophic denitrification. Laboratory shale oxidation batch studies indicate that autotrophic denitrification is a major driver in the release of SeO₄ and sulfate. Similar findings occurred for a shale oxidation flow-through column study, with 70 and 31% more reduced Se and S mass, respectively, removed from the shale material in the presence of NO₃ than in its absence. A final laboratory flow-through column test was performed with shallow soil samples to assess the inhibition of SeO₄ reduction in the presence of NO₃, with results suggesting that a concentration of NO₃ of approximately 5 mg L or greater will diminish the reduction of SeO₄. The inclusion of the fate and transport of NO₃ and dissolved oxygen is imperative when studying or simulating the fate and transport of Se species in soil and groundwater systems.

Simulating variably-saturated reactive transport of selenium and nitrogen in agricultural groundwater systems

Selenium (Se) contamination in environmental systems has become a major issue in many regions world-wide during the previous decades, with both elevated and deficient Se concentrations in groundwater, surface water, soils and associated cultivated crops reported. To provide a tool that can assess baseline conditions and explore remediation strategies, this paper presents a numerical model capable of simulating the reactive transport of Se species in large-scale variably-saturated groundwater systems influenced by agricultural practices. Developed by incorporating a Se reaction module into the multi-species, variably-saturated reactive transport model UZF-RT3D, model features include near-surface Se cycling due to agricultural practices, oxidation-reduction reactions, and the inclusion of a nitrogen (N) cycle and reaction module due to the dependence of Se transformation and speciation on the presence of nitrate (NO₃). Although the primary motivation is applying the model to large-scale systems, this paper presents applications to agricultural soil profile systems to corroborate the near-surface module processes that are vital in estimating mass loadings to the saturated zone in large-scale fate and transport studies. The first application jointly tests the Se and N modules for corn test plots receiving varying loadings of fertilizer, whereas the second application tests the N module for fertilized and unfertilized test plots. Results indicate that the model is successful in reproducing observed measurements of Se and NO₃ concentrations, particularly in lower soil layers and hence in regards to leaching. For the first application, the Ensemble Kalman Filter (EnKF) is used to condition model parameters, demonstrating the usefulness of the EnKF in real-world reactive transport systems.

Estimating the freshwater-lens thickness of atoll islands in the Federated States of Micronesia

The water resources of the 32 atolls of the Federated States of Micronesia (FSM) are under continual threat from El Niño-induced droughts and other natural hazards. With government policies emphasizing sustainable development of atoll-island communities, local managers are in need of tools for predicting changes in the availability of fresh groundwater, which communities depend upon during droughts that incapacitate rain-catchment systems. An application of a recently developed, readily portable algebraic model is demonstrated here, to estimate the freshwater-lens thickness of atoll islands in the FSM, a key component of FSM groundwater resource assessment. Specifically, the model provides estimates of the lens thickness of atoll islands in the FSM during normal and drought conditions. The model was tested for use in the FSM through comparison with available lens data under both average rainfall conditions and intense drought conditions, and then applied to major islands of each atoll within the FSM. Results indicate that out of 105 major islands on FSM atolls, only six would likely retain sufficient groundwater to sustain the local community during an intense drought.RésuméLes ressources en eau des 32 atolls des États Fédérés de Micronésie (EFM) sont sous la menace continue des sécheresses et autres catastrophes naturelles dues à El Niño. Avec les politiques gouvernementales insistant sur le développement durable des communautés d’îles-atoll, les gestionnaires locaux ont besoin d’outils pour prévoir les changements de disponibilité de l’eau douce souterraine dont les communautés dépendent en période de sécheresse qui rend inutiles les systèmes de captage de pluie. Pour estimer l’épaisseur des lentilles d’eau d’îles-atoll du FSM, composante clef de l’évaluation de la ressource en eau des EFM, un modèle numérique transférable facile à mettre en œuvre a été récemment développé, il est présentée ici. Le modèle fournit en particulier l’estimations de l’épaisseur des lentilles d’eu douce d’atolls des EFM dans des conditions normales et de sécheresse. Pour une utilisation dans les EFM, le modèle a été testé par comparaison avec les données disponibles sur des lentilles, à la fois dans des conditions pluviométriques moyennes et de sécheresses intenses, puis appliqué aux îles principales de chaque atoll des EFM. Les résultats indiquent que, sur 105 des principales îles des atolls des EFM, vraisemblablement six seulement retiendraient suffisamment d’eau souterraine pour alimenter eau la communauté locale durant une sécheresse intense.ResumenLos recursos de agua de 32 atolones de los Estados Federados de Micronesia (FSM) están bajo una continua amenaza de sequías inducidas por El Niño y otros riesgos naturales. Con las políticas del gobierno que enfatizan el desarrollo sostenible de las comunidades de las islas atolones, los gestores locales necesitan herramientas para predecir cambios en la disponibilidad de agua dulce, de las cuales dependen las comunidades durante las sequías que se inhabilitan los sistema de captación de la lluvia. Se demuestra que una aplicación recientemente desarrollada de un modelo algebraico fácilmente transportable para estimar los espesores de las lentes de agua dulce de las islas atolones en los FSM, es una componente clave de la evaluación de los recursos de agua subterránea de los FSM. Específicamente, el modelo provee estimaciones del espesor de las lentes de las islas atolones en los FSM durante condiciones normales y de sequía. El modelo fue probado para el uso en los FSM a través de la comparación con datos disponibles de lentes tanto en condiciones de precipitaciones promedio como de sequías intensas, y luego aplicado a las islas principales de cada atolón dentro de los FSM. Los resultados indican que de las 105 islas principales en las islas atolones de los FSM, solo en seis casos probablemente podrían retener suficiente agua subterránea para sostener a la comunidad local durante una intensa sequía.摘要密克罗尼西亚联邦(FSM)的32个环礁岛的水资源持续受到厄尔尼诺现象导致的干旱和其它自然灾害的威胁。在强调环礁岛社区可持续发展的政府政策的指导之下,当地的管理人员正在寻求一种可以预测地下淡水可用性的工具,使环礁岛社区可以在降雨汇流系统失效的旱季对其加以利用。本文应用了一种近期建立起来的便携的代数模型,用来估计FSM的环礁岛中淡水透镜体的厚度,这是FSM地下水资源评价的重要组成部分。具体来说,这个模型估计了正常与干旱条件下FSM环礁岛的透镜体厚度。此模型通过在FSM应用而进行检验测试,对比了在平均降雨条件和严重的干旱条件下可用的透镜体数据,然后应用到了FSM的每个环礁岛的主要岛屿。结果表明,在FSM的105个主要岛屿中,只有6个可以在严重的干旱条件下为当地的社区提供足够的地下水源。ResumoOs recursos hídricos dos 32 atóis dos Estados Federados da Micronésia (EFM) estão sob ameaça contínua das secas induzidas pelo El Niño e outros desastres naturais. Com a ênfase das políticas públicas no desenvolvimento sustentável das comunidades de ilhas-atol os gestores locais têm necessidade de possuir ferramentas para prever as alterações na disponibilidade de água doce subterrânea de que dependem as comunidades durante os períodos de seca que provocam o colapso dos sistemas de captação de água da chuva. Este trabalho apresenta a aplicação de um modelo algébrico recém-desenvolvido, de fácil manuseamento, que permite estimar a espessura da lente da de água doce nas ilhas-atol dos EFM, uma componente crucial para a avaliação dos recursos hídricos subterrâneos dos EFM. Mais especificamente, o modelo fornece estimativas da espessura da lente de água doce nas ilhas-atol no FSM durante condições normais e em eventos de seca. O modelo foi testado para uso nos EFM através de comparação com os dados de lentes de água doce disponíveis para ambas condições médias de precipitação e de seca intensa, sendo posteriormente aplicado nas ilhas principais de cada atol dentro dos EFM. Os resultados indicam que, das 105 principais ilhas em atóis dos EFM apenas seis poderão provavelmente reter água subterrânea suficiente para sustentar a comunidade local durante uma seca intensa.

Estimating spatially-variable first-order rate constants in groundwater reactive transport systems

Numerical reactive transport models are often used as tools to assess aquifers contaminated with reactive groundwater solutes as well as investigating mitigation scenarios. The ability to accurately simulate the fate and transport of solutes, however, is often impeded by a lack of information regarding the parameters that define chemical reactions. In this study, we employ a steady-state Ensemble Kalman Filter (EnKF), a data assimilation algorithm, to provide improved estimates of a spatially-variable first-order rate constant λ through assimilation of solute concentration measurement data into reactive transport simulation results. The methodology is applied in a steady-state, synthetic aquifer system in which a contaminant is leached to the saturated zone and undergoes first-order decay. Multiple sources of uncertainty are investigated, including hydraulic conductivity of the aquifer and the statistical parameters that define the spatial structure of the parameter field. For the latter scenario, an iterative method is employed to identify the statistical mean of λ of the reference system. Results from all simulations show that the filter scheme is successful in conditioning the λ ensemble to the reference λ field. Sensitivity analyses demonstrate that the estimation of the λ values is dependent on the number of concentration measurements assimilated, the locations from which the measurement data are collected, the error assigned to the measurement values, and the correlation length of the λ fields.

Assessing the effectiveness of land and water management practices on nonpoint source nitrate levels in an alluvial stream–aquifer system

The search for ways to allay subsurface nitrate pollution and loading to streams over broad regional landscapes is taken up using a calibrated groundwater model supported by extensive field data. Major processes of transport and chemical reaction are considered in the irrigated vadose zone and the underlying alluvial aquifer in interaction with Colorados Lower Arkansas River and its tributaries. Simulation of a variety of best management practices reveals that there is potential to lower regional nitrate concentrations in groundwater by up to about 40% and mass loading to the river network by up to 70% over a four-decade span. Over the 27BMP scenarios considered in this study, the most effective singular measures are reduction of fertilizer application and sealing of irrigation canals, while combinations of reduced fertilizer application, reduced irrigation application, canal sealing, and enhanced riparian buffer zones are predicted to have the greatest overall impact. Intermittent fallowing of 25% of the land to lease irrigation water also is found to be promising, resulting in a forecasted decrease of about 15% in nitrate groundwater loading to streams. Due to the strong similarity between the study region and other irrigated, fertilized alluvial river valley stream-aquifer systems worldwide, results of this study are expected to be broadly applicable.

Effects of Marine Overwash for Atoll Aquifers: Environmental and Human Factors

Inundation of atoll islands by marine overwash is a serious threat to fresh groundwater, which can be a critical emergency water resource after artificial storage or other water resource infrastructure has been exhausted or destroyed. In contrast to drought, which slowly exhausts water supplies and often can be forecasted in time, overwash can occur with little warning and can ruin both rain catchment storage and groundwater reserves. In this study, a SUTRA-based model is applied to estimate how groundwater contamination by overwash and subsequent recovery of fresh groundwater are influenced by geologic factors (aquifer hydraulic conductivity, dispersivity, and the presence or absence of a reef flat plate), the seasonal timing of the event (wet vs. dry), and the presence of hand-dug wells that penetrate the reef flat plate. Actual tidal and rainfall data from regions in the western Pacific are applied to simulated 30-month recovery periods for hypothetical islands with properties and conditions characteristic of the western Pacific. For all scenarios, results indicate that 12 to 16 months are required to achieve 60% recovery of fresh groundwater. However, the time required to restore useful quantities of groundwater to acceptable salt concentration at depths typical of hand-dug wells is only 3 to 6 months. Of particular interest is the influence of the reef flat plate, which acts as a barrier to infiltrating seawater, thus preserving a pocket of confined freshwater during an overwash event and the recovery, which could probably be utilized if the necessary tools and equipment are on hand.

Review: Selenium contamination, fate, and reactive transport in groundwater in relation to human health

Selenium (Se) is an essential micro-nutrient for humans, but can be toxic at high levels of intake. Se deficiency and Se toxicity are linked with serious diseases, with some regions worldwide experiencing Se deficiency due to Se-poor rocks and soils and other areas dealing with Se toxicity due to the presence of Se-enriched geologic materials. In addition, Se is consumed primarily through plants that take up Se from soil and through animal products that consume these plants. Hence, the soil and groundwater system play important roles in determining the effect of Se on human health. This paper reviews current understanding of Se fate and transport in soil and groundwater systems and its relation to human health, with a focus on alluvial systems, soil systems, and the interface between alluvial systems and Cretaceous shale that release Se via oxidation processes. The review focuses first on the relation between Se and human health, followed by a summary of Se distribution in soil-aquifer systems, with an emphasis on the quantitative relationship between Se content in soil and Se concentration in underlying groundwater. The physical, chemical, and microbial processes that govern Se fate and transport in subsurface systems then are presented, followed by numerical modeling techniques used to simulate these processes in study regions and available remediation strategies for either Se-deficient or Se-toxic regions. This paper can serve as a guide to any field, laboratory or modeling study aimed at assessing Se fate and transport in groundwater systems and its relation to human health.RésuméLe sélénium (Se) est un micro-nutriment essential pour les humains, mais peut être toxique pour des apports à des niveaux élevés. La carence en Se et la toxicité du Se sont associées à des maladies graves, certaines régions du monde connaissant une carence en Se due à des roches et sols pauvres en Se et d’autres régions font face à une toxicité en Se à cause de la présence de matériaux géologiques enrichis en Se. En outre, le Se est consommé principalement par les plantes qui prélèvent le Se du sol et à travers les produits d’origine animale qui consomment ces plantes. Par conséquent, le système sol et eaux souterraines joue un rôle important dans la détermination de l’effet du Se sur la santé humaine. Cet article passe en revue la compréhension actuelle de le devenir et du transport du Se dans les systèmes du sol et des eaux souterraines et leurs relations avec la santé humaine, en mettant l’accent sur les systèmes alluviaux, les systèmes du sol, et l’interface entre les systèmes alluviaux et les schistes du Crétacé qui libèrent du Se via les processus d’oxydation. Cette revue met l’accent tout d’abord sur la relation entre Se et la santé humaine, puis est suivie par un résumé sur la distribution du SE dans les systèmes sol-aquifère, en mettant l’accent sur la relation quantitative entre la teneur en Se du sol et la concentration en Se dans les eaux souterraines sous-jacentes. Les processus physiques, chimiques, et microbiologiques qui gouvernent le devenir du Se et son transport dans les systèmes souterrains sont ainsi présentés, suivis des techniques de modélisation numérique utilisées pour simuler ces processus dans des études régionales et des stratégies disponibles de remédiation pour les régions soit déficitaires ou toxiques en Se. Cet article peut servir de guide pour toutes les études de terrain, de laboratoire et de modélisation visant à évaluer le devenir et le transport du Se dans les systèmes aquifères et sa relation avec la santé humaine.ResumenEl selenio (Se) es un micro-nutriente esencial para los seres humanos, pero puede ser tóxico con niveles altos de ingesta. La deficiencia de Se y la toxicidad Se están asociados con graves enfermedades, con algunas regiones del mundo que sufren deficiencia de Se, debido a la presencia de rocas y suelos pobres en Se y otras cuestiones relacionadas con la toxicidad del Se debido a la existencia de materiales geológicos enriquecidos en Se.Además, el Se se consume principalmente a través de plantas que absorben el Se del suelo y a través de productos animales que consumen estas plantas. Por lo tanto, el suelo y el sistema de agua subterránea juegan un papel importante en la determinación del efecto del Se en la salud humana. En este trabajo se revisa el conocimiento actual del transporte y el destino en sistemas de suelo y agua subterránea y su relación con la salud humana, enfocándose en los sistemas aluviales, sistemas del suelo y la interfase entre los sistemas aluviales y los esquistos cretácicos que liberan Se a través de procesos de oxidación. La revisión se centra primero en la relación entre el Se y la salud humana, seguida por un resumen de la distribución Se en los sistemas suelo-acuífero, con énfasis en la relación cuantitativa entre el contenido de Se en el suelo y la concentración de Se en el agua subterránea. A continuación se presentan los procesos físicos, químicos y microbianos que gobiernan el transporte y el destino en sistemas subsuperficiales, seguidos por técnicas de modelado numérico utilizadas para simular estos procesos en las regiones de estudio y las estrategias de remediación disponibles para regiones deficiente en Se o tóxicas en Se. El presente documento puede servir de guía para cualquier estudio de campo, de laboratorio o de modelización destinado a evaluar el transporte y el en los sistemas de agua subterránea y su relación con la salud humana.摘要硒是人类基本的微量营养成分,但高量摄入是有毒的。硒匮乏和硒中毒与一些严重的疾病相关,世界上有些地区由于贫硒岩石和土壤出现硒匮乏,而有些地区由于存在富硒地质矿物出现硒中毒。另外,硒主要通过从土壤中摄入硒的植物及通过消耗这些植物的动物产品获取的。因此,土壤和地下水系统在确定硒对人类健康的影响方面发挥着重要作用。本文论述了人们目前对土壤和地下水系统中硒的演化和运移的了解和认识,重点是冲积系统、土壤系统、冲积系统和通过氧化过程释放硒的白垩页岩之间的界面。首先重点论述了硒和人类健康之间的关系,然后概述了硒在土壤-含水层系统中的分布,强调了土壤中硒含量和下伏地下水中硒浓度之间的定量关系。随后论述了控制地下系统中硒演化和运移的物理、化学和微生物过程,还论述了用于模拟研究区这些过程的数值模拟技术以及硒匮乏地区和硒中毒地区现有的治理对策。本文可作为任何旨在评价地下水系统中硒演化和运移及其与人类健康关系的室外、室内或模拟研究。ResumoSelênio (Se) é um micronutriente essencial para humanos, mas pode ser tóxico sob altas doses de ingestão. Deficiência de Se e toxicidade de Se são associadas a diversas doenças, com algumas regiões mundo afora vivenciando deficiência em Se devido a rochas e solos pobres em Se e outras áreas lidando com toxicidades por Se devido a presença de materiais geológicos enriquecidos com Se. Além disso, Se é consumido primariamente pelas plantas que extraem Se do solo e pelos produtos animais que consomem essas plantas. Assim, o solo e o sistema de águas subterrâneas desempenham papeis importantes na determinação do efeito do Se na saúde humana. Este artigo analisa a compreensão atual do transporte e do destino do Se no solo e nos sistemas de águas subterrâneas e sua relação com a saúde humana, com foco nos sistemas aluviais, sistemas do solo e interface entre sistemas aluviais e xisto cretáceo que liberam Se via processos de oxidação. A revisão concentra-se primeiro na relação entre o Se e saúde humana, seguida de um resumo da distribuição de Se nos sistemas solo-aquífero, com ênfase na relação quantitativa entre o conteúdo de Se no solo e a concentração de Se nas águas subterrâneas subjacentes. Os processos físicos, químicos e microbianos que regem o destino e o transporte do Se em sistemas subterrâneos são apresentados, seguidos por técnicas de modelagem numérica usadas para simular esses processos em regiões de estudo e estratégias de remediação disponíveis tanto para regiões de deficiente de Se como Se tóxicas. Este trabalho pode servir de guia para qualquer estudo de campo, de laboratório ou de modelagem destinado a avaliar o destino e o transporte do Se em sistemas de águas subterrâneas e sua relação com a saúde humana.

SWATMOD-Prep: Graphical User Interface for Preparing Coupled SWAT-MODFLOW Simulations†

This article presents SWATMOD-Prep, a graphical user interface that couples a SWAT watershed model with a MODFLOW groundwater flow model. The interface is based on a recently published SWAT-MODFLOW code that couples the models via mapping schemes. The spatial layout of SWATMOD-Prep guides the user through the process of importing shape files (sub-basins, hydrologic response units [HRUs], river network) from an existing SWAT model, creating a grid, performing necessary geo-processing operations to link the models, writing out SWAT-MODFLOW files, and running the simulation. The option of creating a new single-layer MODFLOW model for near-surface alluvial aquifers is available, with the user prompted to provide groundwater surface elevation (through a digital elevation model), aquifer thickness, and necessary aquifer parameter values. The option of simulating nitrate transport in the aquifer also is available, using the reactive transport model RT3D. The interface is in the public domain. It is programmed in Python, with various software packages used for geo-processing operations (e.g., selection, intersection of rasters) and inputting/outputting data, and is written for Windows. The use of SWATMOD-Prep is demonstrated for the Little River Experimental Watershed, Georgia. SWATMOD-Prep and SWAT-MODFLOW executables are available with an accompanying users manual at: http://swat.tamu.edu/software/swat-modflow/. The users manual also accompanies this article as Supporting Information.