Bryant C. Jurgens

Comparison of particle-tracking and lumped-parameter age-distribution models for evaluating vulnerability of production wells to contamination

Environmental age tracers have been used in various ways to help assess vulnerability of drinking-water production wells to contamination. The most appropriate approach will depend on the information that is available and that which is desired. To understand how the well will respond to changing nonpoint-source contaminant inputs at the water table, some representation of the distribution of groundwater ages in the well is needed. Such information for production wells is sparse and difficult to obtain, especially in areas lacking detailed field studies. In this study, age distributions derived from detailed groundwater-flow models with advective particle tracking were compared with those generated from lumped-parameter models to examine conditions in which estimates from simpler, less resource-intensive lumped-parameter models could be used in place of estimates from particle-tracking models. In each of four contrasting hydrogeologic settings in the USA, particle-tracking and lumped-parameter models yielded roughly similar age distributions and largely indistinguishable contaminant trends when based on similar conceptual models and calibrated to similar tracer data. Although model calibrations and predictions were variably affected by tracer limitations and conceptual ambiguities, results illustrated the importance of full age distributions, rather than apparent tracer ages or model mean ages, for trend analysis and forecasting.RésuméDes traceurs environnementaux ont été utilisés de différentes façons pour aider à évaluer la vulnérabilité de puits de production d’eau potable à la pollution. L’approche la plus appropriée dépendra de l’information disponible et de celle qui est recherchée. Pour comprendre comment le puits répond à des sources de polluants non localisées et variables, il faut nécessairement une représentation de la distribution des âges de l’eau dans le puits. Une telle information sur des puits productifs est rare et difficile à obtenir, particulièrement dans les zones où manquent des études de terrain détaillées. Dans cette étude, des distributions d’âge déduites de modèles de flux souterrain détaillés avec suivi de particules advectives ont été comparés avec ceux générés par des modèles paramétriques globaux pour examiner les conditions dans lesquelles des estimations tirées de modèles plus simples, de modèles paramétrés moins liés à la ressource pourraient être utilisés à la place d’estimations issues de modèles de suivi de particule. Dans chacun des quatre sites hydrogéologiques contrastées aux USA, des modèles de suivi de particule et des modèles à paramétrage global basées sur des schémas conceptuels similaires et paramétrés pour des traçage similaires ont fourni en gros des distributions d’âge et des tendances de contamination largement semblables. Bien que les paramétrages de modèle et prévisions aient été affectés de façon variable par des limitations du traçage et par des ambiguités conceptuelles, les résultats ont illustré l’importance des distributions de l’âge total, comparativement aux âges apparents de traçage ou âges moyens du modèle, pour l’analyse de tendance et pour la prévision.ResumenLos trazadores de edad ambiental han sido usados de varias maneras para ayudar a evaluar de vulnerabilidad de los pozos de producción de agua potable a la contaminación. El enfoque más apropiado depende de la información que está disponible y que la que es necesaria. Para entender como el pozo responde a la entrada variable de la fuente no puntual del contaminante en la capa freática se necesita alguna representación de la distribución de las edades de agua subterránea en el pozo. Tal información para los pozos de producción está dispersa y es difícil de obtener, especialmente en áreas carentes de un estudio detallado de campo. En este estudio se compararon las distribuciones de edades derivadas de modelos detallados de flujo de agua subterránea con el seguimiento advectivo de partículas para examinar las condiciones en las cuales las estimaciones a partir de modelos de parámetros concentrados más simples y menos intensivamente dependientes de los recursos podrían ser usados en lugar de la estimación de los modelos de seguimiento de partículas. En cada una de las cuatro configuraciones hidrogeológicas contrastantes en EEUU, el seguimiento de partículas y los modelos de parámetros concentrados brindaron a grandes rasgos las distribuciones de edad y las tendencias de contaminación resultan grandemente indistinguibles cuando estaban basadas en modelos conceptuales similares y calibrados con datos de similares trazadores. A pesar que las calibraciones del modelo y las predicciones estuvieron afectadas variablemente por las limitaciones de los trazadores y las ambigüedades conceptuales, los resultados ilustraron la importancia de las distribuciones de edad completa, más bien que las edades aparentes de trazadores o de modelos de edades medias, para los análisis de tendencias y predicciones.摘要用于评价饮用水生产井污染脆弱性的环境年龄示踪已被广泛应用。最适合的方法取决于可利用的信息及所需数据。为了解井对水面上非点源污染物输入变化的响应,必须知道地下水年龄的分布特征。生产井的这些信息较少,且难获得,尤其是在缺少详细野外调查的地区。本次研究,年龄分布来源于详细的地下水流动模型以及平流溶质运移,并与来源于集中参数模拟的年龄分布进行比较,以确定简单少源的加强集中参数模型估计值可用于替代颗粒示踪模型估计值的条件 。在美国的四个对比水文地质设置点,颗粒示踪及集中参数模拟法基于简单概念模型,对简单示踪数据进行验证,得出了大致的简单年龄分布及主要的难分辨的污染物趋势。尽管模型验证与预测受示踪物局限性及概念的不准确性的变化影响,结果表明对趋势分析及预测而言,完整的年龄分布比表观示踪年龄或模型平均年龄更有意义。ResumoTraçadores ambientais de idade têm sido usados de várias maneiras para ajudar a avaliar a vulnerabilidade das captações de produção de água potável à contaminação. A abordagem mais adequada dependerá da informação que está disponível e do que é desejado. Para entender como o poço vai responder às variações das entradas de contaminantes difusos no nível freático, é necessária alguma representação da distribuição das idades das águas subterrâneas no poço. Estas informações são escassas e difíceis de obter para furos de produção, especialmente em áreas carentes de estudos de campo pormenorizados. Neste estudo, as distribuições de idade provenientes de modelos de fluxo de águas subterrâneas detalhados com rastreio de partículas advetivas foram comparadas com aqueles gerados a partir de modelos de parâmetros agregados, para examinar as condições em que as estimativas obtidas a partir de modelos mais simples, com menos recursos, poderiam ser usadas em vez das estimativas a partir de modelos de partículas de rastreio. Uma em cada quatro das configurações hidrogeológicas nos EUA, obtidas por modelos de partículas de rastreio e por modelos de parâmetros agregados, produziram distribuições de idade mais ou menos semelhantes e tendências de contaminação em grande parte indistinguíveis quando baseadas em modelos conceptuais semelhantes e calibrados para idênticos dados de traçadores. Embora a calibração e as previsões do modelo tenham sido afetadas de forma variável por limitações dos traçadores e por ambiguidades conceptuais, os resultados demonstraram a importância das distribuições de idade total, ao invés de modelos de idades aparentes de traçadores ou de modelos de média de idades, para análises de tendências e previsões.

Effects of Groundwater Development on Uranium: Central Valley, California, USA

Uranium (U) concentrations in groundwater in several parts of the eastern San Joaquin Valley, California, have exceeded federal and state drinking water standards during the last 20 years. The San Joaquin Valley is located within the Central Valley of California and is one of the most productive agricultural areas in the world. Increased irrigation and pumping associated with agricultural and urban development during the last 100 years have changed the chemistry and magnitude of groundwater recharge, and increased the rate of downward groundwater movement. Strong correlations between U and bicarbonate suggest that U is leached from shallow sediments by high bicarbonate water, consistent with findings of previous work in Modesto, California. Summer irrigation of crops in agricultural areas and, to lesser extent, of landscape plants and grasses in urban areas, has increased Pco(2) concentrations in the soil zone and caused higher temperature and salinity of groundwater recharge. Coupled with groundwater pumping, this process, as evidenced by increasing bicarbonate concentrations in groundwater over the last 100 years, has caused shallow, young groundwater with high U concentrations to migrate to deeper parts of the groundwater system that are tapped by public-supply wells. Continued downward migration of U-affected groundwater and expansion of urban centers into agricultural areas will likely be associated with increased U concentrations in public-supply wells. The results from this study illustrate the potential long-term effects of groundwater development and irrigation-supported agriculture on water quality in arid and semiarid regions around the world.

Accuracy of travel time distribution (TTD) models as affected by TTD complexity, observation errors, and model and tracer selection

Analytical models of the travel time distribution (TTD) from a source area to a sample location are often used to estimate groundwater ages and solute concentration trends. The accuracies of these models are not well known for geologically complex aquifers. In this study, synthetic data sets were used to quantify the accuracy of four analytical TTD models as affected by TTD complexity, observation errors, model selection, and tracer selection. Synthetic TTDs and tracer data were generated from existing numerical models with complex hydrofacies distributions for 1 public-supply well and 14 monitoring wells in the Central Valley, California. Analytical TTD models were calibrated to synthetic tracer data, and prediction errors were determined for estimates of TTDs and conservative tracer ( NO3−) concentrations. Analytical models included a new, scale-dependent dispersivity model (SDM) for two-dimensional transport from the water table to a well and three other established analytical models. The relative influence of the error sources (TTD complexity, observation error, model selection, and tracer selection) depended on the type of prediction. Geological complexity gave rise to complex TTDs in monitoring wells that strongly affected errors of the estimated TTDs. However, prediction errors for NO3− and median age depended more on tracer concentration errors. The SDM tended to give the most accurate estimates of the vertical velocity and other predictions, although TTD model selection had minor effects overall. Adding tracers improved predictions if the new tracers had different input histories. Studies using TTD models should focus on the factors that most strongly affect the desired predictions.

A ternary age-mixing model to explain contaminant occurrence in a deep supply well

The age distribution of water from a public-supply well in a deep alluvial aquifer was estimated and used to help explain arsenic variability in the water. The age distribution was computed using a ternary mixing model that combines three lumped parameter models of advection-dispersion transport of environmental tracers, which represent relatively recent recharge (post-1950s) containing volatile organic compounds (VOCs), old intermediate depth groundwater (about 6500 years) that was free of drinking-water contaminants, and very old, deep groundwater (more than 21,000 years) containing arsenic above the USEPA maximum contaminant level of 10 µg/L. The ternary mixing model was calibrated to tritium, chloroflorocarbon-113, and carbon-14 (14C) concentrations that were measured in water samples collected on multiple occasions. Variability in atmospheric 14C over the past 50,000 years was accounted for in the interpretation of 14C as a tracer. Calibrated ternary models indicate the fraction of deep, very old groundwater entering the well varies substantially throughout the year and was highest following long periods of nonoperation or infrequent operation, which occured during the winter season when water demand was low. The fraction of young water entering the well was about 11% during the summer when pumping peaked to meet water demand and about 3% to 6% during the winter months. This paper demonstrates how collection of multiple tracers can be used in combination with simplified models of fluid flow to estimate the age distribution and thus fraction of contaminated groundwater reaching a supply well under different pumping conditions.

Large decadal-scale changes in uranium and bicarbonate in groundwater of the irrigated western U.S

Samples collected about one decade apart from 1105 wells from across the U.S. were compiled to assess whether uranium concentrations in the arid climate are linked to changing bicarbonate concentrations in the irrigated western U.S. Uranium concentrations in groundwater were high in the arid climate in the western U.S, where uranium sources are abundant. Sixty-four wells (6%) were above the U.S. EPA MCL of 30μg/L; all but one are in the arid west. Concentrations were low to non-detectable in the humid climate. Large uranium and bicarbonate increases (differences are greater than the uncertainty in concentrations) occur in 109 wells between decade 1 and decade 2. Similarly, large uranium and bicarbonate decreases occur in 76 wells between the two decades. Significantly more wells are concordant (uranium and bicarbonate are both going the same direction) than discordant (uranium and bicarbonate are going opposite directions) (p<0.001; Chi-square test). The largest percent difference in uranium concentrations occur in wells where uranium is increasing and bicarbonate is also increasing. These large differences occur mostly in the arid climate. Results are consistent with the hypothesis that changing uranium concentrations are linked to changes in bicarbonate in irrigated areas of the western U.S.

A hybrid machine learning model to predict and visualize nitrate concentration throughout the Central Valley aquifer, California, USA

Intense demand for water in the Central Valley of California and related increases in groundwater nitrate concentration threaten the sustainability of the groundwater resource. To assess contamination risk in the region, we developed a hybrid, non-linear, machine learning model within a statistical learning framework to predict nitrate contamination of groundwater to depths of approximately 500m below ground surface. A database of 145 predictor variables representing well characteristics, historical and current field and landscape-scale nitrogen mass balances, historical and current land use, oxidation/reduction conditions, groundwater flow, climate, soil characteristics, depth to groundwater, and groundwater age were assigned to over 6000 private supply and public supply wells measured previously for nitrate and located throughout the study area. The boosted regression tree (BRT) method was used to screen and rank variables to predict nitrate concentration at the depths of domestic and public well supplies. The novel approach included as predictor variables outputs from existing physically based models of the Central Valley. The top five most important predictor variables included two oxidation/reduction variables (probability of manganese concentration to exceed 50ppb and probability of dissolved oxygen concentration to be below 0.5ppm), field-scale adjusted unsaturated zone nitrogen input for the 1975 time period, average difference between precipitation and evapotranspiration during the years 1971-2000, and 1992 total landscape nitrogen input. Twenty-five variables were selected for the final model for log-transformed nitrate. In general, increasing probability of anoxic conditions and increasing precipitation relative to potential evapotranspiration had a corresponding decrease in nitrate concentration predictions. Conversely, increasing 1975 unsaturated zone nitrogen leaching flux and 1992 total landscape nitrogen input had an increasing relative impact on nitrate predictions. Three-dimensional visualization indicates that nitrate predictions depend on the probability of anoxic conditions and other factors, and that nitrate predictions generally decreased with increasing groundwater age.

Methane and benzene in drinking-water wells overlying the Eagle Ford, Fayetteville, and Haynesville Shale hydrocarbon production areas

Water wells (n = 116) overlying the Eagle Ford, Fayetteville, and Haynesville Shale hydrocarbon production areas were sampled for chemical, isotopic, and groundwater-age tracers to investigate the occurrence and sources of selected hydrocarbons in groundwater. Methane isotopes and hydrocarbon gas compositions indicate most of the methane in the wells was biogenic and produced by the CO2 reduction pathway, not from thermogenic shale gas. Two samples contained methane from the fermentation pathway that could be associated with hydrocarbon degradation based on their co-occurrence with hydrocarbons such as ethylbenzene and butane. Benzene was detected at low concentrations (<0.15 μg/L), but relatively high frequencies (2.4-13.3% of samples), in the study areas. Eight of nine samples containing benzene had groundwater ages >2500 years, indicating the benzene was from subsurface sources such as natural hydrocarbon migration or leaking hydrocarbon wells. One sample contained benzene that could be from a surface release associated with hydrocarbon production activities based on its age (10 ± 2.4 years) and proximity to hydrocarbon wells. Groundwater travel times inferred from the age-data indicate decades or longer may be needed to fully assess the effects of potential subsurface and surface releases of hydrocarbons on the wells.

Effects of Seasonal Operation on the Quality of Water Produced by Public-Supply Wells

Seasonal variability in groundwater pumping is common in many places, but resulting effects of seasonal pumping stress on the quality of water produced by public-supply wells are not thoroughly understood. Analysis of historical water-quality samples from public-supply wells completed in deep basin-fill aquifers in Modesto, California (134 wells) and Albuquerque, New Mexico (95 wells) indicates that several wells have seasonal variability in concentrations of contaminants of concern. In Modesto, supply wells are more likely to produce younger groundwater with higher nitrate and uranium concentrations during the summer (high) pumping season than during the winter (low) pumping season. In Albuquerque, supply wells are more likely to produce older groundwater with higher arsenic concentrations during the winter pumping season than during the summer pumping season. Seasonal variability in contaminant concentrations in Modesto is influenced primarily by effects of summer pumping on vertical hydraulic gradients that drive migration of shallow groundwater through the aquifer to supply wells. Variability in Albuquerque is influenced primarily by the period of time that a supply well is idle, allowing its wellbore to act as a conduit for vertical groundwater flow and contaminant migration. However, both processes are observed in each study area. Similar findings would appear to be likely in other alluvial basins with stratified water quality and substantial vertical head gradients. Results suggest that even in aquifers dominated by old groundwater, changes to seasonal pumping patterns and/or to depth of well completion can help reduce vulnerability to selected contaminants of either natural or anthropogenic origin.

Educational webtool illustrating groundwater age effects on contaminant trends in wells

Trends in concentrations of nonpoint-source contaminants in wells, springs, and streams are related to the history of contamination in groundwater recharge and the age distribution in the groundwater discharge. The age distribution in discharge depends on the groundwater age distribution in the aquifer and the subset of flowpaths that are sampled by the discharge. Groundwater travel times from recharge to discharge are variable; consequently, responses at discharge locations to changing contaminant loading in recharge can include delayed initial responses, dilution of peak concentrations, and prolonged flushing times. These effects are well understood in principle and have important consequences for water resource management (Eberts et al. 2013), but their implications may not be easy to visualize or communicate.

Using groundwater age distributions to understand changes in methyl tert-butyl ether (MtBE) concentrations in ambient groundwater, northeastern United States

Temporal changes in methyl tert-butyl ether (MtBE) concentrations in groundwater were evaluated in the northeastern United States, an area of the nation with widespread low-level detections of MtBE based on a national survey of wells selected to represent ambient conditions. MtBE use in the U.S. peaked in 1999 and was largely discontinued by 2007. Six well networks, each representing specific areas and well types (monitoring or supply wells), were each sampled at 10year intervals between 1996 and 2012. Concentrations were decreasing or unchanged in most wells as of 2012, with the exception of a small number of wells where concentrations continue to increase. Statistically significant increasing concentrations were found in one network sampled for the second time shortly after the peak of MtBE use, and decreasing concentrations were found in two networks sampled for the second time about 10years after the peak of MtBE use. Simulated concentrations from convolutions of estimates for concentrations of MtBE in recharge water with age distributions from environmental tracer data correctly predicted the direction of MtBE concentration changes in about 65% of individual wells. The best matches between simulated and observed concentrations were found when simulating recharge concentrations that followed the pattern of national MtBE use. Some observations were matched better when recharge was modeled as a plume moving past the well from a spill at one point in time. Modeling and sample results showed that wells with young median ages and narrow age distributions responded more quickly to changes in the contaminant source than wells with older median ages and broad age distributions. Well depth and aquifer type affect these responses. Regardless of the timing of decontamination, all of these aquifers show high susceptibility for contamination by a highly soluble, persistent constituent.