Leon J. Kauffman

Limited Occurrence of Denitrification in Four Shallow Aquifers in Agricultural Areas of the United States

The ability of natural attenuation to mitigate agricultural nitrate contamination in recharging aquifers was investigated in four important agricultural settings in the United States. The study used laboratory analyses, field measurements, and flow and transport modeling for monitoring well transects (0.5 to 2.5 km in length) in the San Joaquin watershed, California, the Elkhorn watershed, Nebraska, the Yakima watershed, Washington, and the Chester watershed, Maryland. Ground water analyses included major ion chemistry, dissolved gases, nitrogen and oxygen stable isotopes, and estimates of recharge date. Sediment analyses included potential electron donors and stable nitrogen and carbon isotopes. Within each site and among aquifer-based medians, dissolved oxygen decreases with ground water age, and excess N(2) from denitrification increases with age. Stable isotopes and excess N(2) imply minimal denitrifying activity at the Maryland and Washington sites, partial denitrification at the California site, and total denitrification across portions of the Nebraska site. At all sites, recharging electron donor concentrations are not sufficient to account for the losses of dissolved oxygen and nitrate, implying that relict, solid phase electron donors drive redox reactions. Zero-order rates of denitrification range from 0 to 0.14 micromol N L(-1)d(-1), comparable to observations of other studies using the same methods. Many values reported in the literature are, however, orders of magnitude higher, which is attributed to a combination of method limitations and bias for selection of sites with rapid denitrification. In the shallow aquifers below these agricultural fields, denitrification is limited in extent and will require residence times of decades or longer to mitigate modern nitrate contamination.

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.

Spatial Variability of Groundwater Recharge and its Effect on Shallow Groundwater Quality in Southern New Jersey

ity in the Glassboro, NJ area (Fig. 1). The Darcian method was used to estimate groundwater recharge Point estimates of groundwater recharge at 48 sediment-coring from water-retention parameters and unsaturated hylocations vary substantially ( 18.5–1840 cm yr 1) in a 930-km2 area of southern New Jersey. Darcian estimates of steady, long-term recharge draulic conductivity on the basis of sediment texture made at depth in the unsaturated zone were estimated using pedoand moisture content data obtained near the water table transfer functions of soil texture and interpolated (mapped) with at 48 locations in the study area. The recharge estimates nonparametric methods to assess aquifer vulnerability in the area. were geostatistically analyzed to evaluate the spatial The probability of exceeding the median recharge (29.1 cm yr 1) is variability of measured sediment properties, to map relow in the southwestern and northeastern portions of the study area charge with respect to land use, and to derive statistical and high in the eastern and southeastern portions. Estimated recharge distributions of recharge at specific locations in the study is inversely related to measured percentage clay and positively related area. The recharge estimates were compared with soils to the percentage of well-drained soils near wells. Spatial patterns of and topographic data to determine whether recharge recharge estimates, exceedance probabilities, and clay content indicate could be accurately predicted from landscape characterthat sediment texture controls recharge in the study area. Relations with land elevation and a topographic wetness index were statistically istics. Finally, recharge estimates were compared with insignificant. Nitrate concentration and atrazine (6-chloro-N2-ethylconcentrations of NO3 and atrazine to evaluate potential N4-isopropyl-1,3,5-triazine-2,4-diamine) percentage detection in sameffects on the quality of shallow, recently recharged ples of shallow groundwater (typically 10 m) are higher for low groundwater. In this study, “shallow groundwater” rerecharge sites ( 29.1 cm yr 1) than for high recharge sites ( 29.1 cm fers to depths 10 m. The depth of the screened interval yr 1) in agricultural and urban areas. Differences between high and below water in observation wells in the area is about low recharge sites in these areas are highly significant for NO3 concen3 m. The objectives of the study were to tration, but not for atrazine concentration. • evaluate the spatial variability of point estimates of ground-water recharge, • map recharge with respect to land use, and N contamination is considered the • compare recharge estimates with NO3 and atrazine single greatest threat to water quality (Corwin et concentrations in shallow groundwater. al., 1997). Preventing contamination of groundwater is crucial in areas where it is a major source of public and domestic supply. Knowing where an aquifer is vulneraMATERIALS AND METHODS ble to surface-derived contaminants would help managDescription of Study Area ers prioritize scarce resources for alternative management practices, monitoring, and cleanup. The study area (Fig. 1) comprises about 930 km within the Coastal Plain Physiographic province of southern New Jersey. Aquifer vulnerability studies at large spatial scales Population in the area has increased from about 50 000 people have used index methods, such as DRASTIC and SEEPin 1940 to about 250 000 in 2000. Groundwater withdrawals AGE (Navulur and Engel, 1996), or overlays made with from the surficial, Kirkwood-Cohansey aquifer system, which geographic information systems (GISs) (Nolan et al., consists of highly permeable unconsolidated sands and gravels, 1997). Both DRASTIC and SEEPAGE underestimated have recently increased to meet the growing demand for drinkcontamination potential by describing areas with high ing water. As of 1986, the Glassboro area comprised 21% NO3 concentration as low risk (Navulur and Engel, urban land, 26% agricultural land, and 39% undeveloped land 1996). Index and overlay methods provide only limited (Stackelberg et al., 1997). understanding of processes controlling the transport of The outcrop of the Kirkwood Formation, a confining unit water and chemicals in the unsaturated zone. Alternaabout 30 m thick, underlies the aquifer and forms the northwest boundary of the study area. Aquifer thickness increases tively, deterministic models can simulate water and to about 75 m at the southeastern boundary (Zapecza, 1989). chemical fluxes, but the spatial variability of sediment Unsaturated zone sediment in the study area consists mainly properties at field scales and above limits accuracy and of the Cohansey Sand, which was deposited during the Mioimposes large uncertainty on model predictions. cene Age on inner shelf, nearshore, and beach areas during In the current study, we used a combined determinisslow retreat of the sea. Sediments in the Cohansey Sand genertic–geostatistical approach to assess aquifer vulnerabilally are coarser at shallower depths, which is consistent with similarly deposited formations in the New Jersey Coastal Plain B.T. Nolan, U.S. Geological Survey, 413 National Center, Reston, (Zapecza, 1989). The Bridgeton Formation overlies the CoVA 20192; A.L. Baehr and L.J. Kauffman, U.S. Geological Survey, West Trenton, NJ. Received 27 Nov. 2002. Original Research Paper. Abbreviations: CCDF, conditional cumulative distribution function; *Corresponding author (btnolan@usgs.gov). DEM, digital elevation model; DO, dissolved oxygen; GIS, geographic information system; IGF, Indicative Goodness of Fit; IK, indicator Published in Vadose Zone Journal 2:677–691 (2003).  Soil Science Society of America kriging; KED, Kriging with external drift; MLR, multiple linear regression; PTF, pedotransfer function. 677 S. Segoe Rd., Madison, WI 53711 USA

On Modeling Weak Sinks in MODPATH

Regional groundwater flow systems often contain both strong sinks and weak sinks. A strong sink extracts water from the entire aquifer depth, while a weak sink lets some water pass underneath or over the actual sink. The numerical groundwater flow model MODFLOW may allow a sink cell to act as a strong or weak sink, hence extracting all water that enters the cell or allowing some of that water to pass. A physical strong sink can be modeled by either a strong sink cell or a weak sink cell, with the latter generally occurring in low-resolution models. Likewise, a physical weak sink may also be represented by either type of sink cell. The representation of weak sinks in the particle tracing code MODPATH is more equivocal than in MODFLOW. With the appropriate parameterization of MODPATH, particle traces and their associated travel times to weak sink streams can be modeled with adequate accuracy, even in single layer models. Weak sink well cells, on the other hand, require special measures as proposed in the literature to generate correct particle traces and individual travel times and hence capture zones. We found that the transit time distributions for well water generally do not require special measures provided aquifer properties are locally homogeneous and the well draws water from the entire aquifer depth, an important observation for determining the response of a well to non-point contaminant inputs.

Modeling the Potential Impact of Seasonal and Inactive Multi-Aquifer Wells on Contaminant Movement to Public Water-Supply Wells

Wells screened across multiple aquifers can provide pathways for the movement of surprisingly large volumes of groundwater to confined aquifers used for public water supply (PWS). Using a simple numerical model, we examine the impact of several pumping scenarios on leakage from an unconfined aquifer to a confined aquifer and conclude that a single inactive multi-aquifer well can contribute nearly 10% of total PWS well flow over a wide range of pumping rates. This leakage can occur even when the multi-aquifer well is more than a kilometer from the PWS well. The contribution from multi-aquifer wells may be greater under conditions where seasonal pumping (e.g., irrigation) creates large, widespread downward hydraulic gradients between aquifers. Under those conditions, water can continue to leak down a multi-aquifer well from an unconfined aquifer to a confined aquifer even when those multi-aquifer wells are actively pumped. An important implication is that, if an unconfined aquifer is contaminated, multi-aquifer wells can increase the vulnerability of a confined-aquifer PWS well.

Modeling the Effects of Naturally Occurring Organic Carbon on Chlorinated Ethene Transport to a Public Supply Well

The vulnerability of public supply wells to chlorinated ethene (CE) contamination in part depends on the availability of naturally occurring organic carbon to consume dissolved oxygen (DO) and initiate reductive dechlorination. This was quantified by building a mass balance model of the Kirkwood-Cohansey aquifer, which is widely used for public water supply in New Jersey. This model was built by telescoping a calibrated regional three-dimensional (3D) MODFLOW model to the approximate capture zone of a single public supply well that has a history of CE contamination. This local model was then used to compute a mass balance between dissolved organic carbon (DOC), particulate organic carbon (POC), and adsorbed organic carbon (AOC) that act as electron donors and DO, CEs, ferric iron, and sulfate that act as electron acceptors (EAs) using the Sequential Electron Acceptor Model in three dimensions (SEAM3D) code. SEAM3D was constrained by varying concentrations of DO and DOC entering the aquifer via recharge, varying the bioavailable fraction of POC in aquifer sediments, and comparing observed and simulated vertical concentration profiles of DO and DOC. This procedure suggests that approximately 15% of the POC present in aquifer materials is readily bioavailable. Model simulations indicate that transport of perchloroethene (PCE) and its daughter products trichloroethene (TCE), cis-dichloroethene (cis-DCE), and vinyl chloride (VC) to the public supply well is highly sensitive to the assumed bioavailable fraction of POC, concentrations of DO entering the aquifer with recharge, and the position of simulated PCE source areas in the flow field. The results are less sensitive to assumed concentrations of DOC in aquifer recharge. The mass balance approach used in this study also indicates that hydrodynamic processes such as advective mixing, dispersion, and sorption account for a significant amount of the observed natural attenuation in this system.