Jeff P. Raffensperger

Basin‐scale hydrogeologic modeling

Mathematical modeling of coupled groundwater flow, heat transfer, and chemical mass transport at the sedimentary basin scale has been increasingly used by Earth scientists studying a wide range of geologic processes including the formation of excess pore pressures, infiltration-driven metamorphism, heat flow anomalies, nuclear waste isolation, hydrothermal ore genesis, sediment diagenesis, basin tectonics, and petroleum generation and migration. These models have provided important insights into the rates and pathways of groundwater migration through basins, the relative importance of different driving mechanisms for fluid flow, and the nature of coupling between the hydraulic, thermal, chemical, and stress regimes. The mathematical descriptions of basin transport processes, the analytical and numerical solution methods employed, and the application of modeling to sedimentary basins around the world are the subject of this review paper. The special considerations made to represent coupled transport processes at the basin scale are emphasized. Future modeling efforts will probably utilize three-dimensional descriptions of transport processes, incorporate greater information regarding natural geological heterogeneity, further explore coupled processes, and involve greater field applications.

Modeling transport of dissolved silica in a forested headwater catchment: Implications for defining the hydrochemical response of observed flow pathways

Groundwater, subsurface stormflow, and overland flow components of discharge, derived from a hydrological model that was applied to a forested headwater catchment in north central Virginia, were used with measured stream water and lysimeter concentrations of dissolved silica to investigate the hydrochemical behavior of the catchment. Concentrations in base flow, taken to be a reflection of groundwater, vary with discharge, an observation in conflict with the typical assumption of constant concentration used in end-member mixing analyses. This observed flow dependence was modeled by considering the concentration in groundwater to be related to the saturation deficit in this zone. A positive correlation between the average groundwater saturation deficit and base flow dissolved silica concentrations is consistent with batch experiments and petrographic analysis of regolith core samples, which both indicate an increase in silica available for dissolution with depth in the groundwater zone. In the absence of subsurface storm flow zone sampling during rainfall events a constant concentration was assumed for this zone. Concentration-discharge (C-Q) paths in the stream were used to evaluate the modeled stream silica concentrations. An inconsistency in the direction of the modeled C-Q rotations suggests that the storm flow zone dissolved silica concentration may also vary with time, because of the “flushing” of high-concentration, preevent soil water on the rising limb of the storm hydrograph. For this catchment in Virginia the assumption of a constant concentration for subsurface storm flow, as well as for base flow, appears to be invalid.

Shallow subsurface storm flow in a forested headwater catchment: Observations and modeling using a modified TOPMODEL

Transient, perched water tables in the shallow subsurface are observed at the South Fork Brokenback Run catchment in Shenandoah National Park, Virginia. Crest piezometers installed along a hillslope transect show that the development of saturated conditions in the upper 1.5 m of the subsurface is controlled by total precipitation and antecedent conditions, not precipitation intensity, although soil heterogeneities strongly influence local response. The macroporous subsurface storm flow zone provides a hydrological pathway for rapid runoff generation apart from the underlying groundwater zone, a conceptualization supported by the two-storage system exhibited by hydrograph recession analysis. A modified version of TOPMODEL is used to simulate the observed catchment dynamics. In this model, generalized topographic index theory is applied to the subsurface storm flow zone to account for logarithmic storm flow recessions, indicative of linearly decreasing transmissivity with depth. Vertical drainage to the groundwater zone is required, and both subsurface reservoirs are considered to contribute to surface saturation.

On the stream function for variable‐density groundwater flow

When the stream function is used to describe variable-density ground water flow, it should be defined in terms of mass flux. The historic use of the stream function describes volume flux and implicitly invokes the Boussinesq assumption. A comparison of the flow equations for mass-based and volume-based stream functions indicates that this assumption can lead to significant errors in the calculated flow field, especially when flow parallels large density gradients.

Old groundwater in parts of the upper Patapsco aquifer, Atlantic Coastal Plain, Maryland, USA: evidence from radiocarbon, chlorine-36 and helium-4

Apparent groundwater ages along two flow paths in the upper Patapsco aquifer of the Maryland Atlantic Coastal Plain, USA, were estimated using 14C, 36Cl and 4He data. Most of the ages range from modern to about 500xa0ka, with one sample at 117xa0km downgradient from the recharge area dated by radiogenic 4He accumulation at more than one Ma. Last glacial maximum (LGM) water was located about 20xa0km downgradient on the northern flow path, where the radiocarbon age was 21.5xa0ka, paleorecharge temperatures were 0.5–1.5u2009u2009°C (a maximum cooling of about 12u2009°C relative to the modern mean annual temperature of 13u2009°C), and Cl–, Cl/Br, and stable isotopes of water were minimum. Low recharge temperatures (typically 5–7u2009°C) indicate that recharge occurred predominantly during glacial periods when coastal heads were lowest due to low sea-level stand. Flow velocities averaged about 1.0 m a–1 in upgradient parts of the upper Patapsco aquifer and decreased from 0.13 to 0.04 m a–1 at 40 and 80xa0km further downgradient, respectively. This study demonstrates that most water in the upper Patapsco aquifer is non-renewable on human timescales under natural gradients, thus highlighting the importance of effective water-supply management to prolong the resource.RésuméLes âges apparents des eaux souterraines le long de deux lignes d’écoulement dans l’aquifère supérieur de Patapsco de la plaine atlantique côtière du Maryland (Etats-Unis d’Amérique) ont été estimés à partir des données du 14C, 36Cl et de l’ 4He. La plupart des âges sont compris entre un âge récent contemporain et environ 500 mille ans, avec un échantillon situé à 117xa0km à l’aval de la zone de recharge daté à partir de l’accumulation radiogénique de l’4He de plus d’un million d’années. L’eau du dernier maximum glaciaire (DMG) est localisée à environ 20xa0km à l’aval de l’écoulement septentrional ; l’âge (déterminé à l’aide du) carbone radioactif est de 21.5 mille ans, les températures de paléorecharge étant comprises entre 0.5 et 1.5u2009°C (un refroidissement maximum d’environ 12u2009°C par rapport à la température moyenne annuelle actuelle de 13u2009°C) , et les concentrations en Cl–, Cl/Br et des isotopes stables de l’eau à leur minimum. Les faibles températures de recharge (typiquement entre 5 et 7u2009°C) indiquent que la recharge a pris place de manière prédominante pendant les périodes glaciaires lorsque les charges hydrauliques côtières étaient les plus basses à cause d’un niveau marin bas. Les vitesses d’écoulement étaient en moyenne d’environ 1xa0m/an dans les secteurs amont de l’aquifère supérieur du Patapsco et diminuaient de 0.13 jusqu’à 0.04 m a–1 à 40 et 80xa0km dans les parties situées à l’aval hydraulique, respectivement. Cette étude montre que la plupart des eaux souterraines de l’aquifère supérieur du Patapsco n’est pas renouvelable à l’échelle des temps humains pour des gradients hydrauliques naturels et met ainsi en évidence l’importance d’une gestion efficace de l’aquifère pour l’alimentation en eau potable pour assurer la pérennité de la ressource.ResumenSe estimaron las edades aparentes de agua subterránea a lo largo de dos trayectorias de flujo en el acuífero del Patapsco superior de la planicie costera Atlántica de Maryland, EEUU, usando datos de 14C, 36Cl y 4He. La mayor parte de la oscilación de edades a partir del moderno a unos 500xa0ka, con una muestra a los 117xa0km gradiente abajo del área de recarga datada por la acumulación de 4He radiogénico en más de 1xa0Ma. El agua del último máximo glacial (LGM) fue localizada 20xa0km gradiente abajo en la trayectoria norte de flujo, donde la edad de radiocarbono fue 21.5xa0ka, las temperaturas de paleorecarga fueron 0.5–1.5u2009°C (un enfriamiento máximo de cerca de 12u2009°C con relación a la temperatura media anual moderna de 13u2009°C), y Cl–, Cl/Br, y los isótopos estables de agua fueron mínimos. Las bajas temperaturas de recarga (típicamente 5–7u2009°C) indican que la recarga ocurrió predominantemente durante los períodos glaciales cuando los promontorios costeros eran más bajos debido a un nivel bajo del mar. Las velocidades de flujo promediaron alrededor de 1.0 m a–1 en las partes gradiente arriba del acuífero Patapsco superior y decrecieron de 0.13 a 0.04 m a–1 a 40 y 80xa0km gradiente abajo respectivamente. Este estudio demuestra que la mayor parte del agua en el acuífero del Patapsco superior es no renovable en escalas de tiempo humano bajo gradientes naturales, resaltando así la importancia del manejo efectivo del abastecimiento de agua para prolongar el recurso.摘要利用14C,36Cl和4He数据估算得到美国马里兰州大西洋滨海平原的Patapsco上段含水层中沿着两条路径运动的地下水的表观年龄。大部分样品的年龄范围为从现代到大约50万年,另外有一个取自由补给区沿水力梯度向下游117km处的样品,用放射性4He的积累的定年方法确定它的年龄为大于100万年。末次盛冰期的地下水位于北部流径沿水力梯度向下游约20km处,它的放射性碳年龄为21500年,古补给温度为0.5u2009∼u20091.5u2009°C(相对于现代年平均温度13u2009°C最多降低了12u2009°C),地下水的Cl–、Cl/Br和稳定同位素为最低值。低补给温度(通常为5u2009∼u20097u2009°C)表明补给主要发生在冰期,此时由于海平面很低导致沿海的地下水水头达到最低。在Patasco上段含水层的上游地区,地下水流速大约为1m/a,分别由沿着水力梯度向下游40km处的0.13xa0m a–1降低为80km处的0.04xa0m a–1。这项研究证明,在人类的时间尺度下,Patapsco上段含水层中的大部分地下水在自然梯度下是不可更新的,这强调了有效的供水管理在延长资源的使用时间方面的重要性。ResumoAs idades aparentes da água subterrânea ao longo de duas linhas de fluxo do aquífero superior de Patapsco, na Planície Costeira Atlântica de Maryland, EUA, foram estimadas, usando dados de 14C, 36Cl e 4He. A maioria das idades varia de moderno a 500xa0ka, com uma amostra a 117xa0km a jusante da área de recarga datada através da acumulação de 4He radiogénico, com mais de um Ma. No Último Máximo Glaciar (LGM), a água localizava-se a cerca de 20xa0km a jusante na linha de fluxo mais a norte, onde a idade de radiocarbono é de 21.5xa0ka, com temperaturas de paleorrecarga de 0.5–1.5u2009°C (um arrefecimento máximo de cerca de 12u2009°C em relação à temperatura média anual moderna de 13u2009°C), e o Cl–, Cl/Br e os isótopos estáveis da água são mínimos. As baixas temperaturas de recarga (tipicamente de 5–7u2009°C) indicam que a recarga ocorreu predominantemente durante períodos glaciares, quando os potenciais hidráulicos costeiros eram os mais baixos, devido ao posicionamento baixo do nível médio do mar. As velocidades médias de fluxo são cerca de 1.0 m a–1 nas zonas mais a montante do aquífero superior de Patapsco e diminuem de 0.13 m a–1 para 0.04 m a–1 a 40 e 80xa0km mais a jusante, respetivamente. Este estudo demonstra que, em condições de gradiente natural, a maioria da água no aquífero superior de Patapsco é não renovável à escala de tempo humana, o que realça a importância de uma gestão eficiente do abastecimento de água, de modo a promover a continuidade deste recurso.

Coupled heat and fluid flow modeling of the CarboniferousKuna Basin, Alaska: implications for the genesis of the Red Dog PbZnAgBa ore district

The Red Dog deposit is a giant 175 Mton (16% Zn, 5% Pb), shale-hosted PbZnAgBa ore district situated in the Carboniferous Kuna Basin, Western Brooks Range, Alaska. These SEDEX-type ores are thought to have formed in calcareous turbidites and black mudstone at elevated sub-seafloor temperatures (120–150 °C) within a hydrogeologic framework of submarine convection that was structurally organized by large normal faults. The theory for modeling brine migration and heat transport in the Kuna Basin is discussed with application to evaluating flow patterns and heat transport in faulted rift basins and the effects of buoyancy-driven free convection on reactive flow and ore genesis. Finite element simulations show that hydrothermal fluid was discharged into the Red Dog subbasin during a period of basin-wide crustal heat flow of 150–160 mW/m2. Basinal brines circulated to depths as great as 1–3 km along multiple normal faults flowed laterally through thick clastic aquifers acquiring metals and heat, and then rapidly ascended a single discharge fault zone at rates ∼ 5 m/year to mix with seafloor sulfur and precipitate massive sulfide ores.

Chapter 3 Numerical simulation of sedimentary basin-scale hydrochemical processes

Abstract Sedimentary basins represent large-scale porous media, are important hosts to a significant portion of the worlds economic energy and mineral resources. Processes occurring in sedimentary basins include groundwater flow, heat transport, and reactive mass transport. Quantitative models of flow and transport in these settings can provide insight into the processes that control the evolution of sedimentary basins by enabling the examination of processes that may occur too slowly to be observed in the field or laboratory. In many cases, such models may be the only available tool for studying processes occurring over geological time and space scales. In addition, it is important to consider the behavior of the processes occurring in sedimentary basins simultaneously, since they are generally coupled. Groundwater flow is controlled by the boundary conditions and the distribution of hydraulic conductivity; as a result, flow velocities vary spatially and temporally. This circulation is capable of transporting thermal energy and dissolved mass. In general, flow rates will be sufficiently small that the water will reach approximate equilibrium with each lithology along the flow path at the ambient temperature and pressure. These successive equilibria produce changes in the chemical composition of the fluid, resulting in reactions with the medium (i.e., precipitation, dissolution), which in turn modify the porosity and permeability. This modification may be insignificant at a human time scale, but very significant at the geological time scale. The hydrogeological flow field then is a coupled hydrological-thermal-geochemical system, requiring solution to three sets of coupled partial differential equations. This paper reviews developments over the past several years in numerical simulation of these coupled processes. The governing conservation equations are presented, and solution procedures discussed; the finite element equations are developed for the case where local chemical equilibrium is assumed. Application of coupled models to a variety of geological problems is discussed, such as the propagation of mineral reaction fronts in one spatial dimension. These studies have noted the importance of hydrodynamic dispersion and its control on the spatial distribution of reaction rates and products. Relatively few two-dimensional simulations are available in the literature, but these few are reviewed, including the formation of uranium ore deposits, mixing-zone reactions in carbonate aquifers, and sandstone diagenesis. These studies note the importance of transport-controlled reaction-front propagation, fluid mixing, and gradient reactions, which all occur to varying degrees in a heterogeneous sedimentary basin. Future developments will require greater computer capability, and are likely to focus on application to well-documented field problems and greater inclusion of natural geological heterogeneity, but results presented to date show promise of enabling quantitative study of coupled hydrological, geochemical, and thermal processes in evolving sedimentary basins.

Predictions of hydrothermal alteration within near-ridge oceanic crust from coordinated geochemical and fluid flow models

Coordinated geochemical and hydrological calculations guide our understanding of the composition, fluid flow patterns, and thermal structure of near-ridge oceanic crust. The case study presented here illustrates geochemical and thermal changes taking place as oceanic crust ages from 0.2 to 1.0 Myr. Using a finite element code, we model fluid flow and heat transport through the upper few hundred meters of an abyssal hill created at an intermediate spreading rate. We use a reaction path model with a customized database to calculate equilibrium fluid compositions and mineral assemblages of basalt and seawater at 500 bars and temperatures ranging from 150 to 400°C. In one scenario, reaction path calculations suggest that volume increases on the order of 10% may occur within portions of the basaltic basement. If this change in volume occurred, it would be sufficient to fill all primary porosity in some locations, effectively sealing off portions of the oceanic crust. Thermal profiles resulting from fluid flow simulations indicate that volume changes along this possible reaction path occur primarily within the first 0.4 Myr of crustal aging.

An Empirical Model of Intrinsic Permeability in Reactive Clay-Bearing Sands

Permeameter experiments were conducted to determine the permeability response of a clay-bearing sand to changes in fluid composition. These experiments examined the effects of varying cation type and concentration using NaCl and CaCl2 solutions, as well as the effects of freshwater flushing of sediments equilibrated with seawater and large versus small stepwise reductions in NaCl concentration. The major goal of the study was to correlate permeability with measured parameters of the pore microstructure. Sediment samples from the tests were analyzed using Brunauer, Emmett, Teller (BET) and ethylene glycol monoethyl ether (EGME) adsorption methods to determine specific surface area. Other samples were examined using scanning electron microscopy and digital image analysis methods. For image analysis, backscattered electron images were acquired and specific measurements of pore structure parameters (porosity, pore-size distribution) were made. Stepwise multiple regression was used to construct an empirical model of permeability, which included terms of porosity, mean pore diameter, and specific surface area. The Kozeny-Carman model was applied to the data and led to predicted permeabilities 2 to 3 orders of magnitude lower than measured values. This was attributed to the presence of dead-end pores and the importance of flow through large conducting channels.

Hydrograph-separation results for 225 streams in the Chesapeake Bay watershed derived by using PART, HYSEP (Fixed, Local minimum, Slide), BFI, and a Recursive Digital Filter with streamflow data ranging from 1913 through 2016

This U.S. Geological Survey (USGS) data release contains daily-mean streamflow and estimated-daily base flow for 225 stream gages in the Chesapeake Bay watershed ranging from 1913 to 2016 (beginning and end dates may vary). There is a table containing hydrograph-separation results by six methods for 225 sites (Hydrograph_separation_results_for_225_streams_in_the_Chesapeake_Bay_watershed) and a summary table with hydrograph-separation results for each site and method (Hydrograph_separation_summary_for_225_streams_in_the_Chesapeake_Bay_watershed). Quantitative estimates of base flow are necessary to address questions of the vulnerability and response of aquatic ecosystems to natural and human-induced change in environmental conditions. Base flow is generally not measured directly, but is estimated from observations of streamflow and/or stream water chemistry. Base flow was estimated using PART (Rutledge, 1998), HYSEP (Fixed, Local minimum, and Slide; Sloto and Crouse, 1996), BFI (Wahl and Wahl, 1988 and Wahl and Wahl, 1995), and a Recursive Digital Filter (Eckhardt, 2005 and Collishonn and Fan, 2013) in selected watersheds throughout the Chesapeake Bay watershed. The references to the above citations are in the Supplemental Information section of this metadata record. n nThese data support the following publication: nRaffensperger, J.P., Baker, A.C., Blomquist, J.D., and Hopple, J.A., 2017, Optimal hydrograph separation using a recursive digital filter constrained by chemical mass balance, with application to selected Chesapeake Bay watersheds: U.S. Geological Survey Scientific Investigations Report 2017-5034, 51 p., https://doi.org/10.3133/sir20175034.