Alan L. Mayo

Solute and isotopic geochemistry and ground water flow in the central Wasatch Range, Utah

Ground water flow systems in the rugged central Wasatch Range, Utah, were investigated by solute and isotopic methods. Six types of ground water systems were identified on the basis of rock type and structure. The systems are broadly grouped into four categories: (1) granitic systems; (2) non-granitic systems: (a) unconsolidated alluvial systems, (b) consolidated sedimentary rock systems, and (c) fault controlled systems; (3) thermal systems; (4) mine drainage systems. These ground water systems have distinctive solute and isotopic chemistries. Based on an analysis of 3H and 14C data, all of the ground waters have a component of post-1952 recharge water, and most of the ground waters are composed almost entirely of post-1952 recharge water. A few of the ground water systems contain some water which is hundreds to perhaps thousands of years old. The δ18O and δ2H data plot on a local meteoric water line, and none of the data exhibit a positive δ18O shift. The absence of a positive δ18O shift suggests maximum aquifer temperatures are approximately 100°C. Granitic terrains are dominated by fracture controlled, local ground water flow systems which respond rapidly to recharge events. The granitic ground water systems have estimated maximum circulation depths of about 160 m below land surface. All the alluvial, most of the consolidated sedimentary bedrock, and some of the fault controlled ground water systems are shallow circulating, local flow systems with short travel times. Ground water circulation of most of the local flow systems is within 200 m of land surface and is bedding plane controlled. Some of the consolidated bedrock and many of the fault controlled ground water systems are of the intermediate type and have circulation depths to 500 m. Four thermal ground water systems have been identified. The thermal systems are heated by the geothermal gradient and have not circulated deeper than 2–2.5 km. The thermal systems are of the intermediate and regional type. The solute chemistry of most of the non-granitic ground waters may be attributed to the dissolution of carbonate minerals and minor amounts of gypsum. Contributions of external CO2(g) to some of the thermal systems from either metamorphic or silicate hydrolysis processes is suspected.

Active and inactive groundwater flow systems: Evidence from a stratified, mountainous terrain

We present a new conceptual model of groundwater flow that describes active and inactive groundwater flow regimes. The model is based on an analysis of interactions between surface water and shallow and deep groundwater in the 240-km-long Wasatch Range and Book Cliffs, Utah, USA. Active zone groundwater flow paths are continuous, responsive to annual recharge and climatic variability, and have groundwater resident times “ages” that become progressively older from recharge to discharge area. Active zone groundwater systems discharge at thousands of springs that issue from the 700+-m-thick, gently dipping, clastic bedrock formations. Springs waters contain appreciable 3 H and anthropogenic 14 C. In contrast, inactive zone groundwater has extremely limited or no communication with annual recharge and has groundwater mean residence times that do not progressively lengthen along the flow path. Groundwater in the inactive zone may be partitioned, occur as discrete bodies, and may occur in hydraulically isolated regions that do not have hydraulic communication with each other. Inactive zone groundwater is encountered in-mines (coal-mines 300–700 m below ground surface) where groundwater discharge rates decline rapidly and the waters have δ 2 H and δ 18 O compositions that are distinguishable from near surface groundwater. In general, deep waters have no 3 H and have mean 14 C residence times of 500 to 20,000 yr (45.9 to 4.9 pmc). Chemical evolution modeling, porosity-permeability core plug analysis, and in-mine hydrographs also indicate hydraulic partitioning.

Enhanced fracture permeability and accompanying fluid flow in the footwall of a normal fault: The Hurricane fault at Pah Tempe hot springs, Washington County, Utah

The Pah Tempe hot springs discharge ~260 L/s of water at ~40 °C into the Virgin River in the footwall damage zone of the Hurricane fault at Timpoweap Canyon, near Hurricane, Utah, USA. Although these are Na-Cl waters, they actively discharge CO2 gas and contain signifi cant quantities of CO2 (~34.6 mmol/kg), predominantly as H2CO3 and HCO3 –. Because of excellent exposures, Pah Tempe provides an exceptional opportunity to observe the effects of enhanced fracture permeability in an active extensional fault. Pah Tempe waters have been deeply circulated (>5 km; >150 °C) into basement rock as illustrated by the clear water-rock exchange of oxygen isotopes. Waters were probably recharged under colder climate conditions than present and therefore have a prolonged subsurface residence. Discharge of both water and gas in the springs correlates to the density of fractures in carbonate rocks above stream level. This observation suggests that clusters of high fracture density in the faultdamage zone act as pathways from the likely regional aquifer, the eolian Queantoweap Sandstone, through the overlying confining unit, the gypsiferous silty Seligman Member of the Kaibab Formation. Mass-balance modeling suggests that the majority of CO2 discharge is the product of the quantitative dissolution of CO2 gas at depth within the fault zone. Upon discharge, most of the carbon is released to the surface as dissolved species. It appears that the subsurface production rate of CO2 is relatively low because Pah Tempe waters are grossly undersaturated in CO2 at inferred minimum circulation depths and temperatures. Geological and geochemical data also suggest that the CO2 is dominated by a crustal component complemented by minor mantle contributions.

Low temperature diagenetic–metamorphic and magmatic contributions of external CO2 gas to a shallow ground water system

Abstract A small ( 2 ), low temperature ( 2 gas overpressured, gently dipping Paleozoic carbonate aquifer rests on Precambrian granite in a narrow canyon along the eastern edge of the Rocky Mountain Front Range, Colorado. The carbonate aquifer is bounded on three sides by granite and in the down dip direction by the Front Range fault. The fault, in a major continental intraplate zone of weakness along which magmatic fluids and gases could migrate to the surface and near surface from great depths. The fault has also overthrusted slices of carbonate and clastic rocks several kilometres below the granitic basement. Stable isotopic ( δ 2 H and δ 18 O) and discharge temperature data suggests that carbonate aquifer ground waters are of meteoric origin and have not circulated to depths greater than the base of the carbonate aquifer (≈650 m). Elevated CO 2 and the δ 13 C of HCO 3 − in the carbonate aquifer suggest an external crustal source of CO 2 gas. 3 He/ 4 He, O 2 /N 2 and Ar/N 2 gas ratios indicate gas contributions from both magmatic and atmospheric sources. Atmosphere contributions account for about 25% of the exsolving gas, whereas magmatic CO 2 accounts for 7 to 14%. Possible external CO 2 sources, which are consistent with the mean HCO 3 − δ 13 C=−2.4‰ (PDB), are clay–carbonate mineral diagenesis or low temperature metamorphism of siliceous–carbonate rocks that have been overthrust by 3 to 6 km of granite. Diagenetic or metamorphic CO 2 , mixed with some magmatic gas, appears to have migrated from the source rock area upward along the Ute Pass thrust fault until it encountered the shallow carbonate aquifer ground water system where it was further diluted with atmospheric gas.

Contributions to the solute and isotopic groundwater geochemistry, Antelope Island, Great Salt Lake, Utah

The 110 km2 Antelope Island is predominantly composed of Precambrian gneiss similar to rocks in the Wasatch Range 32 km to the east. Three types of island groundwater systems have been identified on the basis of aquifer composition and dissolved solute content. The systems include ground waters which discharge from either crystalline or sedimentary rocks. Sedimentary groundwater systems contain ground water which is either Na-Cl rich or poor. All island ground water has anomalously high Na+, Cl, or SO24 concentrations when compared with ground waters that issue from similar bedrock in nearby areas. Island groundwater systems may be statistically distinguished from each other and from other ground water in the region. The solute chemistry of island ground water cannot be accounted for by the simple dissolution of aquifer minerals, and the water does not plot along fault controlled thermal water-precipitation or parent aquifer-precipitation Cl−SO2−4 mixing lines. Discharge elevations and Cl−SO2−4 also rule out mixing with residual salts from paleo Lake Bonneville or brines of the Great Salt Lake for most island ground water. Tritium values (> 40 tritium units (TU)) suggest that island groundwater systems are primarily recharged from modern meteoric water. Stable isotopic δ2H and δ18O data plot along an evaporation line which indicates that recharge water contains a component of evaporated Great Salt Lake water. Temporal variations in spring discharges, tritium concentrations, and discharge temperatures of less than 20°C indicate short flow paths and shallow circulation. Excess Cl− and SO2−4 concentrations are attributed to the deposition of aerosols of wind-blown dust from nearby evaporate deposits and sea spray. Sulfur isotope data suggest contributions to crystalline systems of reduced sulfur from nearby smelter or other industrial plumes.

The role of interbasin groundwater transfers in geologically complex terranes, demonstrated by the Great Basin in the western United States

In the Great Basin, USA, bedrock interbasin flow is conceptualized as the mechanism by which large groundwater fluxes flow through multiple basins and intervening mountains. Interbasin flow is propounded based on: (1) water budget imbalances, (2) potential differences between basins, (3) stable isotope evidence, and (4) modeling studies. However, water budgets are too imprecise to discern interbasin transfers and potential differences may exist with or without interbasin fluxes. Potentiometric maps are dependent on conceptual underpinnings, leading to possible false inferences regarding interbasin transfers. Isotopic evidence is prone to non-unique interpretation and may be confounded by the effects of climate change. Structural and stratigraphic considerations in a geologically complex region like the Great Basin should produce compartmentalization, where increasing aquifer size increases the odds of segmentation along a given flow path. Initial conceptual hypotheses should explain flow with local recharge and short flow paths. Where bedrock interbasin flow is suspected, it is most likely controlled by diversion of water into the damage zones of normal faults, where fault cores act as barriers. Large-scale bedrock interbasin flow where fluxes must transect multiple basins, ranges, and faults at high angles should be the conceptual model of last resort.RésuméDans le Grand Bassin, Etats-Unis, l’écoulement interbassin en domaine de socle constitue un mécanisme par lequel des flux importants d’eaux souterraines s’écoulent à travers plusieurs bassins et seuils associés. Le mécanisme d’écoulement interbassin proposé est basé sur: (1) les déséquilibres de bilans, (2) les différences de charge hydraulique entre bassins, 3) les preuves apportées par les isotopes stables, et (4) des études de modélisation. Cependant, les bilans de nappe sont trop peu précis pour discerner les transferts interbassins, et des différences de charge peuvent exister, avec ou sans transferts interbassins. Les cartes piézométriques dépendent de concepts sous-jacents, conduisant à des déductions erronées quant aux transferts interbassins. L’expertise isotopique est sujette à interprétations multiples et faussée par les effets du changement climatique. Les considérations structurales et stratigraphiques dans une région géologiquement complexe telle celle du Grand Bassin devraient conduire à une compartimentation, où l’augmentation de la taille d’un aquifère augmente les particularités suivant un axe d’écoulement donné. Les hypothèses conceptuelles initiales devraient expliquer un écoulement avec une recharge locale et des chemins d’écoulements courts. Là où l’on suppose un flux interbassin à travers le socle il est très probablement contrôlé par l’écoulement de l’eau à travers les zones altérées de failles normales, dont les plans jouent le rôle de barrières. L’écoulement interbassins à grande échelle à travers le socle, où les flux doivent traverser des bassins multiples, des seuils et des failles à fort pendage, devrait être le modèle conceptuel de dernier recours.ResumenEn la Great Basin, EEUU, el flujo intercuenca en el basamento está conceptualizado como el mecanismo por el cual grandes flujos de agua subterránea fluyen a través de múltiples cuencas y de las montañas interpuestas. Se propuso el flujo intercuenca basado en: (1) desequilibrios en el balance de agua, (2) diferencias potenciales entre cuencas, (3) evidencias de isótopos estables, y (4) estudios de modelación. Sin embargo, los balances de agua son demasiados imprecisos para discernir las transferencias intercuencas y pueden existir diferencias de potencial con o sin flujos intercuencas. Los mapas potenciométricos dependen de fundamentos conceptuales, que conducen a a posibles inferencias falsas en relación a las transferencias intercuencas. La evidencia isotópica es propensa a una interpretación que no es única y puede ser confundida por los efectos del cambio climático. Las consideraciones estructurales y estratigráficas en una región geológicamente compleja como la Great Basin deben producir una compartimentación, donde el tamaño creciente del acuífero incrementa las posibilidades de segmentación a lo larga de una trayectoria de flujo dada. Las hipótesis conceptuales iniciales las deben explicar el flujo de con la recarga local y las trayectorias cortas de flujo. Se sospecha que la existencia del flujo intercuencas en el basamento está muy probablemente controlado por el desvío dentro de las zonas de daño de las fallas normales, donde los núcleos de las falla actúan como barreras. El flujo intercuenca en el basamento a gran escala donde los flujos deben atravesar múltiples cuencas, cordilleras y fallas de alto ángulo deben ser el modelo conceptual del último recurso.摘要在美国大盆地,基岩跨流域水流机理概念化为大的地下水通量流经多个盆地和介于中间的山脉。跨流域水流基于下列原因而提出:(1)水量收支不平衡,(2)流域之间的势差,(3)稳定同位素证据,(4)建模研究。然而,水量收支很不精确以至于不能识别流域间的转移,有流域间通量或没有流域间通量,都可能存在势差。静水压面图取决于概念基础,导致跨流域转可能移虚假的推断。同位素证据倾向于非唯一解译,可能被气候变化影响所混淆。地质复杂地区如大盆地中构造及地层上的考量应该能够对此划分,在此,增大的含水层规模增加了沿水流流径的分割的可能性。最初概念假设可以解释具有当地补给及短流径的水流。在 怀疑有基岩跨流域水流的地方,水流很可能受水转移进入正常断层损伤带的控制,在正常断层,断层核心充当屏障。通量很可能高角度横切多个盆地、山脉和 断层的大型基岩跨流域水流应当是不得已的概念模型。ResumoNa Grande Bacia, nos EUA, concetualiza-se o escoamento no bedrock entre bacias hidrográficas, como o mecanismo pelo qual grandes fluxos subterrâneos atravessam múltiplas bacias e montanhas intercaladas. A suposição da existência de fluxo subterrâneo entre bacias baseia-se em: (1) desequilíbrios no balanço hídrico, (2) diferenças no potencial hidráulico entre bacias, (3) evidências de isótopos estáveis, e (4) estudos de modelação. No entanto, os balanços hídricos são demasiado imprecisos para diferenciar possíveis transferências entre bacias e, relativamente às diferenças de potencial hidráulico, estas podem existir independentemente de haver ou não fluxo entre bacias. Os mapas potenciométricos dependem de pressupostos concetuais, levando a possíveis falsas inferências relativamente às transferências entre bacias. As evidências isotópicas são sujeitas à interpretação não-exclusiva e podem ser confundidas pelos efeitos das alterações climáticas. Numa região geologicamente complexa, como a da Grande Bacia, as considerações estruturais e estratigráficas deverão induzir uma compartimentalização, onde, à medida que aumenta a dimensão de um aquífero, crescem as hipóteses de segmentação ao longo de um determinado caminho de fluxo. As suposições concetuais iniciais devem procurar explicar o escoamento com base em recarga local e caminhos de fluxo curtos. Onde se suspeita de fluxo subterrâneo entre bacias através do bedrock, ele será muito provavelmente controlado pelo desvio de água em zonas afetadas por falhas normais, onde os núcleos atuam como barreiras. O fluxo subterrâneo inter-bacias de grande escala através do bedrock deve atravessar várias bacias, cadeias montanhosas e falhas com ângulos elevados e deverá ser considerado o modelo concetual de último recurso.

Testing the interbasin flow hypothesis at Death Valley, California

Interbasin flow is a process by which groundwater moves from one topographic basin to another through an intervening structural or topographic barrier. For decades, interbasin flow has been the prevailing conceptual paradigm for groundwater movement in the arid southwestern United States wherever carbonate rocks are thought to be in continuous contact [e.g., Anderson, 2002]. This conceptual model of groundwater flow is especially relevant in the Death Valley region where water resources are scarce and where the U.S. government has conducted underground nuclear tests and has planned for the storage of spent nuclear fuel at Yucca Mountain, Nevada. Recent studies of large flux springs (∼10,000 L/min) in the Furnace Creek area of Death Valley, California (Figure 1), however, indicate that the concept of interbasin flow may be fundamentally flawed, or at least not as universally applicable as previously thought. Rather, it appears that aquifers supplying Furnace Creek springs were replenished locally during episodes of wet climate more than 9500 yr ago, a contention supported by extensive regional fossil spring deposits [Quade et al., 2003].

Reply to [“Comment on “Testing the interbasin flow hypothesis at Death Valley, California’”] Winograd et al.

We happily respond to Winograd et al. regarding our recent Eos article concerning interbasin flow [Nelson et al., 2004]. We reiterate that we specifically reject interbasin flow only to Death Valley (DV) from Ash Meadows (AM) through the southern Funeral Mountains (FM), but suggest it should be critically reexamined elsewhere. Clearly, fractured carbonate rocks may be transmissive and deliver much water to a well or spring. We question, however, the spatial scales over which interbasin flow has been invoked, involving as it must continuously connected fracture permeability over tens to hundreds of kilometers.

Imaging the Margins of Pleistocene Lake Deposits with High-Resolution Seismic Reflection in the Eastern Basin and Range: Pilot Valley, Utah (USA)

Abstract A vast area of the northeastern Great Basin of the western USA was inundated by a succession of Pleistocene lakes, including Lake Bonneville. Playa-sediment deposition from these lakes onlapped onto alluvial fans that blanketed the slopes of adjacent mountain ranges to create prominent angular unconformities. Understanding these unconformities is useful for constraining the interpretation of the geologically recent tectonic evolution of the Basin and Range Province, as well as the interaction of lake sedimentation and alluvial fan development. The Pilot Valley playa, located just east of the Utah–Nevada border near Wendover, Utah, represents a remnant of these lakes. High-resolution seismic profiles have been acquired near the base of the bounding mountain ranges. The profiles reveal the stratigraphic relationships between Quaternary pluvial sediments as a shoreline depositional facies and the adjacent bounding fan deposits. On the western side of the basin, these profiles image subhorizontal playa sediments prograding over inclined alluvial fans. The boundary between the playa and fan sediments is marked by a prominent angular unconformity. Seismic images from the opposite side of the basin reveal a more heterogeneous structural and stratigraphic style, including down-to-the-basin normal faulting of shallow Paleozoic bedrock overlain by alluvial fan deposits, which are in turn onlapped by a thin veneer of playa sediments. The new geophysical images, when integrated with available geologic mapping, also aid in constraining how deep aquifers are locally recharged from an adjacent range. The results demonstrate the structural asymmetry of the range and playa system, consistent with a classic half-graben structure. Lastly, this study demonstrates the utility of the method of shallow seismic reflection to provide high-resolution subsurface images in the challenging environment of alluvial fan–playa geology.

Hydrogeology of salt karst under different cap soils and climates (Persian Gulf and Zagros Mts., Iran)

*bruthans@natur.cuni.cz Citation: