David F. Boutt

Present-day principal horizontal stress orientations in the Kumano forearc basin of the southwest Japan subduction zone determined from IODP NanTroSEIZE drilling Site C0009

A 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism.

Systematic diagenetic changes in the grain-scale morphology and permeability of a quartz-cemented quartz arenite

The material properties of sedimentary rocks are controlled by a range of parameters, including grain size, sorting, and modification of the original sediment through the diagenetic processes of compaction and cementation. To isolate the effects of diagenesis and explore how they modify permeability, we quantified changes in grain and pore morphology accompanying progressive diagenesis of a simple system: a well-sorted, variably quartz-cemented quartz arenite of relatively uniform grain size. The most common type of authigenic cement in sandstones, quartz overgrowths, is responsible for significant porosity and permeability reduction. The distribution of overgrowths is controlled by available pore space and the crystallographic orientations of individual quartz grains. We show that progressive quartz cementation modifies the grain framework in consistent, predictable ways. Detailed microstructural characterization and multiple regression analyses demonstrate that both the number and length of grain contacts increase as the number of pores increases and the number of large well-connected pores decreases with progressive diagenesis. The aforementioned changes progressively alter pore shape and reduce pore-size variability and bulk permeability. These systematic variations in the pore network correlate with changes in permeability, such that we can use our data to calibrate the Kozeny-Carmen relation, demonstrating that it is possible to refine predictions of permeability based on knowledge of the sedimentary system.

Quantification of free gas in the Kumano fore-arc basin detected from borehole physical properties: IODP NanTroSEIZE drilling Site C0009

The Kumano fore-arc basin overlies the Nankai accretionary prism, formed by the subduction of the Philippine Sea Plate beneath the Eurasian plate offshore the Kii Peninsula, SW Honshu, Japan. Seismic surveys and boreholes within the framework of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project show evidence of gas hydrates and free gas within the basin. Here we use high-quality borehole sonic data from Integrated Oceanic Drilling Program (IODP) Site C0009 to quantify the free gas distribution in the landward part of the basin. The Brie theory is used to quantify gas content from sonic logs, which are calibrated from laboratory measurements on drill cores. First, we show that the sonic data are mainly sensitive to the fluid phase filling the intergranular pores (effective porosity), rather than to the total porosity that includes water bound to clay minerals. We then compare the effective porosity to lithodensity-derived porosity that acts as a proxy for total porosity. The combination of these two data sets also allows assessment of clay mineralogy of the sediments. Second, we compute free gas saturation and find a gas-rich interval that is restricted to a lithological unit characterized by a high abundance of wood fragments and lignite. This unit, at the base of the fore-arc basin, is a hydrocarbon source that should be taken into account in models explaining gas distribution and the formation of the bottom-simulating reflector within the Kumano fore-arc basin.

Scale dependence of in-situ permeability measurements in the Nankai accretionary prism: The role of fractures

Modeling studies suggest that fluid permeability is an important control on the maintenance and distribution of pore fluid pressures at subduction zones generated through tectonic loading. Yet, to date, few data are available to constrain permeability of these materials, at appropriate scales. During IODP Expedition 319, downhole measurements of permeability within the uppermost accretionary wedge offshore SW Japan were made using a dual-packer device to isolate 1 m sections of borehole at a depth of 1500 m below sea floor. Analyses of pressure transients using numerical models suggest a range of in-situ fluid permeabilities (5E-15–9E-17 m2). These values are significantly higher than those measured on core samples (2E-19 m2). Borehole imagery and cores suggests the presence of multiple open fractures at this depth of measurement. These observations suggest that open permeable natural fractures at modest fracture densities could be important contributors to overall prism permeability structure at these scales.

Regional groundwater flow and accumulation of a massive evaporite deposit at the margin of the Chilean Altiplano

Focused groundwater discharge in closed basins provides opportunities to investigate mechanisms for closing hydrologic and solute budgets in arid regions. The Salar de Atacama (SdA), adjacent to the Central Andean Plateau in the hyperarid Atacama Desert, provides an extreme example of halite (>1800 km3) and lithium brine (~5,000 ppm) accumulation spanning late Miocene to present. Minimum long-term water discharge needed to sustain halite accumulation over this timescale at SdA is 9–20 times greater than modern recharge (and double wet-climate paleo recharge) within the topographic watershed. Closing this imbalance requires sourcing water from recharge on the orogenic plateau in an area over 4 times larger than the topographic watershed. Prolonged water discharge at SdA requires long residence times, deep water tables in recharge zones coupled with persistent near surface water tables in discharge areas, and large contributing areas characterized by strong gradients in landscape and climate resulting from plateau uplift.

A method of estimating bulk potential permeability in fractured-rock aquifers using field-derived fracture data and type curves

A method is devised for estimating the potential permeability of fracture networks from attributes of fractures observed in outcrop. The technique, which is intended as a complement to traditional approaches, is based on type curves that represent various combinations of fracture lengths, fracture orientations and proportions (i.e., intensities) of fractures that participate in flow. Numerical models are used to derive the type curves. To account for variations in fracture aperture, a permeability ratio (R) defined as the permeability of a fracture network in a domain divided by the permeability of a single fracture with identical fracture apertures, is used as a dependent variable to derive the type curves. The technique works by determining the point on the type curve that represents the fracture characteristics collected in the field. To test the performance of the technique, permeabilities that were derived from fractured-rock aquifers of eastern Massachusetts (USA) are compared to permeabilities predicted by the technique. Results indicate that permeabilities estimated from type curves are within an order of magnitude of permeabilities derived from field tests. First-order estimates of fracture-network permeability can, therefore, be easily and quickly acquired with this technique before more robust and expensive methods are utilized in the field.RésuméUne méthode a été élaborée pour estimer la perméabilité d’ensemble de réseaux de fractures à partir de caractéristiques de fractures observées sur affleurement. La technique, qui se veut être un complément aux approches traditionnelles, est basée sur des abaques représentant des combinaisons variables de longueurs, orientations et proportions (i.e. densité de fracturation) des fractures qui contribuent à l’écoulement. On a utilisé des modèles numériques dérivés d’abaques. Pour tenir compte des variations d’ouverture de la fracturation, un ratio de perméabilité (R) défini comme la perméabilité d’un domaine de réseau de fractures divisé par la perméabilité d’une fracture unique d’ouverture identique, a été utilisé comme variable dépendante dérivant des abaques. La technique fonctionne en déterminant le point sur l’abaque type qui représente les caractéristiques de la fracturation relevées sur le terrain. Pour tester la performance de la technique, les perméabilités d’aquifères fracturés de l’Est Massachusetts (USA) ont été comparées avec les perméabilités prévues par la technique. Les résultats indiquent que les perméabilités estimées à partir des abaques sont dans l’ordre de grandeur des perméabilités issues de tests de terrain. Des estimations de premier ordre de la perméabilité du réseau de fractures peut, par conséquent, être aisément et rapidement acquises avec cette technique avant que des méthodes plus lourdes et coûteuses soient utilisées sur le terrain.ResumenSe diseñó un método para estimar la permeabilidad potencial de redes de fracturas a partir de atributos de las fracturas observadas en afloramientos. La técnica, que se intenta como un complemento a los enfoques tradicionales, está basada en curvas tipo que representan varias combinaciones de longitudes, orientaciones y proporciones (por ejemplo intensidades) de fracturas que participan en el flujo. Se utilizan modelos numéricos para desarrollar las curvas tipo. Para tener en cuenta las variaciones en las aperturas de las fracturas, se utilizó una relación de permeabilidad (R) definida como la permeabilidad de una red de fracturas en un dominio dividido por la permeabilidad de una fractura simple con idénticas aperturas de fracturas, como una variable dependiente para desarrollar las curvas tipo. La técnica trabaja determinando el punto sobre la curva tipo que representa las características de la fractura recogidas en el campo. Para probar la performance de la técnica se comparan las permeabilidades que fueron desarrolladas de los acuíferos de rocas fracturadas del este de Massachusetts (EEUU) a las permeabilidades predichas por la técnica. Los resultados indican que las permeabilidades estimadas a partir de las curvas tipo están dentro del orden de magnitud de permeabilidades deducidas de los ensayos de campo. Las estimaciones de primer orden de la permeabilidad de redes de fractura, pueden, por lo tanto, ser fácil y rápidamente adquiridas con esta técnica antes de utilizar métodos de campo más robustos y costosos摘要通过在野外露头观察到的裂隙的特征来评估裂隙网络的潜在渗透性的方法在本文中被提出。这种技术是基于标准曲线的,是对传统方法的一种补充,标准曲线代表了裂隙的长度、裂隙的走向和与地下水流动有关的裂隙的规模(即密度)的不同组合。用数值模型来获得标准曲线。为了解释裂隙开度的变化,定义了渗透性比值(R),渗透性比值是区域上裂隙网的渗透系数与具有相同裂隙开度的单一裂隙渗透系数的比值,渗透性比值也作为获得标准曲线的因变量。这种技术的工作原理是确定标准曲线上能代表在现场获得的裂隙特征的点。为了测试这种技术的效果,把从美国马萨诸塞州东部获得的裂隙含水层的渗透率和通过这种技术预测得到的渗透率进行了对比。结果表明,利用标准曲线评估获得的渗透系数和通过现场测试得到的渗透系数在同一个数量级上。因此,在利用更成熟更昂贵的方法之前,利用这种技术,可以方便快捷地获得裂隙网络渗透系数的一级估计值。ResumoÉ desenvolvido um método para estimar a permeabilidade potencial de redes de fratura através de atributos de fraturas observadas em afloramento. A técnica, que é concebida como um complemento para as abordagens tradicionais, baseia-se em curvas tipo que representam várias combinações de comprimentos de fratura, orientações de fratura e proporções (isto é, intensidades) de fraturas que facilitam o fluxo. São utilizados modelos numéricos para obter as curvas tipo. Para ter em conta as variações de abertura das fraturas, uma relação de permeabilidade (R) definida como a permeabilidade de uma rede de fraturas de um domínio dividida pela permeabilidade de uma fratura simples com idêntica abertura da fratura, é usada como uma variável dependente para obter as curvas tipo. A técnica funciona através da determinação do ponto na curva tipo que representa as caraterísticas das fraturas recolhidas no campo. Para testar o desempenho da técnica, as permeabilidades que foram derivadas a partir de aquíferos rochosos fraturados do leste de Massachusetts (EUA) são comparados com permeabilidades previstas pela técnica. Os resultados indicam que as permeabilidades estimadas a partir das curvas tipo estão dentro da ordem de grandeza da magnitude das permeabilidades obtidas a partir de ensaios de campo. Estimativas de uma primeira ordem de grandeza da permeabilidade da rede de fraturas podem, portanto, ser fácil e rapidamente obtidas com esta técnica, antes de métodos mais robustos e dispendiosos serem utilizados no campo.

Hydrogeological controls on spatial patterns of groundwater discharge in peatlands

Peatland environments provide important ecosystem services including water and carbon storage, nutrient processing and retention, and wildlife habitat. However, these systems and the services they provide have been degraded through historical anthropogenic agricultural conversion and dewatering practices. Effective wetland restoration requires incorporating site hydrology and understanding groundwater discharge spatial patterns. Groundwater discharge maintains wetland ecosystems by providing relatively stable hydrologic conditions, nutrient inputs, and thermal buffering important for ecological structure and function; however, a comprehensive site-specific evaluation is rarely feasible for such resource-constrained projects. An improved process-based understanding of groundwater discharge in peatlands may help guide ecological restoration design without the need for invasive methodologies and detailed sitespecific investigation. Here we examine a kettle-hole peatland in southeast Massachusetts historically modified for commercial cranberry farming. During the time of our investigation, a large process-based ecological restoration project was in the assessment and design phases. To gain insight into the drivers of site hydrology, we evaluated the spatial patterning of groundwater discharge and the subsurface structure of the peatland complex using heat-tracing methods and groundpenetrating radar. Our results illustrate that two groundwater discharge processes contribute to the peatland hydrologic system: diffuse lower-flux marginal matrix seepage and discrete higher-flux preferential-flow-path seepage. Both types of groundwater discharge develop through interactions with subsurface peatland basin structure, often where the basin slope is at a high angle to the regional groundwater gradient. These field observations indicate strong correlation between subsurface structures and surficial groundwater discharge. Understanding these general patterns may allow resource managers to more efficiently predict and locate groundwater seepage, confirm these using remote sensing technologies, and incorporate this information into restoration design for these critical ecosystems.

Effects of Landcover, Soil Property, and Temperature on Covariations of DOC and CDOM in Inland Waters

Significant uncertainty exists in the estimation of dissolved organic carbon (DOC) concentration via remote sensing from colored dissolved organic matter (CDOM) absorption in inland waters pointing to a need for more process-based understanding of the relationship between CDOM and DOC. In this study, we examine the factors affecting the covariations of DOC and CDOM using controlled experiments combined with field measurements at subbasin scale that have varying environmental and biological conditions. Our analysis reveals that the DOC:CDOM ratio is mainly related to landcover types. Higher DOC:CDOM linear regression slopes observed in evergreen leaf litter leachate suggest that CDOM comprises a smaller fraction of the DOC pool in evergreen sites in comparison to agricultural and deciduous leaf litter leachates. Given the same DOC concentrations, the range of CDOM levels from deciduous forest plant varied 3 times greater than that from other plant types. Results indicate that soil narrows the slope differences in the linear regressions of DOC from CDOM for all plant types (by 19% of evergreen, 18% of agriculture, and 77% of deciduous). Raising soil temperature by 5°C could double the range of DOC concentration and CDOM absorption for all scenarios. We present a mathematical model to estimate DOC concentration in freshwater environment via CDOM variations with reference to land cover and soil effects. The model was able to explain 95% of field measurements of multiple years in four subbasins. This improved understanding is critical for the remote sensing of DOC directly via observations of CDOM.

Assessing hydrogeologic controls on dynamic groundwater storage using long-term instrumental records of water table levels

This study analyzes a long-term regional compilation of water table response to climate variability based on 124 long-term groundwater wells distributed across New England, USA screened in a variety of geologic materials. The New England region of the USA is located in a humid-temperature climate underlain by low storage fractured metamorphic and crystalline bedrock dissected by north-south trending valleys filled with glacial and post-glacial valley fill sediments. Uplands are covered by thin glacial till that comprises more than 60% of the total area. Annual and multi-annual responses of the water table to climate variability is assessed to understand how local hydraulic properties and hydrogeologic setting (located in recharge/discharge region) of the aquifer influence the hydrologic sensitivity of the aquifer system to climate variability. This study documents that upland aquifer systems dominated by thin deposits of surface till comprise ~70% of the active and dynamic storage of the region. Total aquifer storage changes of + 5 km3 to -7 km3 occur over the region during the study interval. The storage response is dominated by thin and low permeability surficial till aquifer which fills and drains on a multi-annual basis and serves as the main mechanism to deliver water to valley fill aquifers and underlying bedrock aquifers. While the till aquifer system is traditionally neglected as an important storage reservoir, this study highlights the importance of a process based understanding of how different landscape hydrogeologic units contribute to the overall hydrologic response of a region.