Diverse microbial communities and numerous energy-yielding activities occur in deeply buried sediments of the eastern Pacific Ocean. Distributions of metabolic activities often deviate from the standard model. Rates of activities, cell concentrations, and populations of cultured bacteria vary consistently from one subseafloor environment to another. Net rates of major activities principally rely on electron acceptors and electron donors from the photosynthetic surface world. At open-ocean sites, nitrate and oxygen are supplied to the deepest sedimentary communities through the underlying basaltic aquifer. In turn, these sedimentary communities may supply dissolved electron donors and nutrients to the underlying crustal biosphere.

Unconventional oil and gas production provides a rapidly growing energy source; however, high-production states in the United States, such as Oklahoma, face sharply rising numbers of earthquakes. Subsurface pressure data required to unequivocally link earthquakes to wastewater injection are rarely accessible. Here we use seismicity and hydrogeological models to show that fluid migration from high-rate disposal wells in Oklahoma is potentially responsible for the largest swarm. Earthquake hypocenters occur within disposal formations and upper basement, between 2- and 5-kilometer depth. The modeled fluid pressure perturbation propagates throughout the same depth range and tracks earthquakes to distances of 35 kilometers, with a triggering threshold of ~0.07 megapascals. Although thousands of disposal wells operate aseismically, four of the highest-rate wells are capable of inducing 20% of 2008 to 2013 central U.S. seismicity.

An unprecedented increase in earthquakes in the U.S. mid-continent began in 2009. Many of these earthquakes have been documented as induced by wastewater injection. We examine the relationship between wastewater injection and U.S. mid-continent seismicity using a newly assembled injection well database for the central and eastern United States. We find that the entire increase in earthquake rate is associated with fluid injection wells. High-rate injection wells (>300,000 barrels per month) are much more likely to be associated with earthquakes than lower-rate wells. At the scale of our study, a well's cumulative injected volume, monthly wellhead pressure, depth, and proximity to crystalline basement do not strongly correlate with earthquake association. Managing injection rates may be a useful tool to minimize the likelihood of induced earthquakes.

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Some low-chloride pore waters observed in accretionary complexes are thought to result from clay dehydration and subsequent migration of the released water along faults or sand layers. We test this hypothesis with a two-dimensional flow and transport model for a cross section of the northern Barbados accretionary complex. The model flow system is driven by consolidation of the accreted sediments and by fluids from smectite clay dehydration. Steady state simulations result in concentrations that are too high along the d6collement fault and too low near the seafloor. In a transient model we simulate buildup and release of fluids by assuming that strain or hydrofracture along the fault causes an instantaneous increase in d6collement permeability of 2-3 orders of magnitude. With such an increase, the observed concentrations can be achieved in 100-1000 years.Also pressures along the fault rise to near lithostatic values in 10-100 years and remain high for 1000-10,000 years. This pressure rise may represent a mechanism for sustaining high fault permeabilities long after the initial increase. A steady source of pore fluids enters an accretionary complex with the accreted seafloor sediments. These fluids may then affect a variety of geologic processes. For example, high pore pressures affect fault strength and seismicity [Magee and Zoback, 1993], and fluid-rock interactions influence metamorphic reactions [Peacock, 1990]. In a thorough review of fluid Paper number 95WR02569. 0043-1397/95/95WR-02569505.00 flow in accretionary prisms, Moore and Vrolijk [1992] described two processes that drive the flow system. One is the very high mechanical loading rate on the saturated sediments as they are either incorporated into or underthrust below the complex. This loading rate is up to 15 times the maximum due to sedimentary basin subsidence [Moore and Vrolijk, 1992; Neuzil, 1995]. The resulting consolidation is often limited by low permeabilities, leading to near-lithostatic pore pressures. A second process is the dehydration of minerals and generation of hydrocarbons as sediments buried deep in the complex are subjected to higher temperatures. Anomalously low chloride and high methane concentrations in pore waters suggest that this is occurring in many accretionary complexes [Kastner et al., 1991]. The elevated temperatures and exotic chemical signatures of the pore fluids indicate that the flow systems supplying the vents may extend, in some cases, for 50-100 km. However, the source of heat and solutes is still poorly understood. Figure lc shows pore water chloride and methane concentration changes with depth at Ocean Drilling Program (ODP) site 671 in the Barbados Ridge complex. The pore waters are almost 10% lower in chloride than seawater at the depth of the d6collement fault. In addition, more recent chloride concentrations from the dficollement at ODP site 948 (near site 671) were 20% lower than that of seawater [Kastner et al., 1994]. Gieskes et al. [1990] discussed clay membrane filtration, hydrate dissolution, and the de...

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 N2 from denitrification increases with age. Stable isotopes and excess N2 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 μmol N L−1d−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.

[1] At many sites contaminated with petroleum hydrocarbons, methanogenesis is a significant degradation pathway. Techniques to estimate CH 4 production, consumption, and transport processes are needed to understand the geochemical system, provide a complete carbon mass balance, and quantify the hydrocarbon degradation rate. Dissolved and vapor-phase gas data collected at a petroleum hydrocarbon contaminated site near Bemidji, Minnesota, demonstrate that naturally occurring nonreactive or relatively inert gases such as Ar and N 2 can be effectively used to better understand and quantify physical and chemical processes related to methanogenic activity in the subsurface. In the vadose zone, regions of Ar and N 2 depletion and enrichment are indicative of methanogenic and methanotrophic zones, and concentration gradients between the regions suggest that reaction-induced advection can be an important gas transport process. In the saturated zone, dissolved Ar and N 2 concentrations are used to quantify degassing driven by methanogenesis and also suggest that attenuation of methane along the flow path, into the downgradient aquifer, is largely controlled by physical processes. Slight but discernable preferential depletion of N 2 over Ar, in both the saturated and unsaturated zones near the free-phase oil, suggests reactivity of N 2 and is consistent with other evidence indicating that nitrogen fixation by microbial activity is taking place at this site.

Anaerobic biodegradation of organic amendments and contaminants in aquifers can trigger secondary water quality impacts that impair groundwater resources. Reactive transport models help elucidate how diverse geochemical reactions control the spatiotemporal evolution of these impacts. Using extensive monitoring data from a crude oil spill site near Bemidji, Minnesota (USA), we implemented a comprehensive model that simulates secondary plumes of depleted dissolved O 2 and elevated concentrations of Mn 21 , Fe 21 , CH 4 , and Ca 21 over a two-dimensional cross section for 30 years following the spill. The model produces observed changes by representing multiple oil constituents and coupled carbonate and hydroxide chemistry. The model includes reactions with carbonates and Fe and Mn mineral phases, outgassing of CH 4 and CO 2 gas phases, and sorption of Fe, Mn, and H 1 . Model results demonstrate that most of the carbon loss from the oil (70%) occurs through direct outgassing from the oil source zone, greatly limiting the amount of CH 4 cycled down-gradient. The vast majority of reduced Fe is strongly attenuated on sediments, with most (91%) in the sorbed form in the model. Ferrous carbonates constitute a small fraction of the reduced Fe in simulations, but may be important for furthering the reduction of ferric oxides. The combined effect of concomitant redox reactions, sorption, and dissolved CO 2 inputs from source-zone degradation successfully reproduced observed pH. The model demonstrates that secondary water quality impacts may depend strongly on organic carbon properties, and impacts may decrease due to sorption and direct outgassing from the source zone.