Mark E. Conrad

Subsurface flow paths in a steep, unchanneled catchment

Tracer studies during catchment-scale sprinkler experiments illuminate the pathways of subsurface flow in a small, steep catchment in the Oregon Coast Range. Bromide point injections into saturated materials showed rapid flow in bedrock to the catchment outlet. Bedrock flow returned to the colluvium, sustaining shallow subsurface flow there. The bromide peak velocity of ;10 23 ms 21 exceeded the saturated hydraulic conductivity of intact bedrock. This, and the peak shapes, verify that fractures provide important avenues for saturated flow in the catchment. Deuterium added to the sprinkler water moved through the vadose zone as plug flow controlled by rainfall rate and water content. Ninety-two percent of the labeled water remained in the vadose zone after 3 days (;140 mm) of sprinkling. Preferential flow of new water was not observed during either low-intensity irrigation or natural storms; however, labeled preevent water was mobile in shallow colluvium during a storm following our spiking experiment. In response to rainfall, waters from the deeper bedrock pathway, which have traveled through the catchment, exfiltrate into the colluvium mantle and mix with relatively young vadose zone water, derived locally, creating an area of subsurface saturation near the channel head. This effectively becomes a subsurface variable source area, which, depending on its size and the delivery of water from the vadose zone, dictates the apportioning of old and new water in the runoff and, correspondingly, the runoff chemistry. The slow movement of water through the vadose zone allows for chemical modification and limits the amount of new water in the runoff. Moreover, it suggests that travel time of new rain water does not control the timing of runoff generation.

Deep-sea bacteria enriched by oil and dispersant from the Deepwater Horizon spill

The Deepwater Horizon oil spill resulted in a massive influx of hydrocarbons into the Gulf of Mexico (the Gulf). To better understand the fate of the oil, we enriched and isolated indigenous hydrocarbon-degrading bacteria from deep, uncontaminated waters from the Gulf with oil (Macondo MC252) and dispersant used during the spill (COREXIT 9500). During 20 days of incubation at 5°C, CO(2) evolution, hydrocarbon concentrations and the microbial community composition were determined. Approximately 60% to 25% of the dissolved oil with or without COREXIT, respectively, was degraded, in addition to some hydrocarbons in the COREXIT. FeCl(2) addition initially increased respiration rates, but not the total amount of hydrocarbons degraded. 16S rRNA gene sequencing revealed a succession in the microbial community over time, with an increase in abundance of Colwellia and Oceanospirillales during the incubations. Flocs formed during incubations with oil and/or COREXIT in the absence of FeCl(2) . Synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectromicroscopy revealed that the flocs were comprised of oil, carbohydrates and biomass. Colwellia were the dominant bacteria in the flocs. Colwellia sp. strain RC25 was isolated from one of the enrichments and confirmed to rapidly degrade high amounts (approximately 75%) of the MC252 oil at 5°C. Together these data highlight several features that provide Colwellia with the capacity to degrade oil in cold, deep marine habitats, including aggregation together with oil droplets into flocs and hydrocarbon degradation ability.

Succession of Hydrocarbon-Degrading Bacteria in the Aftermath of the Deepwater Horizon Oil Spill in the Gulf of Mexico

The Deepwater Horizon oil spill produced large subsurface plumes of dispersed oil and gas in the Gulf of Mexico that stimulated growth of psychrophilic, hydrocarbon degrading bacteria. We tracked succession of plume bacteria before, during and after the 83-day spill to determine the microbial response and biodegradation potential throughout the incident. Dominant bacteria shifted substantially over time and were dependent on relative quantities of different hydrocarbon fractions. Unmitigated flow from the wellhead early in the spill resulted in the highest proportions of n-alkanes and cycloalkanes at depth and corresponded with dominance by Oceanospirillaceae and Pseudomonas. Once partial capture of oil and gas began 43 days into the spill, petroleum hydrocarbons decreased, the fraction of aromatic hydrocarbons increased, and Colwellia, Cycloclasticus, and Pseudoalteromonas increased in dominance. Enrichment of Methylomonas coincided with positive shifts in the δ(13)C values of methane in the plume and indicated significant methane oxidation occurred earlier than previously reported. Anomalous oxygen depressions persisted at plume depths for over six weeks after well shut-in and were likely caused by common marine heterotrophs associated with degradation of high-molecular-weight organic matter, including Methylophaga. Multiple hydrocarbon-degrading bacteria operated simultaneously throughout the spill, but their relative importance was controlled by changes in hydrocarbon supply.

Distribution of hydrocarbons released during the 2010 MC252 oil spill in deep offshore waters

The explosion of the Deepwater Horizon oil platform on April 20th, 2010 resulted in the second largest oil spill in history. The distribution and chemical composition of hydrocarbons within a 45 km radius of the blowout was investigated. All available certified hydrocarbon data were acquired from NOAA and BP. The distribution of hydrocarbons was found to be dispersed over a wider area in subsurface waters than previously predicted or reported. A deepwater hydrocarbon plume predicted by models was verified and additional plumes were identified. Because the samples were not collected systematically, there is still some question about the presence and persistence of an 865 m depth plume predicted by models. Water soluble compounds were extracted from the rising oil in deepwater, and were found at potentially toxic levels outside of areas previously reported to contain hydrocarbons. Application of subsurface dispersants was found to increase hydrocarbon concentration in subsurface waters.

Stable isotope and salinity systematics in estuarine waters and carbonates: San Francisco Bay

Salinities, δD and δ18O values of water samples collected bimonthly from two stations in San Francisco Bay estuary during 1991–1993, and along a salinity transect in March of 1992, indicate a linear mixing relation between the isotopic compositions of the waters and their salinities. The salinities and stable isotope compositions of samples from two locations in San Francisco Bay vary in response to changes in freshwater inflow. The data from these locations indicate simple mixtures of Pacific Ocean water (salinity ≈33, δ18O ≈ 0 to −1‰, δD ≈ 0 to −10‰) and Sacramento-San Joaquin River water (salinity ≈ 0, δ18O = −10 to −12‰, δD = −75 to −85‰). Preliminary water balance estimates, using isotopic differences between local and upland runoff, suggest that local runoff (including waste water) comprises less than 20% of total freshwater entering the bay. The average δ18O values of mussel shells (Mytilus edulis) collected live from eight locations in San Francisco Bay primarily reflect the δ18O of the water in which they grew. Shells subsampled along growth bands show that seasonal shifts in salinity and δ18O are recorded in the shells. Therefore, the use of stable isotope measurements should be useful in reconstructing pre-instrumental bay salinity and associated freshwater inflow (both annual average values and seasonal variations) to the San Francisco Bay, as well as potentially other estuarine systems.

Stable isotope record of late Holocene salinity and river discharge in San Francisco Bay, California

Abstract Oxygen and carbon isotopic measurements of fossil mollusks from San Francisco Bay are used to derive a record of paleosalinity and paleostreamflow for the past 5,900 years. The δ 18 O and δ 13 C values of river water (−12‰ and −9‰) are markedly different than seawater (0‰ and 1‰), and vary systematically as a function of salinity in the estuary. The data show that annually averaged salinity in the south-central part of the Bay was very close to the modern ‘diversion-corrected’ value of 26.8‰ over the past 2,700 years, and 4‰ lower than modern between 3,800 and 5,100 yr B.P. Based on those salinities, the average annual river inflow to San Francisco Bay is calculated to have been 1290 m 3 /s over the past 2,400 years, and 1990 m 3 /s between 3,800 and 5,100 yr B.P., 1.8 times greater than the modern ‘diversion-corrected’ value of 1100 m 3 /s, assuming a constant bay volume. The inferred river discharge record generally corroborates independent paleohydrologic records in California, including tree-ring, treeline and lake level records.

Oxygen isotope zoning in garnet.

Oxygen isotope zoning was examined within garnet with the use of the stable isotope laser probe. Four metamorphic garnets from the regional metamorphic terrane in Vermont and the skarn deposit at Carr Fork, Utah, were examined and were found to be concentrically zoned in δ18O values. The largest variations in δ18O values were observed in the regional metamorphic garnets, where δ18O values change by 3 per mil from core to rim. These oxygen isotope zoning profiles reflect the changes in the δ18O values of the rocks during garnet growth, which are caused by infiltration of fluids and by dehydration reactions during metamorphism.

Oxygen-isotope zoning in garnet: A record of volatile transport

Abstract This study examines oxygen-isotope zoning in garnets from a Barrovian metamorphic terrane in eastern Vermont using a CO 2 laser extraction system. Previous strontium isotopic and structural studies of these garnets have shown that they grew over an approximately 10 Ma interval during thrusting and nappe emplacement ( rosenfeld , 1970; Christensen et al., 1989). Our studies show that the garnets are strongly zoned in δ 18 O . This zoning is the result of equilibration of garnet with water derived from dehydration of subjacent pelites during nappe stage deformation. The magnitude and nature of δ 18 O zoning depends upon the garnets location in the outcrop studied. The garnets examined in this study come from an isotopically low δ 18 O paragonitic schist ( δ 18 O whole rock ~ 9‰) that is adjacent to a relatively high δ 18 O schist ( δ 18 O whole rock ~12.5‰). Garnets from the paragonitic schists within 10 m of the contact with the isotopically heavier schists have relatively homogenous δ 18 O values varying from 9.5‰ in the core to 10.5‰ at the rim. Garnet in the paragonitic schists 85 m from the contact are more strongly zoned, with δ 18 O ranging from a low of ~6.0‰ in the cores to a high of ~9.0‰ at the rims of the garnet. These zoning patterns were produced by continuous infiltration of relatively high δ 18 O waters derived from the subjacent schists into the paragonitic schists during garnet-grade metamorphism. It is possible to determine the time-integrated fluid fluxes by comparison of observed δ 18 O zoning profiles in garnet with those calculated from the equation describing combined advection-diffusion of a tracer. Using this method, we calculate time-integrated fluid fluxes of ~ 1.5 × 10 4 cm 3 /cm 2 . Fluxes of this magnitude could have been produced by dewatering of ~ 1.5 km of schist during garnet-grade metamorphism.

Vadose zone infiltration rate at Hanford, Washington, inferred from Sr isotope measurements

[i] Sr isotope ratios were measured in the pore water, acid extracts, and sediments of a 70-m vadose zone core to obtain estimates of the long-term infiltration flux for a site in the Hanford/DOE complex in eastern Washington State. The 8 7 Sr/ 8 6 Sr values for the pore waters decrease systematically with depth, from a high value of 0.721 near the surface toward the bulk sediment average value of 0.711. Estimates of the bulk weathering rate combined with Sr isotopic data were used to constrain the long-term (century to millenial scale) natural diffuse infiltration flux for the site given both steady state and nonsteady state conditions. The models suggest that the infiltration flux for the site is 7 ′ 3 mm/yr. The method shows potential for providing long-term in situ estimates of infiltration rates for deep heterogeneous vadose zones.

Multiphase Reactive Transport Modeling of Seasonal Infiltration Events and Stable Isotope Fractionation in Unsaturated Zone Pore Water and Vapor at the Hanford Site

and diffusive transport). Developing tractable analytical equations for these processes requires simplifying asNumerical simulations of transport and isotope fractionation prosumptions, which lead to analytical methods that are not vide a method to quantitatively interpret vadose zone pore water stable isotope depth profiles based on soil properties, climatic condieasily adapted to field conditions. Previous numerical tions, and infiltration. We incorporate the temperature-dependent models have relied on assumptions such as neglecting equilibration of stable isotopic species between water and water vapor, the temperature dependence of isotope fractionation and their differing diffusive transport properties into the thermodyand treating the isotopic species as nonreactive tracers namic database of the reactive transport code TOUGHREACT. with concentrations defined by fixed partition coeffiThese simulations are used to illustrate the evolution of stable isotope cients. profiles in semiarid regions where recharge during wet seasons disPrior approaches to predicting the impact of infiltraturbs the drying profile traditionally associated with vadose zone pore tion water on stable isotope profiles include a semiemwaters. Alternating wet and dry seasons lead to annual fluctuations pirical model (Barnes and Allison, 1988), a mixing in moisture content, capillary pressure, and stable isotope composischeme (Mathieu and Bariac, 1996b), and an analytical tions in the vadose zone. Periodic infiltration models capture the effects of seasonal increases in precipitation and predict stable isotope model to predict overall average pore water isotope profiles that are distinct from those observed under drying (zero compositions (DePaolo et al., 2004). However, a more infiltration) conditions. After infiltration, evaporation causes a shift general approach is needed to link observed isotope to higher 18O and D values, which are preserved in the deeper pore compositions with dynamic hydrological processes, waters. The magnitude of the isotopic composition shift preserved in where precipitation events or temperature changes afdeep vadose zone pore waters varies inversely with the rate of infilfect the isotopic profile with depth. tration. We use the thermodynamic framework of the TOUGHREACT transport code (Xu and Pruess, 2001; Xu et al., 2003) to develop a general transport model for stable T fraction of precipitation that reaches the deep isotopes in vadose zone soil water and consider the vadose zone, or the net infiltration, is difficult to impact of infiltration processes on measured stable isopredict in arid regions, but important for understanding tope profiles from the Hanford Site. These reactive groundwater recharge and contaminant transport. At transport models of stable isotope transport provide a the USDOE’s Hanford Site in south-central Washingquantitative method to link the observed isotopic proton State, where a large amount of radionuclide contamfiles to soil properties, climatic conditions, and net infilination is present in the vadose zone, it is critical to tration into the vadose zone. know the net water infiltration flux, as this determines how rapidly radionuclides or other contaminants may Background: Stable Isotope Measurements reach groundwater. The vadose zone hydrological proThe isotopic compositions discussed here are meacesses that control net infiltration rate also affect the sured relative to a well-defined standard material (Stanratios of stable isotopes (i.e., 18O/16O and 2H/1H) in water dard Mean Ocean Water [SMOW]). Stable isotope comand water vapor. positions (‰) are calculated as delta values from the The transport of stable O and H isotopes in water isotopic ratio (R 18O/16O or 2H/1H), where within drying soil columns has been studied extensively (e.g., Barnes and Allison, 1983, 1984; Allison et al., 1994; RSample RStandard 1 1000 [1] Shurbaji et al., 1995; Mathieu and Bariac, 1996a; Melayah et al., 1996). Approaches used to predict stable isotope profiles in drying soils must consider the comBased on this system, typical ocean waters have D plex interaction of multiple processes (e.g., drainage, and 18O values near 0‰ relative to SMOW. Meteoric temperature effects on flow and isotope fractionation, precipitation over land varies as a function of temperature, latitude, and altitude, but generally has D and 18O values that are shifted to values less than zero M.J. Singleton, E.L. Sonnenthal, M.E. Conrad, D.J. DePaolo, Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, because of the fractionation of lighter isotopes into the CA 94720; and G.W. Gee, Hydrology Group, Environmental Techvapor phase during the change from liquid to vapor. nology Division, Pacific Northwest National Laboratory, Richland, Craig (1961) documented a linear relationship, known WA. Received 30 Aug. 2003. Special Section: Research Advances in as the global meteoric water line (GMWL), between Vadose Zone Hydrology through Simulations with the TOUGH Codes. *Corresponding author (MJSingleton@lbl.gov). Abbreviations: GMWL, global meteoric water line; LBNL, Lawrence Berkeley National Laboratory; LMWL, local meteoric water line; Published in Vadose Zone Journal 3:775–785 (2004).  Soil Science Society of America PNNL, Pacific Northwest National Laboratory; SMOW, standard mean ocean water. 677 S. Segoe Rd., Madison, WI 53711 USA