We describe the design, calibration, and deployment of a buoy and gas capture assembly for measuring bubbling gas flux in oceans and lakes. The assembly collects gas in a chamber while continuously measuring the position of the gas-water interface that forms as gas accumulates. Interface position is determined from the differential pressure between the chamber and ambient seawater. A spar buoy provides flotation and stability to reduce vertical motions due to surface waves. The gas collection assembly and spar, referred to as a flux buoy, is suitable for deployment from small boats under conditions of light wind and small waves. We are using the flux buoy to determine the spatial distribution of natural hydrocarbon seepage off the south-central California coast. Hydrocarbon seepage from continental shelves may be an important source of atmospheric methane.

The current drought crisis in California highlights the vulnerability of freshwater resources, particularly groundwater reservoirs, which supply up to 60% of California’s water during drought years. Understanding the potential impacts of climate change on groundwater recharge and storage is critical as drought periods become more frequent in the state. Groundwater residence times provide insight into groundwater recharge and transport mechanisms and storage capacities. This study developed and evaluated a new intrinsic tracer method to quantify groundwater recharge and transport using the occurrence of the naturally-produced radioisotope sulfur-35 (35S, half-life 87.5 days) in water as dissolved sulfate (35SO4). Improvements made to established analytical techniques expand the analytical range of 35SO4, which broadens the utility of 35SO4 as a hydrologic tracer. The 35SO4 tracer method was applied to two distinct hydrologic settings: 1) high-elevation Sierra Nevada basins, and 2) low-elevations basins containing managed aquifer recharge (MAR) facilities.

In the Sierra Nevada study, the new 35SO4 method was used to empirically constrain annual groundwater recharge in Sagehen Creek Basin (SCB) and Martis Valley Groundwater Basin (MVGB). Compared to relatively high 5SO4 activity in seasonal snowmelt (5.5 ± 0.3 to 52.9 ± 3.4 mBq/L), groundwater and surface water consistently yielded low 35SO4 activities resulting in a calculated percent new snowmelt (PNS) of <30%. The consistently low PNS suggests that recent (<1 year old) snowmelt represents only a small fraction of the larger aquifer system. As snowpack continues to decline due to climate change, streamflow and springs may respond in a two phase manner: rapid response in discharge followed by more gradual decreases over decades due to declines in groundwater recharge.

The MAR study used 35SO4 to quantify groundwater travel times near MAR operations. MAR sites divert excess surface water, imported water, and reclaimed wastewater into surface-spreading ponds or direct injection wells to replenish groundwater in heavy-usage areas. Identifying groundwater travel times near MAR facilities is critical for determining the fate and transport of potential contaminants, especially for facilities that incorporate a significant portion of reclaimed wastewater. Successful application of the 35SO4 tracer method near MAR sites is dependent on careful characterization of the 35SO4 activity in source waters. Relative to established deliberate tracer experiments, which require extensive field and laboratory effort, the less intensive 35SO4 technique showed comparable groundwater travel times at two MAR facilities located in southern California. Both the Sierra Nevada and MAR studies demonstrate that 35SO4 is a valuable, yet underutilized tracer in hydrologic studies.

A tracer injection experiment was performed in 3.5 Myr old seafloor comprised of sediment-buried abyssal hills oriented N20°E, and located 100 km east of the Endeavor Segment of the Juan de Fuca spreading ridge in the northeastern Pacific. In the summer of 2010, a mixture of tracers (metal salts, dissolved sulfur hexafluoride [SF6], microspheres) was injected into the crust via borehole 1362B as part of a 24-hour injection experiment during IODP Expedition 327. Fluid samples were subsequently collected from 1362B, and from four additional holes (1026B, 1362A, 1301A, and 1301B) located 300 to 550 m away from the injection hole. The borehole array penetrates a hydrothermal fluid flow system thought to be flowing from SW to NE along a buried abyssal hill often referred to as Second Ridge (SR). Hydrothermal flow is thought to be controlled by a series of exposed volcanic outcrops located along the same buried hill. According to the hypothesis, recharge of bottom seawater occurs through Grizzly Bare, an exposed outcrop 52 km south of the borehole array, and discharge seeps through Baby Bare and possibly also Mama Bare outcrops, which are both located within 5 km to the existing boreholes. The goal of this study is to test hypothesized fluid flow direction, flow velocity and crustal permeability using the conservative gas tracer SF6.

Two small cylinders of SF6 were injected at a fluid pumping rate of 6.7 L/s for 20.2 hours, resulting in a mean concentration of 47.6 µM or 47,600 nM (total of ~23 mol of SF6 was injected). Borehole fluid was continuously sampled in 1.8 mm ID copper tubes using osmosamplers (OS) from each of the long-term, subseafloor observatories (known as CORKs) that had previously been installed in the boreholes. The OS spools were recovered using the ROV Jason in 2011 and 2013. Following recovery, fluid samples were transferred into evacuated vials for measuring SF6 via gas chromatography in a shore-based laboratory. Results of samplers recovered in 2011 indicate the first arrival of injected SF6 ~305 days after injection at Hole 1301A, located 550 m south of the injection Hole 1362B. This suggests that the mean lateral transport of tracer is ~1.8 m/day (660 m/yr), a rate at the upper end of previous upper crustal fluid flow velocity estimates.

The first detection of an SF6 patch south of the injection source apparently contradicts the prior hypothesis that crustal fluid is flowing northward from Grizzly Bare to Baby and Mama Bares, but there are extensive leaks in Hole 1301A, which may have inadvertently pulled SF6 southwards. CORKs 1301B and 1026B are also leaking hydrothermal fluid, and any data collected from these holes should be interpreted carefully. Both 1362A and injection Hole 1362B are operating as intended with no known leaks, although a discharge valve was opened at 1362B in 2011 and closed in 2013. Following this perturbation to the flow field, the 1362A valve was opened in 2013 and then closed in 2014. Tracer was detected in both 1362A and 1362B from the beginning of sampling in 2013, one year after injection. The mean tracer concentration for Holes 1362B and 1362A was 4.6 nM and 4.5 nM, respectively. The sustained signal at these CORKs suggests a hovering of an SF6 patch spanning at least 311 meters in the region. The detection of a considerable amount of tracer to the north at 1362A is consistent with the hypothesized SW-NE fluid flow direction. The hydrogeological fabric at our site appears to be heterogeneous, with fluid transport occurring through small, isolated permeable zones found in the upper volcanic portion of the ocean crust.

Natural hydrocarbon seepage from marine environments is an important source of methane and other gases to the atmosphere. Quantifying this flux is necessary for constraining global budgets and understanding local air pollution sources. A field of strong hydrocarbon seepage offshore of Coal Oil Point near Santa Barbara, California produces extensive areas of bursting bubbles at the sea surface. An instrumented buoy was deployed in the Coal Oil Point field to measure directly the atmospheric gas flux from three seeps of varying size and intensity. Spatial scales of continuity of the seeps, quantified by semivariograms, are small, ranging from < 1 m to about 9 m such that flux values on larger scales are uncorrelated. These de-correlation scales are comparable to the horizontal extents of individual bubble plumes estimated visually at the sea surface. Semivariograms for each seep are used in an ordinary Kriging procedure to interpolate flux measurements onto a regular grid and produce objective maps of the spatial distribution of flux. Spatial integrals yield total flux estimates from the three seeps of ~7400 m3 day-1 which amount to 4-13% of the total flux from the Coal Oil Point field based on estimates from previous studies.

Until recently, oceanic crust aquifer properties were estimated from single-borehole experiments and mathematical modeling. This dissertation presents multi-tracer data from a cross-borehole tracer experiment in a hydrothermal system on the eastern flank of the Juan de Fuca Ridge. A rapid tracer breakthrough induced by a ~548 m3 fluid injection suggests that effective porosity is several orders of magnitude lower than bulk material porosity and ridge-parallel anisotropy may have a large effect on fluid flow in shallow ocean crust. Nine years of data suggest that chaotic local convection does not operate on the 102 – 103 m scale encompassing this field site. The low effective porosity precludes application of the conventional point-source model for tracer experiments and necessitates the development of a new conceptual model to explain observed tracer breakthroughs and derive aquifer properties. The average linear flow velocity at this site is found to be ~1 m/d. At the 102 – 103 scale, the representative elemental volume (REV) is constrained for future use in more accurately modeling oceanic crust as a porous medium.