IMPORTANCE OF THE INITIAL CONCENTRATION DISTRIBUTION DURING TRACER RELEASE EXPERIMENTS

A SF₆ and Cs push-pull tracer experiment in sub-seafloor hydrothermal fluid circulating in the eastern flank of the Juan de Fuca Ridge was conducted (2010-2014). The experiment was initiated following the completion of two boreholes (ocean water depth ~2,500 m and overlying sediment aquitard thickness ~250 m) during IODP Expedition 327 with the primary objective to better characterize hydrologic properties of this 3.5 Ma basaltic aquifer. This location was chosen because the northerly direction for regional flow is relatively well known and two additional boreholes constructed during early drilling expeditions could be used. SF₆ was added to the injection water for approximately 22 hr during a 24 hr long multi-borehole pump test, whereas Cs was added during two discrete eight minute long periods. Samples were collected in passive pumps (OsmoSamplers) that continually collect water at a rate of ~1 mL/d into narrow ~1 mm ID tubes, mounted on seafloor platforms for periodic servicing.

Data analyses conducted prior to August 2016 documented the initial breakthrough at a sampling location approximately 532 m up gradient of the injection borehole and 535 m down gradient.Cs tracer results show a very rapid breakthrough at the closest monitoring location down gradient. Using a point-source model, these results require a linear velocity >15 m/d from the injection location to the first sampling location at 311 m, and a linear velocity of 1-2 m/d from there to the second sampling location an additional 224 m down gradient.

A simple model that accounts for both rates is one in which the ~580 m³ injection volume displaced formation fluid a majority of the distance between the injection and the first down gradient sampling locations. This model allows for the rapid breakthrough at the 311 m down gradient borehole while maintaining a uniform background linear velocity of 1-2 m/d, which then produced the later breakthrough at the 535 m sampling location. Such displacement is only possible with some combination of low effective porosity (<10^-3) and high lateral anisotropy (10-100) controlling the spatial extent of the injectate plume. This model also enables refinement of aquifer property estimates using calculations of the stagnation point of a “pumping” borehole that captured a small portion of the tracer plume, which lies up gradient.