Perfluoroalkyl acids (PFAAs), a group of synthetic compounds associated with adverse human health impacts, are commonly found in effluent discharged from wastewater treatment facilities. When that effluent is used for irrigation, the fate of PFAAs depends strongly on vadose zone solute retention properties and loading history. The relative importance of PFAA retention factors under natural conditions remains uncertain, and the historical record of effluent PFAA concentrations is limited. Using soil cores collected from the Penn State Living Filter (irrigated with treated wastewater effluent for nearly 60 years), we evaluated PFAA transport under near-natural conditions, and estimated historical PFAA concentrations in the irrigated effluent. Total perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) masses stored in soils in 2014 were more than 450 times greater than the masses applied during the 2020 effluent irrigation. Equilibrium piston-flow transport models reproduced the observed PFOS and PFOA profiles, allowing us to estimate historical effluent PFOS and PFOA concentrations: 70-170 ng L-1 and 1000-1300 ng L-1, respectively. Estimated concentrations were comparable to concentrations measured in other wastewater effluents in the 1990s and 2000s, indicating that when interpreted with transport modeling, wastewater-irrigated soils function as integrated records of historical PFAA loading. Simulated PFOS breakthrough to groundwater occurred 50 years after the start of wastewater irrigation, while simulated PFOA breakthrough occurred after only 10 years of irrigation. Thus, while wastewater irrigation of soils facilitates retention and reduces effluent PFAA loading to surface waters, the resulting increased PFAA storage in soils potentially creates long-term sources of PFAAs to groundwater.

The Pāpaku fault zone, drilled at IODP Site U1518, is an active splay fault in the frontal accretionary wedge of the Hikurangi Margin. In logging-while-drilling data, the 33 m-thick fault zone exhibits mixed modes of deformation associated with a trend of downward decreasing density, P-wave velocity and resistivity. Methane hydrate are observed from ~30-585 mbsf, including within and surrounding the fault zone. Hydrate accumulations are vertically discontinuous and occur throughout the entire logged section at low to moderate saturation in silty and sandy cm-thick layers. We argue that the hydrate distribution implies that the methane is not sourced from fluid flow along the fault but instead by local diffusion. This, combined with geophysical observations and geochemical measurements from Site U1518, suggests that the fault is not a focused migration pathway for deeply-sourced fluids and that the near-seafloor Pāpaku fault zone has little to no active fluid flow.

We constrain the orientation of the horizontal stress field from borehole image data in a transect across the Hikurangi Subduction Margin. This region experiences NW-SE convergence leading to recurrent slow slip events. The direction of the horizontal maximum stress is E-W at an active thrust fault near the subduction margin trench. This trend changes to NNW-SSE in the Tuaheni Basin in the offshore accretionary wedge, and to NE-SW in the onshore forearc. Multiple, tectonic and geological processes, either individually or in concert, may explain this variability. A general offshore-onshore stress rotation may reflect a change from dominantly compressional tectonics at the deformation front, to a strike-slip and/or extensional stress regime closer to the Taupo Volcanic Zone. In addition, the offshore stress may be affected by topography and/or stress rotation around subducting seamounts, and/or temporal stress changes during the slow slip cycle.

Knowledge of the contemporary in-situ stress orientations in the Earth’s crust can improve our understanding of active crustal deformation, geodynamic processes, and seismicity in tectonically active regions such as the Hikurangi Subduction Margin (HSM), New Zealand. The HSM subduction interface is characterized by varying slip behavior along strike, which may be a manifestation of variation in the stress state and the mechanical strength of faults and their hanging walls, or, alternatively, these variations in seismic behavior may generate variation in the stress state in space and time. In this study, we analyze borehole image and oriented four-arm caliper logs acquired from thirteen boreholes along the HSM to present the first comprehensive stress orientation dataset within the HSM upper plate. Our results reveal a NE-SW SHmax orientation (parallel to the Hikurangi margin) within the central HSM (Hawke’s Bay region) which rotates to a WNW- ESE SHmax orientation (roughly perpendicular to the Hikurangi margin) in the southern HSM. This rotation of SHmax orientation spatially correlates with along-strike variations in subduction interface slip behavior, characterized by creep and/or shallow episodic slip events in the central HSM and interseismic locking in the southern HSM. Observed borehole SHmax orientations are largely parallel to maximum contraction directions derived from geodetic surface deformation measurements, suggesting that modern stress orientations may reflect contemporary elastic strain accumulation processes related to subduction megathrust locking.

Slow slip events (SSEs) have been identified at subduction zones globally as an important link in the continuum between elastodynamic ruptures and stable creep. The northern Hikurangi margin is home to shallow SSEs which propagate to within 2 km of the seafloor and possibly to the trench, providing insights into the physical conditions conducive to SSE behavior. We report on a suite of friction experiments performed on protolith material entering the SSE source region at the Hikurangi margin, collected during the International Ocean Discovery Program Expedition 375. We performed velocity stepping and slide-hold-slide experiments over a range of fault slip rates, from plate rate (5 cm/yr) to ~1 mm/s and quantified the frictional velocity dependence and healing rates for a range of lithologies at different stresses. The friction velocity dependence (a-b) and critical slip distance Dc increase with fault slip rate in our experiments. We observe a transition from velocity weakening to strengthening at slip rates of ~0.3 µm/s. This velocity dependence of Dc could be due to a combination of dilatant strengthening and a widening of the active shear zone at higher slip rates. We document low healing rates in the clay-rich volcaniclastic conglomerates, which lie above the incoming plate basement at least locally, and relatively higher healing rates in the chalk lithology. Finally, our experimental constraints on healing rates in different input lithologies extrapolated to timescales of 1-10 years are consistent with the geodetically-inferred low stress drops and healing rates characteristic of the Hikurangi SSEs.