In a natural analog study of risks associated with carbon sequestration, impacts of CO 2 on shallow groundwater quality have been measured in a sandstone aquifer in New Mexico, USA. Despite relatively high levels of dissolved CO 2 , originating from depth and producing geysering at one well, pH depression and consequent trace element mobility are relatively minor effects due to the buffering capacity of the aquifer. However, local contamination due to influx of brackish waters in a subset of wells is significant. Geochemical modeling of major ion concentrations suggests that high alkalinity and carbonate mineral dissolution buffers pH changes due to CO 2 influx. Analysis of trends in dissolved trace elements, chloride, and CO 2 reveal no evidence of in situ trace element mobilization. There is clear evidence, however, that As, U, and Pb are locally co-transported into the aquifer with CO 2 -rich brackish water. This study illustrates the role that local geochemical conditions will play in determining the effectiveness of monitoring strategies for CO 2 leakage. For example, if buffering is significant, pH monitoring may not effectively detect CO 2 leakage. This study also highlights potential complications that CO 2 carrier fluids, such as brackish waters, pose in monitoring impacts of geologic sequestration.

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As part of the U.S. National Seismic Hazard Model (NSHM) update planned for 2023, two databases were prepared to more completely represent Quaternary-active faulting across the western United States: the NSHM23 fault sections database (FSD) and earthquake geology database (EQGeoDB). In prior iterations of NSHM, fault sections were included only if a field-measurement-derived slip rate was estimated along a given fault. By expanding this inclusion criteria, we were able to assess a larger set of faults for use in NSHM23. The USGS Quaternary Fault and Fold Database served as a guide for assessing possible additions to the NSHM23 FSD. Reevaluating available data from published sources yielded an increase of fault sections from ~650 faults in NSHM18 to ~1,000 faults proposed for use in NSHM23. EQGeoDB, a companion dataset linked to NSHM23 FSD, contains geologic slip rate estimates for fault sections included in FSD. Together, these databases serve as common input data used in deformation modeling, earthquake rupture forecasting, and additional downstream uses in NSHM development.

We use tectonic subsidence patterns from wells and stratigraphic sections to describe the mid-Miocene to present tectonic subsidence history of the Rio Grande rift. Tectonic subsidence and therefore rift opening were quite fast until ca. 8 Ma, with net subsidence rates (~25-65 mm/k.y.) comparable to those of the prerupture phase of rifted continental margins. The rapid subsidence was followed by a late Miocene-early Pliocene unconformity that developed mainly along the flanks of most rift basins. The age of its associated lacuna is spatially variable but falls within 8-3 Ma (mostly 7-5 Ma) and thus is synchronous with eastward tilting of the western Great Plains (ca. 6-4 Ma). Tectonic subsidence rates either remained similar or decreased after the Miocene-Pliocene unconformity. North of 35°N, our analysis of geoid-to-elevation ratios suggests that, at present, topography of the Rio Grande rift region is compensated by a component of mantle-driven dynamic uplift. Previous work has indicated that this dynamic uplift is caused by focused vertical flow in the upper mantle resulting from slab descent and fragmentation of the Farallon slab, and Rio Grande rift opening, which affected the Rio Grande rift area beginning in the late Miocene. The spatial distribution and timing of the unconformity, as well as eastward tilting of the western Great Plains, can be explained by this dynamic mantle uplift, with contributions from variations in rift opening tectonics and climate. The focused mantle upwelling is not associated with increased rift opening rates.