Groundwater flow characterization of an ophiolitic hard-rock aquifer from cross-borehole multi-level hydraulic experiments

Abstract

Ophiolitic formations play a critical role in the groundwater resource of numerous countries and areas. Previous studies show that the structural heterogeneities of these rocks, coming from the presence of both different lithological units and multi-scale discontinuities, result in complex hydrogeological features that are not well characterized yet. In particular, there is a need for understanding how these heterogeneities impact the hydrodynamic properties of ophiolitic aquifers and the highly variable chemical composition of the water. To this end, we conduct various kinds of pumping experiments between two boreholes 15 m apart in the ophiolitic formation of the Batin (BA1) site in the wadi Tayin massif of the Sultanate of Oman. Cross-borehole open pumping experiments, as well as multi-level pumping and monitoring hydraulic tests, are performed in conductive zones that were identified from temperature and flowmeter data, but also in low-permeability zones requiring

Aqueous Geochemical and Microbial Variation Across Discrete Depth Intervals in a Peridotite Aquifer Assessed Using a Packer System in the Samail Ophiolite, Oman

“…The limit of quantitation for  CO 2 c was higher in 2019 than in 2018 because a set of higher volume samples from 2019 was compromised during transport. We usually take two different sample volumes to ensure that, upon acidification, both (Lods et al, 2020).…”

“…Pumping in BA1D was also supplied by horizontal and vertical flows near the pumped borehole, but only horizontal flows further away, near BA1A. Lods et al (2020) speculated that a component of the water pumped during the BA1D test could have been derived from the highly conductive fractures in the shallower regions near BA1A, with this water flowing downward through the BA1A borehole and then horizontally to BA1D via fractures in the 102m and 132m depth interval.…”

“…To provide context for the geochemical results that will follow, we summarize here the findings of Lods et al (2020), who interpreted flowmeter and pumping test data to understand the physical hydrology of boreholes BA1A and BA1D. The present study's samples from 2019 (Table 1) were collected simultaneously with, or immediately following, the pumping tests of Lods et al (2020). Pumping depth intervals targeted hydraulically conductive zones in the subsurface, which were inferred from temperature profiles and flowmeter data (Lods et al, 2020).…”

“…The transmissivity in the shallow regions of BA1A was higher than in BA1D, and an ambient downflow of   1 1L min measured in BA1A from 22m to 59m indicated the displacement of substantial volumes of water from the surficial highly weathered dunites aquifer to lower dunite aquifers at BA1A at least as deep as 59m. Lesser flow may extend to even deeper aquifers at BA1A at rates below the detection limit of the flowmeter used in the experiments of Lods et al (2020) <0.1 NOTHAFT ET AL.…”

Accessing the Subsurface Biosphere Within Rocks Undergoing Active Low‐Temperature Serpentinization in the Samail Ophiolite (Oman Drilling Project)

“…The serpentinites have experienced several prior episodes of alteration and deformation, and are almost fully serpentinized. The rocks are continuing to undergo hydration and oxidation within a fracture‐dominated hydrologic system, with greater density of transmissive fractures within the upper 50–100 m (Dewandel et al., 2005; Lods et al., 2020). Despite the lack of much relict olivine or pyroxene, the primary alteration assemblages contain abundant ferroan brucite, and widespread awaruite and sulfide phases, which serve as pervasive reductants.…”