Surface water availability and field-measured permeability on Santa Rosa Island, Channel Islands National Park, CA

Abstract:

To assess spatial correlation between field-surveyed, discontinuous occurrences of surface water during base-flow conditions on Santa Rosa Island, Channel Islands National Park with geologic substrate, we compared mapped geology to the corresponding permeability at GPS-located sites. We used a portable, handheld air permeameter (NER TinyPerm II) to measure in situ rock permeability on outcrops, under ambient moisture conditions, at the core scale. Sites represent a range of surface water conditions, from wet to dry, and mapped geologic features such as rock type or structure, including unit contacts and Quaternary faults. Along the island-spanning Santa Rosa Island fault and other subsidiary faults, we measured permeability within fault cores and damage zones, and in adjacent rock protolith.

Permeability estimates at 31 sites (>600 measurements) reveal four trends associated with rock type and geologic structure consistent with presence or absence of surface-water occurrence: i) Volcanic rocks and intact, indurated sandstones and shales express lower and intermediate permeabilities (~10^-1 – 10² mD), correlating with more continuous surface water presence. ii) Similar rocks, but with secondary fractures induced by tectonic or topographic stresses, express intermediate to higher permeabilities (~10 ³ – 10⁶ mD) and little correlation with surface water leading to generally dry valley floors. iii) Faults may act as both conduits and barriers to flow with clay-rich cores exhibiting low permeability (~10^-1 – 10² mD) whereas adjacent damage zones contain broken rock with intermediate to higher permeabilities (~10² – 10⁷ mD). Outside the damage zones, non-fractured rock protoliths generally express lower permeability (~10⁰ – 10³ mD). Laterally continuous fault-core gouge, and juxtaposition of rock types with contrasting permeabilities, likely act as effective barriers to cross-fault flow, forcing groundwater to the surface. iv) Bedding, parting, and fracture planes appear to impart local permeability anisotropy as reflected in observed reduced permeability in directions perpendicular to such planes. This research could benefit vegetation restoration strategies in water-limited landscapes where ecosystems are reliant on surface water and where mapped geology can be used to infer permeability.