CORRELATING OBSERVATIONS OF SURFACE WATER WITH FIELD-MEASURED PERMEABILITY ON SANTA ROSA ISLAND, CHANNEL ISLANDS NATIONAL PARK, CA

To evaluate potential causal relations for spatially discontinuous patterns of observed surface water availability on Santa Rosa Island, Channel Islands National Park, we examined mapped geology and measured permeability of earth materials. GPS field-surveyed occurrences of surface-water, base-flow conditions collected by National Park Service personnel demonstrate local correlation with geologic substrate. To evaluate such patterns, we used a portable, handheld air permeameter (NER TinyPerm II) to measure in situ rock and soil matrix permeability on outcrops, under ambient moisture conditions, at the core scale. Sites were selected to represent a range of surface water conditions, from wet to dry, and mapped geologic features such as rock type, map unit contacts, and Quaternary faults. Along the island-spanning Santa Rosa Island fault and other subsidiary faults, we measured permeability within mapped fault cores, fault-related damage zones, and adjacent rock protolith.

Permeability estimates at 27 sites reveal trends associated with rock type and geologic structure that are consistent with presence or absence of surface-water occurrence such that: i) Volcanic rocks and intact sandstones and shales express lower and intermediate permeabilities (~10^-1 – 10² mD), correlating with more continuous surface water presence. ii) Similar rocks with secondary fracture permeability express intermediate to higher permeabilities (~10 ³ – 10⁶ mD) and little correlation with surface water. Valley floors composed of fractured shale are generally dry. iii) Faults appear to act as both conduits and barriers to flow with clay-rich fault cores exhibiting low permeability (~10¹ – 10³ mD) whereas adjacent fault-related damage zones contain fractured rock with intermediate to higher permeabilities (~10⁴ – 10⁶ mD). Outside the damage zones, the non-fault deformed rock protolith expresses lower permeability (~10⁰ – 10² mD). Laterally continuous fault-core gouge, and fault juxtaposition of rock types with contrasting permeabilities, likely act as effective flow barriers forcing groundwater to the surface. This research could benefit vegetation restoration strategies in water-limited landscapes where ecosystems are reliant on surface water and where mapped geology is available to infer permeability.