SURFACE WATER AVAILABILITY, GEOLOGY, AND FIELD-MEASURED PERMEABILITY ON SANTA ROSA ISLAND, CHANNEL ISLANDS NATIONAL PARK, CALIFORNIA

In warm-summer Mediterranean landscapes with heterogeneous geology, rock permeability and structure may dictate connections between groundwater and surface water and hence water availability. To better understand spatial correlations between field-surveyed, discontinuous occurrences of surface water during low-flow conditions on Santa Rosa Island, Channel Islands National Park with geologic substrate, we compared mapped geology and permeability. We used a handheld air permeameter (NER TinyPerm II) to measure in situ rock permeability, under ambient moisture conditions, at the sub-outcrop scale. Measurement sites represent a range of surface-water conditions and mapped geologic features such as rock type or structure, including unit contacts and 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 structure consistent with presence or absence of surface water: i) Volcanic rocks and intact, indurated sandstones and shales express lower to intermediate permeabilities (˜10ˆ-1 – 10ˆ2 mD), correlating with more spatially continuous surface-water presence. ii) Similar rocks, but with secondary fractures induced by tectonic or topographic stresses, express intermediate to higher permeabilities (˜10ˆ3 – 10ˆ6 mD) and typically had no surface water present. iii) Fault zones may act as both conduits and barriers to flow, with clay-rich cores exhibiting low permeability (˜10ˆ-1 – 10ˆ2 mD) whereas adjacent damage zones contain broken rock with intermediate to higher permeabilities (˜10ˆ2 – 10ˆ7 mD). Outside of damage zones, non-fractured rock protoliths generally have lower permeabilities (˜10ˆ0 – 10ˆ3 mD). Laterally continuous fault-core gouge, and structurally juxtaposed rock types with contrasting permeabilities, likely act as effective barriers to cross-fault flow, locally forcing groundwater to the surface. iv) Bedding, parting, and fracture planes appear to impart local permeability anisotropy as reflected in observed increased permeability in directions parallel to such planes. These relations highlight how mapped geology and associated material properties can inform necessary inputs for robust hydrologic modeling.