Mountain headwater streams provide unique ecological habitats that are influenced by underlying geologic structure, including bedrock depth, a characteristic that is often ill-defined. We evaluated the importance of low-permeability bedrock depth for summer stream temperature and channel dewatering patterns using horizontal-to-vertical spectral ratio (HVSR) seismic methods along 8 headwater streams in Shenandoah National Park (Virginia USA). HVSR data collected from 2015-2020 were complimented by spatially continuous surveys of stream channel dewatering during baseflow conditions (128.5 total km of stream length), multiyear stream temperature data from 64 locations, and repeated paired discharge observations using the salt dilution method. Median bedrock depth ranged 1.5 to 3.4 m, with half of the 8 stream corridors showing and average depth < 2 m. Measured bedrock depths were not well represented by existing large-scale geologic datasets or readily predicted based on topography. Two subwatersheds showed a general downstream deepening of the bedrock contact but others showed shallow bedrock throughout or had discrete, deeper bedrock zones (e.g. >20 m depth). The stream with the deepest average underlying bedrock contact supported the coldest summer temperatures, displayed a characteristic thermal signature of deeper groundwater influence, and did not dry during baseflow conditions. Patchy channel dewatering was observed at baseflow throughout the study area, as exemplified by the image below that shows a stream disconnected by a localized deposit of alluvium. Bedrock depth variability along the channel was associated with dewatering observed within some streams, though our results also indicate the importance of the shallow groundwater reservoir in maintaining streamflow and influencing paired streamflow gain and loss patterns. Our study demonstrates the importance of shallow, low-permeability bedrock contacts on stream-groundwater exchange, impacting channel habitat and connectivity within headwater stream networks.

Assessment and management of limited fresh groundwater resources on remote small islands can be complicated by heterogenous geology, natural climate cycles, and a general lack of data. The Palmyra Atoll National Wildlife Refuge is home to one of the few surviving native stands of Pisonia grandis in the central Pacific Ocean, yet these trees face pressure from groundwater salinization, with limited basic groundwater data to guide decision making. Adding to natural complexity, the geology of Palmyra was heavily altered by dredge and fill activities in the 1940s. We combined electromagnetic imaging (EMI) and hydrological field measurements from 2008-2019 with groundwater modeling to map the current distribution of fresh groundwater on the modified main island and a small, more natural islet to better understand potential physical drivers of spatiotemporal variability. Frequency-domain EMI data were collected on the main atoll islands over repeat transects in 2008 following ‘strong’ La Niña conditions (wet) and 2016 during ‘very strong’ El Niño conditions (dry). Shallow monitoring wells were installed adjacent to the geophysical transects in 2013 and screened within the fresh/saline groundwater transition zone. Temporal EMI and monitoring well data showed a strong lateral and vertical contraction of the freshwater lens in response to El Niño conditions, and transient EMI data indicated a thicker lens toward the ocean side, an opposite spatial pattern to that observed for many other Pacific islands. On an outer islet where a stand of mature Pisonia trees exist, EMI surveys revealed only a thin (<3 m from land surface) layer of brackish groundwater during El Niño. Numerical groundwater simulations were performed for a range of permeability distributions and climate conditions at Palmyra. Results revealed that the observed atypical lens asymmetry is likely due to a combination of lagoon dredging and filling with high-permeability material, allowing for more efficient submarine groundwater discharge on the lagoon side. Simulations also predict large negative changes (approximately 40% decrease) in freshwater lens volume during dry cycles and highlight threats to the Pisonia trees, yielding insight for atoll ecosystem management on Palmyra and other small Pacific islands.