Combining multiple electromagnetic methods, direct aquifer measurements,
and modeling to inform ecological management on Palmyra Atoll
Abstract
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.