Global Isotope Hydrogeology―Review

Quantifying groundwater fluxes to and from deep aquifers or shallow sediment is a critical task faced by researchers and practitioners from many environmental science disciplines including hydrology, hydrogeology, ecology, climatology, and oceanography. Groundwater discharge to inland and coastal water bodies influences their water budgets, thermal regimes, and biogeochemistry. Conversely, downward water flow from the land surface or from surface water bodies to underlying aquifers represents an important water flux that must be quantified for sustainable groundwater management. Because these vertical subsurface flows are slow and typically diffuse, they cannot be measured directly and must rather be estimated using groundwater tracers. Heat is a naturally occurring groundwater tracer that is ubiquitous in the subsurface and readily measured. Most of the academic literature has focused on groundwater temperature tracing methods capitalizing on the propagation of diel temperature sine waves into sediment beneath surface water bodies. Such methods rely on temperature–time series to infer groundwater fluxes and are typically only viable in the shallow subsurface and in locations with focused groundwater fluxes. Alternative methods that utilize temperature–depth profiles are applicable across a broader range of hydrologic environments, and point‐in‐time measurements can be quickly taken to cover larger spatial scales. Applications of these methods have been impeded due in part to the lack of understanding regarding their potential applications and limitations. Herein, we highlight relevant theory, thermal data collection techniques, and recent diverse field applications to stimulate further multidisciplinary uptake of thermal groundwater tracing methods that rely on temperature–depth profiles.

Theory, tools, and multidisciplinary applications for tracing groundwater fluxes from temperature profiles

Groundwater discharge generates streamflow and influences stream thermal regimes. However, the water quality and thermal buffering capacity of groundwater depends on the aquifer source-depth. Here, we pair multi-year air and stream temperature signals to categorize 1729 sites across the continental United States as having major dam influence, shallow or deep groundwater signatures, or lack of pronounced groundwater (atmospheric) signatures. Approximately 40% of non-dam stream sites have substantial groundwater contributions as indicated by characteristic paired air and stream temperature signal metrics. Streams with shallow groundwater signatures account for half of all groundwater signature sites and show reduced baseflow and a higher proportion of warming trends compared to sites with deep groundwater signatures. These findings align with theory that shallow groundwater is more vulnerable to temperature increase and depletion. Streams with atmospheric signatures tend to drain watersheds with low slope and greater human disturbance, indicating reduced stream-groundwater connectivity in populated valley settings.

/pdf/continental-scale-analysis-of-shallow-and-deep-groundwater-5ffu6lphx3.pdf

Continental-scale analysis of shallow and deep groundwater contributions to streams.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018WR024236

Empirical Stream Thermal Sensitivities May Underestimate Stream Temperature Response to Climate Warming

Temperature buffering by groundwater in ecologically valuable lowland streams under current and future climate conditions

Inferring watershed hydraulics and cold-water habitat persistence using multi-year air and stream temperature signals.

. Brook trout (Salvelinus fontinalis) spawn in fall and
overwintering egg development can benefit from stable, relatively warm
temperatures in groundwater-seepage zones. However, eggs are also sensitive
to dissolved oxygen concentration, which may be reduced in discharging
groundwater (i.e., seepage). We investigated a 2 km reach of the coastal
Quashnet River in Cape Cod, Massachusetts, USA, to relate preferred fish
spawning habitats to geology, geomorphology, and discharging groundwater
geochemistry. Thermal reconnaissance methods were used to locate zones of
rapid groundwater discharge, which were predominantly found along the central
channel of a wider stream valley section. Pore-water chemistry and temporal
vertical groundwater flux were measured at a subset of these zones during
field campaigns over several seasons. Seepage zones in open-valley
sub-reaches generally showed suboxic conditions and higher dissolved solutes
compared to the underlying glacial outwash aquifer. These discharge zones
were cross-referenced with preferred brook trout redds and evaluated during
10 years of observation, all of which were associated with discrete alcove
features in steep cutbanks, where stream meander bends intersect the glacial
valley walls. Seepage in these repeat spawning zones was generally stronger
and more variable than in open-valley sites, with higher dissolved oxygen and
reduced solute concentrations. The combined evidence indicates that regional
groundwater discharge along the broader valley bottom is predominantly
suboxic due to the influence of near-stream organic deposits; trout show no
obvious preference for these zones when spawning. However, the meander bends
that cut into sandy deposits near the valley walls generate strong oxic
seepage zones that are utilized routinely for redd construction and the
overwintering of trout eggs. Stable water isotopic data support the
conclusion that repeat spawning zones are located directly on preferential
discharges of more localized groundwater. In similar coastal systems with
extensive valley peat deposits, the specific use of groundwater-discharge points
by brook trout may be limited to morphologies such as cutbanks, where
groundwater flow paths do not encounter substantial buried organic material
and remain oxygen-rich.

/pdf/hydrogeochemical-controls-on-brook-trout-spawning-habitats-4c3xhrdbof.pdf

Stephen T. Hurley

Papers

Inferring watershed hydraulics and cold-water habitat persistence using multi-year air and stream temperature signals.

Hydrogeochemical controls on brook trout spawning habitats in a coastal stream.