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Characterizing Hydrology Beneath Rivers & Estuaries — USA
TEM CASE STUDY ● UNITED STATES, 2020
01 – The challenge
The hydrogeology beneath rivers, lakes, and estuaries is almost universally unmapped at any meaningful scale – leaving fresh/saline groundwater interfaces and groundwater–surface water exchange essentially invisible.
Traditional methods can’t economically cover the hundreds of kilometres needed to characterize a river or estuary system, especially to the depths required to resolve real hydrogeological structure. That gap inhibits accurate hydrological modelling nationwide – you can’t predict how groundwater and surface water interact if you can’t see what’s underneath the water in the first place.
02 – The context
A multi-site demonstration of FloaTEM – a floating, towed transient electromagnetic system – to characterize hydrogeology beneath rivers and estuaries across the United States, in partnership with the U.S. Geological Survey (USGS).
PARTNER
U.S. Geological Survey (USGS)
COUNTRY
United States (Connecticut, New York, Mississippi, Massachusetts)
TERRAIN
Riverbed sediment, bedrock, fresh/saline groundwater interfaces
SITES SURVEYED
4
TECHNOLOGY
FloaTEM
YEAR
2020
Demonstrated in partnership with the U.S. Geological Survey (USGS) and the HydroGeophysics Group, Aarhus University.
03 – The solution
Floating Transient Electromagnetic Survey
FloaTEM is a floating transient electromagnetic system, towed behind a boat across the water surface. It’s built to image subsurface structure to depths greater than other towed TEM instruments. It was demonstrated at four sites across the US – the Farmington River (Connecticut), Upper Delaware River (New York), Tallahatchie River (Mississippi), and the Eel River estuary (Massachusetts) – each chosen to test the system against a different hydrogeological question.
Methodology
01
System Deployment
FloaTEM was towed by boat in a 2×4 coil configuration, mapping continuously across each river or estuary surface.
02
Multi-Site Data Collection
Surveys were conducted across four US sites, spanning environments from freshwater rivers to a tidal, nutrient-rich estuary.
03
Cross Validation
At the Tallahatchie River site, FloaTEM results were directly compared against airborne frequency-domain EM data and land-based towed TEM data.
04
Hydrogeological Interpretation
Results were used to resolve bedrock layers, aquifer confining units, sediment-water interface materials, and fresh/saline groundwater boundaries.
04 – The results
FloaTEM mapped bedrock layers and aquifer confining units at several sites, at resolution comparable to airborne methods. At the Eel River estuary, it imaged terrestrial fresh groundwater discharge with flowpaths extending hundreds of metres from shore – improving on previous hydrogeological characterizations of that nutrient-rich coastal exchange zone.
4
US river and estuary sites surveyed
100+m
Freshwater discharge flowpaths imaged at the Eel River estuary
2
Peer-reviewed papers published
The Region
01
Cape Cod’s Only Water Source
The Cape Cod aquifer was designated the sole source of drinking water for the entire Cape by the EPA in 1982, recharged only by rainfall. Understanding exactly where that freshwater meets the surrounding saltwater isn’t academic here – it’s the whole water supply.
02
Four States, One Question
The Farmington, Delaware, Tallahatchie, and Eel River sites span Connecticut, New York, Mississippi, and Massachusetts – different geology, different climates, testing whether one method could reliably answer the same question everywhere.
03
A National Science Agency
This work was carried out with the U.S. Geological Survey, the federal agency responsible for the country’s water resource science – not a private commission, but a genuine research partnership.
04
Under-Mapped at Scale
Hydrogeology beneath rivers and estuaries stays unmapped at the tens-of-kilometres scale hydrologists actually need – a gap that exists nationwide, not just at these four sites.
05 The impact
A new way to see beneath the water.
FloaTEM has since been used beyond these four original sites, including offshore surveys in Denmark. By resolving what was previously invisible – fresh/saline interfaces, buried aquifer structure, groundwater discharge zones – it gives hydrologists a genuinely new way to understand how rivers, lakes, and estuaries exchange water with the ground beneath them.
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