Abstract
Braided river systems are a substantial source of groundwater recharge in New Zealand. These rivers flow from high relief toward the sea across alluvial plain aquifers, across which the primary groundwater recharge occurs. These aquifers are often used for agricultural irrigation, and overexploitation can occur if there is limited knowledge of aquifer recharge mechanisms. To understand these complex recharge patterns, more detailed knowledge of the lithologic controls on the surface water-groundwater exchange is necessary. Here, we develop a case study in which surface nuclear magnetic resonance (SNMR) is used in braided river systems to identify subsurface structures. We find the method to be efficient in identifying permeability contrasts beneath riverbeds and river berms in multiple river settings. We calibrate a local archetype by coincident borehole lithology and SNMR-derived water contents to find the SNMR signature related to lithologic changes. The resulting archetypes are extrapolated to SNMR sites where there is no coinciding borehole information to identify these lithologic transitions at multiple sites. We find that a combined borehole and SNMR survey can map the transitions from aquifers to low-permeability layers efficiently in three case studies. The small 20 m × 20 m SNMR coil enables acquisition on small bars within the river while resolving the thin-saturated units, previously not resolvable with a larger coil size. The laterally constrained inversion improves lateral consistency and the ability to track the features of the braidplain aquifer in all three rivers within the top 10 m. These results demonstrate SNMR as a multisite geophysical method capable of mapping important hydrogeologic layers to provide valuable information on recharge to vulnerable aquifers.
| Originalsprog | Engelsk |
|---|---|
| Tidsskrift | Geophysics |
| Vol/bind | 90 |
| Nummer | 3 |
| Sider (fra-til) | 1-12 |
| Antal sider | 12 |
| ISSN | 0016-8033 |
| DOI | |
| Status | Udgivet - 1 maj 2025 |