EFFICIENT MAPPING OF COMPLEX GROUNDWATER SYSTEMS ASSOCIATED WITH BRAIDED RIVERS USING SMALL COIL SURFACE NUCLEAR MAGNETIC RESONANCE

Braided river systems are a substantial source of groundwater recharge in New Zealand. These rivers flow from high relief towards 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 on aquifer recharge mechanisms. To understand these complex recharge patterns a more detailed knowledge of the lithological controls on surface water-groundwater exchange is necessary. Here we present a case study, where surface nuclear magnetic resonance (SNMR) was employed in braided river systems to identify subsurface structures. We found the method to be efficient in identifying permeability contrasts beneath riverbeds and river berms in multiple river settings. We calibrated a local archetype by coincident borehole lithology and SNMR derived water contents, to find the SNMR signature related to lithological changes. The resulting archetypes were extrapolated to SNMR sites where there is no coinciding borehole information to identify these lithological transitions at multiple sites. We show that a combined borehole and SNMR survey can map transitions from aquifers to low permeability-layers efficiently in three case studies. The small 20 m x 20 m SNMR coil enabled acquisition on small bars within the river while resolving the thin saturated units, previously not resolvable with a larger coil size. The lateral constrained inversion (LCI) improved 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 hydrogeological layers to provide valuable information on recharge to vulnerable aquifers.