Transport and reactivity of nitrous oxide and methane in two contrasting subterranean estuaries

Abstract Continental groundwaters are commonly enriched in nitrous oxide (N 2 O) and methane (CH 4 ), which can discharge into the coast. The contribution of this diffuse source to coastal N 2 O and CH 4 emissions largely depends on the biogeochemical processes of coastal aquifer exit zones, known as subterranean estuaries. Here, we study the role of subterranean estuaries in modulating N 2 O and CH 4 exports toward the coast. The two studied subterranean estuaries are located at Panxón and Ladeira beaches (Ría de Vigo, NW Iberian Peninsula) and had opposite oxygenation at their interior. Groundwater‐borne N 2 O was detected in the oxygenated subterranean estuary of Panxón Beach, although denitrification attenuated N 2 O before porewater was discharged into the coast. An N 2 O hotspot was detected at about 50 cm depth in this subterranean estuary, characterized by the presence of nitrate under suboxic conditions. This N 2 O also seems to be consumed along the flow path before discharging into the coast. The anoxic subterranean estuary of Ladeira Beach completely removed groundwater‐borne N 2 O. Yet, nitrification within its suboxic upper saline plume produced and exported N 2 O toward the coast, hiding the role of this subterranean estuary as an N 2 O sink. The anoxic subterranean estuary exported CH 4 toward the coast. This CH 4 was not sourced by continental groundwater, hence it was produced in situ fuelled by the accumulation of organic matter within the beach. The suboxic upper saline plume in Ladeira Beach and the oxygenated subterranean estuary of Panxón Beach acted as CH 4 sinks.