Abstract. The transformations of chemical constituents in subterranean estuaries (STE) control the delivery of nutrient loads from aquifers to the coastal ocean. It is important to determine the processes and sources that affect nutrient concentrations at a local scale in order to accurately estimate global nutrient fluxes via submarine groundwater discharge (SGD), particularly in boreal environments, where data are still very scarce. Here, the biogeochemical transformations of nitrogen (N) species were examined within the STE of a microtidal boreal sandy beach located in the Îles-de-la-Madeleine (Québec, Canada). This study reveals the vertical and horizontal distribution of nitrate (NO3−), nitrite (NO2−), ammonia (NH4+), dissolved organic nitrogen (DON) and total dissolved nitrogen (TDN) measured in beach groundwater during four spring seasons (June 2011, 2012, 2013 and 2015) when aquifer recharge is maximal after snow melt. Inland groundwater supplied high concentrations of NOx− and DON to the STE, whereas inputs from seawater were very limited. Non-conservative behaviour was observed along the groundwater flow path, leading to low NOx− and high NH4+ concentrations in the discharge zone. The long residence time of groundwater within the beach (~ 82 days), coupled with oxygen-depleted conditions and high carbon concentrations created a favourable environment for N transformations such as heterotrophic denitrification and ammonium production. An estimate of SGD fluxes of N was determined to account for biogeochemical transformations within the STE. Fresh inland groundwater delivers 37.54 mol m−1 y−1 of NOx and 63.57 mol m−1 y−1 of DON to the STE, and NH4+ input was negligible. But the N load to coastal waters is dominated by NH4+ and DON, due to N transformations along the flow path. NH4+ represents 99 % of the DIN flux to coastal waters, at 42.80 mol m−1 y−1. Since N fluxes to the coastal bay (88 mol m−1 y−1) are slightly lower than N fluxes from fresh inland groundwater (102 mol m−1 y−1), the STE appears to be a sink of terrestrially-derived N. The net transformations of N in the STE led to N removal along the groundwater flow path along the groundwater flow path.
Groundwater Flow Path and Evolution in the Nagqu Basin, Northeast Tibet Plateau