Global Nitrogen Mass Flux From the Active Freshwater Aquifer Element Pool

The estimated current global mean nitrogen concentration (geogenic + anthropogenic) in the active continental freshwater aquifer element pool is 1.1 mg/L as N, or between four and five times greater than the assumed geogenic mean. This concentration, combined with groundwater flux, generates a continental mass flux of 17 Tg N/y (teragrams of nitrogen, as N, per year) as a result of direct ocean discharge (0.67 Tg N/y), endorheic basins (1.2 Tg N/y), and cold-wet (0.82 Tg N/y); cold-dry (1.4 Tg N/y); warm-dry (1.6 Tg N/y); and warm-wet (11 Tg N/y) exorheic basins. These values are derived from a geospatial machine learning algorithm and combined groundwater-modeled recharge in an ArcGIS environment. This active continental freshwater aquifer mass flux is between 35% and 40% of the continental integrated riverine system discharge, thus a significant component of the Earth's active continental freshwater nitrogen budget. We estimate the active continental freshwater aquifer volume to be between 1.4 and 2.8 million km³ suggesting a legacy of between 1.5 and 3.1 Pg as N (petagrams nitrogen as N) with mean residences of 90–180 years.