Quantifying atmospheric reactive nitrogen concentrations, dry deposition, and isotope dynamics surrounding a Marcellus Shale well pad

Unconventional natural gas (UNG) extraction activities have become important contributors to regional NO_x emissions inventories. Currently, there is a knowledge gap in the amount of total N deposition surrounding well pads undergoing UNG extraction despite the fact that some areas with extensive natural gas extraction activity are already in exceedance of nitrogen critical loads. In this study, we measured the magnitude of total dry N deposition from NO₂, HNO₃, and NH₃ attributable to the development of two UNG wells at a Marcellus Shale well pad study site. This study documents concentrations, deposition fluxes, and isotope values of NO₂, HNO₃, O₃, and NH₃ up- and down-wind along a 750-m well pad passive sampling transect across a 16-acre well pad containing two unconventional wells during all phases of development and extraction comprising fifteen distinct sampling periods. An access road transect was also utilized to explore reactive N dynamics in a near-road environment on the well pad where NO₂ concentration and isotope dynamics were highly correlated with daily traffic count (r² = 0.78–0.88, p < 0.01). An onsite chemiluminescent NO₂ source apportionment model was compared against δ¹⁵N–NO₂ and δ¹⁸O–NO₂ source apportionment models (r² = 0.57 and 0.82 respectively), demonstrating the possible utility of δ¹⁸O–NO₂ as a source apportionment tool in near-source environments. In addition, the δ¹⁵N–NO₂ source apportionment method compared well against a background-subtraction method (slope = 0.82, r² = 0.88, p < 0.001) during non-wintertime conditions and was used to find N loadings directly attributable to well pad activities. In total, the total N deposition across the transect, attributable to well pad activities, utilizing industry's best management practices, ranged from 0.16 to 0.55 kg N ha⁻¹ yr⁻¹. This magnitude of well pad attributable N deposition is high enough to result in exceedances of nitrogen critical loads in areas with high well count densities and high baseline N deposition.