Photochemical production of dissolved inorganic nitrogen and primary amines from dissolved organic nitrogen in waters of two estuaries and adjacent surficial groundwaters

Abstract

Recent studies have shown that in a number of humic-rich surface waters in North America, NH 4 + is released when dissolved organic matter (DOM) is exposed to sunlight. However, photochemical NH 4 + production has not been observed in all surface waters, and factors that contribute to it are not well understood. We hypothesized that the presence or absence of NH 4 + photoproduction may be affected by the light exposure history of DOM. The present study was undertaken to determine whether DOM from surficial groundwaters, with minimal light exposure history, would produce labile nitrogen (N) photoproducts more consistently. In this study, estuarine surface waters and surficial groundwaters, collected just adjacent to estuaries in Georgia and South Carolina, USA, were exposed to sunlight to quantify the photochemical production of NH 4 + , dissolved primary amines (DPA), and NO 2 - . The photoproduction of NH 4 + was observed in 4 of 5 irradiated estuarine surface water samples but in only 2 of 13 groundwater samples. In contrast, NH 4 + concentrations decreased in 5 of 13 groundwater samples when exposed to sunlight. The results indicate that a small amount of NH 4 + may be lost from waters in which groundwater-derived DOM is first exposed to sunlight. No consistent trends were observed in the photoproduction or loss of DPA and NO 2 - .