Photooxidation of Dissolved Organic Matter in Groundwater and Sediment Porewater in a Subarctic Island

Abstract:

Submarine groundwater discharge (SGD) delivers significant amounts of dissolved organic matter (DOM) to coastal zones where it is subject to photooxidation. To date little is known about the photoreactivity of DOM in groundwater (GW) and sediment porewater (PW), which are the source water of SGD. We studied the photobleaching, photomineralization, and photoammonification of DOM in GW and beach sediment PW collected from the Magdalen Islands in the Gulf of St. Lawrence in the spring of 2012. The PW, with a specific UV absorbance at 254 nm (SUVA₂₅₄) of 12.73 L mg^-1 m^-1, was much more colored than the GW (SUVA₂₅₄: 4.84 L m^-1 mg^-1). Furthermore, the fluorescence index (FI₃₇₀) and humification index (HIX) suggest that DOM in the PW was dominantly of terrestrial and soil origin whereas DOM in the GW primarily originated from aquatic and microbial sources. A 20-d solar-simulated irradiation of the PW resulted in a nearly complete loss of DOM absorbance but only a 47% loss of dissolved organic carbon (DOC). For the GW, the absorbance dropped by 66% while DOC decreased only by 24% after a 4-d irradiation. The photoammonification rate reached 106 µmol N m^-3 d^-1 in the GW and 42 µmol N m^-3 d^-1 in the PW. The absorbed photon-based efficiencies of photobleaching, and photomineralization for the PW were, respectively, 17% and 21% higher than those for the GW ; whereas the efficiency of photoammonification for the PW was 32% lower than that for GW. Irradiation increased the absorption spectral slope (275-295 nm) from 0.014 to 0.025 nm^-1 for the GW and from 0.010 to 0.029 nm^-1 for the PW. Conversely, irradiation decreased the FI₃₇₀ and HIX for both the PW (FI₃₇₀: 1.25→1.09; HIX: 25.90→7.27) and GW (FI370: 1.61→1.29; HIX: 6.97→1.45). This study demonstrates that photooxidation can substantially alter the chemical structure of SDG-derived DOM and potentially impact DOC and nutrient cycling in coastal waters.