Denitrification and isotope fractionation effects under simulated aquifer conditions at a laboratory scale.

Denitrification plays a critical role in controlling the fate of nitrate (NO3-) in groundwater. The nitrogen isotope enrichment factor (εN) during denitrification is essential for quantifying the role of denitrification in nitrate reduction in groundwater. Experimental columns containing groundwater and sediment were established to simulate aquifer conditions. Denitrification rates and the N and O isotope fractionation effects of NO3- in both groundwater and sediment during dentrification were measured in experimental columns using four organic substances as electron donors (ethanol, acetate, glucose and brown sugar) under two different dissolved oxygen (DO) concentrations of 1 and 2 mg·L-1, respectively. Significant variations in denitrification rates and stable isotope enrichment factors were observed in both overlying groundwater (εNo and εOo) and sediment pore water (εNp and εOp), depending on the carbon source and DO concentration. At DO =2 mg·L-1, acetate (εNo, -9.2 ‰; εNp, -7.4 ‰), glucose (εNo, -7.4 ‰; εNp, -4.3 ‰), brown sugar (εNo, -2.6 ‰; εNp, -2.4 ‰) exhibited less negative isotope enrichment factors compared to ethanol (εNo, -11.7 ‰; εNp, -10.0 ‰). For brown sugar, more negative isotope enrichment factors was observed at DO =1 (εNo, -5.0 ‰; εNp, -3.5 ‰) than at DO = 2 mg·L-1. Higher denitrification rates correlated with more negative εNo and εNp values, reflecting stronger nitrogen fractionation. Due to the O-exchange fraction and NO3- reduction processes, positive εOo and εOp values were observed in the denitrification columns, ranging from 5.0 ‰ to 29.2 ‰ for εOo and from 3.9 ‰ to 22.8 ‰ for εOp. Fractionation by diffusive transport of NO3- resulted in higher absolute values of εNo (εOo) compared to those of εNp (εOp). The calculated isotope enrichment factors (εNo and εNp) may be applied to quantify denitrification linked to C sources application under in situ conditions.