Novel insights into source apportionment of dissolved organic matter in aquifer affected by anthropogenic groundwater recharge: Applicability of end-member mixing analysis based optical indices

The composition and main sources of dissolved organic matter (DOM) in groundwater may change significantly under long-term anthropogenic groundwater recharge (AGR); however, the impact of AGR on quantitative sources of groundwater DOM has seldom been reported. This study evaluated the applicability of optical indices combined with mixing stable isotope analysis in R (MixSIAR) in end-member mixing analysis (EMMA) of groundwater DOM. Fluorescent indices, including C1%, C2%, and C3%, were more sensitive to AGR than other absorbance indices, as indicated by the significant difference between the dominant area of artificial groundwater recharged by surface water and the dominant area of natural groundwater recharged by atmospheric precipitation (NGRP). BIX-C1% was selected as the optimal dual index after the screening protocol of groundwater DOM for EMMA. Our results showed that DOM in the aquifer was mainly subject to autochthonous DOM and the contribution of background groundwater to AGRSW and NGRP groundwater accounted for 36.15% +/- 32.41% and 55.46% +/- 37.17% (p < 0.05), respectively. Therefore, AGR significantly changed the native DOM in the groundwater. In allochthonous sources of DOM, sewage and surface water contributed 29.54% +/- 24.87% and 21.32% +/- 28.08%, and 24.79% +/- 15.56% and 15.21% +/- 14.20% to AGRSW and NGRP groundwater, respectively. The contribution of surface water to AGRSW groundwater was significantly higher than that to NGRP groundwater (p < 0.05), indicating that AGR introduced significantly more DOM from surface water to groundwater. This study provides novel insights into the quantitative source apportionment of DOM in groundwater under long-term AGR, which will facilitate the environmental risk assessment of present AGR measures and the sustainable management of clean water.