Groundwater Isotopes Biased Toward Heavy Rainfall Events and Implications on the Local Meteoric Water Line

Stable water isotopes are useful tracers in the groundwater recharge studies, where the local meteoric water line (LMWL) usually serves as a reference line and the amount-weighted average value of precipitation is regarded as the input meteoric signal. Based on a comparison of isotope composition between precipitation and groundwater collected from a groundwater basin in the Beijing area, we verify that small precipitation events do not constitute effective groundwater recharge in a (semi) arid region, causing that the isotopes in groundwater are biased toward the meteoric isotopic signal of heavy rainfall events. The positive deflections in groundwater levels from a phreatic aquifer in Yanqing Basin in the Northwest of Beijing revealed that recharge occurs exclusively during the rainy seasons and is clearly associated with heavy rainfall events (>10mm/day). The isotope composition of groundwater samples that plot far below the mean meteoric input value, further strengthen the dependence of groundwater recharge on heavy rainfall events. We construct the LMWLs with different regression methods using daily stable isotope data from seven hydro-meteorological stations over a hydrological year. When the delta O-18 ranges from -20% to 10%, the lines are quite similar regressed by the weighted monthly values and by heavy rainfall events, so the conventional LMWL can still serve as the reference line for groundwater recharge studies.