In Taiwan’s humid climate, proximal fan groundwater (PFG) is mainly sourced from local precipitation (LP), mountain front recharge (MFR), and mountain block recharge (MBR). This study evaluates the relative importance of the above sources’ respective contributions to the PFG of the Langyang alluvial plain (LAP), northeastern Taiwan. To this end, we first identify stable isotopic characteristics of these target waters and evaluate the hydrological relations among them. Further, we employ ternary end member mixing analysis (EMMA) based on δ18O and electrical conductivity to semi-quantitatively calculate contributing fractions and amounts of water for respective LP, MFR, and MBR end members. EMMA results indicate that the respective contribution fractions of LP, MFR, and MBR to PFG at the LAP are approximately 28, 60, and 12 %, respectively. Further, we employ the obtained contribution fractions to understand the corresponding water amounts of each end-member contributed to PFG. In total, 325 × 106 m3 of water recharges PFG annually; of which, 226 × 106 m3/yr. is from MFR, 76 × 106 m3/yr. from LP, and 23 × 106 m3/yr. from MBR. MFR is clearly the greatest source of water at the LAP and local water resource management and protection authorities should concentrate their energies on this important contributor to groundwater. To keep these results in context, limitations to the EMMA approach are evaluated in the text.