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The use of the sulphate mass balance (SMB) between precipitation and soil water as a supplementary method to estimate the diffuse recharge rate assumes that the sulphate in soil water originated entirely from atmospheric deposition; however, the origin of sulphate in soil and groundwater is often unclear, especially in loess aquifers.This study analysed the sulphur (δ 34 S-SO 4 ) and oxygen (δ 18 O-SO 4 ) isotopes of sulphate in precipitation, water-extractable soil water, and shallow groundwater samples and used these data along with hydrochemical data to determine the sources of sulphate in the thick unsaturated zone and groundwater of a loess aquifer. The results suggest that sulphate in groundwater mainly originated from old precipitation.When precipitation percolates through the unsaturated zone to recharge groundwater, sulphates were rarely dissolved due to the formation of CaCO 3 film on the surface of sulphate minerals. The water-extractable sulphate in the deep unsaturated zone (>10 m) was mainly derived from the dissolution of evaporite minerals and there was no oxidation of sulphide minerals during the extraction of soil water by elutriating soil samples with deionized water. The water-extractable concentration of SO 4 was not representative of the actual SO 4 concentration in mobile soil water.Therefore, the recharge rate cannot be estimated by the SMB method using the water-extractable concentration of SO 4 in the loess areas. This study is important for identifying sulphate sources and clarifying the proper method for estimating the recharge rate in loess aquifers. K E Y W O R D S groundwater recharge, loess tableland, sulphur and oxygen isotopes, unsaturated zone 1 | INTRODUCTION Drylands are distributed worldwide, which account for 45% of the global land area (Pr av alie, 2016) and 38% of the world's population lives in this area (Zhang & Dang, 2014). An increasing number of people would rely on groundwater resources (Scanlon et al., 2006