Origin of sulphate in the unsaturated zone and groundwater of a loess aquifer

Abstract

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

Soil Column Experimental Study on the Effect of Soil Structure Disturbance on Water Chemistry

“…The influent water in the column experiment was a mixture of several groundwater samples (the variation of water chemistry and isotopic compositions was small) reported in our other study [ 21 ]. The concentrations of SO 4 and Cl were 6.8 mg/L and 5.5 mg/L, respectively.…”

“…The soil profile XZ1, with a depth of 55 m drilled on the Xifeng loess platform, reaches a groundwater level of 54 m and penetrates the aquifer within 1 m [ 29 ]. The water-extractable concentration of SO 4 in the soil profile XZ1 showed that SO 4 in the shallow unsaturated zone (<10 m) was mainly derived from anthropogenic sources and the dissolution of evaporite minerals, while SO 4 in the deep unsaturated zone (>10 m) was mainly derived from the dissolution of evaporite minerals [ 21 ]. In order to ensure that the experiment contained no other variables than the control variables (nature and disturbance), the soil used to fill the soil column was from the natural loess with a depth of 42.5–53 m in the soil profile XZ1.…”

“…In recent years, many studies have used isotopes to trace water–rock interactions and the origins of solutes in groundwater under natural conditions [ 16 , 17 , 18 ]. The SO 4 in groundwater can be controlled by atmospheric deposition and the water–rock interaction [ 19 , 20 , 21 , 22 , 23 ]. In terms of the impact of disturbed soil on the environment, many studies have focused on the impact of changes in contaminants transport (organic matter, heavy metals, etc.)…”

“…Therefore, loess is an ideal soil to study the impact of soil disturbance on water quality. Under natural conditions, SO 4 in the groundwater (all were premodern and old water) of the Xifeng Loess Tableland in the Loess Plateau of China was mainly recharged by old precipitation [ 21 , 29 ]. Under disturbance conditions, the soil structure changes.…”

Tracing Sulfate Source and Transformation in the Groundwater of the Linhuan Coal Mining Area, Huaibei Coalfield, China

“…The unsaturated zone (UZ) links surface water and groundwater and plays an important role in the hydrological cycle [ 32 , 33 , 34 ]. The UZ is an important site for SO 4 2− recycling and transformation and is also the main route for surface SO 4 2− pollution to enter groundwater [ 35 , 36 , 37 ]. The groundwater table is low, and the UZ is shallow in the coal mining area.…”