Seasonal effects on contamination characteristics of tap water from rural Beijing: A multiple isotope approach

Abstract

Excessive exploitation of the groundwater and discharge of urban and industrial wastewater into the environment led to a significant decline of the local surface and groundwater quality in many parts of the Beijing urban area. In the present study, tap water derived from deep groundwater (>70 m depth) was collected in two seasons in the rural area around urban Beijing, China, in order to shed light on the hydrochemical composition of the local drinking water. For these, cation (Ca2+, K+, Mg2+, Na+, NH4+) and anion (Cl-, NO3−, SO42-, F-) concentrations were measured. In addition, multiple stable isotope (δ15Nnitrate, δ18Onitrate, δ34Ssulfate, δ18Osulfate, Δ33Ssulfate) analyses were applied for identifying the specific hydrochemical processes and major contamination sources. The result showed that thirty percent of the analyzed tap water in the wet season and twenty-three percent in the dry season do not comply with the national legislation and represent a serious health risk, primarily due to high nitrate concentrations. Moreover, half of tap water could not be drunk due to the pollution of ammonium in wet season. Thereby, the most deteriorated tap water was derived from the highest populated areas in rural Beijing. Multiple isotopes reveal urban sewage as the dominant pollution source for deep groundwater as the principle tap water source. Therefore, it is necessary to control public and private sewage disposal and exchange leaking sewer pipes in order to reduce the risk of anthropogenic contamination of groundwater, representing the prime drinking water source in the Beijing area. This study demonstrates clearly the advantage of multi-isotope approach for evaluating water quality problems in urban groundwater.