Isotopic and chemical evidence for nitrate sources and transformation processes in a plateau lake basin in Southwest China

Yajun Wang; Jianfeng Peng; Xiaofeng Cao; Yan Xu; Hongwei Yu; Gaoqi Duan; Jiuhui Qu

doi:10.1016/j.scitotenv.2019.134856

Isotopic and chemical evidence for nitrate sources and transformation processes in a plateau lake basin in Southwest China

Sci Total Environ. 2020 Apr 1:711:134856. doi: 10.1016/j.scitotenv.2019.134856. Epub 2019 Nov 20.

Authors

Yajun Wang¹, Jianfeng Peng², Xiaofeng Cao¹, Yan Xu¹, Hongwei Yu¹, Gaoqi Duan¹, Jiuhui Qu³

Affiliations

¹ Center for Water and Ecology, School of Environment, Tsinghua University, Beijing 100084, China.
² Center for Water and Ecology, School of Environment, Tsinghua University, Beijing 100084, China. Electronic address: pengjf@mail.tsinghua.edu.cn.
³ Center for Water and Ecology, School of Environment, Tsinghua University, Beijing 100084, China; Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.

PMID: 31818559
DOI: 10.1016/j.scitotenv.2019.134856

Abstract

In recent decades, multiple occurrences of algal blooms have substantially deteriorated water quality, especially for the nutrient budget in plateau lakes. Specifically, NO₃^- pollution has critically threatened groundwater quality, thus increasing human health risk if groundwater serves as a drinking source. To identify the origin and fate of NO₃^- in a plateau lake basin, we utilized nitrate isotope natural abundance and water chemistry information under land use frameworks and groundwater flow information. In December 2018, we collected water samples from aquifers (n = 33), rivers (n = 2), soil (n = 7), and lakes (n = 4) in Chenghai Lake basin, Southwest China. Our results showed that nearly 41% of groundwater samples failed to meet the drinking water standard of WHO and China (GB/T 5749-2006) of 50 mg/L for NO₃^- during the dry season. The high variation of δ¹⁵N-NO₃^- (from -3.3 to +41.3‰) and δ¹⁸O-NO₃^- (from -6.4 to +13.6‰) indicated multiple N sources and N cycling processes. Our analysis revealed that 16%-80% of nitrate in groundwater was derived from accumulated soil N, whereas 13%-76% was contributed from manure/landfill leachate. The contribution from atmospheric nitrogen deposition to aquifers was less than 3%. Manure/landfill and soil nitrogen were the primary N sources, contributing for 38.9% and 35.3% to N loading in lake. As for river water, soil nitrogen contributed for 69.7% and 37.2% in R1 and R2, respectively. The denitrification process significantly affects nitrate attenuation of N sources in aquifers. An increasing trend in NO₃^- concentration was noticed along the groundwater flow path (A-A') from mountain area to lake. Among different pathways, distinct nitrate sources loading downwards to the aquifers were observed in massive farmlands and residential areas. Thus, the information on both land-use and groundwater flow pathways is indispensable for modelling nitrate sources and transformation processes using the dual isotope approach.

Keywords: Chenghai Lake; Denitrification; Groundwater flow pathway; Land use; Nitrate pollution; Stable isotope ratios.

MeSH terms

China
Environmental Monitoring
Groundwater
Lakes*
Nitrates
Nitrogen Isotopes
Water Pollutants, Chemical

Substances

Nitrates
Nitrogen Isotopes
Water Pollutants, Chemical