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

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.