title: Residence time as a key for comprehensive assessment of the relationship between changing land use and nitrates in regional groundwater systems.
abstract: In this study, an approach is put forward to study the relationship between changing land use and groundwater nitrate contamination in the Sanjiang Plain. This approach emphasizes the importance of groundwater residence time when relating the nitrates to the changing land use. The principles underlying the approach involve the assessment of groundwater residence time by CFCs and the Vogel age model and the reconstruction of the land use at the groundwater recharge time by interpolation. Nitrate trend analysis shows that nitrates have begun to leach into the aquifers since agricultural activities boomed after the 1950s. Hydrochemical analysis implies that the possible process relating to the nitrate reduction in the groundwater is the oxidation of Fe(ii)-silicates. However, the chemical kinetics of the oxidation of Fe(ii)-silicates is slow, so this denitrification process contributes little to the nitrate variations. Stepwise regression shows that the nitrate concentrations of samples had no direct relationship with the land use at the groundwater sampling time, but had a relatively strong relationship with the land use at the groundwater recharge time. Dry land is recognized as the dominant factor contributing to the elevated concentration of nitrates. The nitrogen isotope for nitrate (δ(15)N-NO3) gives a more direct result of the identification of nitrate sources: the use of manure in agricultural activities. Principle component (PC) regression shows that the process of the dry land exploitation is the major process that controls the nitrate contamination in the Sanjiang Plain.
authors.affiliation: Key Laboratory of Water Cycle and Related Land Surface Process, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Datun Rd. A11, Chaoyang District, Beijing 100101, China.
authors.forename: Yingjie
authors.initials: Y
authors.lastname: Cao
authors.forename: Changyuan
authors.initials: C
authors.lastname: Tang
authors.forename: Xianfang
authors.initials: X
authors.lastname: Song
authors.forename: Changming
authors.initials: C
authors.lastname: Liu
authors.forename: Yinghua
authors.initials: Y
authors.lastname: Zhang
mesh_terms: D000383:Agriculture; D058440:Denitrification; D004784:Environmental Monitoring; D005296:Ferrous Compounds / Q000737:chemistry; D060587:Groundwater / Q000032:analysis*; D009566:Nitrates / Q000032:analysis*; D010084:Oxidation-Reduction; D017640:Silicates / Q000737:chemistry; D012987:Soil / Q000032:analysis; D013997:Time Factors; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D005296:Ferrous Compounds; D009566:Nitrates; D017640:Silicates; D012987:Soil; D014874:Water Pollutants, Chemical