Tracing the Sources and Fate of NO₃^– in the Vadose Zone–Groundwater System of a Thousand-Year-Cultivated Region

Abstract

Excess nitrate (NO3 –) loading in terrestrial and aquatic ecosystems can result in critical environmental and health issues. NO3 –-rich groundwater has been recorded in the Guanzhong Plain in the Yellow River Basin of China for over 1000 years. To assess the sources and fate of NO3 – in the vadose zone and groundwater, numerous samples were collected via borehole drilling and field surveys, followed by analysis and stable NO3 – isotope quantification. The results demonstrated that the NO3 – concentration in 38% of the groundwater samples exceeded the limit set by the World Health Organization. The total NO3 – stock in the 0–10 m soil profile of the orchards was 3.7 times higher than that of the croplands, suggesting that the cropland-to-orchard transition aggravated NO3 – accumulation in the deep vadose zone. Based on a Bayesian mixing model applied to stable NO3 – isotopes (δ15N and δ18O), NO3 – accumulation in the vadose zone was predominantly from manure and sewage N (MN, 27–54%), soil N (SN, 0–64%), and chemical N fertilizer (FN, 4–46%). MN was, by far, the greatest contributor to groundwater NO3 – (58–82%). The results also indicated that groundwater NO3 – was mainly associated with the soil and hydrogeochemical characteristics, whereas no relationship with modern agricultural activities was observed, likely due to the time delay in the thick vadose zone. The estimated residence time of NO3 – in the vadose zone varied from decades to centuries; however, NO3 – might reach the aquifer in the near future in areas with recent FN loading, especially those under cropland-to-orchard transition or where the vadose zone is relatively thin. This study suggests that future agricultural land-use transitions from croplands to orchards should be promoted with caution in areas with shallow vadose zones and coarse soil texture.

A Critical Review of Groundwater Table Fluctuation: Formation, Effects on Multifields, and Contaminant Behaviors in a Soil and Aquifer System

“…Nitrate (NO 3 − ) from the vadose zone was deposited in the underlying aquifer after GTF, and this behavior became the main way to increase the nitrate concentration in the aquifer. 125,126 The capillary fringe could drive the transportation and reaction of components in both the capillary fringe and GTF; as such, the cycling of dissolved organic carbon and dissolved inorganic nitrogen presented different change trends with the help of the capillary fringe. 127 A field study implied that a nitrate reducer had a high content below the groundwater table and nitrate agents were redistributed during GTF.…”

“…Dynamic variation of the groundwater table plays an important role in regulating nitrogen (N) cycles between unsaturated soil and saturated aquifers (Figure ). Nitrate (NO 3 – ) from the vadose zone was deposited in the underlying aquifer after GTF, and this behavior became the main way to increase the nitrate concentration in the aquifer. , The capillary fringe could drive the transportation and reaction of components in both the capillary fringe and GTF; as such, the cycling of dissolved organic carbon and dissolved inorganic nitrogen presented different change trends with the help of the capillary fringe …”

“…Behaviors of nitrogenous compounds and VOCs during GTF. , With the synergistic effects of microorganisms and oxygen content, nitrogenous compounds undergo nitrification and denitrification in unsaturated soil and saturated aquifers. ,,, VOCs migrate into the surface environment by virtue of the capillary fringe and air–water interfaces. − …”

Spatiotemporal Variation of Groundwater Nitrate Concentration Controlled by Groundwater Flow in a Large Basin: Evidence From Multi‐Isotopes (15N, 11B, 18O, and 2H)

“…On the other hand, groundwater residence time affects groundwater renewability (and thus hydrochemical environment) and the transportation and transformation of $${{\text{NO}}_{3}}^{-}$$ in groundwater (Böhlke & Denver, 1995; Gooddy et al., 2006; Taufiq et al., 2019). These lead to the fact that $${{\text{NO}}_{3}}^{-}$$ in groundwater might not originate from local pollutants or current activities, and even affect the biochemical process of $${{\text{NO}}_{3}}^{-}$$ with changing the redox environment (Niu et al., 2022; Ransom et al., 2016; Yue et al., 2020). For instance, previous studies have stated that groundwater typically exhibited oxygen concentrations of <2 mg/L after long residence time, and $${{\text{NO}}_{3}}^{-}$$ may be attenuated by denitrification (Green et al., 2016; Lindsey et al., 2003; Wolters et al., 2022).…”

“…For instance, previous studies have stated that groundwater typically exhibited oxygen concentrations of <2 mg/L after long residence time, and $${{\text{NO}}_{3}}^{-}$$ may be attenuated by denitrification (Green et al., 2016; Lindsey et al., 2003; Wolters et al., 2022). Some studies also found that $${{\text{NO}}_{3}}^{-}$$ in groundwater with long residence time may originate from the historically accumulated legacy $${{\text{NO}}_{3}}^{-}$$ stores (Basu et al., 2010; Böhlke & Denver, 1995; Niu et al., 2022; Taufiq et al., 2019; Winter et al., 2021). This explains why $${{\text{NO}}_{3}}^{-}$$ concentration in groundwater may remain high even after reducing fertilizer application in source areas.…”