Influence of nitrogen-based fertilization on nitrates occurrence in groundwater of hilly vineyards

Sources and fate of nitrate in the unsaturated zone in an alluvial-lacustrine plain

Nitrate pollution in terrestrial and aquatic ecosystems in global agricultural areas poses an environmental concern. However, there is limited understanding of hydrogeological controls on the behavior of nitrogen compounds in unsaturated zones. Here, Self-Organizing Map and multiple isotopes approaches (δ15N-NO3-, δ18O-NO3-, and δ15N-NH4+) were used to investigate the sources, transport and transformation of N-species in the unsaturated zone in an alluvial-lacustrine plain, southeast China. The results revealed significant spatial heterogeneity in soil texture and physicochemical properties with vertically four soil geochemical and N-species zones (high NO₃⁻, high Fe(Ⅲ) and Mn, low ionic, and high NH₄⁺ contents), dominated by agricultural input, soil minerals and redox conditions. Nitrate in the unsaturated zone primarily originated from fertilizers and soil nitrogen. Excess nitrogen fertilizers infiltrated into the soil, where mineralization, nitrification, and dissimilatory nitrate reduction to ammonium (DNRA) acted as key mechanisms for nitrogen transformation. The change in the depositional environment from the plain to the lakeshore area led to nitrification gradual decrease and DNRA significant increase. Consequently, a conceptual model of reactive transport of N-species, influenced by hydrogeologic conditions and biogeochemical processes, was proposed. This study provides a new insight into the nitrate behaviors in unsaturated zone and contributes to groundwater nitrogen management strategies.

Effect of mixed physical barrier on seawater intrusion and nitrate accumulation in coastal unconfined aquifers

Physical barrier has been proven to be one of the most effective measures to prevent and control seawater intrusion (SWI) in coastal areas. Mixed physical barrier (MPB), a new type of physical barrier, has been shown to have higher efficiency in SWI control. As with conventional subsurface dam and cutoff wall, the construction of MPB may lead to the accumulation of nitrate contaminants in coastal aquifers. We investigated the SWI control capacity and nitrate accumulation in the MPB using a numerical model of variable density flow coupling with reactive transport, and performed sensitivity analysis on the subsurface dam height, cutoff wall depth and opening spacing in the MPB. The differences in SWI control and nitrate accumulation between MPB and conventional subsurface dam and cutoff wall were compared to assess the applicability of different physical barrier. The numerical results show that the construction of MPB will increase the nitrate concentration and contaminated area in the aquifer. The prevention and control efficiency of MPB against SWI is positively correlated with the depth of the cutoff wall, reaching the highest efficiency at the minimum effective dam height, and the retreat distance of the saltwater wedge is positively correlated with the opening spacing. We found a non-monotonic relationship between the change in subsurface dam height and the extent of nitrate accumulation, with total nitrate mass and contaminated area increasing and then decreasing as the height of the subsurface dam increased. The degree of nitrate accumulation increased linearly with increasing the height of the cutoff wall and the opening spacing. Under certain conditions, MPB is 46-53% and 16-57% more efficient in preventing and controlling SWI than conventional subsurface dam and cutoff wall, respectively. However, MPB caused 14-27% and 2-12% more nitrate accumulation than subsurface dam and cutoff wall, respectively. The findings of this study are of great value for the protection of coastal groundwater resources and will help decision makers to select appropriate engineering measures and designs to reduce the accumulation of nitrate pollutants while improving the efficiency of SWI control.

Nitrate with enriched heavy oxygen isotope linked to changes in nitrogen source and transformation as groundwater table rises

Nitrate is a significant constituent of the total nitrogen pool in shallow aquifers and poses an escalating threat to groundwater resources, making it crucial to comprehend the source, conversion, and elimination of nitrogen using appropriate techniques. Although dual-isotope dynamics in nitrate have been widely used, uncertainties remain regarding the asynchronously temporal changes in δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- observed in hypoxic aquifers. This study aimed to investigate changes in nitrogen sources and transformations using temporal changes in field-based NO₃^- isotopic composition, hydro-chemical variables, and environmental DNA profiling, as the groundwater table varied. The results showed that the larger enrichment in δ¹⁸O-NO₃^- (+13‰) compared with δ¹⁵N-NO₃^- (-2‰) on average during groundwater table rise was due to a combination of factors, including high ¹⁸O-based atmospheric N deposition, canopies nitrification, and soil nitrification transported vertically by rainfalls, and ¹⁸O-enriched O₂ produced through microbial and root respiration within denitrification. The strong association between functional gene abundance and nitrogen-related indicators suggests that anammox was actively processed with nitrification but in small bacterial population during groundwater table rise. Furthermore, bacterial species associated with nitrogen-associated gradients provided insight into subsurface nitrogen transformation, with Burkholderiaceae species and Pseudorhodobacter potentially serving as bioindicators of denitrification, while Candidatus Nitrotogn represents soil nitrification. Fluctuating groundwater tables can cause shifts in hydro-chemical and isotopic composition, which in turn can indicate changes in nitrogen sources and transformations. These changes can be used to improve input sources for mixture models and aid in microbial remediation of nitrate.

Nitrate transport behavior behind subsurface dams under varying hydrological conditions

The construction of subsurface dams for controlling seawater intrusion triggers the accumulation of nitrate upstream of a dam. This is raising the concerns about nitrate contamination in those regions of coastal aquifers that are supposed to be used as a fresh groundwater source behind a subsurface dam. Research on this subject has been mostly restricted to the use of a simplified sea boundary (e.g., static and no slope), ignoring sea level fluctuations driven by tides. In this study, the combined effect of tides and subsurface dams on nitrate pollution in upstream groundwater was examined through laboratory experiments and numerical simulations. The results revealed that the difference in the extent of nitrate contamination under various conditions (i.e., static, tidal, static with a dam, and tidal with a dam) was related to the temporal pollution behavior. In the early stage, nitrate contamination in upstream groundwater was essentially identical for different scenarios. Both tides and subsurface dams were found to increase nitrate contamination in upstream aquifers. The extent of nitrate contamination increased with higher tidal amplitudes, whereas the increment was more evident for a large tidal amplitude. The effects of tides and subsurface dams on nitrate contamination were also regulated by the locations and infiltration rates of the pollution source. Interestingly, under the joint action of tides and subsurface dams, the increment in the extent of nitrate pollution was greater than the sum of their individual effects. The increased pollutions caused by subsurface dams and tides were quantified as 9.47% and 37.22%, respectively, whereas the increased value caused by their joint action was measured as 51.10%. These findings suggest that tidal activity should not be overlooked when assessing nitrate contamination in upstream groundwater.

Assessment of Groundwater Quality beneath Agriculturally Advanced Region of Northern Alluvial Plain, India

In the present study, groundwater suitability for domestic and irrigation purposes was analyzed in the alluvial aquifers of the Bist-Doab region of Punjab, India, using various indices such as WQI, WAWQI, MCDA, RSC, SAR, PI, %Na, KR, MH, PS, K, and Ka. Since it is difficult to assess the suitability of groundwater for irrigation based on various indices individually, a composite groundwater quality index for irrigation (CGQII) was used in the study which transforms nine indices to a single value for each sample. Results reveal that the groundwater of a few blocks was found unsuitable for domestic use due to chemical leaching from fertilizers, pesticides, and agricultural and industrial wastes. Whereas, the groundwater of mainly southwestern parts was found unsuitable for irrigation due to long-term water accumulation in aquifers and continuous use of sodium-ion-rich groundwater. The findings conclude that anthropogenic activities have played a significant role in making groundwater unfit for domestic and irrigation purposes in the study area. The present study also emphasizes continuous monitoring and evaluation of groundwater quality, which will help in strategic planning and management for the conservation of groundwater resources in the region.