Nitrate transport behavior behind subsurface dams under varying hydrological conditions

Occurrence of dissimilatory nitrate reduction to ammonium (DNRA) in groundwater table fluctuation zones during dissolved organic nitrogen leaching through unsaturated zone

Nitrate reduction in unsaturated zone is critical for preventing groundwater contamination from anthropogenic nitrogen fertilization. Dissimilatory nitrate reduction to ammonium (DNRA), found in anoxic environments, offers an alternative pathway to denitrification by reducing nitrate while conserving nitrogen. However, the occurrence of DNRA in unsaturated zone remains poorly understood. To address this gap, we conducted numerical simulations to investigate the reactive transport of dissolved organic nitrogen (DON) through unsaturated zone under fluctuating groundwater table conditions, with the focus on the competition between denitrification and DNRA. Our results indicate that DNRA typically gets stronger within capillary fringe, with its intensity varying with groundwater table fluctuations. DNRA competes with denitrification, contributing up to 46.33 % of nitrate reduction, especially when groundwater table drops. The strength of DNRA requires comprehensive consideration of the adsorption characteristics, permeability and porosity of vadose zone, and in our study, silty clay loam-with the relatively weaker adsorptive capacity/lower permeability-exhibits the highest DNRA reaction rates and the largest reaction areas, while DNRA in sandy loam may occur during periods when both DON and NO3--N reserves are relatively low. This study firstly revealed the distribution of DNRA in groundwater table fluctuation zone, exploring its kinetics, controlling factors, and contributions, providing a scientific foundation for assessing the self-purification processes in groundwater contamination.

Effectiveness of double-cut-off walls on seawater intrusion and nitrate concentration in unconfined coastal aquifers

An effective coastal engineering technique for preventing seawater intrusion is constructing a cut-off wall. Nevertheless, the cut-off walls impact on nitrate concentration in downstream aquifers has not been assessed in the previous research that focused on studying a single subsurface physical barrier. In this work, a numerical model was used to examine the effect and mechanisms of the double-cut-off walls on saltwater wedge length and nitrate concentrations transported downstream of them. SEAWAT code has been implemented to simulate seawater intrusion and nitrate transport in unconfined coastal aquifers. Two cases of homogeneous aquifer (Case-H) and heterogeneous aquifer (Case-LH) were studied. The results showed significantly receded in the saltwater wedge and the spread of nitrate contamination increased due to the heterogeneous conditions. A significant effect on nitrate accumulation and an increase in the pollution area between the double-cut-off walls was observed when the second cut-off wall depth was embedded by more than 50 % of the aquifer thickness due to the weak inflow below the cut-off wall. There was no need to raise the second cut-off depth because there was a significant retraction in the saltwater interface after the first cut-off wall was embedded to a depth of more than 70 % of the aquifer thickness. Raising the second cut-off wall depth to more than 30 % of the aquifer thickness when the first cut-off wall depth ratio was less than 50 % significantly impacted the retreating of the saltwater wedge for the short distance between the double-cut-off walls. When the second cut-off wall depth ratio was less than 50 %, raising the first cut-off wall depth significantly lowered the total concentrations of the nitrate that arrived at the double-cut-off walls downstream. In addition, the total concentrations of the nitrate that traveled toward the sea were reduced to 17 % of the original nitrate concentration by raising the depth of both cut-off walls to 70 % of the aquifer thickness and the distance between them to more than 25 m. Although it was shown that the double-cut-off walls substantially reduced saltwater intrusion and nitrate concentrations, they also created a large dispersion area of nitrate pollution, especially in the heterogeneous aquifer. The retreat of the saltwater interface wedge was significantly impacted by the first cut-off wall depth in the heterogeneous aquifer. This study offers useful information for preventing saltwater intrusion and reducing nitrate concentration downstream of the double-cut-off walls, especially, the double-cut-off walls represent a new study for controlling saltwater intrusion and nitrate pollution in a coastal aquifer. The outcomes of this study can be used for the groundwater resources proper management in coastal aquifers.

Effects of organic carbon and subsurface dams on saltwater intrusion and nitrate pollution in sandy coastal aquifers

This study explores the impact of a novel approach on the levels of SWI (saltwater intrusion) and NO3- (nitrate) contamination. Some numerical simulations were conducted utilizing a coupled model that incorporates variably saturation and density, as well as convection diffusion reaction within a sandy coastal aquifer. We verified the reliability of the model for SWI based on comparison lab experiments and for chemical reactions based on a comparison of previous in situ observations. Cutoff walls and subsurface dams cannot simultaneously control SWI and reduce NO3- contamination. A novel approach that combines subsurface dams and permeable CH2O (organic carbon) walls (PC-Wall) is proposed. Subsurface dams are utilized to prevent SWI, while PC-Walls are employed to mitigate NO3- pollution. Results demonstrate that the construction of a PC-Wall with a concentration of 1.0 mM facilitated a transition from nitrification (Ni)-dominated to denitrification (Dn)-dominated. An increase in CH2O concentration to 1.0 mM caused a significant 1942.5 % rise in mDn (the mass of NO3- removed through Dn). Increment of the distance between the PC-Wall and the ocean from 35 to 45 m could result in a 103.7 % mDn increase and reduce mN (the compound mass of NO3- remaining in the aquifer) by 11.7 %. The study offers a detailed comprehension of the intricate hydrodynamics of SWI and NO3- pollution. In addition, it provides design guidance for engineering to mitigate contamination by NO3- and controlling SWI, thus fostering the management of groundwater quality.

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.

Quantifying the impacts of a proposed hydraulic dam on groundwater flow behaviors and its eco-environmental implications in the large Poyang Lake-floodplain system

Dam-induced hydrological alterations and eco-environmental impacts have significant implications, however, these concern issues in large floodplain systems are less well understood. The present study shows a first attempt to adopt a quasi-three-dimensional groundwater flow modeling FEFLOW (Finite Element subsurface FLOW system) to investigate the influences of a proposed hydraulic dam on groundwater dynamics in the largest floodplain lake of the Yangtze River basin (Poyang Lake, China). The FEFLOW model was successfully constructed and has the ability to represent the hydrodynamics of floodplain groundwater flow. Model simulations indicate that, in general, the dam is likely to increase the groundwater levels across the floodplain during different hydrological phases. The responses of floodplain groundwater levels to the dam during the dry and recession phases are stronger (∼2-3 m) than the rising and flooding phases (<2 m). Under the natural condition, the floodplain groundwater may recharge the lake during the dry and recession phases, and discharge the lake during the rising and flooding phases. However, the dam regulation may alter the natural recharge-discharge patterns, forming a generally gaining condition of the floodplain groundwater. The proposed dam is most likely to reduce the groundwater flow velocity (∼<1 m/d) relative to the natural condition (up to 2 m/d) during different hydrological phases, and it may also alter the floodplain groundwater flow direction during the dry and recession phases. Additionally, the floodplain groundwater system is mainly characterized by losing state (-4.5 × 10⁶ m³/yr) under the natural condition, while the dam-induced groundwater system exhibits an overall gaining state (9.8 × 10⁶ m³/yr). The current research findings contribute to future water resources assessment and management by providing a foundation for assessing associated eco-environmental changes of the large lake-floodplain system.