1. Introduction
Since the 1970s, nitrogen pollutants have penetrated into aquifers because of excessive fertilizer use and uncontrolled land treatment. Nitrate contamination has become a global environmental problem. The high level of nitrate concentration threatens human health, causing many diseases such as infant methemoglobinemia, in which hemoglobin is unable to carry oxygen [
1], and adult stomach cancer. Nitrate contamination is an issue in both developed and developing countries. Many countries and organizations have set up a Maximum Concentration Level (MCL) of nitrate. The MCL for nitrate was set to 50 mg/L of nitrate in drinking water by the World Health Organization (WHO), whereas this value was set to 10 mg/L by the US environmental Protection Agency (USEPA) as a result of the risk of methemoglobinemia, and 20 mg/L in China. According to an investigation conducted by the USGS [
2], the concentration of nitrate in drinking water in most parts of the United States has increased year by year, with an average annual increase of 0.8 mg/L. Additionally, nearly 1/4 of water pollution cases are attributed to excessive nitrate nitrogen. In China, 80% of the samples exhibited an increasing trend in nitrate concentration from 1998 to 2013.
The over exploitation of deep groundwater has led to a decline in hydraulic head and increased vertical hydraulic gradient, resulting in a greater risk of nitrate pollution in deep groundwater. The nitrate in shallow groundwater migrates through the low permeability sediment layer, leading to pollution in the deep groundwater. Therefore, it is important to study the migration and transformation processes of nitrate in the low permeability sediment layer. Previous studies have always focused on high permeation aquifers; however, to the best of our knowledge, this is the first investigation quantifying the denitrification process in low permeability sediments. In fact, the low permeability sediment contains mass clay with higher porosity and denitrification bacteria. These sediments provide a suitable environment for the denitrification process, characterized by the complete reduction of nitrate.
Many methods have been used for nitrate removal, including the bioretention method, the microbe method, ion exchange, reverse osmosis, electrodialysis, etc. [
3]. The microbe method and the denitrification process were considered in this research. Denitrification is the main section in the nitrogen cycle. It turns nitrate into nitrogen gas via a chain of microbial reduction reactions [
4,
5]. Denitrification processes in groundwater are controlled by the local biogeochemical and physical conditions, such as the pH values, the groundwater flux rate, the dissolved oxygen concentration, etc. [
6,
7,
8,
9]. Such conditions are spatially distributed, temporal, and variable.
The development of a numerical model of nitrate transport is a prerequisite for addressing nitrate pollution. The implementation of a numerical model of nitrate transport requires fundamental knowledge of the transport processes and reaction kinetics involved in the decomposition of organic matter [
10]. In addition, the field experiment is a tool to ensure the accuracy of the model. Many examples of the establishment and validation of nitrate models have been investigated around the world [
11,
12,
13,
14,
15,
16,
17]; however, there are few studies investigating low permeability conditions, especially when it comes to experimental data that can support the model.
Column experiments are a commonly used method to comprehend solution transport and migration mechanics. Nitrate reduction can be termed as a half equation that illustrates the role of electron (
e−) transfer during the process [
5,
18], as follows:
Equation (1) can be divided into four steps. In the first step, nitrate is reduced to nitrite under the control of nitrate reductase. In the second step, nitrite is reduced to nitric under the control of nitrite reductase (
NiR). In the third step, nitric oxide is reduced to nitrous oxide under the control of
NO oxidoreductase (
NoR). In the last step, nitrous oxide is reduced to nitrogen under the control of nitrous oxide reductase (
N2OR). This process can be terminated at any of the intermediate stages, although denitrification has a stable endpoint with the production of nitrogen gas. The intermediates can be nitrite, nitric oxide, or nitrous in the denitrification process [
5,
18]. Energy is needed in each step. Denitrification bacteria obtain energy, such as optical energy and chemical energy, through many different ways. Some photosynthetic bacteria can grow heterotrophically under light and oxygen-free conditions. Simple organic matter acts as electron donor. Some denitrification bacteria use inorganic materials as an energy source to control the denitrification reaction. The equation can be expressed as follows:
Most denitrification bacteria are heterotrophic, using organic matter as an energy source, such as methanol. The equation can be expressed as follows:
In this study, the fate of denitrification in low permeability sediment layers and the influence of controlling factors, such as pH values, nitrate concentration, flow rate, and sediment particle size, were studied. A nitrate transport model through the silty clay was developed to provide a basis for understanding nitrate migration under natural conditions. The main objectives of this study were to: (1) recognize nitrate transport in low permeation layers; (2) recognize the influence of the factors controlling the reduction of nitrate in low permeation layers; (3) discuss the coefficients used in numerical nitrate transport models in low permeation layers, considering not only laboratory experiments but also their application in in situ and regional-scale scenarios for nitrate pollution abatement.
2. Materials and Methods
2.1. Sediment Preparation
The sediments used in this study were collected from the Tongzhou Site in Beijing (
Figure 1). The red line in
Figure 1b was the outlet of groundwater. Before 2003, the Tongzhou Site was characterized as typical agricultural land. Soil nitrate pollution has become a serious issue at this site due to excessive fertilization. Groundwater pollution affects people’s lives and has become an urgent environmental problem. The site was selected to be a scientific research station in November in 2003. The Ministry of Land and Resources (MLR) of the People’s Republic of China set up a subsidence observation station and an unsaturated zone experimental observation station at the Tongzhou Site from 2004 to 2008. A series of hydrology investigations were conducted in the winter of 2010. In May 2012, five geology wells were drilled. Silty clay was sampled at a depth from 18.2 m to 18.4 m, representing a low permeability soil type. One of the typical geological sections is shown in
Figure 2. The sediments were sieved through mesh with holes that were 2 mm in size in order to remove large blocks, fauna, and shell debris. After air drying, the sediment was ground and then filled into a column using a wet method. The total amount of sediment that filled in the column was 216 g.
2.2. Experimental Setup
A stainless-steel cylindrical column was used in this experiment. The column’s inner diameter and length were 3.5 cm and 15 cm, respectively. The inner surface was smeared with Vaseline to prevent bypass flow. In order to stop the flow scouring the sediment, a layer of 2.5 cm thick quartz was uniformly distributed on the top and bottom surfaces of the column. Then, a 0.22 μm filter was placed on the top surface to prevent the medium from going into subsequent equipment. To keep the soil column saturated, an HPLC pump was used to inject deionized water into the soil column from the bottom before the experiment. This process lasted for over 50 h, continuing until no nitrate was detected in the outflow. The influence of nitrate in the soil was removed in this step. A total of 100 mL of a mixture of KNO3 and KBr was injected into the column after saturating and leaching, and the fraction collector took aqueous samples from the top of the soil column. Nitrate, nitrite bromide, and ammonia concentrations in the samples were measured. The nitrate, nitrite, and bromide concentrations were determined using Ion Chromatography and ammonia was determined using ultraviolet spectrophotometry.
Five groups of various experimental conditions were set up:
The flow rate was 0.1 mL/min. The sediment in the column was silty clay.
The pH values were 5, 7, and 10. The nitrate concentration was 100 mg/L. The solution was a mixture of HCI, KBr, and KNO3. H2SO4 and NaOH were used to regulate the pH.
The flow rate was 0.1 mL/min or 0.2 mL/min. The nitrate concentration was 100 mg/L. The sediment in the column was silty clay.
A total of 100 mg/L of a nitrate and bromide mixture solution was injected into the column for 17 h at a rate of 0.1 mL/min. The injection was stopped for 48 h. Then, the mixture solution continued to be injected until the bromide concentration in the outflow reached its initial concentration. Deionized water was then injected into the column. The sediment in the column was silty clay.
Silty clay and fine sand filled in the column. A total of 100 mg/L of a nitrate and bromide mixture solution was injected into the column at a rate of 0.1 mL/min.
The five groups of experimental conditions are listed in
Table 1.
2.3. Transformation Modeling
In order to quantify denitrification in the low permeation layer, a one-dimensional nitrate transport model was developed by MODFLOW and MT3D. MODFLOW was developed by USGS [
19] and MT3D was developed by Zheng in 1993 [
20]. MT3D is a transport model based on MODFLOW. The water inlet was defined as a fixed flux boundary in the transport model, and the water outlet was defined as a constant head boundary. We speculated that adsorption and denitrification reactions occurred in the soil. Adsorption was defined as a liner action and denitrification was defined as a first depth decay action. The governing equation is as follows:
The initial condition is as follows:
The boundary conditions are as follows:
where
C is the dissolved concentration of nitrate,
t is time,
t0 is the injection time of a solution,
DL is the vertical hydrodynamic dispersion coefficient tensor,
L is the length of the column,
vx is the velocity of groundwater flux,
K is the coefficient of the first depth of the denitrification reaction,
Kd is the adsorption distribution coefficient,
θ is the porosity of the sediment in the column, and
C* is the nitrate concentration of the soil.
The porosity was determined in the laboratory and the dispersion coefficient was calculated according to the results of bromide. The experimental equation used to calculate the dispersion coefficient can be expressed as follows:
In this study, the concentration solution column experiment was combined to calculate the dispersivity.
4. Conclusions
This study focused on the mechanisms that affect nitrate transport in the low permeability layer. The low permeability layer plays an important role in slowing down the migration of nitrate in the groundwater system. A column soil experiment was designed and an MT3D transport model of nitrate was developed in order to understand the mechanism of nitrate migration in a low permeability system. This study aimed to establish a foundation for simulating the migration of nitrate pollution from shallow groundwater to deep groundwater through lateral flow under field conditions. The conclusions of this study are as follows:
Nitrate concentration can decrease dramatically in the low permeation layer. When nitrate passes through a low permeability layer, it undergoes processes such as adsorption and denitrification. Among these processes, denitrification is the primary factor, and it is enhanced with the progression of microbial reactions. Additionally, the low permeability layer exhibits a weak adsorption capacity for nitrate ions.
The concentration of nitrate has little influence on its migration and transformation in low permeability layers. The rate of denitrification remains almost the same. However, high concentrations of nitrate can inhibit nitrite reductase and promote the reduction of nitrate to ammonium, resulting in the accumulation of nitrite and ammonium.
Under acidic or alkaline conditions, both the denitrification and adsorption of nitrate in low permeability layers are greatly reduced. At a pH of 5 or a pH of 10, the denitrification rate of nitrate decreased by approximately 70% and the adsorption rate decreased by approximately 90%. Acidic or alkaline conditions inhibit nitrite reductase, leading to the accumulation of nitrite. Alkaline conditions promote the reduction of nitrate to ammonium, resulting in an increase in ammonium.
Different velocities had a strong effect on nitrate adsorption and denitrification. A higher velocity resulted in a decrease in nitrate adsorption and the denitrification rate. When the velocity was doubled, the denitrification rate of nitrate decreased by approximately 30% and the adsorption rate decreased by approximately 90%.
The sediment particle had a significant effect on nitrate transport, and smaller grain content had a weaker ability to remove nitrate. The adsorption rate and denitrification rate of nitrate in fine sand were only 50% of those in the low permeability layer.
The model results can be used to explain the experimental phenomena and verify the theory. In turn, the experimental results can also verify the validity of the model, contributing to modifications in the structure of the model. In our experiments, the model results were validated. Although this study focused on a one-dimensional soil column, the conclusions drawn from this study can be considered universal. Future investigations can study three-dimensional solute transport and promote the application of the model in managing actual nitrogen pollution.
A one-dimensional model was chosen for this study due to the typically thin nature of the low permeability layer. The level of the horizontal migration of pollutants in actual observations can be ignored. This study clarified the laws of migration and transformation of nitrate in low permeability soils, as well as the factors that influence these processes. The results not only provide a basis for conducting larger-scale nitrate simulations, but also provide a reference for the treatment of nitrate in low permeability layers.