Role of ponds in mitigating contaminants (nitrate and trace metals): a synthetical study in agricultural critical zone (southwestern France)

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Ammonia adsorption

Ammonia adsorption is a physical adsorption and/or ion exchange process, which can occur in some specific media, such as biochar and zeolite (Saeed and Sun, 2012; Yong et al., 2016). A variety of specific media have been deployed in constructed wetlands to optimize the nitrogen retention ability (Yong et al., 2016; Kizito et al., 2017). Three modes of ammonia adsorption rate in a number of different media have been discovered (Yong et al., 2016). The different trends of ammonia adsorption depend on the adsorption mechanism of a given substrate (Fig. I-18). Therefore, the desorption of adsorbed ammonia should be considered in order to promote the efficiency of ammonia retention in specific media. Studies have shown that volcanic rock, porcelain ceramist, zeolite, and biochar are good media to optimize ammonia adsorption in agricultural constructed wetlands due to their high ion exchange capacity and relatively small desorption rate (Lu et al., 2009; Yong et al., 2016; Kizito et al., 2017).

Organic nitrogen burial and accretion

In some cases, some part of the organic nitrogen combined with detritus could become an unavailable phase, which can no longer participate in the nitrogen cycling, as a consequence of sediment formation and burial. Nevertheless, the data on the nitrogen burial in the constructed wetland is very limited. A study shows that the contribution of nitrogen burial to the total N removal in a constructed wetland is very variable, ranging from 1 to 46% (Chen et al., 2014).

Anaerobic ammonium oxidation

Anaerobic ammonium oxidation (ANAMMOX) converts NO2- and NH4+ to N2 (Mulder et al., 1995). During ANAMMOX, nitrite/nitrate acts as the electron acceptor.
ANAMMOX is a promising technology to treat nitrogen in wastewater. However, to date, the study of ANAMMOX in constructed wetlands is still not abundant compared to the studies on other nitrogen removal reactions, such as nitrification and denitrification.
In general, the nitrogen cycle infiltrates vast spheres in a constructed wetland in CZ, which is from the near-surface layer of atmosphere (volatilization and fixation) to the bottom sediment (assimilation, denitrification, burial, etc.).

Nitrate transfer, transformation, and controlling factors

Although nitrogen exists in various forms as stated above, nitrate is generally the most abundant N substance in agricultural aquatic environment. The knowledge of nitrate has accumulated through the worldwide studies over past 100 years. However, the pathways of nitrate removal in agricultural waters and streams are still not totally clarified since novel mechanisms of nitrate removal have been discovered in recent years with the development of new methods and analytical techniques. Meanwhile, although nitrate is indispensable, it also exerts burdens on the environment and brings adverse effects on the living organisms. In such a condition, many organizations and countries have set several upper limits on the concentration of nitrate in drinking water, which is based on the consideration of human health (Powlson et al., 2008). The European Union (EU), for example, regulates an upper limit of 50 mg of nitrate L-1 and 44 mg L-1 in the United States. Considering nitrate as a double-edged sword, it is of great importance to know where nitrate comes from and its main removal pathways in the agricultural aquatic system.

Anthropogenic sources of nitrogen

Galloway et al (2004) reported that the nitrogen availability has been greatly increased by human activities at most regional scales. Though the increased use of inorganic fertilizers supports the augmented crop yields, a large fraction of N applied to crops finally enters the freshwater system and cause several environmental issues. A recent global study has revealed that around 75% of N loads derived from agricultural diffuse sources (Mekonnen and Hoekstra, 2015). Meanwhile, the cereals showed the largest contribution to the N-related contaminated freshwater (Mekonnen and Hoekstra, 2015). To prevent the N-related contamination in freshwater in France is then much important since she is the 5th largest wheat producer. In fact, the excessive nitrate concentration (> 50 mg L-1) has been already observed in some French agricultural catchments due to the intensive fertilizer application (Paul et al., 2015).

Nitrate transfer from soil to water

Nitrate is highly soluble in water, which contributes to its considerable mobility from one compartment to another. The agricultural activities, especially the fertilizer spreading, have been regarded as the major source of nitrate in agricultural water channels (Bur et al., 2009; N’guessan et al., 2009; Guo et al., 2010). Meanwhile, other fertilizers (e.g., manure, home-made fertilizers, etc.) also introduce some non-nitrate nitrogen compounds into the water. For instance, NH4+-N can be released after the application of manure in croplands. It can be transported to the downstream water channels due to the runoff and subsequent soil erosion. Once entered the water, NH4+-N is transformed to nitrite and/or nitrate by the microbiological nitrification (see Section 2.1.4).
Soil erosion and leaching are responsible first for nitrate transfer from soil to the aquatic system. Leached nitrate passes a variety of compartments and landscapes prior to discharge of the aquatic system. Two kinds of landscape components have been identified by Haag and Kaupenjohann (2001): retention compartments and corridors. Corridors, including macropores, preferential paths, drainage tiles and streams, direct nitrate to the aquatic systems very fast, while retention compartments can delay the movement of leach nitrate to the aquatic system or even remove nitrate during the retention by various nitrate removal pathways. The retention compartments are capillary tubes, saturated zones, and macrophytes, etc.
Meanwhile, Paul et al (2015) revealed a lag phenomenon of nitrate increase in stream water during the storm flood event. The stream nitrate concentration did not increase in the first stage of the flood due to the dilution of rainfalls and surface water. When the subsurface water and soil solution reached the stream in the second stage of the flood event, the stream nitrate began to increase.

Nitrate removal pathways in aquatic system

The biogeochemical cycle of nitrogen has been presented in Section 2.1. In consideration of the high abundance of nitrate in the agricultural aquatic system, to investigate the nitrate pathways is critical to remove excessive nitrate and then maintain the sustainable development of an ecosystem, as well as to ensure a healthy environment for the living organisms. These removal pathways include various processes.

Pathways for nitrate removal

Current studies agree that the removal of heavy nitrate loads in the aquatic system is largely due to several biological transformations, including assimilation into biomass, or to respiratory denitrification by bacteria (Burgin and Hamilton, 2007). However, according to the direct assays for denitrification, in some cases, the denitrification process only accounted for less than half of the total nitrate removal (Seitzinger et al., 1993; Burgin and Hamilton, 2007), which indicated that part of the nitrate removal could be a result of other processes except assimilation or respiratory denitrification. Indeed, some novel pathways of nitrate have been discovered, such as dissimilatory nitrate reduction to ammonium (DNRA) (Friedl et al., 2018), anaerobic ammonium oxidation (ANAMMOX) (Dong et al., 2009; Humbert et al., 2012), nitrate reduction coupled to iron oxidation (Davidson et al., 2003; Smith et al., 2017), denitrification coupled to sulfide oxidation (Brunet and Garcia-Gil, 1996), etc. However, in agricultural constructed wetlands, the denitrification still accounts for the most proportion of the total nitrate removal since other processes mentioned above normally require strict conditions, such as high sulfidic or ferrous environments (Vymazal, 2007; Saeed and Sun, 2012).

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Potential toxic metals (PTE) in sediments

In contrary to industrial regions or mining sites, PTEs in agricultural areas may not trigger acute toxic responses to living organisms, and their concentrations are normally much lower than heavily metal-contaminated regions. However, the long-term accumulation of PTEs in sediments from cultivated catchments can result in the potentially environmental risks to surrounding ecosystem and various life forms (see Section 1.2.5 for the potential risks and tragic consequences). The transfer and distribution of PTEs thus cannot be neglected.

Brief of sediments

In agricultural areas, sediment is a solid material, which is broken down from local upstream soil by the weathering process and is then transported and deposited in river by the fluvial process. Agricultural practices particularly lead to unsustainable soil losses due to the erosion rate greater than the soil production rate (Montgomery, 2007). In such a condition, agricultural soil has been considered as an irreversible resource.
Studies have found that the anthropogenic activities can largely affect the sediment yield and transportation into the river or coastal systems (Syvitski et al., 2005; Wang et al., 2016). Anthropogenic activities have increased the sediment yield through soil erosion by 2.3 ± 0.6 billion metric tons per year. However, constructed reservoirs (from small ponds to large dams) have sequestered 1.4 ± 0.3 billion metric tons per year of sediment flux (Syvitski et al., 2005).

Characteristics and toxicity of PTEs

Potential toxic elements (PTE) are those metal(loid)s which can cause adverse effects to the living organisms. The nomenclature of these metals is not unified since some researchers call them “potentially hazardous metals” or “toxic metals”. In this study, eight PTEs have been analyzed, e.g., arsenic (As), copper (Cu), lead (Pb), cobalt (Co), zinc (Zn), chromium (Cr), nickel (Ni), and cadmium (Cd) according to their toxicities and the presence in agricultural areas.

Table of contents :

Chapter I State of the art
1. Overview of the Critical Zone (CZ)
1.1 Current state of CZ
Definition
Compartments and interacts within the CZ
Interactions within CZ
Influence of CZ on the environment
Critical Zone Observatories (CZO)
1.2 Agriculture in the CZ
Share of land use and cover
Land occupation types in France
Fertilizer input
Pesticide input
Consequences of intensively agricultural activities
1.3 Constructed wetlands and ponds in agricultural catchments
Definition of catchments, constructed wetlands, constructed ponds, and sediments
History and types of CWs
Effects of CWs on contaminants
2. Nitrogen in aquatic ecosystem
2.1 Nitrogen in biogeochemical cycle
Fixation
Volatilization
Ammonification
Nitrification
Nitrate reduction
Assimilation and plant uptake
Ammonia adsorption
Organic nitrogen burial and accretion
Anaerobic ammonium oxidation
2.2 Nitrate transfer, transformation, and controlling factors
Anthropogenic sources of nitrogen
Nitrate transfer from soil to water
Nitrate removal pathways in aquatic system
3. Potential toxic metals (PTE) in sediments
3.1 Brief of sediments
3.2 Characteristics and toxicity of PTEs
3.3 Sources of PTEs in agricultural soils and sediments
Natural source
Anthropogenic source
3.4 Anthropogenic PTE transportation and distribution in sediments
From soil to sediment
Major pathways and controlling factors
Chapter II Materials and methods
Introduction
1. Sampling site
1.1 Pond selection
1.2 Catchment and pond descriptions
Catchment characteristics
Pond characteristics
2. Sampling strategies
2.1 Water
2.2 Sediment
3. Physicochemical and biological analyses
3.1 Water
Basic physicochemical analyses
Stable isotopes
3.2 Sediment
Physicochemical analyses
Denitrification analyses
Total digestion
single EDTA extraction
Metal analyses
Molecular analysis
4. Data analyses
Chapter III Role of constructed ponds in denitrification and nitrate behavior: key controlling factors in streams
and ponds at a catchment scale
Introduction
Materials and methods
Part I. Denitrification and its controlling factors
1. Results
1.1 Water characteristics
1.2 Sediment characteristics
Physicochemical characteristics
Potential denitrification rate (PDR)
1.3 Relationships between variables
Principal component analysis (PCA)
Relationship between PDR and N2O emission rate
1.4 qPCR assay for denitrification genes in pond sediments
1.5 Multilinear regression model for PDR
2. Discussion
2.1 PDR magnitude and spatial variation
2.2 Controlling factors of PDR
2.3 Predictive models and interest from a management perspective
Part II. Nitrate behavior and the role of ponds
1. Results
1.1 NO3
– concentration, discharge, and NO3
– flux patterns
1.2 Stable isotopes
1.3 Multilinear regression model for NO3
– removal efficiency
2. Discussion
Summary
Chapter IV Influence of ponds on hazardous metal distribution in sediments at a catchment scale (agricultural critical zone, S-W France)
Introduction
1. Materials and methods
1.1 Geoaccumulation index (Igeo)
1.2 Enrichment factor (EF)
1.3 Anthropogenic contribution (AC, %)
1.4 Statistical analysis
2. Results
2.1 Concentrations of major elements and eight potential toxic elements (PTE) in sediments
2.2 Sediment texture
2.3 Contamination indices
Geoaccumulation index (Igeo)
Enrichment factor (EF)
2.4 Principal component analysis (PCA) and correlation matrix (CM)
All sediments
Stream sediments
Pond sediments
2.5 EDTA extraction
3. Discussion
3.1 Natural vs. anthropogenic origins of PTEs in sediments, and their distributions
3.2 Distribution of PTE concentration and its controlling factors
3.3 Availability of PTE in sediments
3.4 Effect of ponds on PTE transfer in catchments
Summary
Chapter V Role of ponds in mitigating contaminants (nitrate and trace metals): a synthetical study in agricultural critical zone (southwestern France)
Introduction
1. Materials and methods
1.1 Multi-element contamination indices
Contamination degree (CD)
Modified contamination degree (mCD)
Pollution load index (PLI)
Nemerow’s pollution index (PI)
Modified pollution index (mPI)
Potential ecological risk index (PERI)
1.2 Sediment quality guidelines (SQGs)
1.3 Statistical analysis
2. Results and discussion
2.1 Overall metal contamination in sediments
Integrated contamination magnitude
Potential ecological risk in sediments and the effect of metal contamination on
denitrification
2.2 Role of ponds in types of contaminants simultaneously
All sediments
Stream and pond sediments
Summary
Conclusions and perspectives
References .

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