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Volume 31 Issue 4
Aug.  2020
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Rui Liu, Teng Ma, Wenkai Qiu, Ziqi Peng, Chenxuan Shi. The Environmental Functions and Ecological Effects of Organic Carbon in Silt. Journal of Earth Science, 2020, 31(4): 834-844. doi: 10.1007/s12583-020-1349-z
Citation: Rui Liu, Teng Ma, Wenkai Qiu, Ziqi Peng, Chenxuan Shi. The Environmental Functions and Ecological Effects of Organic Carbon in Silt. Journal of Earth Science, 2020, 31(4): 834-844. doi: 10.1007/s12583-020-1349-z

The Environmental Functions and Ecological Effects of Organic Carbon in Silt

doi: 10.1007/s12583-020-1349-z
More Information
  • Silt is a kind of unconsolidated sediment consisting of fine particles; silt is generally deposited across wide areas on the surfaces of drainages and in oceans under static or slow-hydrodynamic conditions. The organic carbon (OC) in silt has multiple essential environmental functions. This paper elaborates the morphological and environmental indication functions of OC in silt, and the effect of its own migration and transformation on environmental deterioration. Organic carbon exists in silt in two forms, free and mineral-binding. Meanwhile, based on its formation and structure, OC can be divided into light and heavy fraction of OC. Environmental information including data related to paleoclimates, ancient levels of productivity level, and variations in regional organism abundance can be discovered from total organic carbon, the C/N ratio, and OC isotope content. Degradation of OC is believed to participate in the emission of greenhouse gases, release of heavy metals and other contaminants. Finally, from the view of silt deposition, the possible influence of complex water-rock interaction in which OC is involved during the evolution of silt to a clayey aquitard on the hydrochemical composition of groundwater is discussed, which provides a new perspective for future research on the carbon cycle in nature.
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The Environmental Functions and Ecological Effects of Organic Carbon in Silt

doi: 10.1007/s12583-020-1349-z

Abstract: Silt is a kind of unconsolidated sediment consisting of fine particles; silt is generally deposited across wide areas on the surfaces of drainages and in oceans under static or slow-hydrodynamic conditions. The organic carbon (OC) in silt has multiple essential environmental functions. This paper elaborates the morphological and environmental indication functions of OC in silt, and the effect of its own migration and transformation on environmental deterioration. Organic carbon exists in silt in two forms, free and mineral-binding. Meanwhile, based on its formation and structure, OC can be divided into light and heavy fraction of OC. Environmental information including data related to paleoclimates, ancient levels of productivity level, and variations in regional organism abundance can be discovered from total organic carbon, the C/N ratio, and OC isotope content. Degradation of OC is believed to participate in the emission of greenhouse gases, release of heavy metals and other contaminants. Finally, from the view of silt deposition, the possible influence of complex water-rock interaction in which OC is involved during the evolution of silt to a clayey aquitard on the hydrochemical composition of groundwater is discussed, which provides a new perspective for future research on the carbon cycle in nature.

Rui Liu, Teng Ma, Wenkai Qiu, Ziqi Peng, Chenxuan Shi. The Environmental Functions and Ecological Effects of Organic Carbon in Silt. Journal of Earth Science, 2020, 31(4): 834-844. doi: 10.1007/s12583-020-1349-z
Citation: Rui Liu, Teng Ma, Wenkai Qiu, Ziqi Peng, Chenxuan Shi. The Environmental Functions and Ecological Effects of Organic Carbon in Silt. Journal of Earth Science, 2020, 31(4): 834-844. doi: 10.1007/s12583-020-1349-z
  • Researchers in various disciplines such as engineering geology, marine science, and sedimentology have defined silt in different ways. In the Soil Classification Standard of China (GBJ145-09, 2002), silt is defined as fine or granular unconsolidated sediment generated through physiochemical and biochemical processes under a hydrostatic or weak hydrodynamic sedimentary environment (GBJ145-09, 2002). Generally speaking, silt is mainly deposited in the discharge areas of groundwater systems (e.g., oceans, lakes, deltas, wetlands and rivers) and pertains to water retention and chemical energy storage zones, gene pools of species (Liu et al., 2003), information bases of environmental change (Liu, 2002), and carbon sinks of greenhouse gases (Zhao et al., 2008; Fang and Chen, 2001). The average particle size of silt is small and usually less than 0.062 5 mm (Aplin et al., 1999) and the original porosity of mud can be as high as over 70%. The water content of natural silt is also high, generally over 75%, with diverse forms of pore water including gravitational, bound, capillary and mineral bound-water. Furthermore, clay minerals take up the majority of silt mineral composition, although other detrital minerals such as quartz and feldspar are also found in silt (Aplin et al., 2011). More than 2% of silt components are formed by organics matter (OM) (Judd and Hovland, 2007) that largely originate from the remains of higher plants or plankton. Apart from these, abundant microorganisms such as aerobic bacteria, sulfate reductive bacteria and methanogens are found in silt; the gaseous composition of silt includes O2 and CO2 (Potter et al., 2005). A variety of low permeability media distributed in wetlands, rivers, lakes, reservoirs, and oceans, including silt and its successors such as shale and mudstone, usually come from a wealth of sources and are characterized as intricate in composition and structure with sluggish hydraulic exchange; this plays an irreplaceable role in water storage, contaminant detoxification, maintaining of biodiversity, climate mediation and the formation of fossil fuels (Wang et al., 2013; Parker et al., 2008).

    The carbon in silt undergoes an incessant mineralization from the moment of burial, namely early diagenesis, which involves the degradation of organic carbon (OC) and the dissolution and precipitation of inorganic carbon (Sand-Jensen et al., 2019). Inorganic carbon is mainly reserved in different forms of carbonate minerals (such as calcite, aragonite, smithsonite, and magnesite). In addition, only a small proportion of inorganic carbon with high binding strength contributes to the carbon cycle of the early diagenesis (Helmke and Bauch, 2001). Compared to inorganic carbon, the regeneration cycle of the OC pool is short. The OC pool of silt is a part of the entire carbon pool of soil. Globally, around 1.4×1018–1.5×1018 g of carbon storage of soil is in the form of OC (Houghton, 2007), which accounts for a bulk of soil mass and serves as an essential part of carbon cycle research. Organic carbon is an important component of biogenic argillaceous sediments. Not only is OC a fundamental media of research on the transformation and transfer of aquatic contaminants, but it also is an electron donor during biogeochemical processes. Moreover, OC is also a medium and an evolutionary indicator of deep sediments, water- rock interaction and biogeochemical processes (Olk et al., 2019; Yu et al., 2018). Organic carbon is mainly stored in organic matter (OM), and the latter is in control of the ecological toxicity, transformation and transfer of organic pollutants and heavy metals in sediments through adsorption and co-precipitation (Xing et al., 2020; Xu et al., 2020; Reszat and Hendry, 2007). The forms of OC play an integral part in the transformation and transfer of carbon; they are crucial for carbon flux and carbon transformation on the water-sediments interface.

    At present, the spotlight has focused more on the engineering and geological characteristics of silt and dredging technique research worldwide (Lewan and Roy, 2011; Nygård et al., 2004). However, little research has focused on the change of environmental information indicated by silt and related to silt being "pollution source" during environmental degradation under different conditions (Ai et al., 2019; Cao et al., 2019; Yang et al., 2019). Restricted by the slow deposition process, low permeability and difficulties in obtaining effective data on site, there is insufficient knowledge about the mechanism involved in the evolution of silt and of its ecological functions. In this article, the OC in silt is the focus because of its abundance and the nature of its activity. By extensive literature researches and tests on the physical and chemical properties of silt in different geological environments, we summarize the forms and characteristics of OC in silt and present the progress in research related to the application of the C/N ratio, OC isotope, and so on, for implications in the sedimentation environment. In addition, the relationship between silt and water pollution is discussed from the perspective of rivers, lakes, and oceans. The influence of OC mineralization in silt on the greenhouse effect is introduced in combination with the release of heavy metals and other pollutants, the eutrophication of water, and other environmental problems related to surface water. Finally, from a novel perspective of silt evolution, the influence of OC as an electron donor and the carrier of pollutants on the groundwater quality is introduced. During the process of burial and evolution, silt is constantly covered by new sediments and consequently endures compaction and diagenesis (Hesse and Schacht, 2011). With intricate water-rock interactions, the porosity of silt decreases, and the squeezed pore water is pulsed to adjacent aquifers which changes the volume and composition of groundwater storage; as a result, silt changes into a clayey aquitard (Aplin and Macquaker, 2011). In this process, OC plays the roles of an electron acceptor, serves as a carbon source of microorganisms, and participants in the formation of sediment aggregates. The exploration of the structure and function of OC and the involved biochemical reactions in aquifers during the evolution of silt to clayey aquitard is still in the initial stages (Liu et al., 2020). Hence, research on OC in silt has a wide range of environmental significance, understanding the functions of which is a fundamental step in attaining basic theory related to further research on the environment changes occurring during surface flow and in groundwater. Likewise, research on the OC of silt is also of great scientific significance for treatments on silt-driven pollution.

  • Most researchers have defined sediment OM as the combination of humus, animal and plant remains, and microorganisms formed by microbial action (Gregorich et al., 1994); meanwhile, the carbon contained in OM is defined as OC. The source of OC in silt sediment is generally divided into OC from internal and external sources. The internal sources mainly include the remains of endophytes (plankton, emergent plants, submerged plants, and various algae). The external sources mainly include terrestrial plant debris brought in surface water systems or ocean transportation; external OM is greatly affected by temperature and precipitation (Lutzow et al., 2006; Six et al., 2002). The form of OC found in silt can be classified in many ways, which are mainly divided into OC in a free state with high activity, that is easily to be decomposed and OC that is mineral-bound and in a very stable state. In addition, silt under natural sedimentary conditions is not affected or is minimally affected by human activities. The composition of total OC (TOC), the carbon : nitrogen ratio (C/N), and OC isotope (δ13C) in sediment stores the information of the ecosystem environment around the water body along with data related to past climate and environmental changes, which can better reflect the initial productivity status and environmental climate change of water environment (Zuo et al., 2018; Wu et al., 2006; Lucke and Brauer, 2004).

  • In the 1980s, proportion grouping technology of sediment OM appeared for the first time. According to the deposition of sediment in a certain proportion (1.6–2.5 g·mL-1) solution, OC was divided into the light fraction of OC (LFOC) and heavy fraction of OC (HFOC) (Janzen et al., 1992). The HFOC mainly exists in the organic-inorganic complex, which is composed of humus (HS). Its biological activity is relatively low and microorganisms can only use it with some difficulty (Christensen, 2001). The HFOC serves as a stable carbon pool in soil. The LFOC is mainly composed of plant remains, roots, and charcoal that is not completely decomposed, and also includes a small number of living microorganisms and their secretions (Christensen, 1992). The LFOC is easily decomposed and used by microorganisms, which represents the easily decomposed OC pool; the LFOC provides an intermediate carbon pool between plant remains and humus. In recent years, many studies have used the term "easily oxidized OC" to describe easily oxidized and unstable while "inactive OC" or "stable OC" have been used to describe the stable OC, that becomes oxidized with difficulty; this type of OC can be further divided into loose, stable, and tight state OC (Mao et al., 2011; Wu F C et al., 2004). This classification method mainly focuses on humic-like substances. In addition to the form of OC in solid media, OC of silt also includes dissolved OC (DOC) in pore water. The general classification of OC in silt is shown in Table 1.

    Morphological classification
    Free state Easily oxidized OC/LFOC. Composed of plant remains, roots, charcoal, living microorganisms and so on, which is easily decomposed and used by microorganisms
    Mineral-bound state Inactive OC/HFOC; composed of humus Loose state Easily decomposed and oxidized by microorganisms
    Stable state The stability is moderate between the loose and tight states
    Tight state Closely bound with minerals and is difficult decomposed by microorganisms
    Dissolved state Organic molecular mixture with a particle size of < 0.45 μm (Thurman, 1985)
    Environmental indication functions
    TOC The TOC content can reflect the input and storage conditions of OM, the level of lake primary productivity, and the change of temperature
    C/N Sources of OM in silty and clayey sediments
    OC isotope (δ13C) The evolution of paleoclimate and paleoenvironment, and the primary productivity and nutritional status of ancient lakes

    Table 1.  Morphological classification and environmental indication of OC in silt

    The decomposition of unstable OC, causes a large amount of carbon (about 3.5 Gt per year) to be released into the atmosphere from the soil globally, resulting in an increase in the concentration of greenhouse gases (Houghton, 2007). The stability of SOM is very important not only for the assessment and prediction of the biogeochemical cycling of carbon, but also for the effective management of the soil ecosystem under global climate change (Adhikari et al., 2016; Zhao et al., 2016). The adsorption of OC and its association with minerals can protect SOM from degradation through physical and chemical isolation that enhances the stability of SOM. The retention of DOC in silt is mainly controlled by the adsorption of OC by minerals. Therefore, the stability of mineral-bound-OC determines the amount and turnover of OC in silt (Pan et al., 2016). The stabilization of OM by adsorption into minerals is believed to be caused by (Ⅰ) the adsorption of OC into pores (50 nm), preventing hydrolase from approaching and decomposing organic substrates, or (Ⅱ) the formation of strong multiple bonds through the complexation of organic ligands on the mineral surface, resulting in reduced availability of organic molecules (Kaiser and Guggenberger, 2003). Meanwhile, molecules closely linked by multiple complex bonds may be located in a small orifice, which makes it almost impossible for them to be desorbed allowing these molecules to resist the attack of chemical agents and perhaps enzymes. Weak crystalline iron (Fe) oxide in silt has become an important adsorbent for DOC due to its high specific surface area (SSA) and the presence of surface hydroxyls with different charges, such as ferrihydrite, which has an adsorption coefficient of up to 7.1 mol·C/kg (Kaiser and Guggenberger, 2007). Based on our tests and data collection related to OC in different surface water systems, the TOC content of silt is 15.8–21.3 mg·g-1, and the proportion in TOC gradually increases with depth from 63% near the silt surface to over 90% at depth. The OC of Fe oxide combined by adsorption and coprecipitation accounted for 30.3% and 31.6% of TOC on average, respectively, which occupied a certain scale and had a stronger ability or anti-microbial decomposition (Liu et al., 2020). Therefore, the mineral bound-OC in silt is an inert component participating in the carbon cycle in nature.

  • The stability of OC enriched in silt is an important factor affecting greenhouse gas emissions. Meanwhile, the content and isotopic composition of OC in silt contains a variety of environmental change information (Zakharov et al., 2018). To some extent, the content of OC can reflect the input of OM in sediments and the productivity characteristics of lakes (Machiwa, 2010). The C/N ratio is used to identify the source of organic matter in silt sediments (Table 1). Generally speaking, due to the rich protein and low cellulose content of aquatic plants, the C/N ratio of sediment organic matter is low, generally between 4 and 10, while the C/N ratio of terrestrial plants can reach 20 or higher. Therefore, the C/N ratio of lacustrine silt sediment can be used to distinguish whether the organic matter comes from the endogenous aquatic plants in a lake or from the exogenous terrestrial plants at the lake. The OC isotope (δ13C) in silt is affected by many factors, including the primary organic productivity of a water body, the main sources of OM and the changes in internal and external sources, and the concentration of atmospheric CO2.

    The OC isotope (δ13C) plays an important role in reflecting the changes of productivity of ancient lakes (Liu, 2018; Zhang et al., 2013; Brenner et al., 1999). In a relatively warm climate, the productivity of lake is higher. The content of OC in sediment increases and more dissolved HCO3- is contributed to the lake to maintain high productivity. Aquatic plants will increase the selective absorption of 12CO2, thus increasing the absorption level of H12CO3- in the water body, resulting in a higher δ13C value of OC in sediment. In a cold climate, the primary level of lake is low, and the absolute carbon content in sediment is reduced, resulting in a relative loss of δ13C and a lower value of δ13C in sediment (Stuiver, 1975). The source of OM in lake sediments will affect the δ13C composition of OC isotopes. The δ13C concentration of terrestrial C4 plants is more positive than that of C3 plants, so the δ13C concentration will also change when the source of sediment OM changes. The isotopic composition of OC in some lacustrine silts is also affected by the concentration of CO2 in the atmosphere (Broecker, 1982). For example, annual convective mixing and the rapid dissolution of biogenic carbonates in some lakes help to maintain the balance between atmospheric CO2 and lake dissolved CO2. Because the δ13C concentration of aquatic organisms, mainly comprised of dissolved CO2, depends on the concentration of dissolved CO2 in water, and the changes of concentration of CO2 in the atmosphere will affect the carbon isotope fractionation found in aquatic organisms. This will cause a change in the δ13C concentration, while it will also change the OC isotope composition of lake sediments in the later transformation of diagenesis (Zhang et al., 2013).

    Organic carbon has been widely used to study change information related to the paleo-environmental worldwide (Table 1). Tenzer et al. (1999) found that an increase of the C/N ratio of OM was related to an increase in terrigenous OM content. Andreev et al. (2005) found that TOC and TN increased and peaked in the period of 10.7–7.8 cal ka BP, indicating that the level of productivity in the lake increased and peaked at this time; the change trend of the δ13C concentration tended to decrease, which also reflected that the OM of the lake sediment mainly came from a change in plankton biomass. Therefore, the OC content of silt can be used as an important measurement to reveal environmental changes. Xiao et al. (2008) pointed out that a fluctuation of TOC and total inorganic carbon (TIC) indicating the change of lake water level by studying the sediments of boreholes of Dali Lake in Inner Mongolia, and thought that the change of lake water level in the Early Holocene was mainly caused by melting water from ice and snow in the northern hemisphere. Morellón et al. (2009) concluded that a change of OC δ13C ratio mainly reveals a change of lake productivity and nutrient level, but not a change of material source.

  • Through adsorption and complexation, the OC in silt has a decisive control over the ecotoxicity and environmental migration of heavy metals and toxic organic compounds in the sediment; the mineralization of OC will cause significant changes in the Eh-pH field of the overlying water, which will lead to the degradation of plant species, eutrophication of water, release of heavy metal pollutants, which on a global scale will exacerbate the greenhouse effect and other environmental problems. The mineralization of OC occurs when OC is decomposed into small molecules or is completely mineralized into CO2 as an electronic donor under the action of microorganisms (Su et al., 2014). Silt is generally lying in a reducing environment, so the mineralization rate of OC is relatively low. Moreover, due to the weak alkalinity of silt pore water, OC can be involved in fixing heavy metals and organic pollutants. However, under man-made or natural disturbance, the original structure of the sediment could be destroyed, so the organic-rich silt and sediment particles at the bottom of a lake, river, or ocean will be suspended in the upper water body, and a reductive environment may be transformed to an oxidative environment. The OC will be decomposed when exposed to oxygen, and the heavy metals and organic pollutants fixed by OC will be released into the water, which can considerably degrade the environment of the water body (Smit et al., 2008; Neff, 2005).

  • Rivers provide an important link between terrestrial and marine ecosystems, as well as between the two active carbon pools of ocean and land. The lateral and vertical erosion of a river causes a large amount of land-based materials to enter the river channel and when the velocity slows down, so erosion becomes an important source of OC in the silt of the river.

    Among all different kinds of water bodies, rivers are most closely associated with human, and human industrial production and agricultural activities have imported a large amount of polluting elements into rivers. The OC in the bottom silt mainly exists in the form of mineral-bound-OC acquired through adsorption, which enriches the pollutants to a certain extent and increases the risk of contaminant release. Dissolved organic carbon is very reactive, and it directly affects the dissolved oxygen content and Eh-pH field of water; it also has an important effect on the content and migration capacity of contaminants in water (Świetlik et al., 2004; Wu Y T et al., 2004). The hydrodynamic conditions near a river estuary are not stable. The dramatic changes of flow in wet and dry periods disturb the silt that was originally deposited on the riverbed and seafloor, resulting in the suspension of silt particles in the upper water body. The changes in flow result in sudden changes in redox conditions and pH causing the OC in the silt to mineralize, releasing a variety of toxic substances and causing serious damage to the aquatic environment (Leenheer and Croué, 2003); this also helps to explain the abnormal levels of observed heavy metal pollution and eutrophication in many river estuaries and reclamation areas.

    The release of various heavy metals and pollutants has been found to reach a balance in a disturbance experiment. However, some pollutants (such as Cu2+) can reduce the concentration of these pollutants and mitigate the damage they cause by constantly changing (cleaning) the upper water body. Meanwhile, a decrease in the concentrations of most pollutants (such as Zn2+, COD, N, P) only occurs at the beginning of change in the upper water body. Later, these pollutants remain at equilibrium with a relatively high concentration, so the effect of water exchange on cleaning is very limited (Zhu, 2001). This proves that silt has a strong ability to release pollutants, and it cannot release pollutants rapidly through simple water exchange cleaning. Pollutants in the form of mineral combinations can cause long-term environmental damage. Even if clean water from the upstream areas can continuously wash the contaminated sediment, the pollution caused by the internal release of sediment is often very serious.

    Under both natural conditions and human disturbance, the OC of silt will release DOC into the overlying water body continuously. The release rate depends on the DOC concentration gradient between the silt pore water and the overlying water body. Therefore, dredging a river may not effectively inhibit the release of DOC (Zhu, 2001; Murphy et al., 1999). Because of the effect of the concentration gradient, dredging the surface sediment with low OC content also causes exposure of the subsurface sediment with a high OC content, which results in an explosive increase in DOC into the water body. The consumption of O2 in the water body causes decomposition of OM and a release of more heavy metal ions, which often causes serious environmental damage (Fig. 1).

    Figure 1.  Schematic diagram of explosive release of OC caused by dredging silt.

  • Lakes can be divided into non-urban and urban lake types based on the distance of a lake from a city and the degree of human impact. The main difference between the two types of lakes is that urban lakes lie near convenient transportation corridors, so urban lakes in a city often undertake additional functions such as providing for tourism, and helping to mitigate the effects of droughts and floods (Peng et al., 2004). Compared with non-urban lakes, the problems created by industrial and domestic sewage in urban lakes are frequently more serious with point source pollution centered on areas of sewage release.

    Non-urban lakes are usually more independent water bodies than rivers where the OC mainly comes from the remains of aquatic plants and animals are deposited internally, while they receive less external input. Nutrient elements and DOC have difficulty migrating in lake water because of their closed water body characteristics. Under the hydrostatic conditions, a large number of nutrients and OC are often deposited in the silt layer at the bottom of a lake, and these are slowly released into the water body, which can create an important internal load (Sand-Jensen et al., 2019). However, a large number of submerged plants often grow in a lake, which can effectively limit the suspension of bottom silt particles, preventing the release of OC into the overlying water and reducing its mineralization rate while purifying the water and improving water quality. Plants can also prevent eutrophication caused by the re-suspension of silt particles. In arid and semi-arid climates, water discharge often takes the form of evaporation, which leads to the enrichment and concentration of nutrient elements and OC in some regions (e.g., Wuliangsu Sea, China) where an abundance of submerged plants grow on the bottom of a lake which may experience extreme variations in the climatic conditions over time. Meanwhile, a large amount of residue generated by submerged plants may increase the thickness of the silt layer in a lake and accelerate the evolution of lake to wetland to saline alkali flat to desert (Tuikka et al., 2011; Kniskern et al., 2010).

    Urban lakes often provide more ecological services than non-urban lakes, which often include the functions of urban sewage treatment and discharge, landscape greening, flood storage, and drought mitigation. When pollutants and nutrient elements are imported from the external sources and exceed the capacity of lakes to manage them, the lakes become turbid, and the submerged plants relying on photosynthesis die out. At the same time, the silt at the bottom of these lakes may become unstable so that a large number of suspended particles may release DOC and heavy metals to the lakes causing degradation of the aquatic ecosystem. With increased levels of pollutants, phytoplankton became the dominant population of aquatic plants while aquatic oligochaetes with a strong tolerance for pollutants become the main benthic species; mollusks that cannot tolerate toxicity disappear (Hallare et al., 2005). With the effects of urban heat islands and the creation of landscape- scale entertainment facilities created by and for humans, the air convection above an urban lake typically becomes more intense, which makes it easy to stir up the silt particles at the bottom of the lake. This situation becomes even more serious when no submerged plants are present to fix the lake bottom silt. The suspension of silt particles that are rich in OC provides appropriate conditions for the propagation of algae in water, often causing outbreaks of algal blooms. Furthermore, the consumption of dissolved oxygen which blocks benthic organisms from obtaining oxygen and the blocking of sunlight result in a vicious cycle (Zhang et al., 2009) (Fig. 2). As often occurs in river silt, the problem of pollution caused by the release of OC and nutrient elements from silt to the overlying water body cannot be completely solved by dredging the silt. The silt creates a more serious pollution problem because silt, as an internal source of pollutants, accounts for a larger proportion of a lake's pollution load than in a river's pollution load, so its impact cannot be ignored.

    Figure 2.  A sketch map of the progress of lake eutrophication.

  • Marine silt mainly refers to the silt deposited in an estuary of a river or in a seaport. A river estuary occurs at the intersection of a river and an ocean, while a seaport lies at the intersection of land and sea. The slow flow velocity in these locations causes a large amount of fine-grained materials to be deposited and form marine silt.

    Along the shoreline, a river entering the sea and coastal wind will affect the nature of seawater. The silt on the bottom can easily become re-suspended. These suspended particles are often toxic to aquatic organisms. In this situation, suspended OC will consume dissolved oxygen, often causing losses to aquaculture facilities near the coast, but this type of harm is generally not as serious as the effects of silt in a lake or river. However, the depth of seawater typically becomes relatively deep farther offshore. In addition, the change of seasons can cause nutrients (phosphates, nitrates) brought by rivers to enter the seawater in some areas. When fresh water enters the sea, it reduces the salinity of the surface seawater, resulting in a saline type of stratification of the upper and lower seawater. When this occurs, it may be easy for a red tide to occur (Wei et al., 2007). During a red tide, large populations of algae species propagate in the surface layer, consuming nutrients and generating OM particles and dissolved oxygen. At the same time, a red tide further causes a saline stratification in the upper and lower layers of seawater. This stratification results in oxygen depletion when decomposition consumes dissolved oxygen that cannot be supplemented by the silt at the bottom of the ocean; thus, a large-scale undersea anoxic layer, can form, causing the death of fish and shellfish (Bolam, 2014). With the change of seasons and ocean currents, red tides may occur frequently. Because the Yangtze River carries a large burden of domestic sewage and industrial waste water as it flows into the East China Sea, the river carries a large amount of land-based nutrients, leading to the formation of frequent red tides in the Sea. Correspondingly, when dissolved oxygen fails to circulate in the upper and lower layers because of seawater stratification, this causes the decomposition of OM in the sea bottom silt. This results in a serious loss of dissolved oxygen (Chen et al., 2007) and the formation of anoxic layers of different scales and in large quantities, which are not conducive to the survival of aquatic organisms and may result in a large loss of marine resources.

    The needs of the navigation industry often require the reclamation of land from the sea to create harbors and raise the shoreline elevation to facilitate the berthing and movement of ships. During land reclamation, a sea blocking dam is normally built to cut off the flow of water between the coastal zone and the sea. Initially, seawater behind the dam will be drained so that the filling work can be carried out. This process will expose the previously alkaline submarine silt that was in reduced condition to the atmosphere under the action of continental groundwater; as a result, the silt is converted into silt in oxidized condition. During this process, the change of the Eh-pH field causes the originally stable heavy metal ions and organic pollutants to become desorbed from the state of OC adsorption. This change not only causes serious damage to the coastal groundwater environment and marine waters but also causes the diffusion of heavy metals and organic pollutant into the surrounding soil environment (Bolam, 2011; Smit et al., 2008). Therefore, engineering activities in the reclaimed area will cause sediments on the sea floor to be brought up and suspended in the water. Meanwhile, the engineering activities normally disrupt the originally stable continental shelf. The anthropogenic overburden is frequently lethal to the benthos in the sea. A deeper overburden often causes a higher death rate. The higher the concentration of suspended particulate OC, the more serious pollution will occur from heavy metals and eutrophication.

  • Aside from the effect of the atmosphere and surface water, the silt layer has an important effect on water storage and quality of the aquifer during the deposition process. Clay layer is a common kind of aquitard in plains. An aquitard is not only a crucial water storage zone but also an important area for filtering pollutant and cleaning water source (Parker et al., 2008). Under the process of natural burial, the silt has been piled up constantly by new sediments and the complex physical, chemistry and biological reactions occur among the water, microorganism, organic substance and mineral of the silt with the continuously rising of temperature and pressure. As a result, the porosity and permeability of silt continuously decreases while the pore water is continuously released, which eventually makes silt evolve into a clayey aquitard (Potter et al., 2005). A large number of pore water originally stored in the silt, carries plenty of chemical substances into the adjacent aquifer under the pore pressure, which changes the quality of groundwater. This process happens not only in the geological history, but also in the clay aquitard with different depths under the influence of contemporary human activities. More and more studies show that aquitard can provide water quantity for the groundwater, in the same time, it also can provide ion and nutrition sources for the groundwater. Konikow and Kendy (2005) and Wicks and Herman (1994) found that the total amount of groundwater by long-term exploitation from the aquifer system was far greater than the estimated amount of groundwater resources stored in the aquifer; In the investigation and research of high-arsenic and low-quality groundwater, it was found that the high levels of arsenic in aquifers are always associated with dark muddy clay layer (Wang et al., 2009). Polizzotto et al. (2008) clearly estimated that the As contribution of the 5–20 m thick clay near the surface of the wetland to the underlying groundwater with high levels of As is as high as 600–2 000 kg/a.

    The deposition and burial of silt can be divided into physical and chemical processes. The physical process is mainly mediated by pressure, which compacts and consolidates silt under the continuously loading of overlying water and sediments. This process has its strongest effect on the reduction of porosity and water content in silt that occurs along with a massive release of pore water. After the silty sediments are buried, the pore water existing outside a crystal structure of clay mineral will initially be released under the effects of pressure from overlying stratum. With an increase of burial depth, the combined water, interlayer water and even structural water can be released in sequence (Potter et al., 2005; Jiang, 2003). The chemical processes that occur during the deposition and burial of silt are relatively complex. The chemical processes in the shallow burial stage of silt are a complex of water-rock interaction process mediated by microorganisms, which will experience the evolution of the oxygen bearing, anaerobic, and methane producing zones from surface to subsurface (Hesse and Schacht, 2011). Meanwhile, the chemical process in the deep burial stage of silt is a water-rock interaction process under the influence of high temperatures and high pressures. The most representative change is the transformation of clay minerals and the thermal maturation of OM.

    Most of OC within an aquitard is part of the inert carbon pool, but OC loses the protection of metallic or clay minerals and becomes transformed a part of the active carbon pool under changing of natural conditions and human activities, which leads to the emission of greenhouse gases and the deterioration of water quality caused by the decomposition of OC and the release of pollutants (Fig. 3). The chemical composition of pore water in a clayey aquitard is always different from that in an adjacent aquifer. This is mainly due to low permeability of the aquitard, resulting in a slow alternation of pore water circulation and affected by the sedimentary environment, adsorption alternation, sulfate reduction, and biodegradation (Bufflap and Allen, 1995).

    Figure 3.  Main environmental behaviors of OC during the evolution of silt to clayey aquitard (taking Fe bound-OC as a representative of mineral bound-OC).

    As often occurs in surface and ocean water, OC also mediates the migration and transformation of typical pollutants in groundwater systems (Reszat and Hendry, 2007). Figure 4 shows that the main redox reactions of dissolved organic matter (DOM) occur in neutral groundwater, where DOM is represented by CH2O. The main redox functions of DOM in groundwater are respiration, denitrification, manganese reduction, ammonifying, iron reduction, and methanogenesis. When the available DOM (active) is exhausted, the above series of biogeochemical processes will stop. As a very important electron donor, OC is promoted into an aqueous solution catalyzed by Fe-reducing bacteria. In addition, microorganisms can use OM to act as a carbon source for various biogeochemical reactions, thus affecting the migration and transformation of arsenic in varying degrees (Niggemyer et al., 2001). In the study of the migration of heavy metals in groundwater systems in the Pearl River Delta area, Wang et al. (2016) found that the migration of V, Ba, Cr, Rb, and Cs is closely related to the degradation of OM, while the migration of Co, Ni, Cu, Zn, Pb, and Cd is closely related to the reduction and dissolution of ferromanganese oxides. When studying the source of natural high groundwater with high levels of NH4-N in the Pearl River Delta, Jiao et al. (2010) found that abundant NH4-N is preserved in OM, which becomes a potential source of NH4-N in groundwater. We found that TOC of sediments and the DOC of pore water are significantly correlated with the As, Cr, NH4-N content of pore water in varying degrees in the study of the hydrogeochemical characteristics of the clayey aquitard in Jianghan Plain, China (Liu et al., 2020). Therefore, the change of OC form and content in the evolution of silt to clay is important to reveal the effects of an aquitard on an aquifer.

    Figure 4.  Major redox reactions involving DOM in neutral groundwater.

  • Silt is a common and widely distributed sediment, and this paper analyzed the morphological characteristics of OC within silt and several major environmental effects. The rich OC in silt of various surface water bodies is closely related to the release of pollutants, which makes it necessary to consider the effects of OC mineralization caused by environmental changes when carrying out engineering activities or dredging (Zhu, 2001). Moreover, corresponding treatment models need to be developed based on the characteristics of different water bodies. For example, an enclosed lake can gradually recover with biological treatment when the ecosystem is damaged by eutrophication; meanwhile, for an area of ocean or a river with rapid circulation, the input of pollutants and nutrients should be reduced, and composite means for treatment should simultaneously be adopted.

    Moreover, the fluctuation of surface water level has a strong influence on the burial environment (undercurrent zone) when silt is deposited in shallow water. When the surface water level responds to a change in the upstream and downstream water levels and to seasonal rainfall, the depositional environment of shallow silt alternates between an oxidation and reduction environment. In oxidized environment, heterotrophic microorganisms consume a large amount of OM in the silt for growth. They degrade the OM, activate the heavy metals previously fixed by OM, and produce small molecules as by-products, resulting in an increase the content of DOC and heavy metals in pore water (Smit et al., 2008; Neff, 2005). When the surface water level rises, the environment in silt returns to a reducing condition so that the activity of heterotrophic microorganisms is inhibited. The high content of ions and DOC in pore water allows the ions and DOC to diffuse to adjacent aquifers or surface water in a process that is driven by the concentration gradient, the mechanism of which is worthy of further study; see the conceptual model shown in Fig. 5.

    Figure 5.  Schematic diagram of interaction pattern between groundwater and silt in subsurface flow zone: (a) profile of silt-groundwater system; (b) interaction pattern at low water level; (c) interaction pattern at high water level.

    At present, the influence of a silty aquitard on an aquifer has been studied to some extent worldwide, but almost all previous studies have been concerned with the effects of clayey aquitard on the hydrochemistry of an aquifer (Alvarez et al., 2012; Cuevas et al., 2012). Few studies have addressed the physical, chemical and biological processes and reactions mediated by OC during the evolution of a deposition of silt transformed into a clayey aquitard. Researchers should consider using TOC, C/N, and δ13C to indicate the nature of environmental changes during the process of silt deposition and to compare the effects of different sedimentary environments on the transformation of OC forms, such as the differences between alluvial lacustrine and marine facies. In addition, the compaction process is an indispensable driving force for the clayey aquitard when it provides a water source for adjacent aquifers, while previous research on chemical compaction has mainly focused on hydrocarbon generation simulation and rock formation simulation of OM (Nygård et al., 2004). It is necessary to further explore the interaction between OC and other important sedimentary elements, such as nitrogen, sulfur, and iron (Hunter et al., 1998), to quantify their contribution to the water content and quality of aquifers, to reveal the sedimentary stages of changes in OC and the types of biogeochemical reactions, to establish a geochemical model, and to provide a new understanding of the formation of natural inferior groundwater. Finally, the terminal point of silt evolution must not be in the clayey aquitard, and the study of carbon bearing materials or other staged products (such as coal, oil, and metal minerals) should be carried out.

  • This study was jointly supported by the National Natural Science Foundation of China (Nos. 41630318 and 41521001), Research Program for Geological Processes, Resources and Environment in the Yangtze River Basin (No. CUGCJ 1702), and the China Geological Survey Project (Nos. 2019040022, DD20190263). The final publication is available at Springer via

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