**1. Introduction**

Metals are naturally occurring non-biodegradable substances in the environment, existing in both natural ways and man-made pollution. Generally, they occur by the geo-chemical weathering of rocks in an environment.

Anthropogenic interventions such as mining, smelting industries, paints made of metals and batteries, and several other inventions have caused severe metal pollution. Metals that are distributed to other places through metal mobility and that get stored in the places where they are deposited pose a serious threat to humans and the overall ecosystem's health.

Though some heavy metals like iron, copper, and zinc are essential for both humans and animals to a certain extent for the proper functioning of the body, those that present in quantities exceeding their limits are extremely hazardous, causing physiological damages in both humans and animals. For example, excess copper, through contaminated vegetables and fruits and other sources, is linked with anemia and liver and kidney damage.

Mercury poisoning at Minamata, Japan, is one example of such contamination that reached humans through fish consumption from the affected Yatsushiro Sea and the Agano River. Lead poisoning of drinking water is another common problem. Arsenic contamination of groundwater has been reported in recent years.

As a result of scientific advancements, large quantities of raw materials using various chemical elements are produced, and the resulting wastes end up as pollutants in major aquatic and terrestrial ecosystems, mainly in rivers and streams as untreated industrial effluents and hazardous chemical wastes. Many freshwater ecosystems around the world have been studied for such pollution effects in the past years.

### **1.1 Heavy metal pollution**

Among the inorganic pollutants, metal pollutants seems to have gained importance as they stay forever in the environment present and tend to bio-magnify when present in a food chain. Metals tend to bioaccumulate in sediments, bio-concentrate in water, and deposit as free radicals in the air and cause damage to the respective ecosystems. Accumulation happens mainly through the food source and prey–predator relationships. From one trophic level to the other, metals tend to bio-magnify, reaching the highest level in the organism at the highest trophic level.

In an aquatic ecosystem, metals generally are found as free metal ions in the water or the sediment mostly as a metal complex bound with soil constituents. They may be colloidal in nature or as suspended solids in an aquatic ecosystem. In colloidal and particulate phases, they exist as oxides, hydroxides, silicates or sulphides and as metal complexes combined with organic matter. The soluble forms are generally ions or non-ionized chelated metallic complexes. pH, ligand concentration, oxidation state, and redox potential are all solubility-controlling factors in an aquatic environment.

Heavy metal contamination of drinking water sources is extremely dangerous for human consumption. Metals in the dissolved phase are more toxic to aquatic organisms. Surface run-off attributes to the pollution of water sources.

Heavy metals also find their way into the agricultural soils in the form of heavily loaded pesticides and fertilizers which eventually end up in the soil, and crops grown in that soil are also affected. In sediments, metals have an affinity towards certain chemicals and, hence, are present as metal complexes.

In sediments, controlling factors include metal speciation, acid volatile sulphides, particulate organic carbon, and Fe and Mn oxyhydroxides. The bioavailability of heavy metals in soil highly depends on factors other than sediment texture and physio-chemical properties such as soil pH, temperature and other organic soil constituents like humus. Sand, silt, and clay also play a major part in the metal's behavior in each sediment.

Heavy metal toxicity in sediments can be measured with bioaccumulation tests. Metal bioaccumulation has recently achieved more concentration from researchers due to its feasibility to conduct both field and laboratory experiments with indicative organisms. Bioaccumulation can be measured directly or using models.

Direct measurement includes assessment of the biota, the organisms present, water, or sediment. Models include simple regression models to mechanistic and empirical models. With recent advances in bioaccumulation models, there is a huge scope for studying metal bioaccumulation at the pollution level and applying the model elsewhere to get results without disturbing the environment. For example, measuring bioaccumulation in a particular sediment can be compared with sediment elsewhere with a simple one-compartment bioaccumulation model.

Bioaccumulation endpoints can be considered for environmental impact assessments. Here, we will concentrate on the recent bioaccumulation models and the relative importance of one compartment model using earthworms as an indicator organism to measure bioaccumulation in a sediment biota.

### **1.2 Heavy metal toxicity in sediments**

Sediments are considered a mixture of various sorbent phases such as organic matter, oxides, sulphides, carbonates, and clay or stilt minerals in which their abundance is based on pH, redox conditions, hydrological level, and the depositional environment [1].

Sediments can be a long-term source of metal pollution. Sediment contamination with metals poses a severe threat to the soil ecosystem, particularly the organisms that are in direct contact with the environment, affecting their survival, reproduction, and growth. Heavy metal toxicity in sediments can be present for a longer period since sediments act as sinks for all kinds of pollution, both organic and inorganic.

Sediments not only store potentially toxic metals but also enhance the transportation of the same. Especially, aquatic sediments are likely to attract heavy metals due to their finer texture and store metals through adsorption, complexation and precipitation processes. Most of the metals present as free metal ions remain in waters surrounding the sediment. The form of the metals present (speciation) impacts the toxicity level of the metals in sediments.

Thus, speciation and distribution of heavy metals in sediment are of major concern when indicating metal toxicity in sediments. Physio-chemical factors such as temperature, hydrodynamic conditions, redox state, the content of organic matter and microbes, salinity and particle size affect the chelation process at the sediment level [2].

However, the distribution of heavy metals in the sediment is highly based on the composition of the sediment, particle size and organic matter content. In finer sediments, organic carbon plays a vital role in the binding of the metals, and higher organic carbon reduces metal solubility and toxicity [3, 4]. Especially humus in soil binds metals more than any other organic matter. This is due to the affinity the humic substances have towards heavy metals, due to their chelating properties.

This may serve the purpose of protection as the toxicity is absorbed to a considerable amount. The aquatic sediments relate to surrounding waters and overlying interstitial water. Hence, any disturbance leads to changes in the sediment biota and therefore may alter its biodiversity. Under aerobic conditions, microorganisms may break down the periphytons and the other aquatic plants and release phosphorous, but in anaerobic conditions, the phosphorus may enter the water column resulting in eutrophication [5].

Any alteration in the sediment composition affects the benthic organisms, severely disrupting the habitat's normal functions such as the exchange of organic matter or nutrients. In terrestrial sediments, the toxicity depends largely on the sulphides that are present in the sediment layers.

These sulphides, termed acid volatile sulphides (AVS), are formed by sulfur-reducing bacteria from organic matter, which are more in anaerobic sediments. Other factors include soil pH, particle size, carbonates and Fe and Mn-oxy hydroxides [6].

### **2. Metal speciation and bioavailability**

The form in which a metal exists in the sediment can be defined as metal speciation. It determines the level of toxicity in the environment in which the metal is present, whether aquatic or semi-aquatic or terrestrial. Most of the metals exist as free metal ions, metal complexes and metal species that occur in an undissolved state.

Speciation affects the transportation of metals from the sediment to the overlying water. Thus, the mobility, fate and transport of the metals stored in the sediment are highly influenced by speciation, particularly in an aquatic environment [7].

In an aquatic environment, sedimentation can occur if the composition of the above sediment is altered, and the normal functioning of the stream or river is severely affected [8]. In the hyporheic zone, where the surface water and groundwater mix, a unique set of microbes and macroinvertebrates is present. But due to severe toxicity, the oxygen supply may be suspended along with the organic matter, due to which the hyporheic organisms disappear.

Understanding metal speciation is crucial to know metal bioavailability and toxicity. Bioavailability is the amount of potentially available forms of metals for uptake by the organisms living in that environment. Hence, in recent studies, bioavailability has been used as a criterion for measuring metal pollution in an aquatic environment.

The bioavailability of free metal ions is the best predictor of metal uptake and toxicity. In sediments, this is further affected by the presence of soil-dwelling organisms through their nature of burrowing. Burrowing gives way to bioturbation, in which a considerable amount of water flows in and out of the burrows of the organisms.

Bioturbation leads to an upward transport of pollutants embedded in deeper soil layers. This may affect the measurement of the free metal ions transported through the course. But, in recent days, a substantial amount of bioavailability tools has been developed by a computer programmed to give more accurate results.

Differences in metal bioavailability are also affected by several other environmental factors such as pH and redox, thereby affecting metal solubility and metal complexation with organic matter present in the soil such as hummus. pH is the most important factor governing metal speciation. It affects the solubility of metal hydroxide minerals and the adsorption and desorption processes.

Metal hydroxides have very low solubilities under pH conditions in water. Other factors include water hardness, organic carbon content and dissolved oxygen content. Due to this, there exist differences in metal bioavailability, leading to metal toxicity. There are also substantial uncertainties in the data collected when bioavailability is considered one of the tools to measure soil bioaccumulation [9].

### **2.1 Key factors determining bioavailability in an aquatic sediment**

Sorption: It is a process by which a solute becomes physically or chemically attached to a solid sorbent regardless of the mechanism (e.g., chemisorption, adsorption, and absorption).

Desorption: It involves the removal of a chemical from a solid to which it is attached or a liquid to which it is dissolved.

Adsorption: It occurs when dissolved metals are attached to surfaces of particulate matter such as iron, Mn and Al oxides, clay and organic matter.

Chelation: Chelation of metal ions happens with soil constituents, and chelated metal complexes are formed. It is the binding of molecules with metal ions. Chelation often reduces metal toxicity by reducing the concentration of free metal ions.

### **2.2 Sediment–water–column interactions**

The water in and around the sediment plays an important role in the movement of free metal ions in and out of the sediment. Pore-water measurement should be considered when measuring bioavailability.

### **2.3 Organism behavior**

The sediment-dwelling organisms mostly are burrowing in nature. Their burrows are constantly irrigated or immersed with the surrounding pore water. This may lead to the movement of metals from the sediment to the porewater and vice versa. Complex metal ions are dissipated in this process. Due to this process, the organism's exposure routes may also vary. The sediment biotic ligand model approach (sBLM) is used to predict the bioavailability of metals in overlying water in such cases [10].

### **3. Bioaccumulation in sediments**

Bioaccumulation refers to an increased level of metal concentration in a living organism than present in its environment. Many such pollutants are taken up by the organisms and stored or metabolized and excreted. In the case of metals, although some are essential for the organism's survival, many are hazardous. They are equally hazardous to the environment in which they are prone to accumulate since they cannot be degraded. Such non-degradable metals accumulate in the sediments in soil and water and contaminate the ecosystem forever. Since aquatic sediments are finer, they tend to accumulate more metals than in land. But in a terrestrial environment, the agricultural soil is the most affected due to pesticides and chemical fertilizers applied over a period regularly stored up in the soil layers. This is taken up by the plants grown in that environment and by the benthic biota present in that place. Thus, sediment heavy metal contamination happens and cannot be redone, thereby affecting the entire flora and benthic fauna. Accumulated metals are stored in different soil fractions. Hence, measuring sediment-associated contaminants requires sequential extraction procedures. The various binding sites in an organism's tissue enhance metal binding and lead to improper function of the organism.

### **3.1 Bioaccumulation in benthic organisms**

Benthic ecosystem often refers to the bottom of the ocean floor and in some cases the bottom sediment in any aquatic ecosystem, both fresh and salt water. The organisms that are present in such sediments are referred to as benthic organisms. These may include a variety of species from micro- to macro-organisms representing insects, polychaete worms, earthworms and snails. Benthic macroinvertebrates readily accumulate contaminants and have been suggested to be reliable indicators of metal bioavailability in metal-contaminated aquatic ecosystems. They are mostly sessile, have long life cycles and represent a range of ecological niches [11]. Variations in season, functional feeding group and size of the organisms should be considered while measuring bioaccumulation in these organisms. They are also an important part of the food web in an aquatic ecosystem, serving as a prey for many fish and birds and are potential candidates for biomagnification. The sediment-dwelling organisms can easily bioaccumulate as they are highly exposed to pollution through the ingestion of contaminated soil and food (**Figure 1**).
