**4. Methodology**

### **4.1 Measuring bioaccumulation**

Bioaccumulation can be used as a measurement tool for analyses of soil toxicity. Bioaccumulation data involve field and laboratory analyses of test organisms exposed to spiked metal concentrations. Bioaccumulation data are metal and organism specific [14].

### *4.1.1 AVS/SEM*

The sulphide minerals in sediments with more iron sulphide appear to be a controlling factor for certain divalent cationic metals affecting the metal activity and toxicity in sediments.

The sulphides termed acid volatile sulphides and metals that are combined with AVS are extracted through a process called fractionation and are termed sequentially extracted metals (SEM) and are used in measurements that can assess the potentially bioavailable metals in the sediment as the metals tend to bind well with the AVS content.

Hence, the SEM/AVS theory assumes that, if the AVS concentration is less in sediment than the concentration of SEM, toxicity will be observed.

In other words, if the SEM/AVS ratio is >1, sufficient AVS is not available to bind all the SEM, and therefore, benthic organisms might be exposed to toxic metals. If the ratio is <1, sufficient AVS exists to bind all SEM, and toxicity in benthic organisms is less expected.

### *4.1.2 Bioaccumulation tests*

Bioaccumulation tests are conducted either in situ as field monitoring studies or using an indicator organism in a closed environment under controlled conditions with an artificially spiked substrate where the organisms are kept for certain days for uptake and elimination phases and are monitored during those days and the data are stored.

The degree to which bioaccumulation occurs can be expressed as follows:

BAF (bioaccumulation factor) or,

BSAF (biota-sediment accumulation factor),

BAF/BSAF is the ratio of the chemical concentration in an organism to the concentration in the sediment.

$$\text{BSAF} = \text{C}\_{\text{biota}} / \text{C}\_{\text{sediment}} \tag{1}$$

Bioaccumulation endpoints include organisms'survival rate, mortality, growth or reproduction, or loss in growth and reproduction.

Sediment toxicity tests include the physicochemical characterization of sediment, toxicity-level assessments and benthic community surveys. This is called a sediment quality triad which is extensively used in decision-making frameworks for contaminated sediments.

### **4.2 Bioaccumulation models**

Bioaccumulation endpoints in sediments can also be expressed using models such as those discussed in the following sections.

### *4.2.1 Equilibrium-partitioning model and kinetic model (regression model)*

Equilibrium portioning models (EqP) assume a steady state concentration, which is achieved due to the thermodynamic equilibrium that exists among the organism and the sediment or the biota or substrate where it is present. Therefore, the fugacity of the compound (the particles' movement or their escape from their current phase) is assumed to be equal to the other compartments in the same environment [15].

And at equilibrium, bioaccumulation can be expressed as a simple partition coefficient or biota-sediment factor (BSAF). However, this model applies to organic contaminants since the lipid content in the organism is necessary to measure the hydrophobicity of the compound.

Based on the interconnections with the hydrophobicity of a compound and its lipid content, a portioning coefficient of 1.7 has been suggested for all compounds [16].

Kinetic models are mathematical models that need uptake and elimination data, the rates of which are modeled independently. The advantage of this model includes no assumption of equilibrium conditions, and hence, non-steady-state concentrations can be predicted.

The model also uses multiple exposure routes and different ways of bioaccumulation in the organism. Compartment-based models describe the movement of the chemicals through first-order equations.

### *4.2.2 One-compartment bioaccumulation model*

A one-compartment bioaccumulation model assumes the organism as a single homogenous unit and follows a first-order reaction. It is expressed as:

Rate of metal accumulation dx*=*dt ¼ Rate in uptake ð Þ–Rate out excretion ð Þ (2)

Where the concentration of the metal in the body of the organism is directly proportional to the concentration of the metal in the soil.

The exchange of matter between the compartments is due to flux, which is given as *K*a. The flux of the metal *K*<sup>a</sup> in the earthworm is the product of an uptake rate constant *K*in and the external metal conc *C*<sup>e</sup> [17].

The parameters which affect the bioaccumulation of a substance include BAF, the uptake rate constant (*K*in) and the elimination rate constant (*K*out).

This model is gaining popularity and is used in environmental risk assessments (ERAs) extensively. It describes the organism as a single homogenous unit. This model is suitable for compounds that distribute rapidly throughout the body.

Two parameters govern the kinetics of the compounds in a one-compartment model [18].

The uptake or accumulation rate

$$K\_{\rm in} \,\mathrm{day}^{-1} \tag{3}$$

The elimination or excretion rate

$$K\_{\text{out}} \,\text{day}^{-1} \tag{4}$$

*Heavy Metal Bioaccumulation in Sediment and Benthic Biota DOI: http://dx.doi.org/10.5772/intechopen.110015*

The uptake rate is proportional to the exposure concentration in the environment (*C*exp, mg/kg), and the elimination rate is proportional to the concentration in the organism (*C*org, mg/kg).

Therefore,

$$d\mathbf{C}\_{\rm org}/d\_T = K\_{\rm in.} \mathbf{C}\_{\rm exp} - K\_{\rm out.} \mathbf{C}\_{\rm org} \tag{5}$$

The rate of the contaminant is given as

$$C\_{\text{ew}}/d\_T = \text{Rate of uptake} - \text{Rate of elimination}$$

When the rate of absorption or intake is absent, the equation becomes.

$$d\mathbf{C}\_{\text{org}}/d\_T = -\mathbf{Rate out} \tag{6}$$

If the excretion rate follows first-order kinetics, then,

$$d\mathcal{C}\_{\text{org}}/dT = -K\_{\text{out}}\mathcal{x} \tag{7}$$

Where *K*out is the first-order elimination rate constant, and *x* is the amount of contaminant in the organism at a given time.

At initial time *t*, the concentration in the organism is 0 and the concentration in the substrate is constant. Eq. (1) has the following solution:

$$\mathbf{C}\_{\rm org} = \mathbf{C}\_{\rm exp} \left( K\_1 / K\_2 \right) \left( 1 - e - k\_{2t} \right) \tag{8}$$

Where

*C*org = Concentration of contaminant in the organism.

*C*exp = Concentration of contaminant in the substrate.

*K1* = uptake rate constant/day.

*K*<sup>2</sup> = elimination rate constant/day.

*t* = time/day.

And as the exposure time approaches infinity, the equation for the steady state condition becomes:

$$\mathbf{C\_{org}}/\mathbf{C\_{exp}} = K\_1/K\_2 = \mathbf{BAF} \tag{9}$$

If uptake and elimination rate constants are determined, a BAF can be calculated using the above equations (**Figures 2** and **3**).

### **5. Results and discussion**

Bioaccumulation by earthworms is non-linear, that is, decreases as the concentration increases. The biota to soil accumulation factor (BSAF) assumes that accumulation is linear and constant across all soil concentrations, and hence, the use of the loglinear regression model is used in this study to explain the bioaccumulation in the selected earthworm species. The log-log regression model explains that the bioaccumulation of metals or any other pollutants by earthworms decreases as soil concentration increases. As soil pollutant concentration and elimination rate increase,

### **Figure 2.**

*Relationship between the concentrations of heavy metals (Zn, Cu and Pb) in soil and internal concentrations in earthworms of the earthworm species* L*.* mauritii *and* P*.* excavatus*.*

### **Figure 3.**

*Regression model applied on the study site between heavy metal concentrations in soil and those in earthworms* L*.* mauritii *and* P*.* excavatus *(R2 = 0.97). (a) and (c) Cu and Zn in soil vs. Cu and Zn in earthworm tissues in* L*.* mauritii*; (b) and (d) Cu and Zn in soil vs. Cu and Zn in earthworm tissues in* P*.* excavatus*.*

*Heavy Metal Bioaccumulation in Sediment and Benthic Biota DOI: http://dx.doi.org/10.5772/intechopen.110015*

accumulation may decrease. In our study, heavy metal concentrations in the tissues of the earthworm species collected from the study area were generally not so high and were significantly different across the study sites (p < 0.05), especially for Pb and Cu. The earthworms showed increased accumulation pattern for Zinc. It is observed that Zn is the most bioaccumulated; Zn > Cu > Pb, with mean values in the range of Zn (0.13–0.7), Cu (0.28–0.82 Cu) and Pb (0.08–0.89) in *L. mauritii* and Zn (0.09–0.32), Cu (0.18–0.73) and Pb (0.05–0.3) in *P. excavatus*. The accumulation of zinc can be attributed to the readily available metal form in the soil. The order of accumulation of heavy metal BAF in the present study is Zn > Cu > Pb, indicating that zinc is a potentially high accumulating metal compared to Cu and Pb. Although zinc is an essential metal when present at higher levels, it causes cellular disruptions such as mitochondrial dysfunction and limits population growth by affecting reproduction in earthworm species. Copper and lead when present at higher levels are also known to cause higher mortality and reduction in the growth size of the organisms. *P. excavatus* being an epigeic species, which mostly feed on decomposed leaf, are comparatively less affected than the anecic *L. maurtii* that deeply burrow in the soil and are likely to be more exposed to soil metal pollution.

### **6. Conclusion**

Heavy metals in sediments are the main cause of bioaccumulation in benthic organisms and in plants due to the uptake of water, minerals and other nutrients through direct body contact and via roots.

Soil-dwelling micro- and macro-organisms are in direct contact with metal pollution and are exposed to irreversible damage as sediments are very difficult to be remediated. Hence, sediment metal pollution should be considered a serious threat, and strict sediment quality guidelines should be applied.

Reducing the form of metal wastes that are generated through human activities can serve as a start. Going organic in food production can save the benthic biota and our future generation. Natural bioremediation techniques such as phytoremediation should be used to reduce the bioavailability to soil organisms.

Aerobic and anaerobic microorganisms can also be used to treat highly contaminated soils for biodegradation. However, organisms are adapted to high metal concentrations. In that case, the food web should be considered for biomagnification, and the organism that is much affected should be monitored closely.

Natural chelating substances can be used to bind metals that form organometallic complexes, which may be less hazardous for the soil-dwelling species. Also, some detoxifying mechanisms are present in benthic organisms, and hence, bioaccumulation measurements should include those as well.

The ADME process (adsorption, detoxification, metabolism and ejection) is common in all organisms. The level of absorbed pollutants should be considered only for toxicity tests. Identification of bio-accumulative metals may help in enhanced remediation processes.

*Heavy Metals – Recent Advances*
