Demographical Identification of Trace Metals Found in Soil Samples from India

Abstract

Soil has various trace metals, which help to identify the demographical origin of the soil. The formation of soil undergoes changes due to several external factors. However, certain trace metals are not affected by these external factors. This chapter considers two approaches for the detection of these trace elements; first, it highlights the usefulness of the trace elements present in the soil whose presence in deficiency or excess affects the soil quality; second, the analysis of soil transferred from various surfaces, to detect the presence of these trace elements. This chapter involves various instrumental techniques used to study its elemental composition and morphological characteristics. Due to the heterogeneous nature of the soil, the information from this chapter can be used as a database to narrow down the area of search and objects under study. It also provides insights into understanding the presence of trace metals in soil, their effects, and their role in forensic soil science. The use of soil in the search for trace evidence, which gives background knowledge on the importance of comprehending soil from the topographical scale to the crime scene, has been overviewed. This aids law enforcement agencies in investigations.

1. Introduction

Formerly, all elements were believed to be trace metals, with the exception of the eight common rock-forming elements: O, Si, Al, Fe, Ca, Na, K, and Mg. Elements with concentrations of 0.1% and lower were categorized as trace elements. Therefore, “Any element having an average concentration of less than about 100 parts per million atoms” is defined as trace element as per the IUPAC Gold Book [1, 2]. Due to their occurrence in trace amounts (ppb range to less than 10 ppm) in a wide range of natural environments, heavy metals are also categorized as trace elements. Natural elements with a high atomic weight and a density at least five times greater than that of water are known as heavy metals. Arsenic, cadmium, chromium, lead, and mercury are among the priority metals of public health concern due to their high degree of toxicity. Even at modest exposure levels, these metallic elements are known to cause numerous organ damage and are regarded as systemic toxicants [3] (Figure 1).

Figure 1.

Soil and its analysis play an important role in soil science and forensic soil science.

As a result of human activities such as mining, metal smelting, combustion agriculture, improper deposition of industrial waste, other metal-based industrial activities, and use of commercial fertilizers and pesticides, heavy metals such as cadmium (Cd), iron (Fe), lead (Pb), zinc (Zn), cobalt (Co), copper (Cu), nickel (Ni), etc., enter the soil. These heavy metals also occur in the soil as a consequence of natural processes. However, the presence of a few of these trace metals in the soil in high or low concentrations is beneficial for plant growth. Plants utilize the vital micronutrients present in the soil for their growth. For example, copper is a necessary heavy metal that actively contributes to photosynthesis, and several metabolic enzymes, like malic dehydrogenase and oxalosuccinic decarboxylase, have manganese as an essential component. This chapter aims to offer an overview of the various trace metals present in the soil, their origin, the extent of contamination, and their influence on plant growth and development [4].

2. Soil and its formation

Soil is primarily composed of mineral particles, organic materials, air, water, and living organisms, all of which interact slowly but consistently with one another as rocks gradually deteriorate via weathering resulting in soil formation. It takes hundreds to thousands of years for soil formation. However, the soil is destroyed more quickly than it is formed due to negligent land reclamation, man-made erosion, acidification, pollution of water, air, and other resources, and takeover of land for residence, construction, transportation, and setting up of industries. The spatial arrangement of numeric microscopic particles determines their physical properties. All terrestrial organisms depend on soil, as it is a unique and vital natural resource essential for living and sustaining an ecosystem. The topmost thin layer of the soil that makes up the earth’s crust and where plant roots embed themselves to get water and essential nutrients for their growth is not the sole part. These are complex and sophisticated natural formations that emerge from their parent materials, such as sediments or solid rock, as a result of the interaction between soil organisms, water, air, plants, bacteria, and other soil-dwelling organisms. In terms of their physical, chemical, and mineralogical characteristics, they typically differ significantly from the parent material, which makes them best suited as a rooting media for plants. The hydrological cycling of water, carbon, nitrogen, and other components is significantly influenced by soils. In addition to acting as a foundation for plant growth, this affects the chemical composition of various materials found in the hydrosphere and the atmosphere [5].

3. Factors affecting soil formation

Soil formation is attributed to several factors. External environmental factors, including climate, biological factors, biota, time, parent material, temperature gradients, and variations, are some of the key physical processes affecting soil formation. The unique characteristics of soil are contributed by these factors (Figure 2).

Figure 2.

Representation of various factors affecting the soil formation.

3.1 Climate

The most crucial elements in the development of soil are temperature and rainfall, which influence how well the parent material weathers, how much water percolates through the soil, and what kinds of microorganisms are present. In the same type of climate, the same soil may form from two different parent materials. Similarly, two distinct soil types in two distinct climates can result from the same parent material. In moderately moist sections of the monsoonal zone, the crystalline granites generate laterite soil and non-laterite soil in drier places. Regardless of the parent rock, hot summer and low rainfall create black soil, as is the case in several areas of Tamil Nadu [6]. Sandstone and granite both produce sandy soil in Rajasthan’s desert environment. In arid and semi-arid regions in India like Rajasthan, Gujarat and, Punjab, respectively, the evaporation rates are usually greater than precipitation due to which they lack much vegetation and are highly humus deficient which in turn gives a pale color to the soil. Excessive evaporation causes lime accumulation in soils in Rajasthan and the surrounding dry and semi-arid regions, making the soil pedocal in nature, which has minimal organic matter and is high in calcium carbonate. The deterioration of vegetation occurs slowly in the Himalayan region due to its frigid winters, and the soils are acidic in nature.

3.2 Temperature effects

As soils have a large heat capacity and a low heat conductivity, which significantly minimizes temperature variations, the magnitude of daily and seasonal temperature changes decreases dramatically with depth. The diversity and activity of the biota, chemical changes, the mechanical degradation of rocks, and the distribution of small and large particles are the four key factors that temperature has on soil formation processes.

3.3 Biological factors (biota)

The soil formation has been significantly influenced by biological agents present in the soil such as bacteria or gophers. These consist of flora and fauna. The expansion of vegetation has a significant impact on how soil forms and develops. The decomposed leaf matter enriches the soil with much-needed humus, enhancing its fertility. Some of India’s finest soils are found in the densely forested areas, and there is a direct connection between the different types of vegetation and the soil. Soil erosion is lessened by vegetation. Through the action of carbon dioxide and other acidic substances, they stimulate percolation, drainage, and stronger mineral dissolution. Due to the continual mixing of the soil profile by the animals that dig underground, such as rats, earthworms, ants, and others which are crucial in the development of soil, the soil formation has enhanced [7].

3.4 Time

Numerous researches have been carried out regarding the type and degree of changes that the soil undergoes over time. These studies compare distinct properties of soils with various backgrounds but generated from the same type of parent material in the same environment. The outcome of these studies demonstrated a significant rise in the concentration of the organic components in the soil initially followed by its degradation with passage of time. Climate, the composition of the parent material, the prevalence of burrowing species, etc. are just a few of the many associated factors formation of various layers of the soil. Clay content in the soil rises when weathering progresses as a result of primary minerals being physically and chemically altered. The soil region containing the clay concentration may undergo transformations with additional chemical changes. The climatic factors and chemical environment of the soil determine the nature of the clay particles generated [8].

3.5 Parent material

Parent materials are the bedrock that soils are created from. The parent material has a great influence on and governs the color, composition, texture, and properties of soil including physical and chemical properties. The soil that is created under varying climatic conditions may differ in its physical characteristics from the parent rock. Weathering that takes place at the surface layers of the rocks turns them into tiny granular particles that act as a foundation for the development of soil. For instance, in India, the red soils found in ancient crystalline and metamorphic rocks are caused by the presence of iron oxides. As the Gondwana rocks have immature sedimentary rocks, the soils evolved are less fertile, 2hereas the basalts from the Deccan region are known as “black soils” because they are rich in minerals like titanium, magnetite, aluminum, and magnesium [9, 10].

4. Constituents of soil

Solids, liquids, and gaseous phases all make up soils to form a complex medium. The important properties of soil comprise texture, density, structure, consistence, porosity, temperature, color, and aggregate stability. These parameters affect the properties of soil to a vast extend. The constituents of soil can be broadly classified as inorganic and organic components.

4.1 Inorganic components of soil

The majority of the soil’s constituents are typically made up of inorganic material comprising aluminosilicates. The inorganic constituents of the soil can be further classified into crystalline and non-crystalline components. Primary minerals (formed from the weathering of rock) and secondary minerals (transformed as fine particles) are the two categories under which inorganic minerals fall. A significant amount of the sand and silt fraction is made up of primary minerals.

4.2 Organic components of soil

In accordance with the proportion of organic content they contain, soils are categorized as either organic or mineral. Mineral soils, which can have an organic matter content of only a trace to 30%, make up the vast portion of agricultural land on earth. Organic soils naturally contain a large amount of organic compounds, largely as a result of environmental factors. Non-humic and humic materials make up the organic matter of the soil. Polysaccharides, proteins, lipids, and low-molecular-weight organic acids are some of the non-humic compounds. Most of the organic matter in soil is made up of humic materials that play a vital role in improving the properties of the soil, growth of plants, etc. Humic compounds are divided into fluvic acids, humic acids, and humins based on how differently they dissolve in acid or base [11].

5. Trace elements present in the soil

Various databases obtained from previous literatures were surveyed and reviewed in an effort to evaluate the state of the soil pollutions caused by human activities and industrial processes. The findings show that pollution metal pollution in soils was particularly severe. Few elements that are often found in extremely minute concentrations in soils are necessary for the growth and development of flora and fauna. They are referred to as trace elements due to their low presence and minute quantity in naturally occurring geological formations or land surfaces [12]. As reported by Kabir et al., Pb, Zn, Ni, Cu, Fe, and As were produced as a result of industrial activities such as metal manufacturing, Mn and Cd from textile manufacturing, and Cr from the leather manufacturing. The study also discovered that metal levels in the tested locations exceeded the standard regulation guideline values that are applied by several countries. To control and minimize this scenario, it is crucial to maintain regular and ongoing monitoring of heavy metals. Natural processes cannot break down metals, particularly when elemental metallic content is taken into account. In contrast, organic pollutants may undergo biological or chemical processes that cause them to breakdown into less harmful components. As a result, metal contamination may have significant and long-lasting effects on local habitats and other living species [13].

6. Contamination of soil with trace metals

All the possible sources of soil contamination arising from heavy metals include pain containing lead, residues from mine, the discarding and dumping of waste containing heavy metals, overuse of chemical fertilizers and pesticides, and spilling of petrochemicals containing metals such as cadmium (Cd), lead (Pb), nickel (Ni), zinc (Zn), chromium (Cr), arsenic (As), mercury (Hg), and copper (Cu) [14]. The aforementioned heavy metals are most frequently discovered in contaminated sites (Figure 3).

Figure 3.

Schematic representation showing the transfer of trace elements from soil to the human body and causing adverse effects like metabolic disorders which can be fatal.

The major soil contaminants arise mostly from two factors, namely natural and man-made factors. The former includes soil erosion, volcanic eruption, etc., and the latter includes industrial development, hospital waste, etc. Metal contamination causes the metal concentration in the soil to rise above the acceptable threshold, making it harmful to plants and animals. Metals, in general, are a potential long-term source of soil contaminants. In a study conducted by Zhang et al., they have calculated the average pesticide usage per year globally, between the years 2010–2014. The results of the study predicted that by 2020, the yearly global use of pesticides would reach 3.5 million tons [15].

As depicted in Figure 4, soil and plants are affected by the high concentrations of heavy metals. It is assumed that metals with high allowable limits are safe. Pb has the greatest acceptable levels in soil, followed by Zn and Cu, and Cd has the lowest permissible values. These data indicate that the accumulation of Cd in the soil is more hazardous than Cu, Zn, and Pb even at lower concentrations. However, Pb, Zn, Cd, and Cu have the greatest limits in plants, respectively. In contrast to soil, which has the safest limits for Cu, Pb, and Zn, buildup of Cd in plants poses the greatest threat. Figure 5 shows the statistical representation of the results obtained in their study for pesticide-consuming countries.

Figure 4.

Representation of the admissible upper limits for heavy metal concentrations in plants and soil as per WHO; reprinted from [16].

Figure 5.

Statistical representation of averaged pesticide use per year (kg/ha) in India and other countries between 2010 and 2014; reprinted from [16].

These heavy metals cause adverse effects on human health as well as the environment. The risk of heavy metal contamination in the environment has recently been rapidly increasing and causing havoc, especially in the agricultural industry, as a result of the accumulation of heavy metals in the soil and their plant uptake. An array of elements in the ecosystem is what is referred to as heavy metal buildup. The primary site of interaction for heavy metal ions transported from the soil is plant roots. They frequently bind and link the contaminants in the soil, which lowers their bioavailability [16].

The trace metals present in the soil eventually turn into soil contaminants as their rates of transfer via artificial cycles are faster than natural ones and are easily transferred from mines to random environmental locations where there is a higher risk of direct exposure. The higher rate of concentration of these trace metals in discarded products than in the surrounding environment is also a major factor contributing to soil contamination [17]. Trace elemental concentrations of heavy metals such as Pb, Zn, Mn, Ni, Cn, Cr, Fe, Cd, and As arising from various industries involving textiles, tannery, plastic, furniture, chemicals, dyes, paint, pharmaceuticals, batteries, and fertilizers from various parts of the country have been listed below in Table 1.

Sl No.	Type of industry	City	Trace element concentration (mg kg^−1)
			Pb	Zn	Mn	Ni	Cn	Cr	Fe	Cd	As
1.	Textile	Haridwar	191	_	668	_	109	568	308	83.6	_
2.	Tannery	Haridwar	_	_	097	_	0.04	744	37.7	0.04	_
3.	Textile, plastic, furniture, industries	Thane-Belapur	—	191	—	184	105	521	—	—	—
4.	Chemicals, dyes, textile, paint industries	Rajasthan	293	—	—	136	298	240	—	—	_
5.	Chemicals, pharmaceutical, batteries	Hyderabad	65.0	313	—	45.0	193	433	—	0.70	33.0
6.	Tannery, textile, fertilizer, rerolling and casting, chemicals paints plastics	Jajmau, Unnao	38.3	159.9	—	—	42.9	265	—	—	—

Table 1.

Representation of trace element concentrations in soils impacted by diverse industrial operations in several cities across the nation; reprinted from [13].

7. Methods used for analysis of soil samples for trace metal analysis

The most commonly used methods for preparation of soil samples for trace metal analysis are as follows:

1. ISO 11466: Air-dried soil of 1 gram is weighed precisely to the nearest 0.001 g. With 1 ml of water, it is moistened. About 21 ml of HCl and 7 ml of HNO₃ are added. The sample is allowed to stay at room temperature for around 16 hours. The sample is then annealed for a further 2 hours at 180 to 200°C. The remaining is filtered and transferred quantitatively to a 50-ml measuring flask.

2. EPA Method 3051: In polytetrafluoroethylene (PTFE) containers, 1 g of air-dry soil is precisely weighed to within 0.001 gram. HNO₃ (10 ml) is then added. The samples are processed by a microwave system before being analyzed.

3. BDS ISO 14869-1: A platinum pot containing 0.250 g of air-dry soil is weighed, and then it is heated in an electric oven. The oven’s temperature is progressively raised to 450°C for an hour before being maintained for 3.5 hours before being lowered to room temperature. The cooled sample is mixed with 5.0 ml HF and 1.0 ml HCIO₄, and it is then heated on a hotplate for 30 to 40 minutes until the steams from the HCIO₄ and SiFa vanish. After the melting pot has cooled, 1.0 ml of nitric acid and 5.0 ml of water are added. Careful heating can assist in the sediment’s disintegration [18].

To determine the heavy metal content in the soil samples, the samples prepared using any of these above-mentioned methods must be introduced to desired instrumental techniques.

8. Forensic soil science

Soil science or pedology involves the study of soil as a naturally occurring resource material which enables farming and building and provides structural support. The term pedology has been coined from a Greek word “pedon” which means soil, whereas forensic soil science can be used to address legal interrogations by studying the morphological features of the soil, soil mapping, and its chemical and biological composition. The biological, chemical, physical, mineralogical, and hydrological aspects of soils are intricate and dynamic. Forensic interpretations and comparisons can be made by the determination of the origin of the soil and its characterization. Understanding crime scene protocols, evidential requirements for forensic personnel, and the different types of legal bounds that pertain to forensic work are all crucial steps in the move from conventional soil research to forensic soil science. Understanding the wide variety of naturally occurring and artificial soils, their formation and thorough analysis are crucial since it enables precise forensic interpretations [19]. This chapter discusses the standardized procedures including the conventional as well as modern methodologies used for the forensic testing of soils.

9. Role of soil in forensic science

The use of soil as trace evidence is crucial in forensic investigations. These trace pieces of evidence are strong proof or corroborative evidence when they clearly link one or more individuals to the crime. Soil, which is a physically transferable material, has a tendency to be carried away from one place to another by wind or objects such as shoes, textiles, tools. In forensic crime scene investigations, it can be used to link particular crime scenes to criminal suspects by determining the nature and point of contact (Figure 6).

Figure 6.

Collection of soil samples from a crime scene for forensic analysis.

The presence of foreign material, such as trace elements and paint chips, may provide a degree of individuality to soil collected from a crime scene and may provide vital information to forensic soil investigations in addition to the study of rocks and minerals [20].

The importance of soil as forensic trace evidence in allied cases has been listed below:

1. It provides additional information in criminal cases involving

1. Hit and run cases

2. Sexual abuse and sexual assaults

3. Abduction

4. Murder

5. Wildlife crimes

6. Illegal mining of land

7. Theft.

1. It provides additional information in civil cases involving

1. Drug shipment pathway

2. Construction.

   1. It provides additional information for intelligence work involving

3. Identification of geographical location of the crime scene

4. Narrowing down the area of search

5. Determination of the presence or absence of a suspect or a vehicle at a particular location.

10. Trace evidential value of soil transferred from one surface to the other

As Dr. Edmond Locard in the early twentieth century stated that “Every contact leaves a trace” which later came to be known as Locard’s principle of exchange, which holds that when two surfaces make physical contact with one another, there is a possibility of mutual material transfer. Trace evidence is used to link persons or things to places, other people, or things, and it frequently serves as the initial point for a search or as a lead for a specific path of inquiry. Forensic experts often spot soil components on the surface of objects such as vehicle tires, shoes, tools, clothing, carpets. Such soil samples must be first spotted and identified on the enquiry-related objects followed by the systematic documentation, collection, and preservation of the soil samples in order to safeguard the integrity of the evidence for reanalysis and characterization. The soil samples collected from questioned objects, such as vehicles and shoes, are then matched with the control samples [21].

11. Forensic protocol and methods for comparison of soil traces

This chapter offers guidance on systematic approaches to be used in particular examinations but does not include any standard procedures followed universally. It offers a summary of the essential criteria and suggestions for choosing the most suitable analytical methods and the optimal sequence in which they should be used in forensic cases. A flowchart describing the steps involved in the analysis of forensic soil samples and presence of trace metals in them is depicted below [22] (Figure 7).

Figure 7.

Flowchart describing the steps involved in the analysis of forensic soil samples and presence of trace metals in them.

12. Instrumental techniques used for the soil sample analysis

Particularly soil found on polluted sites is a very complex material to analyze. Sand, limestone, clay, microorganisms, trace metals, etc., or a combination of several minerals may make up the actual soil matrix. From hazardous waste to highly toxic medical waste, mercury, explosives, etc., to comparatively safe construction materials, the variety of pollutants is likewise wide. Significant innovations in trace evidence have been made possible by improvements in microscopy, chemical analysis, improved instrumental procedures, and database technology for evidence comparison. The increase in accessibility and availability of these techniques has made it easy to compare trace evidence samples like soil against established standards to produce reliable results with compatible classification rates. The World Reference Base (WRB) and Soil Taxonomy are the two most extensively used international soil classification standards. Various facets of granular soils or soil samples such as their color, texture, consistency, moisture content, and the structural arrangement of each particle vary for soil samples from different geographical regions. These soil samples are complex and have varying mineralogy, composition, and quantity of organic and inorganic components. The soil material or earth material is often subjected to change with the passage of time and exposure to external environmental conditions due to which they exhibit variation at different time intervals. There is a need to monitor these variations for effective forensic soil analysis which is achieved by the use of various characterization techniques that are often performed in the laboratory. To distinguish soil from various geographical places and to assess its color, texture, particle size, etc., a range of methodologies can be utilized, a few of which are discussed in this chapter [23].

12.1 Microscopic examination

The main mineral components of soil specimens are frequently identified and quantified via microscopic inspection. A forensic expert may be able to positively identify a plant that produced the pollen grains, for instance, if a soil sample under investigation contains pollen grains or seeds. If the obtained plant mixture seems to be unusual, it may be possible to signify with high confidence level that the questioned specimen originated from the same location. This can be done by comparing it to known or control soil samples collected from an area where those plants are present (Figure 8).

Figure 8.

Microscopic set up for the examine soil specimens.

A stereo microscope is often used for the preliminary visual examination of the specimens at low magnification ranging from 10X to 40X. A forensic expert can get a general sense of the soil sample from this visual inspection and record the existence and abundance of several foreign material such as metal fragments, glass particles, and pollen grains. Particles ranging from a size of down to 10 micrometers to large as the physical limits of the microscope stage can be seen under the stereo microscope. The grid features often built in the eyepieces make it easier to count individual particles in the sample and provide information regarding the particle size distribution.

12.2 X-ray fluorescence (XRF)

The extend of soil degradation or weathering is obtained by assessing the elemental composition of the soils. XRF, which is a non-destructive technique, enables the rapid analysis of elemental composition of the soil consisting of heavy metals and other trace elements (Figure 9).

Figure 9.

Graphical representation of soil spectrum obtained using XRF showing the ranges of heavy metals present in the soil; reprinted from [24].

For heavy metals present in the soil, XRF offers multi-elemental analysis with good sensitivity [25]. However, due to insufficient sensitivity for some trace elements, X-ray fluorescence spectrometry has been restricted in its application for trace element identification.

12.3 XRD

Over the past two decades, there has been a significant advancement in X-ray-based techniques for elemental analysis. There are now many different scientific fields that utilize X-ray-based methodologies. As these techniques can quantify trace components in situ, they are among the most potent analytical techniques available. Unlike many other analytical techniques, there is no requirement for chemical extraction for this technique. Moreover, secondary minerals created by weathering processes could have crystallographic properties that have a significant impact on the chemical and physical properties of soils. Certain soil minerals, such as phyllosilicates, are referred to as expansible because they adsorb water and cations on internal surfaces within the crystal structure itself. The method that is most frequently used in soil mineralogical analysis and study of the crystal structure is X-ray diffraction (XRD) [26, 27].

12.4 SEM-EDX

SEM-EDX is a rapid surface analysis method that gives surface morphological features in the form of photos in high resolution showing the topography of the sample analyzed. The macro-elements present in the soil such as nitrogen, carbon, phosphorous, oxygen, sulfur, potassium, chlorine, sodium, magnesium, aluminum, silicon, calcium, titanium, iron, manganese, and nickel can be detected and quantified using this instrumental technique (Table 2).

S. No.	Na	Mg	Al	Si	P	K	Ca	Ti	Fe	Mn	Ni
1	0.39	0.33	6.09	19.16	0.34	1.27	1.87	0.57	4.81	0.17	0.07
2	0.13	0.24	5.61	16.45	0.25	0.99	1.53	0.46	4.31	0.11	0.04
3	0.11	0.31	4.43	12.78	0.2	1.02	0.86	0.25	2.32	0.08	0.06
4	0.25	0.36	6.22	19.09	0.39	2.13	1.63	0.41	4.75	0.18	0.08
5	0.37	0.48	6.79	24.42	0.46	2.57	2.34	0.61	6.4	0.21	0.14
6	0.21	0.28	5.96	17.37	0.33	2.01	1.87	0.5	4.71	0.18	0.07
7	0.24	0.25	5.23	15.53	0.18	0.92	0.84	0.38	2.77	0.09	0.04
8	0.12	0.26	5.9	13.77	0.26	0.74	0.9	0.44	4.05	0.08	0.05
9	0.26	0.17	5.42	16.4	0.12	0.82	0.84	0.35	3.88	0.10	0.12
10	0.17	0.23	5.14	14.04	0.28	1.77	1.48	0.48	4.74	0.21	0.06
11	0.33	0.27	5.36	18.4	0.26	1.49	0.83	0.45	4.77	0.16	0.05
12	0.14	0.44	6.09	15.99	0.29	1.29	1.41	0.34	3.02	0.12	0.04
13	0.16	0.33	5.09	17.26	0.21	1.7	1.57	0.37	3.95	0.09	0.05
14	0.35	0.38	9.5	16.15	0.23	1.37	1.64	0.48	5.78	0.16	0.06
15	0.36	0.49	7.36	17.66	0.37	2.18	1.57	0.58	5.15	0.21	0.12
16	0.18	0.27	5.98	17.37	0.16	1.41	1.33	0.25	3.81	0.12	0.08
17	0.25	0.33	8.49	15.42	0.18	1.24	1.26	0.33	3.04	0.11	0.07
18	0.11	0.28	5.21	17.57	0.17	1.69	1.39	0.36	3.44	0.12	0.05
19	0.26	0.38	4.53	12.86	0.29	1.95	1.65	0.56	4.06	0.19	0.09
20	0.13	0.35	3.68	19.15	0.16	1.71	1.58	0.32	3.96	0.17	0.06
21	0.19	0.32	4.76	13.97	0.11	0.68	1.65	0.36	3.25	0.18	0.06
22	0.27	0.28	7.59	14.59	0.21	1.26	1.47	0.31	5.94	0.11	0.07
23	0.17	0.29	5.27	16.2	0.29	2.17	1.01	0.41	5.91	0.09	0.07
24	0.1	0.26	5.85	17.86	0.35	1.06	1.63	0.41	4.41	0.22	0.16
25	0.24	0.35	6.19	17.09	0.26	1.84	1.11	0.36	4.99	0.1	0.05
26	0.27	0.12	4.56	12.19	0.16	1.35	1.27	0.31	2.42	0.12	0.04
27	0.31	0.31	9.85	17.61	0.34	1.6	2.3	0.46	5.63	0.18	0.16
28	0.19	0.33	5.44	17.88	0.30	1.86	0.85	0.32	4.53	0.11	0.05
29	0.21	0.32	6.19	20.34	0.60	1.62	2.17	0.35	4.26	0.12	0.10
30	0.36	0.48	10.31	22.31	0.49	3.22	2.72	0.53	5.22	0.21	0.15

Table 2.

Concentration of trace elements in soil, percentage of soil samples from Korba; reprinted from [28].

The table above depicts the various trace metals present in soil samples from Korba along with their concentration in percentage. The elemental composition, size, and morphology of soil minerals are all highly variable due to which they vary in metal concentration from region to region depending upon the activities occurring in that region. For example, due to extensive coal mining and burning, the soil found at the surface of the Korba basin was discovered to have a higher metal concentration than other parts of the nation [28].

12.5 IR spectroscopy

The physicochemical characteristics of soil can be analyzed using this spectroscopic technique. As reported in the literature [29], the total carbon, total nitrogen, their ratio, etc., can all be investigated using infrared spectroscopy. Highly detailed spectrum information may be a benefit of the high-resolution data. For more precise object recognition and identification, the sensing capabilities of visible and near-infrared spectral imaging systems can be utilized. These techniques have significantly increased in recent years. The complex optical features from the sample can be captured with higher spectral and spatial resolution using this spectroscopic technique [30].

12.6 Atomic absorption spectrometry

Atomic absorption spectrometry developed is an analytical technique widely used to identify metals in materials. It is incredibly dependable and easy to use. The quantity of trace metals present in the soil samples is also measured. It is the most common method used to analyze metals in soil. The solvent is vaporized, and the sample is broken up into its constituent atoms using an acetylene flame. Light from a hollow cathode lamp passes through the cloud of atoms created by the atomization process and is absorbed by the target atoms. In a study conducted by Senthamilselvi P et al., they collected and analyzed soil samples from three regions of a steel plant in Tamil Nadu state, using this technique, the results of which are listed below in Table 3.

Sample No.	Lead (Pb) concentration range ppm	Chromium(Cr) concentration range ppm	Cadmium (Cd) concentration range ppm x10–4	Manganese (Mn) concentration range ppm
Sample 1	0.00438	0.00401	4.33	0.247
Sample 2	0.00168	0.00591	3.17	0.221
Sample 3	0.00129	0.00592	3.04	0.272

Table 3.

Heavy metal concentration in soil samples from three regions of Tamil Nadu; reprinted from [31].

The findings of the study demonstrated that the amount of Cr in the soil sample was within the WHO-permitted level. All three samples have levels of Cd, Mn, and Pb that are less than the acceptable limit and lower than those of Cr. Assessing the concentration of heavy metals in soil samples helps to identify if they are present in amounts that may cause a threat to human health [31].

12.7 ICP-OES/MS

There is a need for predictive soil extraction techniques for pollutants including lead, mercury, arsenic, and other toxic elements due to the rise in toxic heavy metal that leads to pollution of soils from industrial sources, mining, disposal of waste on fields or bare land, etc. There is no such extraction process for several of these components. The plant uptake of these trace metals such as lead and cadmium in trace concentration beyond the detection limit of some instruments can be analyzed using mass spectroscopic techniques such as ICP-MS or ICP-OES [32]. The device employs temperatures as high as 10,000°C to effectively atomize even the most refractory materials for ICP/ICP-AES analysis. As a result, the limits of detection of the system can be in orders of magnitude lower than FAAS methods (usually at the 1–10 parts-per-billion level). In a single sample run of less than a minute, the ICP method may simultaneously screen for up to 60 elements without any loss of precision or detection limits [13].

13. Conclusion

It is vital to understand how trace metals affect the environment and living organisms. A well-established area of forensic science that is crucial to both criminal and environmental forensics is soil science, which analyzes trace metals found in soil. In sophisticated criminal investigations where traditional forensic techniques fall short of providing sufficient evidence, soil samples are often used as a physical evidence. The obtained soil data can be utilized for intelligence work or as an important evidence in the court. In order to locate and stop environmental crimes, environmental forensics is essential. In order to locate and prevent environmental crimes, environmental forensics is essential. In order to identify and locate the crime scene, experts working in the field of environmental forensics compare the questioned soil samples with the existing soil databases, control samples, or natural soil samples from the scene of occurrence. In the majority of the crime scenes, soil samples and controlled samples are collected from the crime scene and control sites, respectively, since these are the locations from which dirt or soil can be transferred to footwear, tools, clothes, tires, etc. The chapter highlights the significance of trace metals present in the soil found at a crime scene, the analysis of which helps; to narrow down the area of search, to locate the crime scene, to determine the origin of soil, aids in intelligence work and can be produced as an evidence in court. To validate the testimony of soil as proof, a comparative analysis of soil samples taken from several sites around a crime scene will be beneficial.

Conflict of interest

The authors declare no conflict of interest.