# Heavy Metal Pollution and Assessment of the Bioaccumulation Potential of Earthworms from the Soil of Punjab, Pakistan

## TL;DR

This study assesses heavy metal pollution in Punjab, Pakistan, and evaluates how earthworms absorb metals, finding that cadmium poses the greatest risk and earthworms show limited effectiveness as pollution indicators.

## Contribution

The study provides new insights into the bioaccumulation potential of earthworms in a specific region and highlights cadmium as a major soil contamination risk.

## Key findings

- Cadmium was found to pose the greatest risk with high contamination levels in many areas of Punjab.
- Earthworms accumulated certain metals like calcium, copper, and manganese but not highly toxic ones like cadmium and lead.
- Bioaccumulation showed weak relationships with soil metal concentrations and environmental factors like pH and moisture.

## Abstract

The presence of heavy metals in soil can create a serious problem for soil life and can reduce agricultural productivity and have serious negative impacts on human health. Here, the soil contamination of Punjab, Pakistan, was assessed based on the contaminants of heavy metals and was assessed based on how much these heavy metals move into the earthworm’s body. This study assessed soil contamination across Punjab, Pakistan, and examined how toxic metals move from the soil into earthworms, which are commonly used as indicators of the soil condition, although their effectiveness can vary depending on the environmental conditions. Soil and earthworm samples were collected from nineteen different locations and tested for several metals, including cadmium, lead, copper, zinc, iron, and nickel. The findings revealed that cadmium posed the greatest risk, with many areas showing high to very high levels of contamination, while most other metals remained within safe limits. Earthworms were able to absorb certain metals from the soil, particularly calcium, nickel, manganese, strontium, and copper, although the degree of uptake varied among sites. However, they did not show strong accumulation of highly toxic metals such as cadmium and lead. Environmental factors such as soil moisture, pH, and salinity showed only weak relationships with metal uptake, indicating that these factors explained only a small portion of the variation. The study concludes that cadmium pollution represents a major threat to soil quality in Punjab. Because bioaccumulation showed weak relationships with soil metal concentrations and physicochemical properties, earthworms in this region demonstrated limited effectiveness as indicators of toxic heavy metal accumulation. The limited internal accumulation of toxic metals suggests possible physiological regulation or tolerance, although the long-term ecological effects require further investigation. Further controlled studies are needed before confirming their suitability for soil bioremediation, waste management, and vermicompost production.

Earthworms are widely used as biological indicators of soil contamination and are often referred to as the ‘intestine of the soil’; however, their effectiveness can vary across regions and environmental conditions. The performance of earthworms as indicators of soil pollution may vary among regions, and there is a need to assess their reliability under specific environmental conditions. So, in this study, soil and earthworm samples were collected from 19 locations across Punjab, Pakistan, to assess heavy metal pollution and bioaccumulation using geoaccumulation indices (Igeo) and bioaccumulation factors (BAF). Soil and earthworm tissues were digested and analyzed for heavy metals such as Sr, Zn, Ca, Cu, Fe, Mn, Co, Cr, Pb, Cd and Ni using ICP-MS. According to Igeo, cadmium frequently classified soils as strongly to extremely polluted, with values up to 4.5 in Bahawalnagar, whereas most other metals fell within the unpolluted to moderately polluted classes. Earthworms showed clear bioaccumulation (BAF > 1) for several essential or poorly regulated elements, particularly Ca, Mn, Cu and Sr; the maximum BAF for Ca reached 8.18 in Faisalabad. In contrast, group-1 carcinogenic metals (Pb, Cd and Cr) generally did not exhibit strong bioaccumulation. Relationships between BAF and soil metal concentrations or abiotic properties (moisture, pH, electrical conductivity, total dissolved and suspended solids) were generally weak (R2 typically ≤ 0.32), indicating that these factors explain only a modest fraction of the variation in metal uptake. Overall, the results highlight cadmium as the primary soil contamination risk in the surveyed areas. Because bioaccumulation showed weak relationships with soil metal concentrations and physicochemical parameters, earthworms in this study demonstrated limited effectiveness as bioindicators of toxic heavy metal accumulation. The limited internal accumulation of highly toxic metals suggests possible physiological regulation or tolerance, although the long-term ecological implications require further investigation. Further controlled studies are necessary to better evaluate their ecological role and potential applications in soil remediation, waste management, and vermicompost production.