Report on Zinc Bioaccumulation and Health Risks in Vegetables Irrigated with Different Water Sources in Punjab, Pakistan

Abstract

Zinc (Zn) is an essential micronutrient for plant growth and productivity, but its excessive accumulation poses environmental and health risks. This study evaluated Zn uptake in kitchen vegetables irrigated with wastewater (WW), canal water (CW), and tube well water (TW) across three ecological sites in Punjab, Pakistan. Significant differences in Zn concentrations were observed in water, soil, and vegetables, with the order of Zn concentration being TW

Introduction

The increasing use of untreated wastewater for irrigation in water-scarce developing regions, including Pakistan, is driven by urban population growth and food demand. While wastewater provides nutrients, it introduces heavy metals like zinc, which can bioaccumulate in soils and crops, posing risks to plant health and human safety. Zinc is vital for enzymatic processes in plants and immune function in humans but becomes toxic at high concentrations, affecting crop yield and causing health issues. This dual role necessitates monitoring Zn levels to ensure sustainable agriculture and food safety, supporting SDG 12 (Responsible Consumption and Production) and SDG 15 (Life on Land).

Materials and Methods

Study Sites

1. Three sites in Punjab, Pakistan: Sargodha (Site 1), Sahiwal (Site 2), and Shahpur (Site 3).
2. Three irrigation sources at each site: tube well water (TW, control), canal water (CW), and wastewater (WW).
3. Selected vegetables: Spinacia oleracea, Brassica campestris, Coriandrum sativum, and Mentha spicata.

Sampling and Analysis

• Collected 27 water samples, soil samples at 5–30 cm depth, and 108 vegetable samples during the crop season (Nov-Dec 2022).
• Water samples preserved with HNO3 and stored under controlled conditions.
• Soil and vegetable samples oven-dried, homogenized, digested using acid digestion methods.
• Zinc concentrations measured by Atomic Absorption Spectrophotometry (AAS) with quality control using certified reference materials.

Pollution and Health Risk Indices

1. Transfer Factor (TF): Ratio of Zn concentration in vegetables to soil.
2. Single Pollution Index (SPI): Ratio of Zn concentration in soil to reference threshold.
3. Enrichment Factor (EF): Assessment of Zn enrichment in soil and vegetables.
4. Daily Intake of Metals (DIM): Estimated daily Zn intake from vegetables.
5. Health Risk Index (HRI): Ratio of daily metal intake to oral reference dose.
6. Target Hazard Quotient (THQ): Non-carcinogenic risk assessment.
7. Geoaccumulation Index (Igeo): Soil contamination classification.

Results

Zinc Concentration in Water

• Zn concentrations ranged from 0.7757 to 1.4521 mg/L, significantly differing by site and irrigation source (p
• Highest Zn levels found in wastewater, followed by canal water, and lowest in tube well water.
• All Zn levels were below FAO/WHO permissible limits, supporting SDG 6 (Clean Water and Sanitation).

Zinc Concentration in Soil

• Zn concentrations in soil ranged from 5.36 to 41.3 mg/kg, significantly affected by site and irrigation source (p
• Wastewater-irrigated soils showed the highest Zn accumulation, up to 31.12 mg/kg at Site 1.
• Zn accumulation in wastewater-irrigated soils was 199% higher than in tube well water-irrigated soils.
• Geoaccumulation index indicated soils were unpolluted under TW and CW but moderately polluted under WW irrigation.

Zinc Concentration in Vegetables

• Zn concentrations in vegetables ranged from 4.11 to 24.86 mg/kg, significantly influenced by vegetable type, site, and irrigation source (p
• Highest Zn found in Spinacia oleracea irrigated with wastewater at Site 1.
• Wastewater irrigation increased Zn levels in vegetables by up to 118% compared to tube well water.
• All Zn levels were below FAO/WHO permissible limits, indicating food safety.

Pollution and Health Risk Indices

• Transfer Factor (TF) ranged from 0.46 to 1.21, indicating bioaccumulation of Zn in vegetables.
• Single Pollution Index (SPI) values ranged from 0.12 to 0.93, with highest contamination risk in Spinacia oleracea.
• Enrichment Factor (EF) ranged from 0.2 to 0.54, showing moderate to high soil enrichment.
• Health Risk Index (HRI) ranged from 0.06 to 0.38, and Target Hazard Quotient (THQ) ranged from 0.014 to 0.085, both below risk thresholds.
• These results indicate no significant health risks to consumers, contributing to SDG 3 (Good Health and Well-being).

Correlation and Statistical Analysis

• Strong positive correlations observed between Zn concentrations in water, soil, and vegetables across sites and treatments.
• Principal Component Analysis (PCA) showed human activities, especially wastewater irrigation, as major contributors to Zn accumulation.
• Cluster analysis and heatmaps confirmed the relationship between soil contamination and vegetable Zn content.

Discussion

The study highlights the dual role of zinc as an essential nutrient and a potential contaminant. The use of untreated wastewater for irrigation contributes to Zn accumulation in soils and vegetables, posing long-term environmental and health risks. Although current Zn levels are below permissible limits, continuous use of wastewater may lead to excessive accumulation, threatening soil health and food safety. These findings underscore the importance of sustainable water management and regular monitoring to align with SDG 2 (Zero Hunger), SDG 3 (Good Health and Well-being), SDG 6 (Clean Water and Sanitation), and SDG 12 (Responsible Consumption and Production).

Conclusion

• Zinc concentrations in water, soil, and vegetables irrigated with wastewater, canal water, and tube well water were below FAO/WHO safety limits.
• Wastewater irrigation resulted in significantly higher Zn accumulation in soils and vegetables compared to other water sources.
• Health risk assessments indicate no immediate toxic risk to consumers; however, long-term wastewater use may cause Zn buildup exceeding safe thresholds within 10–20 years.
• Recommendations include avoiding exclusive reliance on wastewater for vegetable irrigation and implementing regular monitoring to prevent Zn bioaccumulation.
• These measures support sustainable agricultural practices and public health, contributing to the achievement of multiple SDGs.

1. Sustainable Development Goals (SDGs) Addressed or Connected

1. SDG 2: Zero Hunger
   • The article discusses the impact of wastewater irrigation on vegetable crops, which relates to food security and sustainable agriculture.
2. SDG 3: Good Health and Well-being
   • The study assesses health risks associated with zinc bioaccumulation in vegetables, addressing food safety and human health concerns.
3. SDG 6: Clean Water and Sanitation
   • The article highlights the use of untreated wastewater for irrigation and its implications on water quality and soil contamination.
4. SDG 12: Responsible Consumption and Production
   • Monitoring and managing zinc accumulation in agricultural systems promotes sustainable production practices.
5. SDG 15: Life on Land
   • The article addresses soil health, contamination, and ecosystem impacts due to zinc accumulation from wastewater irrigation.

2. Specific Targets Under the Identified SDGs

1. SDG 2: Zero Hunger
   • Target 2.4: By 2030, ensure sustainable food production systems and implement resilient agricultural practices that increase productivity and production.
2. SDG 3: Good Health and Well-being
   • Target 3.9: By 2030, substantially reduce the number of deaths and illnesses from hazardous chemicals and air, water, and soil pollution and contamination.
3. SDG 6: Clean Water and Sanitation
   • Target 6.3: Improve water quality by reducing pollution, minimizing release of hazardous chemicals and materials, and substantially increasing water recycling and safe reuse globally.
4. SDG 12: Responsible Consumption and Production
   • Target 12.4: By 2020, achieve the environmentally sound management of chemicals and all wastes throughout their life cycle, in accordance with agreed international frameworks.
5. SDG 15: Life on Land
   • Target 15.3: By 2030, combat desertification, restore degraded land and soil, including land affected by desertification, drought, and floods.

3. Indicators Mentioned or Implied to Measure Progress

1. Zinc Concentration Levels
   • Measured in water (mg/L), soil (mg/kg), and vegetables (mg/kg) to assess contamination and accumulation.
2. Transfer Factor (TF)
   • Indicator of bioaccumulation of zinc from soil to vegetables.
3. Single Pollution Index (SPI)
   • Evaluates the degree of zinc pollution in soil relative to threshold values.
4. Enrichment Factor (EF)
   • Assesses the level of zinc enrichment in soil and vegetables compared to background levels.
5. Health Risk Index (HRI)
   • Estimates potential health risks to consumers from zinc intake through vegetables.
6. Target Hazard Quotient (THQ)
   • Assesses non-carcinogenic health risks associated with zinc exposure.
7. Geoaccumulation Index (Igeo)
   • Determines the contamination level of zinc in soils.
8. Daily Intake of Metals (DIM)
   • Quantifies daily zinc intake from vegetable consumption.

4. Table of SDGs, Targets, and Indicators

Source: nature.com