Low mercury risks in paddy soils across the Pakistan.

Mercury (Hg) is a globally distributed heavy metal. Here, we study Hg concentration and isotopic composition to understand the status of Hg pollution and its sources in Pakistan's paddy soil. The collected paddy soils (n = 500) across the country have an average THg concentration of 22.30 ± 21.74 ng/g. This low mean concentration suggests Hg pollution in Pakistan was not as severe as previously thought. Meanwhile, samples collected near brick kilns and industrial areas were significantly higher in THg than others, suggesting the influence of Hg emitted from point sources in certain areas. Soil physicochemical properties showed typical characteristic of mineral soils due to the study area's arid to semi-arid climate. Hg stable isotopes analysis, depicted mean Δ199Hg of -0.05 ± 0.12‰ and mean δ202Hg -0.45 ± 0.35‰, respectively, for contaminated sites, depicting Hg was primarily sourced from coal combustion by local anthropogenic sources. While uncontaminated sites show mean Δ199Hg of 0.15 ± 0.08‰, mean Δ200Hg of 0.06 ± 0.07‰ and mean δ202Hg of -0.32 ± 0.28‰, implying long-range transboundry Hg transport through wet Hg(II) deposition as a dominant Hg source. This study fills a significant knowledge gap regarding the Hg pollution status in Pakistan and suggests that the Hg risk in Pakistan paddies is generally low.

Effects of selenium on biogeochemical cycles of cadmium in rice from flooded paddy soil systems in the alluvial Indus Valley of Pakistan.

This study delves into the pollution status, assesses the effects of Se on Cd biogeochemical pathways, and explores their interactions in nutrient-rich paddy soil-rice ecosystems through 500 soil-rice samples in Pakistan. The results showed that 99.6 % and 12.8 % of soil samples exceeded the World Health Organization (WHO) allowable Se and Cd levels (7 and 0.35 mg/kg). In comparison, 23 % and 6 % of the grain samples exceeded WHO's allowable Se and Cd levels (0.3 and 0.2 mg/kg), respectively. Geographically Weighted Regression (GWR) model results further revealed spatial nonstationarity, confirming diverse associations between dependent variables (Se and Cd in rice grain) and independent variables from paddy soil and plant tissues (root and shoot), such as Soil Organic Matter (SOM), pH, Se, and Cd concentrations. High Se:Cd molar ratios (>1) and a negative correlation (r = -0.16, p < 0.01) between the Cd translocation factor (Cd in rice grain/Cd in root) and Se in roots suggest that increased root Se levels inhibit the transfer of Cd from roots to grains. The inverse correlation between Se and Cd in paddy grains was further characterized as Se deficiency, no risk, high Cd risk, Se risk, Cd risk, and Se-Cd co-exposure risk. There was no apparent risk for human co-consumption in 42.6 % of grain samples with moderate Se and low Cd. The remaining categories indicate differing degrees of risk. In the study area, 31 % and 20 % of grain samples with low Se and Cd indicate Se deficiency and risk, respectively. High Se and low Cd levels in rice samples suggest a potential hazard for severe Se exposure due to frequent rice consumption. This study not only systematically evaluates the pollution status of paddy-soil systems in Pakistan but also provides a reference to thoroughly contemplate the development of a scientific approach for evaluating human risks and the potential dangers associated with paddy soils and rice, specifically in regions characterized by low Se and low Cd concentrations, as well as those with moderate Se and high Cd concentrations. SYNOPSIS: This study is significant for understanding the effects of Se on Cd geochemical cycles and their interactions in paddy soil systems in Pakistan.

DOM influences Hg methylation in paddy soils across a Hg contamination gradient.

Rice paddies provide optimum conditions for Hg methylation, and paddy soil is a hot spot for Hg methylation and the predominant source of methylmercury (MeHg) accumulated in rice grains. The role of dissolved organic matter (DOM) in controlling Hg bioavailability and methylation in rice paddy systems remains unclear. Paddy soils from eight various cultivation sites in China were chosen to investigate the variations in soil DOM and the influence of DOM concentration and optical characteristics on Hg methylation in rice paddy systems. In the present study, 151 rhizosphere soil samples were collected, and UV-Vis absorption and fluorescent spectroscopy were used to identify the optical properties of DOM. The relationship between MeHg and DOM's optical property indices revealed the production of MeHg consumes lower molecular weight DOM. Moreover, the correlation between DOM concentration and its optical characteristics highlighted the significant role of humic components on MeHg variability in paddy soil. Variation and correlation results demonstrated the allochthonous origin of DOM in the Hg-contaminated soil, with a higher molecular weight and humic character of DOM, as well as the dominant role of autochthonous DOM in promoting Hg methylation in uncontaminated soil. The current study indicated that soil organic matter and its dissolved fractions tend to limit Hg bioavailability and subsequently diminish MeHg production in contaminated paddy soils. Furthermore, the leading roles of allochthonous DOM in protecting MeHg from degradation and autochthonous DOM signatures in enhancing MeHg production in paddy soils. Overall, these findings provide insight into the correlative distributions of DOM and Hg along a Hg concentration gradient in paddy soil, thereby highlighting their potential role in controlling Hg bioavailability and regulating Hg methylation in the soil ecosystems.

Chromium contamination in paddy soil-rice systems and associated human health risks in Pakistan.

Chromium (Cr) contamination in paddy soil-rice systems threatens human health through the food chain. This study used a new dataset of 500 paddy soil and plant tissue samples collected in the rice-growing regions of Sindh and Punjab Provinces of Pakistan. Overall, 97.4% of grain samples exceeded the Cr threshold values of 1.0 mg kg-1, determined by the China National Food Standard (CNFS). The Cr in paddy soil, 62.6% samples exceeding the China natural background threshold value (90 mg kg-1) for Cr concentration in paddy soil, and lower than the (pH-dependant > 7.5 threshold value for Cr 350 mg kg-1) as determined by China Environmental Quality Standards (EQSs) for paddy soil (GB15618-2018). Geographically weighted regression (GWR) modelling showed spatially nonstationary correlations, confirming the heterogeneous relationship between dependent (rice grain Cr) and independent paddy soil (pH, SOM, and paddy soil Cr) and plant tissue variables (shoot Cr and root Cr) throughout the study area. The GWR model was then used to determine the critical threshold (CT) for the measured Cr concentrations in the paddy soil system. Overall, 38.4% of paddy soil samples exceeding CT values confirm that the paddy soil Cr risk prevails in the study area. Furthermore, the GWR model was applied to assess the loading capacity (LC), the difference between the CT, and the actual concentration of Cr in paddy soil. Loading capacity identified potential paddy soil Cr pollution risk to rice grain and assessed the risk areas. Overall LC% of samples paddy soil Cr risk areas grade: low-risk grade I (34.6%); moderate-risk grade II (15.8%); high-risk grade III (11.2%); and very high-risk grade IV (38.4%) have been assessed in the study area. The human health index, total hazard quotient (THQ ≪ 1), indicates no potential health risk originating from Cr exposure to the population. However, the excess Cr level in paddy soil and rice grain is still a concern. The current study's results are also valuable for the national decision-making process regarding Cr contamination in the paddy soil-rice system.

Unravelling the interactive effect of soil and atmospheric mercury influencing mercury distribution and accumulation in the soil-rice system.

Mercury (Hg) accumulation in rice is an emerging health concern worldwide. However, sources and interactions responsible for Hg species accumulation in different rice tissues are still uncertain. Four experimental plots were carefully designed at an artisanal Hg mining site and a control site to evaluate the effect of atmospheric and soil Hg contents on Hg accumulation in rice. We showed that inorganic Hg (IHg) contents in rice tissues grown either in contaminated or control site soil (non-contaminated soil) were higher at Hg artisanal mining site than those at the control site. Elevated total gaseous mercury (TGM) levels in ambient air were the predominant source of IHg to rice at the Hg mining area. Methylmercury (MeHg) concentrations in rice plant tissues increased in proportionality with MeHg contents in paddy soil. Our results suggest that both atmosphere and soil Hg sources have been impacted the IHg accumulation in rice. Above ground rice tissues, grains, leaves, and stalk accumulated IHg from both atmosphere and soil to varying degrees. Nonetheless, the study also provides the first direct evidence that atmospheric Hg accumulated by above-ground rice tissues could be translocated to below-ground tissues (roots). However, the extent to which atmosphere or soil Hg contributes to IHg in rice tissues may vary with each source's concentration gradient at the given site. No evidence of in planta Hg methylation was found during the current study. Hence, paddy fields are potential MeHg production sites, whereas paddy soil is a unique MeHg accumulation source in rice plants. This study expands and clarifies the contribution of various sources involved in Hg accumulation in the soil rice system. The findings here provide the basis for future research strategies to deal with the global issue of Hg contaminated rice.

Variations in the profile distribution and protection mechanisms of organic carbon under long-term fertilization in a Chinese Mollisol.

Long term fertilization may have a significant effect on soil organic carbon (SOC) fractions and profile distribution. However, previous research mostly explored the SOC in the topsoil and provided little or no information about its distribution in deeper layers and various protection mechanisms particularly under long-term fertilization. The present study investigated the contents and profile distribution (0-100 cm) of distinct SOC protection mechanisms in the Mollisol (black soil) of Northeast China after 35 years of mineral and manure application. The initial Organic Matter content of the topsoil (0-20 cm) ranged from 26.4 to 27.0 g kg-1 soil, and ploughing depth was up to 20 cm. A combination of physical-chemical fractionation methods was employed to study various SOC fractions. There were significant variations throughout the profile among the various fractions and protection mechanisms. In topsoil (to 40 cm), mineral plus manure fertilization (MNPK) increased the total SOC content and accounted for 16.15% in the 0-20 cm and 12.34% in the 20-40 cm layer, while the manure alone (M) increased the total SOC by 56.14%, 48.73% and 27.73% in the subsoil (40-60, 60-80 and 80-100 cm, respectively). Moreover, MNPK and M in the topsoil and subsoil, respectively increased the unprotected coarse particulate organic carbon (cPOC) (48% and 26%, respectively), physically protected micro-aggregate (µagg) (20% and 18%, respectively) and occluded particulate organic carbon (iPOC) contents (279% and 93%, respectively) compared with the control (CK). A positive linear correlation was observed between total SOC and the cPOC, iPOC, physico-biochemically protected NH-µSilt and physico-chemically protected H-µSilt (p < 0.01) across the whole profile. Overall, physical, physico-biochemical and physico-chemical protection were the predominant mechanisms to sequester carbon in the whole profile, whereas the biochemical protection mechanisms were only relevant in the topsoil, thus demonstrating the differential mechanistic sensitivity of fractions for organic carbon cycling across the profile.

Elucidating various geochemical mechanisms drive fluoride contamination in unconfined aquifers along the major rivers in Sindh and Punjab, Pakistan.

The present study aims to investigate the spatial distribution and associated various geochemical mechanisms responsible for fluoride (F⁻) contamination in groundwater of unconfined aquifer system along major rivers in Sindh and Punjab, Pakistan. The concentration of F⁻ in groundwater samples ranged from 0.1 to 3.9 mg/L (mean = 1.0 mg/L) in Sindh and 0.1-10.3 mg/L (mean = 1.0 mg/L) in Punjab, respectively with 28.9% and 26.6% of samples exhibited F⁻ contamination beyond WHO permissible limit value (1.5 mg/L). The geochemical processes regulated F⁻ concentration in unconfined aquifer mainly in Sindh and Punjab were categorized as follows: 1) minerals weathering that observed as the key process to control groundwater chemistry in the study areas, 2) the strong correlation between F⁻ and alkaline pH, which provided favorable environmental conditions to promote F⁻ leaching through desperation or by ion exchange process, 3) the 72.6% of samples from Sindh and Punjab were dominated by Na⁺- Cl⁻ type of water, confirmed that the halite dissolution process was the major contributor for F⁻ enrichment in groundwater, 4) dolomite dissolution was main process frequently observed in Sindh, compared with Punjab, 5) the arid climatic conditions promote evaporation process or dissolution of evaporites or both were contributing to the formation of saline groundwater in the study area, 6) the positive correlation observed between elevated F⁻ and fluorite also suggested that the fluorite dissolution also played significant role for leaching of F⁻ in groundwater from sediments, and 7) calcite controlled Ca2⁺ level and enhanced the dissolution of F-bearing minerals and drive F⁻ concentration in groundwater. In a nut shell, this study revealed the worst scenarios of F⁻ contamination via various possible geochemical mechanisms in groundwater along major rivers in Sindh and Punjab, Pakistan, which need immediate attention of regulatory authorities to avoid future hazardous implications.

A Review on the Status of Mercury Pollution in Pakistan: Sources and Impacts.

Mercury (Hg) contamination in environmental matrices and associated human exposure has been recognized as a critical long-lasting issue worldwide. However, studies are still elusive that summarized the overall status of Hg pollution and its impacts on public health in Pakistan. Hence, this review encompasses the environmental prevalence, potential sources, and human exposure tendencies to Hg contamination in Pakistan. Reviewed literature revealed jolting levels of Hg in various environmental samples, such as dust, soil, water, and air collected from the residential and industrial areas. Inhalation of Hg via dust particle was identified as the primary pathway for human exposure, while atmospheric deposition and gold mining are identified as the two primary sources of Hg contamination in the environment. Considering human exposure, the highest bioaccumulation of Hg was ranged from 5885 to 8698 µg/kg in hair samples collected from the residents of the Kashmir Valley, Pakistan. However, in the lower Himalayan regions, including Islamabad and Swabi, the concentration of Hg in hair samples was reported at 1085 µg/kg, slightly beyond WHO devised reference dose (RfD) of Hg (1000 µg/kg). This review revealed the worst scenario of Hg contamination in human biomatrices and environmental compartments in Pakistan, which needed immediate rehabilitation measures.

Unraveling prevalence and public health risks of arsenic, uranium and co-occurring trace metals in groundwater along riverine ecosystem in Sindh and Punjab, Pakistan.

The current study focuses on the understanding of contamination status, distribution, source apportionment and health perspectives of arsenic (As), uranium (U) and other co-occurring trace metals in the groundwater samples collected along the major rivers in Sindh and Punjab provinces, Pakistan. ICP-MS analysis revealed that the concentrations of As in the groundwater in Sindh and Punjab ranged from 0.2 to 81.1 µg/L (n = 38) and 1.1 to 501.1 µg/L (n = 110), respectively. Importantly, this study is the first evidence of U contamination in the groundwater samples in Pakistan, which revealed the concentrations of U at from 0.8 to 59.0 and 0.1 to 556.0 µg/L respectively, in Sindh and Punjab. Moreover, the concentrations of Sr and Mn exceeded the WHO limits in the current study area. Anthropogenic activities such as urbanization, direct dispose of industrial, agricultural waste into waterways and extensive use of pesticides and fertilizers might be the main sources of elevated levels of total dissolved solids and electrical conductivity, which increased the mobilization of As, U and Sr in the groundwater samples. Human health risk assessment parameters such as average daily dose, hazard quotient (HQ) and cancer risk indicated severe risks of As and U in the study area. The HQ values of As and U in Punjab were observed at 69.6 and 7.7, respectively, implying the severity of the health risks associated with consumption of contaminated groundwater for drinking purposes. In a nutshell, proactive control and rehabilitation measures are recommended to eradicate trace metals associated groundwater contamination in the targeted areas to avoid future worst scenarios.