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Arsenic irrigated vegetables : risk assessment for South Asian horticulture

TitleArsenic irrigated vegetables : risk assessment for South Asian horticulture
Publication TypeThesis
Year of Publication2014
AuthorsBhatti, Saleem Maseeh
Academic DepartmentMassey University
DegreeDoctor of Philosophy in Soil Science

Arsenic (As) contaminated water is often used in South Asia to irrigate vegetables. These vegetables accumulate As in their edible tissues and once ingested, increase As burden in humans. Despite the apparent risk, the As uptake potential of vegetable species when irrigated with As-contaminated water is not well defined. Most research on As-irrigated vegetables are monitoring surveys that only describe the As concentration levels in various vegetable species from affected areas. Because of the great variability in As concentration of irrigation water, soil type, vegetable species and cultivars, agronomic practices and climatic factors, As uptake potential of an individual vegetable species cannot be described from the monitoring data. Identifying vegetable species and soil conditions that result in high As concentrations in the edible tissues of vegetables is prerequisite for risk assessment and proposing As mitigation strategies. The objectives of this study were to (i) determine the As uptake response of common vegetable species when irrigated with As-contaminated water, (ii) calculate the risk to humans upon ingestion of As-contaminated vegetable species, (iii) elucidate factors that may increase As concentrations in vegetable species, and (iv) propose management strategies for South Asian countries where As-contaminated water is used for vegetable cultivation. In the first glasshouse experiment (Chapter 4), four common vegetables, carrot (Daucus carota), radish (Raphanus sativus), spinach (Spinacia oleracea), and tomato (Solanum esculentum) were irrigated with a range of AsV enriched water (50 to 1000 µg L-1) using two irrigation techniques. These irrigation techniques were (i) non-flooded, where soil moisture was maintained to 70% field capacity (Fc) of soil, and (ii) flooded, where the water was maintained at 110% Fc initially followed by drainage and onset of aerobic conditions until the next irrigation event. Only the 1000 µg As L-1 treatment showed a significant increase of As concentration in the vegetables compared to all other treatments. There was a higher concentration of As in the vegetables grown under flood irrigation relative to non-flood irrigation. The trend of As uptake among vegetable species was spinach > tomato > radish > carrot. Only in spinach leaves, the As concentration was above the Chinese food safety standard for inorganic As (0.05 µg g-1 fresh weight) by a factor of 1.6 to 6.4 times, when irrigated with 100, 200, and 1000 µg As L-1 under flood irrigation and with 1000 µg As L-1| ii under non-flood irrigation. The USEPA carcinogenic and non-carcinogenic risk parameters for the scenario where vegetables are consumed 500 grams per day were calculated. The USEPA Hazard Quotient (HQ) value for spinach leaves ranged from 0.32 to 1.26 for adults and 0.38 to 1.51 for adolescents while the Cancer Risk (CR) value ranged from 1.4 x 10-4 to 5.7 x 10-4 for adults and 1.7 x 10-4 to 6.8 x 10-4 for adolescents for treatment water concentrations 100 µg As L-1 or greater. An HQ value greater than 1 represents an unacceptable non-carcinogenic risk and a CR value greater than 10-4 represents an unacceptable carcinogenic risk. A laboratory batch experiment (Chapter 5) was conducted using four soils to determine their As adsorption behavior and the soil properties that control As retention in these soils. Soils used in this study were (i) Rangitikei silt loam (the soil which was used in glasshouse experiment 1), (ii) Rangitikei silt loam soil amended with calcium hydroxide to raise the pH to 7.5, to model the soil pH level of South Asian countries, and (iii) two New Zealand soils, Korokoro silt loam and Tokomaru silt loam. Both arsenate (AsV) and arsenite (AsIII) were investigated in the experiment because these As species are mainly present in irrigation water. The results showed that the AsV was adsorbed to a greater degree than AsIII as defined by high adsorption maxima, bonding energy and As partition coefficient values of Langmuir and Freundlich isotherms. Adsorption of both AsV and AsIII was mainly controlled by amorphous Al, total C and Olsen P content of selected soils. A glasshouse experiment (Chapter 6) was conducted to explore those factors which can promote As concentration in plants. The following factors which are likely to affect horticulture in South Asia were included: two As species (AsV and AsIII), four As concentration levels of irrigation water (50 to 1000 µg L-1), two soil pH levels (6.1 and 7.5), and two soil amendments (biochar and cattle manure). The control treatment for this experiment was no As in irrigation water and no soil amendment. Spinach was selected for this work due to its high uptake potential described in the earlier glasshouse experiment (Chapter 4). The findings of this experiment showed that the As concentration in spinach leaves was dependent on As concentrations in water and soil amendments and was independent of soil pH and As species under flood irrigation. Spinach plants grown in biochar and cattle manure amended soils had significantly higher As concentration in their leaves when compared with spinach plants grown with no amendment. In both biochar andcattle manure amended soils, the As concentration in spinach leaves exceeded the Chinese food safety standard (0.05 µg g-1 fresh weight) by a factor of 1.6 to 8.3 times, where the concentration of As in irrigation water was 200 µg L-1 or greater. The CR values for spinach grown in cattle manure amended soil was greater than the critical value of 1 x 10-4 for the scenarios where vegetable consumption is 205 grams and/or 500 grams per day. This increase was found where the As concentration in irrigation water was 200 µg L-1 or greater. The HQ value was above the critical value of 1 for the scenario where the vegetable consumption is 500 grams per day. This increase was observed for spinach grown in cattle manure amended soil with an As concentration in irrigation water 500 µg L-1 or greater. Arsenic daily intake (mg kg-1 body weight) associated with the ingestion of spinach leaves corresponds to proposed ATSDR (Agency for Toxic Substances and Disease Registry) and drinking water daily intake values that may lead to development of cancer (bladder, lung and skin), skin lesions, and intellectual impairment in children. The As intake through ingestion of spinach correlates to an As concentration in drinking water that is 10 µg L-1 or greater. Overall, the results of glasshouse studies indicate that the As concentrations greater than 50 µg L-1 should be avoided for spinach cultivation where flood irrigation is practiced. Addition of cattle manure can further intensify the risk by increasing the As concentration in plant tissues, therefore its usage in South Asian horticulture is questionable. I propose that the As concentration in vegetables should not be overlooked as they can alone be a major source of As poisoning in humans.