Arsenic irrigated vegetables : risk assessment for South Asian horticulture : a thesis presented in fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, Massey University, Palmerston North, New Zealand
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Date
2013
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Massey University
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Abstract
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.
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Keywords
Vegetables, Arsenic content, Irrigation, Water, Environmental aspects, Pollution, South Asia