Determination of Arsenic Forms in Wastewater Irrigated Rice (Oryza sativa L.) Soils

Abstract

Title: Determination of arsenic forms in wastewater irrigated rice (Oryza sativa L.) soils The “Indus” river water carries the drainage and wastewater of adjoining cities on the way and additionally receives the waste of Hyderabad city, toxic runoff of industrial waste and sewage in Fuleli and Akram canals, which irrigate the agricultural lands in the study area. This study evaluated arsenic in irrigation water (As-III and As-V), soil (Ex-As, Al-As, Fe-As and Ca-As and total) and rice shoots (grain and straw+husk) of the study area. Composite shoots and associated soils (0-15 and 15-30 cm) were randomly taken from fourty eight rice fields of Tando Muhammad Khan and Badin. While, the irrigation water samples were collected at the entrance of each rice field. The data was subjected to descriptive statistics. The arsenic was determined Hydride Vapor Generation Assembly coupled with Atomic Absorption Spectrometer. Arsenic in irrigation water (5.02-102.18 µg L-1 ) data showed that only 11% sample were above the recommended limit (50 µg L-1 ) for irrigating the rice crop. The soil arsenic data also depicted that only 11% surface (0-15 cm) samples were contaminated (>50 mg kg-1 ), from which only 2% of rice grain samples were contaminated (>1 mg kg-1 ). Similar picture emerged on the basis of arsenic intake of 2.00, 2.39 and 2.52 μg kg-1 body weight day-1 , for each 300 and 150 g, respectively for adults (18-70 years age), adolescent (12- 18 years age) and children (5-12 years age). All three categories were safe from cancer risk, which was lower than WHO permissible limit of 3.0 μg kg-1 body weight day-1 arsenic. Both inorganic species (As-III and As-V) from irrigation water contributed equally to arsenic fractions in soil and the contribution trend of fractions followed the order of Ca-As>Ex As>Al-As>Fe-As. The contribution was less in case of lower (15-30 cm) soil depth. The contribution of arsenic from soil fractions to the arsenic in grain and straw+husk was of the order Ca-As >Ex-As>Fe-As>Al-As. The irrigation water species (90%) and soil fractions (95.5%) were close to the total arsenic concentration in each. The soil properties (silt, clay, EC, organic matter, AB DTPA P and Fe) also showed positive relationship with arsenic. Based on flood irrigation, 0.19-0.26 mg kg-1 of arsenic will be added annually to current arsenic of 25.5 mg kg-1 already present in soil. The straw+husk had relatively more arsenic (0.75 mg kg-1 ) over grain (0.40 mg kg-1 ) and is generally used as a feed for animals. This study also highlighted, that other than rice intake, arsenic can enter in the human food chain by utilizing animal products such as milk, meat and other dairy products.