Arsenic Contamination of Soils and Agricultural Plants through Irrigation Water in Nepal

Abstract

This study was conducted in the irrigation area of Nawalparasi district which is known as highly contaminated by arsenic among the Terai districts of Nepal. The purpose of this study was to analyse the arsenic of agricultural plants due to irrigation with arsenic contaminated water and the role of phosphorus fertilization. It is hoped that the result of this study will help to develop guidelines to protect the food chain by arsenic contamination in Nepal which is an important issue of the Millennium Development Goal (MDG) regarding the food security. 65 % of all 52 wells showed an arsenic concentration higher than 0.05 mg L-1, which is the Nepal interim standard for drinking water. Overall, 36 % of the irrigation wells exceeded the FAO guideline value (0.1 mg L-1) for arsenic in irrigation water. A wide range of arsenic concentrations was observed in the irrigation water, ranging between < 0.05 and 1.014 mg L1 . The As(III) dominates in the water with a mean percentage of about 76%. The arsenic concentrations in the irrigated soils ranged from 6.1-16.7 mg kg-1. The arsenic content was significantly (p < 0.05) higher in the soils of the vegetable fields than in the soils of the rice fields. The arsenic content in different parts of plants decreased in the order of roots > shoots > leaves > edible parts. The mean arsenic content of edible parts (dry weight) decreased in the order of onion leaves (0.55 mg kg-1) > onion bulb (0.45 mg kg-1) > cauliflower (0.33 mg kg-1) > rice (0.18 mg kg-1) > brinjal (0.09 mg kg-1) > potato (< 0.01 mg kg1 ). The arsenic contents of different soil aggregate fractions and its correlation with Fe, Mn, Al and Ca were studied in a rice and a vegetable field. The investigated aggregate size fraction was 1 – 2 mm. Arsenic content was highest in the smallest aggregates. Under rice no distinct correlation of arsenic with Fe, Mn, Al and Ca was found. Under vegetables As showed a good correlation with Fe, Mn and Ca in the smaller aggregate fractions (≤ 0.63 mm). The laboratory experiment showed that the arsenic solubility was highly influenced by changes in the soil redox potential. The As solubility was increased about 5 fold after the addition of Di-Ammonium Phosphate (DAP) fertilizer under redox potential conditions (aerobic to anaerobic) while solubility of Fe and Ca was not affected. This indicates that Pfertilizer can solublize As through ion exchange processes.