Cobalt Concentration in Rice Cultivars and Soil Irrigated with Untreated Wastewater

Abstract

The assessment of quality of untreated wastewater is prerequisite before its application for crop husbandry. A field survey was organized to assess cobalt (Co) concentration in wastewater, irrigated rice cultivars and soil. Plant tissue and soil samples were collected from experimental field of Nuclear Institute of Agriculture (NIA), Tando Jam, where the untreated wastewater is used for irrigation. For comparison, water, rice crop and soil samples were also collected from canal water irrigated field. In wastewater irrigated field, three rice cultivars (Shandar, Shua-92 and Sarshar) and in canal water irrigated field, one rice cultivar (Shandar) was grown. The irrigation water analysis results indicate that the EC was above permissible limit for irrigation and pH was strongly alkaline in reaction. The Co concentration in wastewater (0.11±0.02 mg L-1) and canal water (0.15±0.01 mg L-1) was high and unfit for irrigation. The highest Co concentration in paddy (12.32±0.77 mg kg-1) and straw (9.43±1.97 mg kg-1) was found in cultivar Shandar irrigated with canal water and Shua-92 (12.13±0.76 mg kg-1) irrigated with wastewater. The cultivar, Sarshar accumulated the least Co in its straw and paddy (5.93±1.29 mg kg-1 and 5.40±0.26 mg kg-1 respectively). Cobalt concentration in soils irrigated with canal water and wastewater was more or less equal at both soil depths (0-15 cm and 15-30 cm) and ranged from 23.53±2.30 to 30.24±1.82 mg kg1. These values are above the Target value for cobalt in soil (9.0 mg kg-1). It is suggested that the wastewater should not be used for irrigation in the selected area because of its high levels of Co, pH and EC, and subsequently build-up of Co in rice tissues and soil. Canal water should be monitored to find out the source of Co input, and its discharge into canal water should be minimized.