ARSENIC CONTAMINATION OF PADDY RICE FIELDS IN SOUTHERN PUNJAB, PAKISTAN

The study builds on our assessment of arsenic (As) in a matched set of irrigation water and paddy soil which documented large variations in the As accumulation of paddy soil irrigated with As-contaminated ground in the Punjab plains. Therefore, this research work was designed to investigate the rice fields of District Muzaffargarh and Multan, Southern Punjab irrigated with As contaminated water. From a total of 18 rice fields, irrigation water and rice grains as well as 180 soil samples (10 from each field) were collected and analyzed in the laboratory using Atomic Absorption Spectrometry (AAS). Irrigation water and 180 soil samples were also analyzed in the field using a field kit. Results shows that concentrations of As in 12% of irrigation water samples contains less than 50 μg/L of As whereas 72% contains As in the range of 50-100 μg/L and 16% of the water samples contains As > 100 μg/L. In case of paddy soil, the As concentration in 33.3% of samples were in the range of 0-5 mg/kg whereas 55 % soil samples were in the range of 5 to 10 mg/kg and 12 % of the samples exceeds 10 mg/kg. The distribution of As in irrigation water clearly shows the high As near the river Chenab and the similar trend of high As was observed in paddy soil As. In the individual rice field, however, the soil As decreases from inlet to the opposite side of the field. Systematic intercalibration of laboratory measurements with field kit measurement yields a positive correction (r²= 0.65946 for water) and (r²=0.699 for soil) which reveals that field-kit measurements should be expanded to other areas of Pakistan susceptible to arsenic. The persistent As distribution due to unchanged cultivation practices shows net arsenic accumulation of 3.465 mg/kg in the field and ideally soil accumulates 0.486 mg/kg of arsenic every year. In case of rice, overall, 44% of samples exceeds 0.2 mg/kg of As which reveals that paddy rice is taking up the As from soil. Using matched water, soil, and rice As data, this study confirms that groundwater pumping is the main driver of As concentrations in paddy soil of southern Punjab and with continued irrigation of rice paddies with ground water will result in further increases in As concentrations in paddy soil and consequently uptake by rice crop and will result human exposure.