Pollution of River Mahaweli and farmlands under irrigation by cadmium from agricultural inputs leading to a chronic renal failure epidemic among farmers in NCP, Sri Lanka

摘要

Chronic renal failure (CRF) associated with elevated dietary cadmium (Cd) among fanning communities in the irrigated agricultural area under the River Mahaweli diversion scheme has reached a significantly higher level of 9,000 patients. Cadmium, derived from contaminated phosphate fertilizer, in irrigation water finds its way into reservoirs, and finally to food, causing chronic renal failure among consumers. Water samples of River Mahaweli and its tributaries in the upper catchment were analyzed to assess the total cadmium contamination of river water and the possible source of cadmium. Except a single tributary (Ulapane Stream, 3.9 ug Cd/1), all other tested tributaries carried more than 5 ug Cd/1, the maximum concentration level accepted to be safe in drinking water. Seven medium-sized streams carrying surface runoff from tea estates had 5.1-10 ug Cd/1. Twenty larger tributaries (Oya), where the catchment is under vegetable and home garden cultivation, carried 10.1-15 ug Cd/1. Nine other major tributaries had extremely high levels of Cd, reaching 20 ug Cd/1. Using geographic information system (GIS), the area in the catchment of each tributary was studied. The specific cropping system in each watershed was determined. The total cadmium loading from each crop area was estimated using the rates and types of phosphate fertilizer used by the respective farmers and the amount of cadmium contained in each type of fertilizer used. Eppawala rock phosphate (ERP), which is mostly used in tea estates, caused least pollution. The amount of cadmium in tributaries had a significant positive correlation with the cadmium loading of the cropping system. Dimbula Tea Estate Stream had the lowest Cd loading (495.9 g/ha/ year), compared with vegetable-growing areas in Uma Oya catchment with 50,852.5 g Cd/ha/year. Kendall's t rank correlation value of total Cd loading from the catchment by phosphate fertilizer used in all crops in the catchment to the Cd content in the tributaries was +0.48. This indicated a major contribution by the cropping system in the upper catchment area of River Mahaweli to the eventual Cd pollution of river water. Low soil pH (4.5-5.2), higher organic matter content (2-3%), and 18-20 cmol/kg cation exchange capacity (CEC) in upcountry soil have a cumulative effect in the easy release of Cd from soil with the heavy surface runoff in the upcountry wet zone. In view of the existing water conveyance system from upcountry to reservoirs in North Central Province (NCP) through diversion of River Mahaweli, in addition to their own nonpoint pollution by triple superphosphate fertilizer (TSP), this demands a change in overall upper catchment management to minimize Cd pollution through agriculture inputs to prevent CRF due to elevated dietary cadmium among NCP fanners.