BIOLOGICAL OXYGEN DEMAND AND SUGARS IN IRRIGATION WATER RUNOFF FROM SUGARCANE FIELDS

The occurrence of fish-kills in waterways in sugarcane areas is of growing concern. To investigate if cane growing practice could be a contributing factor, runoff from irrigation water from burnt cane (2 weeks after harvest/furrow irrigation), as well as from cane under a green trash blanket (8 days after harvest/trickle irrigation), was analysed for biological oxygen demand (BOD5), total organic carbon (TOC), sucrose, glucose, fructose and total water-soluble carbohydrates (TWSC). BOD5 concentrations in the runoff water were high (up to 300 mg/L) compared to average background concentrations in natural stream water (2–3 mg/L) and the State licensing limit for the discharge of sewage and industrial effluents (20 mg/L). The major contributor to these high values was the presence of sugars (averages: sucrose 45 mg/L, glucose 60 mg/L and fructose 90 mg/L). An artificial mixture of these sugars (values as above) in irrigation water resulted in BOD5 of 130 mg/L. Hence, the presence of sucrose, glucose and fructose explained approximately 50% of the BOD5 values. TWSC values of up to 400 mg/L fructose equivalents showed that other carbohydrates were also present. All carbohydrates contributed about 66% of the BOD5 values. If runoff with this high BOD5 was allowed to reach local waterways, it could cause potentially serious environmental hazards, namely fish kills due to oxygen depletion.