Fate of urea fertilizers in sandy aquifers: Laboratory and field case study from Kalpitiya, Sri Lanka

Abstract

High nitrate (NO3-) levels in groundwater are attributed to fertilizer leaching from sandy soil. A simulation laboratory study was performed to investigate the fate of leached urea in an agricultural field in Kalpitiya and the enhancement of fertilizer retention in sandy soil. A field experiment was conducted to investigate the effectiveness of using nutrient rich groundwater to reduce the nitrate levels in the Kalpitiya aquifer. Nitrate retention experiment revealed that increasing clay up to 15% increased the retention by > 50% with an efficiency of 3.7% and the retention is conversely correlated with the quantity of clay. A soil column experiment showed that nitrate concentration of leachate increases rapidly for an initial period of 250 hours and then started to decrease gradually. Initial nitrification followed by denitrification reactions may be the causative factors for this behaviour. The study further revealed that clay and organic manure are potential sinks of nitrate while the latter is also a source of nitrate. Gradual building up of nitrate in the groundwater of the aquifer system appears to be due to available favourable conditions for continuous nitrification while cycling through the aquifer due to intensive irrigation. This field case study reveals that a polluted aquifer can probably be recovered by reusing the nitrate contaminated groundwater as a source of fertilizer for the crops cultivated on the land above the aquifer allowing longer contact time for plant uptake. The process can gradually purify the groundwater while allowing farmers to use less nitrate fertilizers.