SF1. A Lachat QuickChem 8000 flow injection analyser, the schematic diagram of which is shown in Fig. 1, was used for the determination of nitrate. The FIA manifold is equipped with peristaltic pumps (P), an injection valve (V), a Cu–Cd reduction column (C) and a two state switching valve (SV). The teflon tubes R1, R2 and R3 (1.07 mm ID) were used for the carrier water, buffer and reagent flow respectively whilst teflon tubes used for the manifold connections, the delay coil (DC, 70 cm) and the reaction coil (RC, 70 cm) were of 0.8 mm ID. The FIA is equipped with an auto-sampler (sample volume 200 lL) with a 10 mm path length flow cell, a 60-position sample rack and a colorimetric detector (D) with UV filter of 520 nm.