A modified DGT technique for the simultaneous measurement of dissolved inorganic nitrogen and phosphorus in freshwaters.

A modified diffusive gradients in thin films (DGT) technique uses both a mixed binding layer (PrCH and A520E resins for NH-N and NO-N, respectively) and multiple binding layers (Metsorb binding layer for PO-P overlying the mixed binding layer) for the simultaneous measurement of dissolved inorganic nitrogen (nitrate and ammonium) and phosphate in freshwater (INP-DGT). High uptake and elution efficiencies were determined for a mixed (PrCH/A520E) binding gel for dissolved inorganic nitrogen and an agarose-based Metsorb binding layer for PO-P. Diffusion coefficients (D) obtained from DGT time-series experiments (conductivity 180 μS cm) for NH-N, NO-N and PO-P agreed well with those measured using individual DGT techniques in previous studies, but were characterised over a wider range of ionic strengths here. D for NO-N and PO-P were constant over a range of ionic strengths (between 100 and 800 μS cm) while the diffusion coefficient for NH-N decreased with increasing ionic strength, as reported previously. The measurement of NH-N, NO-N and PO-P using the INP-DGT was independent of pH (3.5-8.5) and quantitative over varying ionic strength ranges (up to 0.004 mol L NaCl for NH-N, up to 0.014 mol L NaCl for NO-N and over 0.1 mol L NaCl for PO-P) for a 24 h deployment time. Performance of INP-DGT in synthetic freshwaters with differing conductivity indicated the three nutrients were affected differently, with NH-N measurements being most sensitive. Representative performance was determined for NO-N (90-330 μS cm) and PO-P (all tested conductivities) over a 72 h deployment period and for NH-N (<330 μS cm) over a 24 h deployment period. Field validations showed that the ratios of INP-DGT concentrations to the average concentrations from grab samples were generally between 0.80 and 1.13 over 24 and 48 h deployment periods. To ensure the representative performance of INP-DGT for all three nutrients, the conductivity should not exceed 400 μS cm and deployment times should be no longer than 24 h. The results of this study have demonstrated that INP-DGT could provide a cost-effective monitoring technique for measuring time-weighted average concentrations of dissolved inorganic nutrients in many freshwaters.