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 NH₄-N and NO₃-N, respectively) and multiple binding layers (Metsorb binding layer for PO₄-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 PO₄-P. Diffusion coefficients (D) obtained from DGT time-series experiments (conductivity 180 μS cm⁻¹) for NH₄-N, NO₃-N and PO₄-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 NO₃-N and PO₄-P were constant over a range of ionic strengths (between 100 and 800 μS cm⁻¹) while the diffusion coefficient for NH₄-N decreased with increasing ionic strength, as reported previously. The measurement of NH₄-N, NO₃-N and PO₄-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⁻¹ NaCl for NH₄-N, up to 0.014 mol L⁻¹ NaCl for NO₃-N and over 0.1 mol L⁻¹ NaCl for PO₄-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 NH₄-N measurements being most sensitive. Representative performance was determined for NO₃-N (90–330 μS cm⁻¹) and PO₄-P (all tested conductivities) over a 72 h deployment period and for NH₄-N (<330 μS cm⁻¹) 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⁻¹ 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.