Development and evaluation of a diffusive gradients in a thin film technique for measuring ammonium in freshwaters

“Diffusive Gradients in Thin Films” Techniques Provide Representative Time-Weighted Average Measurements of Inorganic Nutrients in Dynamic Freshwater Systems

“…Independent t tests showed that only two DGT results (NH 4 −N from March 3 to 6 and PO 4 −P from February 28 to March 3) were significantly different to the average grab sample (p < 0.05) concentrations for the same periods; although the DGT results were still quite similar, the grab samples had a low standard deviation for these deployments. These differences are also apparent in Creek section and previous studies, 21,22 in which DGTs have been demonstrated to provide highly representative measurements, it is probable that there were changes in nutrient concentrations that were not accurately captured by the daily grab samples. For instance, the heavy rainfall event on 28th February could readily have produced a higher peak PO 4 −P concentration than was measured with the grab samples.…”

“…Previous studies have reported very high concentrations of NO 3 −N and NH 4 −N at this site. 21,22 The Botanic Gardens is bordered by a golf club and a large residential area and contains a series of artificial ponds, dense with lilies and algae, which receive stormwater runoff from the surrounding areas and the gardens. Previous measurements at this site have found high NO 3 −N and NH 4 −N concentrations.…”

“…DGT samplers were assembled as described previously. 21,22,34 The probes were stored in sealed plastic bags with 1−2 mL of deionized water prior to deployment.…”

“…where M is the mass of NH 4 −N, NO 3 −N, or PO 4 −P bound to the adsorbent (ng) corrected with the appropriate elution efficiency, 21,22,34 Δg is the diffusive layer thickness, D is the diffusion coefficient of inorganic nutrient ion through the diffusive layer (cm 2 s −1 ), A is the area of the probe exposed to solution (3.14 cm 2 ), and t is the deployment time (s…”

“…Most importantly, all conductivities were less than 1 mS cm −1 , which is the recommended maximum for use of the NO 3 −N and NH 4 −N DGT techniques. 21,22 Dissolved oxygen saturation (%) ( Figure 1D) also varied considerably between sites, with mean values ranging from 49% (Saltwater Creek stream) to 103% (Currumbin Creek stream). All sites, except Currumbin Creek stream had an average DO below the Queensland Water Quality Guidelines lower value of 85% in lowland streams and 90% in freshwater lakes.…”