Evaluation of DGT as a long-term water quality monitoring tool in natural waters; uranium as a case study

Abstract

The performance of the diffusive gradient in thin film technique (DGT) was evaluated as a tool for the long-term monitoring of water quality, using uranium as a case study. DGTs with a Metsorb™ (TiO2) sorbent were deployed consecutively at two alkaline freshwater sites, the River Enborne and the River Lambourn, UK for seven-day intervals over a five-month deployment period to obtain time weighted average concentrations. Weekly spot samples were taken to determine physical and chemical properties of the river water. Uranium was measured in these spot samples and after extraction from the DGT devices. The accuracy of the DGT device time weighted average concentrations to averaged spot water samples in both rivers was 86% (27 to 205%). The DGT diffusive boundary layer (DBL) (0.037-0.141 cm - River Enborne and 0.062-0.086 cm - River Lambourn) was affected by both water flow and biofouling of the diffusion surface. DBL thicknesses found at both sites were correlated with flow conditions with an R(2) value of 0.614. Correlations were also observed between the DBL thickness and dissolved organic carbon (R(2) = 0.637) in the River Lambourn, indicating the potential presence of a complex zone of chemical interactions at the surface of the DGT. The range of DBL thicknesses found at the River Lambourn site were also attributed to of the development of macro-flora on the active sampling surface, indicating that the DBL thickness cannot be assumed to be water flow dependant only. Up to a 57% under-estimate of uranium DGT concentration was observed compared to spot sample concentrations if the DBL was neglected. This study has shown that the use of DGT can provide valuable information in environmental monitoring schemes as part of a 'tool-box' approach when used alongside conventional spot sampling methods.

Coupling DGT passive samplers and multi-collector ICP-MS: A new tool to measure Pb and Zn isotopes composition in dilute aqueous solutions

“…The thicknesses of DBL for each column 1, 2 and 3 were estimated by previous study (Berho et al, 2015) and were respectively 0.18, 0.22 and 0.24 cm. The DBL thicknesses measured in this experiment (0.18-0.24 cm) are large compared to those calculated in surface waters, for example 0.062 to 0.082 cm in river with a flow rate of 0.1m/s (Turner et al, 2014), but are in agreement with those estimated in unstirred solutions: 0.15±0.01 cm (Warnken et al, 2006). For DGT immersed in this pilot, the diffusion zone, defined by the thickness of diffusive gel (+filter membrane) plus the thickness of the DBL, is thus important (up to 0.45 cm).…”

Interpretation of diffusion gradients in thin films (DGT) measurements: a systematic approach

“…[49] Future work to establish the impact of the Nernst diffusion layer approximation is timely. Experiments with varying gel thickness have been performed to estimate the physical length of the DBL [32,52,53] . Several works [12,25,50,51,54,55] tackled the determination of δ (superscript "w" indicates that the diffusion coefficient correspond to the solution -water phase; no superscript is used here for parameters in the diffusive gel although in the SI of ref [25] the superscript "gel" was used).…”

Weekly water quality monitoring data for the River Thames (UK) and its major tributaries (2009–2013): the Thames Initiative research platform

“…Since its commencement in March 2009, the Thames Initiative data have been used to quantify nutrient dynamics (Bowes et al, 2015a;Skeffington et al, 2015;Wade et al, 2012), nutrient sources (Bowes et al, 2014), phytoplankton dynamics (Bowes et al, 2012a(Bowes et al, , 2016Read et al, 2014) and bacterioplankton biodiversity . The resulting data sets have been extensively used as a modelling resource (Bussi et al, 2017(Bussi et al, , 2016Hutchins et al, 2016;Whitehead et al, 2015).…”