Application of Diffusive Gradients in Thin Films for Monitoring Groundwater Quality

Abstract

Diffusive gradients in thin films (DGTs) provide time-weighted solute concentrations in well-mixed fluvial environments. However, the DGT method has rarely been applied to relatively slowly moving groundwater systems, where the increased length of the diffusive boundary layer (DBL) must be accounted for, to give accurate concentration measurements. Because the DGT method pre-concentrates trace-level solutes (e.g., Cd2+) and averages out fluctuations in more dynamic contaminants (e.g., NO3–), it has potential advantages over conventional methods. This study evaluated the application of DGT methods for monitoring trace contaminants in groundwater, which are often present at levels below standard instrumental detection limits. We tested three different DGT deployment approaches using NO3– to estimate the DBL thickness: a static DGT suspension, a DGT shaker, and a pumped flow-cell unit coined the “universal DGT monitoring system” (UDMS). Unlike static and DGT shaker approaches, the UDMS reduced the DBL thickness (∼0.02 cm) to values within the range of well-mixed waters (DBL ≤ 0.03 cm). Comparable DBL thicknesses for NO3– and Cd2+ and correlations with grab sample measurements (R2 = 0.99 and 0.98, respectively) demonstrate the suitability of DGTs combined with the UDMS for groundwater monitoring. This approach is particularly suitable for analysis of trace-level solutes including organic contaminants.