Monitoring efficiency of urban stormwater treatment devices: discrete-sampling vs. diffusive gradient in a thin film technique

Monitoring the hydrochemical efficiency of urban stormwater treatment devices is not straightforward as the traditional, automated, discrete-sample approach is time-consuming and logistically expensive. An alternative monitoring technique, diffusive gradient in thin film (DGT), was tested alongside the traditional approach at an urban stormwater treatment basin. Pb concentrations in runoff were low to negligible, and the basin had little measurable effect. Using log-transformed Event Mean Concentration (EMC) the basin was calculated to reduce the Cu and Zn concentration by 39 and 51% respectively, but high variability meant there was no statistical evidence of a reduction. The DGT measurements not only suggested a higher efficiency for the basin (77% for Cu, 72% for Zn), but were also less variable providing confidence in the result. Simple modelling implied that the concentration in baseflow fluctuated. The cost effectiveness and low variability of the DGT technique may make it suited to the monitoring requirements of the stormwater industry.