Assessing contaminated groundwater discharge to a marine environment can be difficult given the complex nature of groundwater – surface water interaction. The objective of the study was to undertake single-well fluorescent dye tracer tests along a transect of wells with decreasing distance from the inferred connection with surface water to characterise the groundwater and contaminant mass flux towards the environment.
The methodology consisted in applying the Finite Point Volume Dilution Method (FVPDM), (Brouyere et al, 2008). The FVPDM involves injecting a tracer in a well at a low and continuous rate. The rate of tracer dilution resulting from groundwater flow through the well is an in-situ measurement of the Darcy flux. In-well contaminant sampling is then used to derive the contaminant mass flux. The measurement does not depend upon estimate of hydraulic conductivity and hydraulic gradient.
The ten FVPDM tests undertaken at the study location enabled an accurate quantification of groundwater and contaminant mass fluxes along the investigated transect. The main benefit of FVDPM was to measure how the Darcy flux responded to variable hydraulic conditions and to show that these fluxes can differ in magnitude according to each specific tidal event (i.e. neap and spring tide).
Direct measurement of groundwater and contaminant fluxes enabled the planning and development of more effective risk mitigation measures to protect the marine environment.
Brouyere et al, 2008. A new tracer technique for monitoring groundwater fluxes: Finite Volume Point Dilution Method. Journal of Contaminant Hydrology 95(121-140).