Multitracing of artificially recharged Rhine River water in the coastal dune aquifer system of the Western Netherlands

Pieter J. Stuyfzand
1KWR Watercycle Research Institute, PO Box 1072, 3430 BB Nieuwegein, Netherlands
Email: pieter.stuyfzand@kwrwater.nl.
2VU University, Amsterdam, Netherlands

Abstract
The coastal dunes of the Western Netherlands are locally recharged by pretreated Rhine River water in order to supply drinking water to large cities like Amsterdam. This happens on a large scale since 1955, when the drawdown of groundwater tables and aquifer salinization became unacceptable. Ecological interests and both national and EU legislation now compel the water utilities to monitor the expansion of the resulting artificial groundwater bodies amidst the surrounding, natural dune groundwater.

That monitoring requires, however, multitracing techniques to be applied in order to unambiguously identify the infiltrated Rhine water amidst coastal dune groundwater, because both watertypes show a large variation in water quality yielding overlapping tracer contents. In addition, the identification is becoming more difficult due to a reducing contrast between both waters.

In this contribution the performance of various environmental tracers is shown, both single and in various combinations (multitracing), for diverse hydrogeochemical compartments of the aquifer system. The following useful tracer types are considered: conservative (2H, 18O, Cl, Br), reactive with retardation factor R = 1 (SEC, HCO3, SO4, 3H), and reactive with R > 1 (temperature, F, I, Mo, B, Li, Sr, Na, K, Mg).

Results of mapping the extension of the infiltrated Rhine water are shown, with special attention to socalled rain water lenses on top of laterally migrating Rhine water, groundwater dating and hydrological information (hydraulic conductivity, dispersivity) as derived from the observed spatial patterns.
This study does not only reveal how multitracing works in the dutch delta, but also offers methods with general applicability, namely how to: (a) combine various tracers; (b) rank tracer performance, and (c) quantify and reduce uncertainties in hydrochemically discriminating 2 water bodies and in determining end member contributions to their mixture on the basis of known analytical errors and expected uncertainties in end member concentrations.

Subject Area:             Geochemistry during Infiltration and Flow