Hydrogeochemical processes after oxidation of sea sediment at mega-scale suppletion The Sand Motor. Comparison between a mega-scale suppletion site, a traditional suppletion site and a control site.

Abstract

The Sand Motor is a megasuppletion pilot project constructed in sea next to the beach between The Hague and Hoek van Holland. The application of 21*10^6 m^3 of sea sediment will prevent ecosystem disturbance by suppletion for the next 20 years and the sand will be distributed over the coast by natural processes. This research focusses on hydrogeochemical processes on the Sand Motor, since oxidation of former sea sediments can influence the soil and water quality. The mineral pyrite (FeS­­2) is formed on the seafloor and is expected to dissolve in ground- and pore water upon oxidation. The sulphide mineral is of interest, because oxidation could acidify the groundwater and possibly mobilise adsorbed trace contaminants. In order to evaluate whether geochemical processes may form a negative aspect of the Sand Motor, fieldwork was on performed on the Sand Motor, a traditional suppletion site and a control site. The groundwater and pore water samples were measured for alkalinity, ions, major elements and trace elements.

Soluble sulphate in combination with iron oxide confirms the oxidation of pyrite on the Sand Motor, but large-scale presence of pyrite is not proven. The oxidation effects are most prominent in the unsaturated zone in the middle of the Sand Motor where freshening has advanced the furthest. The buffering capacity is generally high, however, a selection of samples shows acidification or decreased alkalinity due to pyrite oxidation in the unsaturated zone. This results in trace elements exceeding target values and occasionally intervention values for shallow groundwater. The impact of pyrite oxidation on the traditional suppletion site is moderate. However, the control site indicates, contrary to expectations, more significant pyrite oxidation.

Long-term monitoring of the Sand Motor is recommended to see whether the elevated trace element concentrations are temporal or will persist for longer periods of time.