OBSERVATIONS OF SALINITY PATTERNS IN SHALLOW GROUNDWATER AND DRAINAGE WATER FROM AGRICULTURAL LAND IN THE NORTHERN PART OF THE NETHERLANDS

In studies concerning rainwater lenses on a parcel scale in brackish polder areas it is assumed that infiltrated rainwater flows through the rainwater lens and seeps to the watercourses. This freshwater system is supposed to be superimposed on the brackish diffuse upward seepage system. This study investigates the influence of field drainage on the dynamic behaviour of fresh rainwater lenses and the risk of salinization of the root zone. Observations presented in this paper show that shallow water and salt movements in clayey polders are more complex and more dynamic than previously thought. On 18 parcels 2D geo-electrical resistivity imaging profiles were measured, which give a good spatial and temporal representation of the salinity distribution of the groundwater. It appears that during wet periods upconing of brackish groundwater appears under pipe drains and ditches, which drain a mixture of fresh meteoric water and deep brackish groundwater. Between drainpipes small rainwater lenses develop. During the dry season, when the groundwater table drops below the drainage level, brackish soil moisture stays behind. Transpiration of the crops rooted in the clayey polder soils result in strong capillary rise of salts, which may even reach the root zone in some places. Copyright (C) 2011 John Wiley & Sons, Ltd.