Factsheet: Freshwater and Saltwater in Zeeland

Generally speaking, fresh water has a chloride concentration of less than 150 mg/l, whilst North Sea water has a concentration of between 16,000 and 17,000 mg/l. In Zeeland, water is termed ‘agricultural fresh water’ when its chloride concentration is below 1,000 mg/l, making it suitable for crops such as potatoes, onions and wheat. On sandy creek ridges and dunes, rainwater forms a freshwater lens up to 30 metres thick due to differences in density; in dunes, this can reach almost 100 metres. These layers form because fresh water is heavier than salt water, causing it to float on top of the salty groundwater. In polders with clay layers, infiltration is limited, meaning that the rainwater lens is often thin (less than 2 metres) and the shallow groundwater is brackish to saline. External drainage of groundwater affects the stability of the rainwater lens.