Effects of flushing of surface water on salinization have been modelled in a small subcatchment in polder Haarlemmermeer. Results show that availability of freshwater can be increased by small interventions relatively easily.
Improvement of freshwater management in polder Haarlemmermeer to reduce salinization of surface water by exfiltration of saline groundwater
Exfiltration of saline groundwater causes strong salinization of surface water in polder Haarlemmermeer. In order to limit the salinization of the surface water and allow agriculture in the area, freshwater is flushed into the ditches of the polder. However, the current flushing regime is less efficient than previously thought. Diverted freshwater only reaches a limited number of ditches and rapidly becomes saline due to mixing of exfiltrating groundwater. It is expected that in the future availability of freshwater will decrease, while exfiltration of saline groundwater will still continue to increase. To get understanding of the functioning of the flushing regime and the distribution of chloride on a local scale, a detailed SOBEK model has been made of the flow and chloride concentrations of surface water in a subcatchment of 10 km2 in the polder Haarlemmermeer. For several scenarios, the effects of change of the flushing regime have been investigated: smaller flushing rates, more effective flushing (better use of the same amount of flushed water), and more efficient flushing (only use water for flushing if needed). The results show that, when flushing has been resumed after a period without flushing, it takes about one week until the availability of freshwater is similar to the situation as it would be if flushing was continued throughout the whole season. Also, the availability of freshwater can be increased relatively easily by small interventions, such as separation of saltwater and freshwater, and increase of water level in certain ditches.