Natural saltwater upconing by preferential groundwater discharge through boils

Abstract

Natural saltwater upconing caused by the preferential groundwater discharge of boils is a key proce ss in the salinization of Dutch deep polders. The factors controlling upconing by boil discharge and boil water salinities are poorly constrained and have not been previously documented. We addressed this knowl- edge gap by investigating upconing mechanisms using field measurements and numerical simulation s of simplified situations. Boils occur as conduits in the upper aquitard connecting the underlying aquifer to the surface and allowing groundwater to discharge at rates up to 100 m3 d?1 with Cl concentrations up to5gL?1. Boils are found as isolated features or clustered in small areas of 20–100m2. Field observations show that preferential flow through boils creates localized and narrow saltwate r upconing spikes, caus- ing the elevated boil water salinities. Modeling results indicate that boil water in Dutch polders com- prises mixtures of groundwater from a wide range of depths and salinities with larger contributions from shallower and less saline groundwater than from the deeper and more saline water. Similar to pre- vious numerical studies of pumping-in duced upconing, the numerical results show that the most impor- tant factors controlling the boil salinity in Dutch polders are boil discharge, the horizontal hydraulic conductivity of the aquifer, the depth of the transition zone and the salinit y (or density) contrast within the aquifer. When boils are clustered, natural saltwater upconing is a function of the total discharge of a boil cluster, whereas the boil-to-boil salinity variations within a cluster are determined by the discharge of individual boils and their position relative to neighboring boils. Regional lateral flow significantly mod- ifies flow patterns by dividing the groundwater flow system into a local boil system overlying the regio- nal flow system. Despite this, regional flow has only a minor effect on the relative contribu tions of saline and fresh groundwater to boil discharge and thus on boil salinity as well.