Natural saltwater upconing by preferential groundwater discharge through boils

Natural saltwater upconing caused by the preferential groundwater discharge of boils is a key process 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 knowledge gap by investigating upconing mechanisms using field measurements and numerical simulations 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 to 5 g L−1. Boils are found as isolated features or clustered in small areas of 20–100 m2. Field observations show that preferential flow through boils creates localized and narrow saltwater upconing spikes, causing the elevated boil water salinities. Modeling results indicate that boil water in Dutch polders comprises 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 previous numerical studies of pumping-induced upconing, the numerical results show that the most important 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 salinity (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 modifies flow patterns by dividing the groundwater flow system into a local boil system overlying the regional flow system. Despite this, regional flow has only a minor effect on the relative contributions of saline and fresh groundwater to boil discharge and thus on boil salinity as well.