Holocene dynamics of the salt–fresh groundwater interface under a sand island, Inhaca, Mozambique

Abstract

The configuration of coastal groundwater systems in southeast Africa was strongly controlled by the Holocene sea-level changes, with an Early Holocene transgression ∼15 m (10,000–5000 cal BP), and two assumed high-stand events in the Middle and Late Holocene with levels higher than the present. The fluctuation of the salt–fresh groundwater interface under Inhaca Island in Mozambique during the Holocene has been studied using an adapted version of the numerical code SUTRA (Saturated-Unsaturated Transport). In this study, small-scale variations such as tidal effects have not been considered. A number of transient simulations were run with constant boundary conditions until the steady state condition was reached in order to study the sensitivity of response time, salt–fresh interface position, and thickness of the transition zone to different parameters such as hydraulic conductivity, porosity, recharge, and dispersivity. A 50% increase in horizontal hydraulic conductivity yields a rise in the location of the interface of >15 m, while an increase in recharge from 8% to 20% of mean annual precipitation (MAP) causes a downward shift in the interface position of >40 m. A full transient simulation of the Holocene dynamics of the salt–fresh groundwater interface showed a response time of several hundred years, with a duration sensitive to porosity, hydraulic conductivity and recharge and a position determined by the recharge rate and the hydraulic conductivity. Dispersivity controls the thickness of the transition zone in this non-tidal model. Physical processes, such as changes in recharge and/or the sea level, may cause rapid shifts in the interface position and affect the thickness of the transition zone.