Paleo-modeling of coastal saltwater intrusion during the Holocene: an application to the Netherlands

Abstract

Abstract. Coastal groundwater reserves often reflect a complex evolution of marine transgressions and regressions, and are only rarely in equilibrium with current boundary conditions. Understanding and managing the present-day distribution and future development of these reserves and their hydrochemical characteristics therefore requires insight into their complex evolution history. In this paper, we construct a paleo-hydrogeological model, together with groundwater age and origin calculations, to simulate, study and evaluate the evolution of groundwater salinity in the coastal area of the Netherlands throughout the last 8.5 kyr of the Holocene. While intended as a conceptual tool, confidence in our model results is warranted by a good correspondence with a hydrochemical characterization of groundwater origin. Throughout the modeled period, coastal groundwater distribution never reached equilibrium with contemporaneous boundary conditions. This result highlights the importance of historically changing boundary conditions in shaping the present-day distribution of groundwater and its chemical composition. As such, it acts as a warning against the common use of a steady-state situation given present-day boundary conditions to initialize groundwater transport modeling in complex coastal aquifers or, more general, against explaining existing groundwater composition patterns from the currently existing flow situation. The importance of historical boundary conditions not only holds true for the effects of the large-scale marine transgression around 5 kyr BC that thoroughly reworked groundwater composition, but also for the more local effects of a temporary gaining river system still recognizable today. Model results further attest to the impact of groundwater density differences on coastal groundwater flow on millennial timescales and highlight their importance in shaping today's groundwater salinity distribution. We found free convection to drive large-scale fingered infiltration of seawater to depths of 200 m within decades after a marine transgression, displacing the originally present groundwater upwards. Subsequent infiltration of fresh meteoric water was, in contrast, hampered by the existing density gradient. We observed discontinuous aquitards to exert a significant control on infiltration patterns and the resulting evolution of groundwater salinity. Finally, adding to a long-term scientific debate on the origins of groundwater salinity in Dutch coastal aquifers, our modeling results suggest a more significant role of pre-Holocene groundwater in the present-day groundwater salinity distribution in the Netherlands than previously recognized. Though conceptual, comprehensively modeling the Holocene evolution of groundwater salinity, age and origin offered a unique view on the complex processes shaping groundwater in coastal aquifers over millennial timescales.

Revised conceptualization of the North China Basin groundwater flow system: Groundwater age, heat and flow simulations

“…The saline water is generally located at depth <60 m, except in the coastal area where saline water occurs to a depth of 1000 m (Chen and Xu, 1986). Density-driven groundwater flow may drive saline groundwater to move a considerable distance in the opposite direction to the originally present fresh groundwater flow (e.g., Duffy and Al-Hassan, 1988;Delsman et al, 2014). Whether density-driven flow could play an important role in shaping the present groundwater salinity distribution could be evaluated by further paleo-hydrogeological modeling, to simulate the groundwater solute transport over a geologic timescale.…”

Groundwater impacts on surface water quality and nutrient loads in lowland polder catchments: monitoring the greater Amsterdam area

“…The five regions are: (1) the Zuider Zee margin region, with shallow brackish groundwater, lies directly adjacent to the former saltwater Zuider Zee, which was dammed in the 1930s and transformed into the freshwater Lake IJssel (connected to Lake IJ), which is now the biggest freshwater reservoir of the Netherlands; (2) the deep polders Groot Mijdrecht and Horstermeer, which are reclaimed lakes with clayey lake sediments at the surface. These polders are characterized by upconing of salt groundwater from deeper layers (Oude Essink et al, 2005;Delsman et al, 2014) and intensive arable farming; (3) the central Holland region, where the polders are characterized by a relatively thick sequence of marine clays and intercalated peats; (4) the Vecht lakes region at the western margin of the ice-pushed ridge, characterized by shallow peat soils over a sandy subsoil and large shallow lakes and wetlands resulting from peat excavations (van Loon, 2010), mostly used for dairy farming; and (5) the icepushed ridge in the eastern part of the study area, which is characterized by permeable sandy soils, recharge of freshly infiltrated water, and the near absence of draining water courses. Our a priori expectation was that the groundwater quality of these five regions is significantly different, because of their specific paleo-hydrological situations and present-day groundwater flow patterns.…”

“…80), two former lakes that were formed after peat excavations. Drainage for lake reclamation and groundwater extraction (Schot and Molenaar, 1992) caused further subsidence and increased seepage of paleo-marine brackish groundwater from deep aquifers (Delsman et al, 2014).…”

“…The salt loading towards these polders is expected to increase, mainly due to the further lowering of surface water levels in response to subsidence (e.g. Oude Delsman et al, 2014). Draining the peat polders has also led to subsidence and repetitive lowering of surface water and groundwater levels.…”

Dynamics of rainwater lenses on upward seeping saline groundwater

“…Pauw et al (2015a, b) continued the studies on thickness variation to man-induced variation, aimed at optimizing sustainable use of fresh water by means of infiltration in elevated creek ridges and the impact of pumping on rainwater lenses. Delsman et al (2014a) established a model that shows the origin and development of saline groundwater water during the Holocene in the Dutch Coastal zone, providing the hydrogeological context on a regional scale. The differences between local and regional scale (saline) ground water systems and their influence on the salt water load in a polder field were investigated by them as well (2014b).…”