Observations and analytical modeling of freshwater and rainwater lenses in coastal dune systems

Abstract

Observations are reported on (i) groundwater recharge rates under various types of vegetation as measured with megalysimeters in the dunes, (ii) freshwater lenses along the Dutch North Sea coast in the early 1900s, and (iii) rainwater lenses that develop on top of laterally migrating, artificially recharged riverwater. Subsequently analytical methods are presented to estimate annual natural groundwater recharge as function of rainfall and vegetation, and to calculate the size, shape and transition zone of freshwater lenses on saline groundwater and rainwater lenses on infiltrated riverwater. An empirical correction factor, based on the hydraulic resistance of an aquitard within the freshwater lens, is proposed to account for the frequently observed reduction of the GhybenHerzberg ratio of 40. This factor raises the groundwater table, reduces the depth of the fresh/salt interface and increases the lens formation time. The suite of methods offers a tool box for knowledge based water management of dune systems, by rapidly predicting: (i) more or less autonomous changes due to sealevel rise, climate change and vegetation development; and (ii) the potential (side) effects of interventions. Knowing what happened or will happen to the fresh water lens or a rainwater lens is important, because changes impact on important natural habitat parameters such as salinity, depth to groundwater table, decalcification rate (and thus on pH, Ca/Al, PO 4 , NH 4 ) and nutrient availability, and on drinking water supply. The analytical models are applied to predict effects of sealevel rise, coastal progradation, vegetation changes, and increased temperature of coastal air and river water to be infiltrated.

Sediment availability provokes a shift from Brownian to Lévy‐like clonal expansion in a dune building grass

“…Differences in grain size, for instance, can affect soil moisture content and nutrient availability, but in our experiment we excluded these effects using sand from the same source (Table ). Although differences in soil moisture in the field can be caused by proximity to sea or due to the formation of freshwater lenses under embryonic dunes (Stuyfzand, 2017; Silva et al , 2018), these effects were absent in our experimental setting. Therefore, observed differences in soil moisture likely result from differences in infiltration and evaporation depending on sediment deposition rates (Table ).…”

Modeling the Evolution of a Freshwater Lens under Highly Dynamic Conditions on a Currently Developing Barrier Island

“…Even the larger zone characterized by salinities ≤20 g L -1 does not expand further after 100 years due to downward migrating saltwater fingers. However, transient conditions of the geomorphology, e.g., further dune growth due to accretion or dune erosion-which are likely to occur in such a highly dynamic environment studied here-will most likely affect the extent of the investigated FWL in the future, since variations of the coastal landscape have impact on the shape and volume of a FWL 11 Geofluids [77,78]. This was also visible in the onset of the FWL formation in the VG-model (Figure 4).…”

Investigating the Land-Sea Transition Zone

“…FWL on barrier islands evolve in dune areas that are not prone to inundation events (Röper et al 2013;Holt et al 2017). The formation process is mainly driven by density differences between infiltrating freshwater and saline groundwater (Drabbe and Badon Ghijben 1889;Herzberg 1901;Fetter 1972;Vacher 1988), while factors such as tides, groundwater recharge rates, land propagation, or climate change may play an additional role (Röper et al 2013;Stuyfzand 2017;Holt et al 2019). Being the only natural freshwater resource on barrier islands, FWL are precious for both drinking water supply and freshwater ecosystems.…”