Deltares
SOil and groundwater
As upward seepage of saline groundwater from the upper aquifer is leading to surface water salinization of deep polders in the Netherlands, we monitored the processes involved in the Noordplas Polder, a typical deep polder. Our results... more
As upward seepage of saline groundwater from the upper aquifer is leading to surface water salinization of deep polders in the Netherlands, we monitored the processes involved in the Noordplas Polder, a typical deep polder. Our results show three types of seepage: (1) diffuse seepage through the Holocene confining layer, (2) seepage through paleochannel belts in the Holocene layer, and (3) intense seepage via localized boils. They differ with regard to seepage flux, chloride concentration, and their location in the polder; thus, their contributions to surface water salinization also differ. Permeable, sandy paleochannel belts cut through the lower part of the Holocene layer, resulting in higher seepage fluxes than the diffuse seepage through the Holocene layer where there are no paleochannels. The average chloride concentration of paleochannel seepage is about 600 mg/l, which is sixfold higher than the average concentration of diffuse seepage. The highest seepage fluxes and chloride concentrations are found at boils, which are small vents in the Holocene layer through which groundwater preferentially discharges at high velocities. This results in upconing of deeper and more saline groundwater, which produces an average chloride concentration of 1100 mg/l. Despite the fact that seepage fluxes are difficult to measure, we were able to calculate that boils contribute more than 50% of the total chloride load entering the Noordplas Polder and they therefore form the dominant salinization pathway.
- by Gualbert Oude Essink and +1
- •
- Hydrogeology, Salinization, Polders, Sand Boils
Climate change in combination with increased anthropogenic activities will affect coastal groundwater systems throughout the world. In this paper, we focus on a coastal groundwater system that is already threatened by a relatively high... more
Climate change in combination with increased anthropogenic activities will affect coastal groundwater systems throughout the world. In this paper, we focus on a coastal groundwater system that is already threatened by a relatively high seawater level: the low‐lying Dutch Delta. Nearly one third of the Netherlands lies below mean sea level, and the land surface is still subsiding up to 1 m per century. This densely populated delta region, where fresh groundwater resources are used intensively for domestic, agricultural, and industrial purposes, can serve as a laboratory case for other low-lying delta areas throughout the world. Our findings on hydrogeological effects can be scaled up since the problems the Dutch face now will very likely be the problems encountered in other delta areas in the future. We calculated the possible impacts of future sea level rise, land subsidence, changes in recharge, autonomous salinization, and the effects of two mitigation countermeasures with a three‐dimensional numerical model for variable density groundwater flow and coupled solute transport. We considered the effects on hydraulic heads, seepage fluxes, salt loads to surface waters, and changes in fresh groundwater resources as a function of time and for seven scenarios. Our numerical modeling results show that the impact of sea level rise is limited to areas within 10 km of the coastline and main rivers because the increased head in the groundwater system at the coast can easily be produced though the highly permeable Holocene confining layer. Along the southwest coast of the Netherlands, salt loads will double in some parts of the deep and large polders by the year 2100 A.D. due to sea level rise. More inland, ongoing land subsidence will cause hydraulic heads and phreatic water levels to drop, which may result in damage to dikes, infrastructure, and urban areas. In the deep polders more inland, autonomous upconing of deeper and more saline groundwater will be responsible for increasing salt loads. The future increase of salt loads will cause salinization of surface waters and shallow groundwater and put the total volumes of fresh groundwater volumes for drinking water supply, agricultural purposes, industry, and ecosystems under pressure.
Thin rainwater lenses (RW-lenses) near the land surface are often the only source of freshwater in agricultural areas with regionally-extensive brackish to saline groundwater. The seasonal and inter-annual dynamics of these lenses are... more
Thin rainwater lenses (RW-lenses) near the land surface are often the only source of freshwater in agricultural areas with regionally-extensive brackish to saline groundwater. The seasonal and inter-annual dynamics of these lenses are poorly known. Here this knowledge gap is addressed by investigating the transient flow and mixing processes in RW-lenses beneath two tile-drained agricultural fields in the Netherlands. Evidence of RW-lens dynamics was systematically collected by monthly ground and soil water sampling, in combination with daily observations of water table elevation, drain tile discharge and drain water salinity. Based on these data, and numerical modeling of the key lens characteristics, a conceptual model of seasonal lens dynamics is presented. It is found that variations in the position of the mixing zone and mixing zone salinities are small and vary on a seasonal timescale, which is attributed to the slow transient oscillatory flow regime in the deepest part of the lens. The flow and mixing processes are faster near the water table, which responds to recharge and evapotranspiration at a time-scale less than a day. Variations of drain tile discharge and drain water salinity are also very dynamic as they respond to individual rain events. Salinities of soil water can become significantly higher than in the groundwater. This is attributed to the combined effect of capillary rise of saline groundwater during dry periods and incomplete flushing by infiltrating freshwater due to preferential flow through cracks in the soil. The results of this study are the key to understanding the potential impact of future climate change and to designing effective mitigating measures such as adapting tile-drainage systems to ensure the future availability of freshwater for agriculture.
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... more
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 m
3
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 m
2
. 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.
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 m
3
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 m
2
. 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.
The forecast of climate change effects on the groundwater system in coastal areas is of key importance for policy makers. The Dutch water system has been deeply studied because of its complex system of low-lying areas, dunes, land won to... more
The forecast of climate change effects on the groundwater system in coastal areas is of key importance for policy makers. The Dutch water system has been deeply studied because of its complex system of low-lying areas, dunes, land won to the sea and dikes, but nowadays large efforts are still being done to find out the best techniques to describe complex fresh-brackish-saline groundwater dynamic systems. In this paper, we describe a methodology consisting of high-resolution airborne electromagnetic (EM) measurements used in a 3-D variable-density transient groundwater model for a coastal area in the Netherlands. We used the airborne EM measurements in combination with borehole-logging data, electrical conductivity cone penetration tests and groundwater samples to create a 3-D fresh-brackish-saline groundwater distribution of the study area. The EM measurements proved to be an improvement compared to older techniques and provided quality input for the model. With the help of the built 3-D variable-density groundwater model, we removed the remaining inaccuracies of the 3-D chloride field and predicted the effects of three climate scenarios on the groundwater and surface water system. Results showed significant changes in the groundwater system, and gave direction for future water policy. Future research should provide more insight in the improvement of data collection for fresh-brackish-saline groundwater systems as it is of high importance to further improve the quality of the model.
Groundwater-surface water (GW-SW) interaction in numerical groundwater flow models is generally simulated using a Cauchy boundary condition, which relates the flow between the surface water and the groundwater to the product of the head... more
Groundwater-surface water (GW-SW) interaction in numerical groundwater flow models is generally simulated using a Cauchy
boundary condition, which relates the flow between the surface water and the groundwater to the product of the head difference between the node and the surface water level, and a coefficient, often referred to as the ‘‘conductance.’’ Previous studies have shown that in models with a low grid resolution, the resistance to GW-SW interaction below the surface water bed should often be accounted for in the parameterization of the conductance, in addition to the resistance across the surface water bed. Three conductance expressions that take this resistance into account were investigated: two that were presented by Mehl and Hill (2010) and the one that was presented by De Lange (1999). Their accuracy in low-resolution models regarding salt and water fluxes to a dense drainage network in a confined aquifer system was determined. For a wide range of hydrogeological conditions, the influence of (1) variable groundwater density; (2) vertical grid discretization; and (3) simulation of both ditches and tile drains in a single
model cell was investigated. The results indicate that the conductance expression of De Lange (1999) should be used in similar hydrogeological conditions as considered in this paper, as it is better taking into account the resistance to flow below the surface water bed. For the cases that were considered, the influence of variable groundwater density and vertical grid discretization on the accuracy of the conductance expression of De Lange (1999) is small.
boundary condition, which relates the flow between the surface water and the groundwater to the product of the head difference between the node and the surface water level, and a coefficient, often referred to as the ‘‘conductance.’’ Previous studies have shown that in models with a low grid resolution, the resistance to GW-SW interaction below the surface water bed should often be accounted for in the parameterization of the conductance, in addition to the resistance across the surface water bed. Three conductance expressions that take this resistance into account were investigated: two that were presented by Mehl and Hill (2010) and the one that was presented by De Lange (1999). Their accuracy in low-resolution models regarding salt and water fluxes to a dense drainage network in a confined aquifer system was determined. For a wide range of hydrogeological conditions, the influence of (1) variable groundwater density; (2) vertical grid discretization; and (3) simulation of both ditches and tile drains in a single
model cell was investigated. The results indicate that the conductance expression of De Lange (1999) should be used in similar hydrogeological conditions as considered in this paper, as it is better taking into account the resistance to flow below the surface water bed. For the cases that were considered, the influence of variable groundwater density and vertical grid discretization on the accuracy of the conductance expression of De Lange (1999) is small.
in Dutch: GO-FRESH is een Kennis voor Klimaat project in de Zuidwestelijke Delta. Een consortium bestaande uit Deltares (trekker), Alterra, KWR, Acacia Water en HZ University of Applied Sciences onderzoekt in hoeverre lokale maatregelen... more
in Dutch:
GO-FRESH is een Kennis voor Klimaat project in de Zuidwestelijke Delta. Een consortium bestaande uit Deltares (trekker), Alterra, KWR, Acacia Water en HZ University of Applied Sciences onderzoekt in hoeverre lokale maatregelen de zoetwaterbeschikbaarheid voor de landbouw kunnen vergroten in gebieden die onafhankelijk zijn van het hoofdwatersysteem. Hierbij wordt de ondergrond gebruikt voor opslag van zoet water in periodes van wateroverschot, om het water vervolgens te gebruiken in droge tijden.
Dit project richt zich op twee typen gebieden die door klimaatverandering waarschijnlijk sneller onder druk komen te staan in termen van zoetwaterbeschikbaarheid: a. gebieden met kreekruggen met een wat diepere zoetwaterlens en b. lager gelegen gebieden met zoute kwel en dientengevolge dunne regenwaterlenzen.
Binnen GO-FRESH zijn 3 maatregelen gestart:
1. Kreekrug Infiltratie Proef: toename zoetwatervoorraad in een kreekrug door verhoging van de grondwaterstand via peilgestuurde drainage in combinatie met infiltratie van oppervlaktewater
2. Freshmaker: toename zoetwatervoorraad in een kreekrug door injectie zoet water én onttrekking zout grondwater via horizontale putten
3. Drains2Buffer: vergroten/behouden zoetwatervoorraad van dunne regenwaterlenzen door diepe drainage
Twee van de drie maatregelen hebben de dikte van de zoetwaterlens vergroot tijdens de duur van het KvK project (dat liep van 1 maart 2013 tot en met 28 februari 2014) en kunnen op de proeflocaties droogteschade voorkomen. In de derde maatregel is de meetperiode niet lang genoeg geweest om te onderscheiden of de dunne zoetwaterlens is gegroeid als direct gevolg van de maatregel of als onderdeel van de natuurlijke variabiliteit van het weer. Binnen het werkpakket Sociaaleconomische haalbaarheid is -op basis van een multicriteria analyse- een rekentool in Excel gemaakt om de netto opbrengst te berekenen bij implementatie van de 3 maatregelen. Het werkpakket Kennistransfer heeft alle opgedane kennis in een zogenaamde VUE-Graph omgezet.
GO-FRESH is een Kennis voor Klimaat project in de Zuidwestelijke Delta. Een consortium bestaande uit Deltares (trekker), Alterra, KWR, Acacia Water en HZ University of Applied Sciences onderzoekt in hoeverre lokale maatregelen de zoetwaterbeschikbaarheid voor de landbouw kunnen vergroten in gebieden die onafhankelijk zijn van het hoofdwatersysteem. Hierbij wordt de ondergrond gebruikt voor opslag van zoet water in periodes van wateroverschot, om het water vervolgens te gebruiken in droge tijden.
Dit project richt zich op twee typen gebieden die door klimaatverandering waarschijnlijk sneller onder druk komen te staan in termen van zoetwaterbeschikbaarheid: a. gebieden met kreekruggen met een wat diepere zoetwaterlens en b. lager gelegen gebieden met zoute kwel en dientengevolge dunne regenwaterlenzen.
Binnen GO-FRESH zijn 3 maatregelen gestart:
1. Kreekrug Infiltratie Proef: toename zoetwatervoorraad in een kreekrug door verhoging van de grondwaterstand via peilgestuurde drainage in combinatie met infiltratie van oppervlaktewater
2. Freshmaker: toename zoetwatervoorraad in een kreekrug door injectie zoet water én onttrekking zout grondwater via horizontale putten
3. Drains2Buffer: vergroten/behouden zoetwatervoorraad van dunne regenwaterlenzen door diepe drainage
Twee van de drie maatregelen hebben de dikte van de zoetwaterlens vergroot tijdens de duur van het KvK project (dat liep van 1 maart 2013 tot en met 28 februari 2014) en kunnen op de proeflocaties droogteschade voorkomen. In de derde maatregel is de meetperiode niet lang genoeg geweest om te onderscheiden of de dunne zoetwaterlens is gegroeid als direct gevolg van de maatregel of als onderdeel van de natuurlijke variabiliteit van het weer. Binnen het werkpakket Sociaaleconomische haalbaarheid is -op basis van een multicriteria analyse- een rekentool in Excel gemaakt om de netto opbrengst te berekenen bij implementatie van de 3 maatregelen. Het werkpakket Kennistransfer heeft alle opgedane kennis in een zogenaamde VUE-Graph omgezet.
- by Gualbert Oude Essink and +2
- •
- Hydrogeology, Groundwater Hydrology, Water Supply, ASR
In zoute kwelgebieden zijn dunne regenwaterlenzen van groot belang voor de landbouw omdat ze vaak de enige zoetwaterbron zijn en voorkomen dat (te) zout kwelwater de wortelzone bereikt. Veldonderzoek in Zeeland heeft voor het eerst in... more
In zoute kwelgebieden zijn dunne regenwaterlenzen van groot belang voor de landbouw omdat ze vaak de enige zoetwaterbron zijn en voorkomen dat (te) zout kwelwater de wortelzone bereikt. Veldonderzoek in Zeeland heeft voor het eerst in beeld gebracht hoe deze lenzen er precies uitzien, hoe ze veranderen in de tijd en welke processen daarbij een rol spelen. De lenzen blijken erg kwetsbaar voor klimaatverandering. De opgedane systeemkennis maakt het mogelijk hiervoor mitigerende maatregelen te formuleren.
- by Perry Louw and +3
- •
- Hydrogeology, Groundwater modeling
Although the recent history of human colonisation and impact on Mauritius is well documented, virtually no records of the pre-human native ecosystem exist, making it difficult to assess the magnitude of the changes brought about by human... more
Although the recent history of human colonisation and impact on Mauritius is well documented, virtually no records of the pre-human native ecosystem exist, making it difficult to assess the magnitude of the changes brought about by human settlement. Here, we describe a 4000-year-old fossil bed at Mare aux Songes (MAS) in south-eastern Mauritius that contains both macrofossils (vertebrate fauna, gastropods, insects and flora) and microfossils (diatoms, pollen, spores and phytoliths). With >250 bone fragments/m 2 and comprising 50% of all known extinct and extant vertebrate species (n s ¼ 44) of Mauritius, MAS may constitute the first Holocene vertebrate bone Concentration-Lagerstä tte identified on an oceanic volcanic island. Fossil remains are dominated by extinct giant tortoises Cylindraspis spp. (63%), passerines (w10%), small bats (7.8%) and dodo Raphus cucullatus (7.1%). Twelve radiocarbon ages [four of them duplicates] from bones and other material suggest that accumulation of fossils took place within several centuries. An exceptional combination of abiotic conditions led to preservation of bones, bone collagen, plant tissue and microfossils. Although bone collagen is well preserved, DNA from dodo and other Mauritian vertebrates has proved difficult. Our analysis suggests that from ca 4000 years ago (4 ka), rising sea levels created a freshwater lake at MAS, generating an oasis in an otherwise dry environment which attracted a diverse vertebrate fauna. Subsequent aridification in the south-west Indian Ocean region may have increased carcass accumulation during droughts, contributing to the exceptionally high fossil concentration. The abundance of floral and faunal remains in this Lagerstä tte offers a unique opportunity to reconstruct a pre-human ecosystem on an oceanic island, providing a key foundation for assessing the vulnerability of island ecosystems to human impact.
In the light of the currently increasing drought frequency and water scarcity on oceanic islands, it is crucial for the conservation of threatened insular vertebrates to assess how they will be affected. A 4000 yr old fossil assemblage in... more
In the light of the currently increasing drought frequency and water scarcity on oceanic islands, it is crucial for the conservation of threatened insular vertebrates to assess how they will be affected. A 4000 yr old fossil assemblage in the Mare Aux Songes (MAS), southwest Mauritius, Mascarene Islands, contains bones of 100 000+ individual vertebrates, dominated by two species of giant tortoises Cylindraspis triserrata and C. inepta, the dodo Raphus cucullatus, and 20 other vertebrate species Mid-Holocene vertebrate bone Concentration-Lagerstätte on oceanic island Mauritius provides a window into the ecosystem of the dodo (Raphus cucullatus). Quaternary Science Reviews 28: 14-24). Nine radiocarbon dates of bones statistically overlap and suggest mass mortality occurred between 4235 and 4100 cal. yr BP. The mortality period coincides with a widely recognized megadrought event. Our multidisciplinary investigations combining geological, paleontological and hydrological evidence suggests the lake was located in a dry coastal setting and had desiccated during this period. Oxygen isotope data from a Uranium-series dated stalagmite from Rodrigues, an island 560 km east of Mauritius, supports this scenario by showing frequently alternating dry and wet periods lasting for decades between 4122 and 2260 cal. yr BP. An extreme drought resulted in falling water-tables at MAS and elsewhere on the island, perhaps deprived these insular vertebrates of fresh water, which led to natural mass mortalities and possibly to extirpations. In spite of these events, all insular species survived until at least the seventeenth century, confirming their resistance to climatic extremes. Despite this, the generally exponential increase of combined human impacts on islands including loss of geodiversity, habitats, and stocks of fresh water, there will be less environmental safe-haven options for insular endemic and native vertebrates during future megadrought conditions; and therefore will be more prone to extinction.
Although the recent history of human colonisation and impact on Mauritius is well documented, virtually no records of the pre-human native ecosystem exist, making it difficult to assess the magnitude of the changes brought about by human... more
Although the recent history of human colonisation and impact on Mauritius is well documented, virtually no records of the pre-human native ecosystem exist, making it difficult to assess the magnitude of the changes brought about by human settlement. Here, we describe a 4000-year-old fossil bed at Mare aux Songes (MAS) in south-eastern Mauritius that contains both macrofossils (vertebrate fauna, gastropods, insects and flora) and microfossils (diatoms, pollen, spores and phytoliths). With >250 bone fragments/m 2 and comprising 50% of all known extinct and extant vertebrate species (n s ¼ 44) of Mauritius, MAS may constitute the first Holocene vertebrate bone Concentration-Lagerstä tte identified on an oceanic volcanic island. Fossil remains are dominated by extinct giant tortoises Cylindraspis spp. (63%), passerines (w10%), small bats (7.8%) and dodo Raphus cucullatus (7.1%). Twelve radiocarbon ages [four of them duplicates] from bones and other material suggest that accumulation of fossils took place within several centuries. An exceptional combination of abiotic conditions led to preservation of bones, bone collagen, plant tissue and microfossils. Although bone collagen is well preserved, DNA from dodo and other Mauritian vertebrates has proved difficult. Our analysis suggests that from ca 4000 years ago (4 ka), rising sea levels created a freshwater lake at MAS, generating an oasis in an otherwise dry environment which attracted a diverse vertebrate fauna. Subsequent aridification in the south-west Indian Ocean region may have increased carcass accumulation during droughts, contributing to the exceptionally high fossil concentration. The abundance of floral and faunal remains in this Lagerstä tte offers a unique opportunity to reconstruct a pre-human ecosystem on an oceanic island, providing a key foundation for assessing the vulnerability of island ecosystems to human impact.
In the light of the currently increasing drought frequency and water scarcity on oceanic islands, it is crucial for the conservation of threatened insular vertebrates to assess how they will be affected. A 4000 yr old fossil assemblage in... more
In the light of the currently increasing drought frequency and water scarcity on oceanic islands, it is crucial for the conservation of threatened insular vertebrates to assess how they will be affected. A 4000 yr old fossil assemblage in the Mare Aux Songes (MAS), southwest Mauritius, Mascarene Islands, contains bones of 100 000+ individual vertebrates, dominated by two species of giant tortoises Cylindraspis triserrata and C. inepta, the dodo Raphus cucullatus, and 20 other vertebrate species Mid-Holocene vertebrate bone Concentration-Lagerstätte on oceanic island Mauritius provides a window into the ecosystem of the dodo (Raphus cucullatus). Quaternary Science Reviews 28: 14-24). Nine radiocarbon dates of bones statistically overlap and suggest mass mortality occurred between 4235 and 4100 cal. yr BP. The mortality period coincides with a widely recognized megadrought event. Our multidisciplinary investigations combining geological, paleontological and hydrological evidence suggests the lake was located in a dry coastal setting and had desiccated during this period. Oxygen isotope data from a Uranium-series dated stalagmite from Rodrigues, an island 560 km east of Mauritius, supports this scenario by showing frequently alternating dry and wet periods lasting for decades between 4122 and 2260 cal. yr BP. An extreme drought resulted in falling water-tables at MAS and elsewhere on the island, perhaps deprived these insular vertebrates of fresh water, which led to natural mass mortalities and possibly to extirpations. In spite of these events, all insular species survived until at least the seventeenth century, confirming their resistance to climatic extremes. Despite this, the generally exponential increase of combined human impacts on islands including loss of geodiversity, habitats, and stocks of fresh water, there will be less environmental safe-haven options for insular endemic and native vertebrates during future megadrought conditions; and therefore will be more prone to extinction.
In deltaic areas with saline seepage, freshwater availability is often limited to shallow rainwater lenses lying on top of saline groundwater. Here we describe the characteristics and spatial variability of such lenses in areas with... more
In deltaic areas with saline seepage, freshwater availability is often limited to shallow rainwater lenses lying on top of saline groundwater. Here we describe the characteristics and spatial variability of such lenses in areas with saline seepage and the mechanisms that control their occurrence and size. Our findings are based on different types of field measurements and detailed numerical groundwater models applied in the south-western delta of the Netherlands. By combining the applied techniques we could extrapolate measurements at point scale (groundwater sampling, temperature and electrical soil conductivity (TEC)-probe measurements, electrical cone penetration tests (ECPT)) to field scale (continuous vertical electrical soundings (CVES), electromagnetic survey with EM31), and even to regional scale using helicopter-borne electromagnetic measurements (HEM). The measurements show a gradual mixing zone between infiltrating fresh rainwater and upward flowing saline groundwater. The mixing zone is best characterized by the depth of the centre of the mixing zone D mix , where the salinity is half that of seepage water, and the bottom of the mixing zone B mix , with a salinity equal to that of the seepage water (Cl-conc. 10 to 16 g l −1 ). D mix is found at very shallow depth in the confining top layer, on average at 1.7 m below ground level (b.g.l.), while B mix lies about 2.5 m b.g.l. The model results show that the constantly alternating upward and downward flow at low velocities in the confining layer is the main mechanism of mixing between rainwater and saline seepage and determines the position and extent of the mixing zone (D mix and B mix ). Recharge, seepage flux, and drainage depth are the controlling factors.
A controlled artificial recharge and drainage (CARD) system was used to increase freshwater lenses below creek ridges to increase freshwater supply. Creek ridges are typical geomorphological features that lie up to 2 m higher than the... more
A controlled artificial recharge and drainage (CARD) system was used to increase freshwater lenses below creek ridges to increase freshwater supply. Creek ridges are typical geomorphological features that lie up to 2 m higher than the surroundings in the reclaimed tidal flat landscape of the southwestern Netherlands. The 5–30-m thick freshwater lenses below the creek ridges are a vital source for irrigation, as the groundwater and surface waters are predominantly saline. However, freshwater supply from these lenses is commonly not sufficient to meet the irrigation demand, which leads to crop damage. The CARD system was tested in the field and the development of the freshwater lens was monitored during the period May 2013 to May 2014. Numerical models, which were used to investigate a long-term effect of theCARD system, predicted that below the center of the creek ridge, the 13–15-m thick freshwater lens increased 6–8 m within 10 years. The total volumetric increase of the freshwater lens was about 190,000 m3 after 10 years, which was about 40 % of the total recharge (natural and artificial recharge). From this increased freshwater lens, up to three times more water can be extracted using horizontal wells, compared to the initial size of the freshwater lens. A higher water table in the CARD system leads to a thicker freshwater lens but a lower storage efficiency. A lower water table has the opposite effect.
Since 2005, excavations at Mare aux Songes, Mauritius, have revealed the presence of a very rich, ∼4,200-year-old fossil bone bed including dodo (Raphus cucullatus) bones and bone fragments. The recently excavated dodo assemblage... more
Since 2005, excavations at Mare aux Songes, Mauritius, have revealed the presence of a very rich, ∼4,200-year-old fossil bone bed including dodo (Raphus cucullatus) bones and bone fragments. The recently excavated dodo assemblage comprises at least 17 individuals and is characterised by the presence of small and fragile skeletal elements, a dominance of leg elements and an absence of juveniles. The hydrology of the area suggests that dodos, like many other species, were probably lured to Mare aux Songes by the presence of freshwater during times of drought. The most likely scenario for the origin of the fossil deposit is that animals became trapped in the sediment in repeated miring events, which would favour the conservation of hindlimbs. Such a scenario is fully in accordance with the taphonomic characteristics of the bone assemblage.
- by Julian Hume and +2
- •
- Extinction
In the European TOPSOIL project, countries around the North Sea are searching for solutions for climate related threats. They explore the possibilities of using the topsoil layer to solve current and future water challenges. The main... more
In the European TOPSOIL project, countries around the North Sea are searching for solutions for climate related threats. They explore the possibilities of using the topsoil layer to solve current and future water challenges. The main objective is to improve the climate resilience of the water management of the topsoil and shallow aquifers in the North Sea region. TOPSOIL is supported by the Interreg VB North Sea Region program in line with priority 3 of the program: ‘Sustainable North Sea Region, protecting against climate change and preserving the environment’. The Belgian part of this project, called FRESHEM for GO-FRESH Vlaanderen (‘FREsh Salt groundwater distribution by Helicopter ElectroMagnetic survey for Geohydrological Opportunities FRESH water supply’), focuses on mapping the salinity distribution of groundwater using airborne electromagnetics and aims to look into a number of measures that could increase the availability of freshwater for agriculture in the polder area. Tw...
- by Perry Louw
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Several extended field-monitoring programmes were set up in the Netherlands to study hydrological and surface water quality impacts of agricultural drainage activities for clay soils. At the field scale, agricultural land use data and... more
Several extended field-monitoring programmes were set up in the Netherlands to study hydrological and surface water quality impacts of agricultural drainage activities for clay soils. At the field scale, agricultural land use data and hydrological measurements were combined with analysis of drainage water quality data, and related to the loading of the surface water system with salts and nutrients. Water, salt, and nutrient balances of the soil and/or surface water system were set up to analyse the hydrological behaviour of the systems studied and to quantify solute sources. Field observations were performed by field data analysis and numerical modelling, using the 1D F1W-MultiSWAP code, a 2D dual-porosity code, and the MODFLOW code. Drainage activities showed a significant impact on hydrology by influencing subsurface drainage water flow paths and regional-scale groundwater seepage. Through solute balances it was shown that the salt and nutrient loading of the surface water were clearly affected by both agricultural and drainage activities. The conclusion was that the drainage activities and their surface water quality aspects need to be studied in an integrated way. Changes in subsurface drainage system redesign, surface water level management, and the field-scale nutrient management need to be integrated. Copyright # 2006 John Wiley & Sons, Ltd. key words: agricultural drainage; solute loads; field experiments; numerical modelling; physical and chemical processes RÉ SUMÉ Au cours de la dernière décennie, de nombreux et vastes programmes de surveillance de terrain ont été mis en place aux Pays-Bas pour étudier l'impact des activités de drainage agricole en sols argileux sur l'hydrologie et la qualité des eaux de surface. À l'échelle du terrain, les données sur l'utilisation de la terre agricole et les mesures hydrologiques ont été combinées avec l'analyse des données sur la qualité des eaux de drainage par rapport au chargement de sels et d'éléments nutritifs dans le système d'eaux de surface. Des bilans eau, sels, éléments nutritifs du sol et/ou le système d'eaux de surface ont été réalisés en vue d'analyser le comportement hydrologique des systèmes étudiés et de quantifier les sources de soluté. Les observations de terrain ont été réalisées par analyse des données de terrain et modélisation numérique, à l'aide du code 1D F1W-MultiSWAP, d'un code à double porosité 2D et du code MODFLOW. Les activités de IRRIGATION AND DRAINAGE
Nationaal Onderzoekprogramma Kennis voor Klimaat (KvK). Alle rechten voorbehouden. Niets uit deze uitgave mag worden vermenigvuldigd, in geautomatiseerde bestanden opgeslagen en/of openbaar gemaakt door middel van druk, fotokopie,... more
Nationaal Onderzoekprogramma Kennis voor Klimaat (KvK). Alle rechten voorbehouden. Niets uit deze uitgave mag worden vermenigvuldigd, in geautomatiseerde bestanden opgeslagen en/of openbaar gemaakt door middel van druk, fotokopie, microfilm, geluidsband of op welke andere wijze ook, zonder voorafgaande schriftelijke toestemming van het Nationaal Onderzoekprogramma Kennis voor Klimaat. In overeenstemming met artikel 15a van het Nederlandse auteursrecht is het toegestaan delen van deze publicatie te citeren, daarbij gebruik makend van een duidelijke referentie naar deze publicatie. Aansprakelijkheid Hoewel uiterste zorg is besteed aan de inhoud van deze publicatie aanvaarden de Stichting Kennis voor Klimaat, de leden van deze organisatie, de auteurs van deze publicatie en hun organisaties, noch de samenstellers enige aansprakelijkheid voor onvolledigheid, onjuistheid of de gevolgen daarvan. Gebruik van de inhoud van deze publicatie is voor de verantwoordelijkheid van de gebruiker. Fresh Water Options Optimizer-Fase 1 Auteurs Jan van Bakel (De Bakelse Stroom) Perry de Louw (Deltares) Lodewijk Stuyt (WUR) Lieselotte Tolk (Acacia Water) Jouke Velstra (Acacia Water) Marco Hoogvliet (Deltares)
- by Perry Louw
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