Reistijden in de bodem en aanvulling van het grondwater uit het Landelijk (LMG) en de Provinciale Meetnetten Grondwaterkwaliteit (PMG)
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Series / Report no.
Open Access
Type
Report
Language
nl
Date
2003-05-14
Research Projects
Organizational Units
Journal Issue
Title
Reistijden in de bodem en aanvulling van het
grondwater uit het Landelijk (LMG) en de Provinciale Meetnetten
Grondwaterkwaliteit (PMG)
Translated Title
Travel times and groundwaterrecharge determined
from the national and provincial monitoring
networks
Published in
Abstract
De reistijden van het grondwater in de bodem (in jaren)
zijn onmisbaar bij het verklaren van verontreinigingen in het grondwater
voor milieukundige overzichten. Gegevens over de concentraties aan tritium
(3H) in monsters water uit 332 filters van 187 putten van provinciale
meetnetten grondwaterkwaliteit (PMG) in Drenthe, Gelderland, Zuid-Holland en
Brabant zijn gebruikt voor bepalingen van reistijden in de bodem en de
aanvulling van het grondwater in de zandgebieden. Eerdere resultaten uit
het landelijk meetnet zijn nogmaals samengevat. De filters van PMG liggen
op een diepte van minder dan 10 tot ongeveer 25 m onder maaiveld. De
tritiumconcentraties leverden waarden op voor de reistijden in de bodem en
de aanvulling door de neerslag van het bemonsterde grondwater. In 45
monsters was de 3H concentratie lager dan de detectiegrens. In bepaalde
gebieden komt oppervlakkige afvoer van de neerslag voor, zodat de aanvulling
van het grondwater kleiner is dan het neerslagoverschot. De belangrijkste
oorzaak is het voorkomen van slecht doorlatende lagen in de ondiepe bodem.
De analyse van de PMG gegevens toont echter aan dat nog andere factoren een
rol kunnen spelen zoals het geringe doorlaatvermogen van de ondergrond in
Oost-Gelderland en in delen van De Peel en het voorkomen van Holocene
kleilagen in het kustgebied. Het betreft relatief kleine gebieden, zodat de
eerder gegeven beelden van de reistijden en de aanvulling van het grondwater
gebaseerd op het landelijk meetnet in het algemeen geldig
blijven.
Travel times of groundwater in the soil are necessary for describing and explaining data on groundwater pollution as needed for environmental surveys. Tritium (3H) concentrations in groundwater samples derived from provincial groundwater monitoring systems (PMG) in the Netherlands were made available by the provinces of Drenthe, Gelderland, Zuid-Holland and Noord-Brabant. Preceding results from the national monitoring system were summarised. The PMG samples came from 187 wells in regions with a sandy soil, where 332 screens were placed. The shallowest screens were at a depth of less than 10 m and the deepest ones of roughly 25 m below land surface. Elaboration yielded values for travel times in the soil and recharge by precipitation of the sampled groundwater. No determination was possible in 45 out of the 332 samples, where concentrations were below the detection limit. The investigation showed that groundwater sampled from PMG had predominantly been recharged near the observed wells (within a radius of some kilometres). Excess precipitation is discharged by surficial runoff components in certain areas, thus reducing the water available for groundwater recharge. Surficial recharge is caused by specific factors, the main reason being the presence of low permeability layers in the topsoil. Additionally, the elaboration of PMG data showed that also areas having a subsurface of low transmissivity, or the presence of Holocene clay layers in the coastal zone, might influence the occurrence of surficial runoff. The areas concerned are relatively small and their situation is well known, so that the general representation of groundwater recharge and travel times in sandy regions given before does not need revisions.
Travel times of groundwater in the soil are necessary for describing and explaining data on groundwater pollution as needed for environmental surveys. Tritium (3H) concentrations in groundwater samples derived from provincial groundwater monitoring systems (PMG) in the Netherlands were made available by the provinces of Drenthe, Gelderland, Zuid-Holland and Noord-Brabant. Preceding results from the national monitoring system were summarised. The PMG samples came from 187 wells in regions with a sandy soil, where 332 screens were placed. The shallowest screens were at a depth of less than 10 m and the deepest ones of roughly 25 m below land surface. Elaboration yielded values for travel times in the soil and recharge by precipitation of the sampled groundwater. No determination was possible in 45 out of the 332 samples, where concentrations were below the detection limit. The investigation showed that groundwater sampled from PMG had predominantly been recharged near the observed wells (within a radius of some kilometres). Excess precipitation is discharged by surficial runoff components in certain areas, thus reducing the water available for groundwater recharge. Surficial recharge is caused by specific factors, the main reason being the presence of low permeability layers in the topsoil. Additionally, the elaboration of PMG data showed that also areas having a subsurface of low transmissivity, or the presence of Holocene clay layers in the coastal zone, might influence the occurrence of surficial runoff. The areas concerned are relatively small and their situation is well known, so that the general representation of groundwater recharge and travel times in sandy regions given before does not need revisions.
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