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Determination of denitrification parameters in deep groundwater. A pilot study for several pumping stations in the Netherlands
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Series / Report no.
Open Access
Type
Report
Language
en
Date
2004-05-10
Research Projects
Organizational Units
Journal Issue
Title
Determination of denitrification parameters in deep
groundwater. A pilot study for several pumping stations in the
Netherlands
Translated Title
Bepaling van denitrificatieparameters in diep
grondwater. Een 'pilot-study' voor enkele pompstations in
Nederland
Published in
Abstract
Nitraatmetingen in het grondwater in midden en oost
Nederland geven aan dat op een groot aantal locaties denitrificatie
optreedt. Bij modelstudies ter ondersteuning van beleidsbeslissingen dient
men derhalve rekening te houden met denitrificatie. Er is echter weinig
informatie over de denitrificatiesnelheid en de ruimtelijke verdeling ervan.
In dit rapport wordt een modelconcept voorgesteld om denitrificatie te
simuleren. De onbekende parameters van dit concept worden bepaald aan de
hand van een kalibratieprocedure met hulp van het programma PEST. Voor de
kalibratie zijn nitraatmetingen gebruikt van het Landelijk en Provinciaal
Meetnet Grondwater, alsmede nitraatgegevens van het grondwater dat bij een
aantal drinkwater-pompstations wordt onttrokken. De kalibratie is
uitgevoerd voor negen pompstations in het midden en oosten van Nederland.
Voor zones waar organisch materiaal mag worden verwacht kan denitrificatie
worden beschreven als een exponentieel verval met een halfwaardetijd van
ongeveer 500 dagen. In zones waar organisch materiaal niet voorkomt is de
halfwaardetijd veel hoger (ca 2750 dagen). In de buurt van het freatisch
vlak wordt een 'instantane' denitrificatie verondersteld. Hier treedt een
gemiddelde reductie op van 50%. Dit getal doet echter tevens dienst als
compensatie voor een mogelijke onder- of overschatting van de hoeveelheid
nitraat die bij de waterspiegel het systeem ingaat. De gevonden
parameterwaarden bezitten een grote onzekerheid. Enkele verklaringen worden
geopperd en bediscussieerd.
Groundwater nitrate measurements in the central and eastern part of the Netherlands point out that at many locations denitrification occurs. Therefore, models applied for analysis of management decisions concerning the groundwater nitrate distribution, need to take denitrification into account. Little information is available on the rate of denitrification and its spatial distribution. In this report a model concept is proposed to simulate denitrification. The unknown parameters in this concept are determined by a calibration procedure using the program PEST. For calibration nitrate measurements from the Monitoring Network are used as well as nitrate data from the abstracted water in a number of public drinking water pumping stations. Calibration is carried out for nine groundwater-pumping stations in the central and eastern part of the Netherlands. In zones where the presence of organic substances is likely, denitrification may be described as an exponential decay with a half-life time of around 500 days. In zones where organic material is absent, the half-life time is much higher (2750 days). At or near the phreatic surface an instantaneous nitrate reduction is assumed. Here, an average reduction of 50% is found. This figure, however, may also represent a compensation for over- or underestimation of the nitrate input at the water table. The parameter values found in this report appear to have a high measure of uncertainty. Several explanations are suggested and discussed.
Groundwater nitrate measurements in the central and eastern part of the Netherlands point out that at many locations denitrification occurs. Therefore, models applied for analysis of management decisions concerning the groundwater nitrate distribution, need to take denitrification into account. Little information is available on the rate of denitrification and its spatial distribution. In this report a model concept is proposed to simulate denitrification. The unknown parameters in this concept are determined by a calibration procedure using the program PEST. For calibration nitrate measurements from the Monitoring Network are used as well as nitrate data from the abstracted water in a number of public drinking water pumping stations. Calibration is carried out for nine groundwater-pumping stations in the central and eastern part of the Netherlands. In zones where the presence of organic substances is likely, denitrification may be described as an exponential decay with a half-life time of around 500 days. In zones where organic material is absent, the half-life time is much higher (2750 days). At or near the phreatic surface an instantaneous nitrate reduction is assumed. Here, an average reduction of 50% is found. This figure, however, may also represent a compensation for over- or underestimation of the nitrate input at the water table. The parameter values found in this report appear to have a high measure of uncertainty. Several explanations are suggested and discussed.
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VROM-DGM-BWL