1. Philippe Orban1, Serge Brouyère1 & Francis Delloye2
The GWB BE-Meuse-RWM040 “Chalk of the Geer basin”: delayed effect for nitrate in deep, old groundwater
Philippe Orban1, Serge Brouyère1 & Francis Delloye2
1Université de Liège, Urban & Environmental Engineering, Hydrogeology & Environmental Geology, Liège, Belgium
2Service public de Wallonie, DGO3 Department Water & Environment, Groundwater Direction, Jambes, Belgium
32nd CIS-Groundwater Working Group Meeting
Kempiński Hotel, Triq ir-Rokon, Gozo, Malta
25-26 April 2017
Good afternoon. Pleased to be here today to present our case study.
I would like to acknowledge Philippe Orban who did most of the work I am presenting today in the scope of his PhD at the University of Liège

2. GWB BE-Meuse-RWM040 “Chalk of the Geer basin”
Groundwater abstraction
Groundwater abstraction 30 millions m³/year of drinking water to supply approx. 600,000 people in the region of Liège
Draining galleries
465 km²
SOUTH
NORTH
Geology
soils and loess deposits
residual flint conglomerate and locally sands
fractured, dual-porosity chalk (aquifer reservoir)
clay (aquifer base)
So, the case study of interest correspond to an aquifer located in the Geer basin in Eastern Belgium, close to the city of Liège.
The groundwater resource is strategic for the region because 30 million m³ of groundwater is abstracted annually using pumping wells and 40 kms of draining galleries from a chalk aquifer. This aquifer is overlain by a thick unsaturated zone where water infiltrates across eolian loess deposits and the unsaturated part of the chalk. No preferential flow is observed.
Image adapted from Programme Action Hesbaye
32nd CIS Groundwater Working Group Meeting - Malta /04/2017

3. 32nd CIS Groundwater Working Group Meeting - Malta - 25-26/04/2017
GWB BE-Meuse-RWM040 “Chalk of the Geer basin”
Intensive agriculture (65% of the basin)
Groundwater quality nitrate concentrations close to and even above the 50mg/L NO3 threshold
This thick layer of eolian loess provides the Geer basin with very good soils for agriculture. As a consequence, 65% of the basin is covered by intensive agriculture.
And, of course, as observed in many other regions, this has resulted in the deterioration of groundwater quality with a continuous increase in nitrate concentrations observed during the last 60 years and the 50 mg/L threshold excessed in different parts of the aquifer.
32nd CIS Groundwater Working Group Meeting - Malta /04/2017

4. 32nd CIS Groundwater Working Group Meeting - Malta - 25-26/04/2017
GWB BE-Meuse-RWM040 “Chalk of the Geer basin”
To face this problem, different measures taken:
GWB BE-Meuse-RWM040 classified in 1994 as vulnerable to NO3 in the sense of the EU Nitrate Directive (91/676/CEE)
Action programme in accordance with the Code of Good Agricultural Practice
Investigations on fate and management of NO3 in the basin (Programme Action Hesbaye (Dautrebande et al. 1996, Hallet, 1998…)
More recently, NO3 treatment plant built by CILE (water company)
Despite these efforts, key remaining questions:
Any natural mechanisms contributing to delay groundwater quality improvement in response to the applied measures?
How long do we have to wait until NO3 trend reversal is observed in groundwater of the Geer basin?
Different measures have been taken in the years 90s to try to reduce the nitrate pressure in the Geer basin. The basin was classified as vulnerable to nitrate, different investigations and programmes of measures were set up in the basin with farmes and water managers. Despite all these efforts, NO3 concentrations are still increasing and recently, the main water company exploiting the galleries in the basin has built a nitrate treatment plant.
So, there are key questions that remained addressed: (1) is there any « natural » explanation to the fact that we do not see yet the effect of measures that started 20 years ago? (2) how long do we have to wait still until NO3 trend reversal is observed?
To contribute to answer to these questions, complementary investigations and modelling was performed, mostly in the scope the the FP6 IP AquaTerra project.
32nd CIS Groundwater Working Group Meeting - Malta /04/2017

5. 32nd CIS Groundwater Working Group Meeting - Malta - 25-26/04/2017
GWB BE-Meuse-RWM040 “Chalk of the Geer basin”
NO3
Spatial distribution of nitrate content
S : NO3 =~ 30 – 90 mg/L
NE : NO3 =~ 20 – 25 mg/L
N : NO3 =~ 0 mg/L
Spatial distribution of tritium content
S : UT =~ 10, young water
NE : UT =~ 5, mixing between old/young
N : UT =~ 1, old water
tritium
Recharge area
→ young water
One of the key result of these investigations is the observation of a strong correspondance between nitrate and tritium spatial patterns in groundwater. This allows proposing a conceptual model for groundwater and NO3 dynamics at thr regional scale, with (1) a zone where recharge processes dominates (highest NO3 and 3H vaklues), in the south part of the basin, (2) a zone where mixing between older and younger water occurs (north east) and the confined part of the aquifer, in the Flamish region of Belgium where old, non polluted groundwater is found.
Discharge area
→ mixing between old/young
Confined area
→ old water
32nd CIS Groundwater Working Group Meeting - Malta /04/2017

6. 32nd CIS Groundwater Working Group Meeting - Malta - 25-26/04/2017
Modeling nitrate trends in groundwater
SUFT3D variably saturated GW flow and transport finite element model (Orban 2006)
Calibration with 3H data measured in 2004
These data were used to develop a regional scale groundwater flow and transport model using the finite element code SUFT3D. This model includes both the unsaturated zone and the saturated zone. It was first calibrated to reproduce 3H concentration data. One of the interesting first conclusion of the model is that we come to a travel time of 1 m/year for tritium across the unsaturated zone. This was confirmed by tracer experiments performed in the unsaturated chalk.
Mean observed velocity across the unsaturated zone=~ 1m/y
32nd CIS Groundwater Working Group Meeting - Malta /04/2017

7. 32nd CIS Groundwater Working Group Meeting - Malta - 25-26/04/2017
Modeling nitrate trends in groundwater
Using the calibrated model, we ran different scenarios on NO3 recharge, from the unrealistic case of no NO3 spread on the agricultural lands to thr case of a more realistic reduction to 50 mg/L in the recharge water.
For the historical data, results show that the model reproduces quite well the different NO3 concentration evolutions. For the prediction period, the model shows that trend reversal is progressively but slowly observed in the recharge zone of the aquifer. However, no trend reversal is expected in the north-east area until tens of years because of the very long residence time and slow travel time of water and dissolved compounds across the thick unsaturated zone and the dual porosity chalk aquifer.
32nd CIS Groundwater Working Group Meeting - Malta /04/2017

8. 32nd CIS Groundwater Working Group Meeting - Malta - 25-26/04/2017
Conclusions and lessons learned
In the Geer basin, spatial distribution of NO3 strongly linked to hydrodynamic conditions and groundwater mixing
Good description of regional hydrodynamic conditions is essential
Strong correspondence between NO3 and 3H patterns in GW
Combined NO3 – gw age dating tracers useful to distinguish between NO3 pollution corresponding to very different time scales
NO3 trend assessment indicative as a first warning However regional GW flow and transport models are essential for reliable prediction of future NO3 concentrations in GW
What can we learn from this case study?
First, as shown here, when dealing with diffuse pollution such as nitrate, one can expect that the regional dynamics of groundwater flow, in both the unsaturated and saturated zones play an essential role and they have to be clearly understood.
Second, the fact that dating groundwater may be also essential to explain the concentration patterns and evolution and to argue on the importance of natural conditions.
And finally regional scale groundwater flow and transport models, when well constructed and calibrated, are essential to in order to obtain reliable predictions for the future
Observations and modelling results indicate that Article 4(4) can be used to extend the time to achieve good chemical status in the GWB BE-Meuse-RWM040 “Chalk of the Geer basin”
32nd CIS Groundwater Working Group Meeting - Malta /04/2017