Origin of nitrate pollution in the Walloon part of the Brusselian groundwater body: Crossing a modelling approach with an isotopic analysis.

Transcription

1 Origin of nitrate pollution in the Walloon part of the Brusselian groundwater body: Crossing a modelling approach with an isotopic analysis. Samuel MATTERN, Marnik VANCLOOSTER Introduction Status of the Brusselean groundwater body in Wallonia with respect to the nitrate problem : The Brusselean groundwater body in Wallonia is subjected to spatially distributed nitrate contamination. At many locations, nitrate concentrations continue to increase. The Région wallonne was condamned in 2005 by the European Court of Justice for the observed absence of nitrate contamination reduction in the Brusselean groundwater body in Wallonia. 1

2 Introduction Norme potabilité Sables Bruxelliens Introduction Mean concentrations ( ) [NO 3- ] (mg/l) 2

3 Introduction Sustainable groundwater management Understanding of the nitrate contamination process Identifying contamination sources Objectives Improving the understanding of the nitrate contamination problem of the groundwater body of the Brusselean aquifer in Wallonie Implementing novel hybrid techniques (statistical modelling and isoptopic analysis) for identifying the origin of nitrate contamination 3

4 Study area Groundwater body of the Brusselean aquifer in wallonia Study area : Hydrography 4

5 Study area : Soil types Source: Tavernier and Maréchal (1958). Study area: Geology 5 m Limon 40 m Sables Bruxelliens Socle 5

6 1/23/2010 Study area : Piezometry Study area : Land use 51 % 17 % 10 % 13 % Source: PCNOSW (2005). 6

7 Study area : Available data Data ~ 100 stations Water production companies Regional administration Private persons Bore holes Galleries Wells Study area: Available data 7

8 Study area : Available data Methods - Statistical analysis correlation between observed contamination and different environmental attributes multiple regression vs. regression tree analysis - Isotopic analysis information about the processes that control nitrate contamination 8

9 1/23/2010 Statistical analysis Land use Depth of the aquifer Nirate concentrations within groundwater body Height Slope... Statistical analysis 9

10 Statistical analysis: Multiple regression Conc. nitrate Conc. nitrate % forest % agriculture Statistical analysis: Multiple regression Yˆ = bˆ 0 + bˆ x bˆ x ˆ b j x1x2 + bˆ k x x

11 Statistical analysis: Multiple regression Variable b pval Residential area 4,1 5,2 x 10-6 Arable land 3,8 3,5 x 10-5 Forest and hedgerow area -2,5 1,1 x 10-2 Height 1,9 1,0 x 10-4 Slope 1,2 6,6 x 10-3 Statistical analysis: Multiple regression 11

12 Statistical analysis: Multiple regression Methodological limitation - Poor explanation of the total variance (R 2 =0,52) - A priori selection of variables and variable interaction Yˆ = bˆ 0 + bˆ x bˆ x ˆ b j x1x2 + bˆ k x x Poor consideration of the local specific situation Statistical analysis: regression trees 12

13 Statistical analysis: regression trees Statistical analysis: regression trees - Acceptable explanation of observed nitrate contamination (R 2 = 0,85) - No clear and unique relation between the observed contamination and a specific land use class, although residential areas seem to have more impact than arable land - Interactions between dependent variables are important - 13

14 Isotopic analysis Isotopes of N : 14 N : 7 neutrons, 7 protons ( % of N on earth) 15 N : 8 neutrons, 7 protons ( %) Isotopes of O : 16 O : most abundant ( %) 17 O : least abundant ( %) 18 O : % Definition δ 15 N ( ) = (R sample R standard ) / R standard x 1000 with R the isotopic ratio 15 N / 14 N Isotopic analysis 14

15 Isotopic analysis Isotopic analysis 15

16 Isotopic analysis Isotopic analysis Flow direction 16

17 Isotopic analysis Flow direction Isotopic analysis For some sampling points, the origin of the nitrate pollution could be clearly identified: - Mineral fertilisation (agriculture, golf yard) - Manure storage For most sampling points: - A mixture of pollution source explain the observed isotopic ratio 17

18 Conclusion Statistical analysis: The observed nitrate contamination of the walloon part of the Brusselean groundwater body, can only statistically be explained by an interaction of different land and land use attributes. Within the study area, residential and agricultural land use explains to a large extent the observed contamination. Isotopic analysis: The isotopic signature (N and O) of groundwater samples allowed to identify the origin of the observed contamination for only a limited amount of samples. Most samples exhibits an isotopic signature that corresponds to a mixture of possible pollution sources References Mattern S., P. Bogaert and M. Vanclooster, Introducing time variability and sampling rate in the mapping of groundwater contamination by means of the Bayesian Maximum Entropy (BME) method. CRC, selected papers in hydrogeology 14: Mattern S., D. Fasbender and M. Vanclooster, Discriminating sources of nitrate pollution in a sandy aquifer. Journal of hydrology 376: Mattern S and M. Vanclooster, Estimating travel time of recharge water through the unsaturated zone using transfer function model Environmental Fluid Mechanics. DOI /s Mattern S., Mapping and source identification of groundwater pollution by nitrate., PhD. Thesis. 233 pp. 18

19 Acknowledgement F.R.I.A. 19