1 SENSITIVITY ANALYSIS OF THE STABILITY OF GROUNDWATER FLOW SYSTEMS M.J.M. Vissers M. van der Perk, P.F.M. van Gaans Universiteit Utrecht, Centre for Geo-Ecological Research, Faculty of Geographical Sciences, P.O. Box 80115, 3508 TC Utrecht, The Netherlands, phone: , fax ,

2 Groundwater flow is important for spatial planning and groundwater quality assessment. Configuration of pathways of point and diffuse source pollution: -Temporal -Spatial

Hydrological systems analysis! Toth (1962): A flow system is defined as a set of flow lines in which any two flow lines adjacent at one point of the flow region remain adjacent throughout the whole region How to operationalize this definition? How to quantify stability?

4 Manual delineation of groundwater flow systems by interpretation of particle tracks (Engelen and Kloosterman, 1996) Automatic? Uncertainty ?

5 METHODS -Groundwater model -Define groundwater flow systems -Monte Carlo analysis of stability

6 Study area Ice-pushed ridge in the east, “flat” in the west Mainly Agricultural land use, eastern part cultivated in the 1920’s Forest City / Industry Heathland Wetlands Grass Stream / Ditch Height lines on moraine Observation well Drinking water well Overijssels Canal

7 Study area Drenthe clay in the west forms base at –30m NAP Sandy unconsolidated aquifer

8 Groundwater model MODFLOW 18 x 12 km 50 x 50 m cells 6 layers Drainage network Calibration

9 Define groundwater flow systems When a discharge area is “continuous”, the flow lines towards it will remain adjacent throughout the region Identified by a 3-cell buffer around discharge cells

10 Define groundwater flow systems Size of recharge area may be too small  take minimum of 50 cells Groundwater divide can be present “within” a groundwater flow system

11 Groundwater flow systems and transit distance X-coordinate Y - c o o r d i n a t e (km)

12 Transit time X-coordinate Y - c o o r d i n a t e

Vertical transect: age and distance

14 Time

15 Transit time X-coordinate Y - c o o r d i n a t e

16 Groundwater flow systems can be identified Configuration of pathways: –In space: groundwater flow system & characterization –In time: travel distance and age at specified depths Uncertainty….

17 Sensitivity analysis Recharge K H /K V Drainage resistance Drainage level Recharge & Drainage level (Monte Carlo)

18 Drainage resistance Parametric analysis X-coordinate Y - c o o r d i n a t e

19 Recharge Parametric analysis X-coordinate Y - c o o r d i n a t e

X-coordinate Y - c o o r d i n a t e Monte Carlo Analysis (recharge & drainage level)

21 Conclusions Hydrological systems analysis can be applied to groundwater model results Uncertainty in flow is not only caused by parameter errors, heterogeneity etc. Climatic variations and human factors are very important in natural flow systems Uncertainty can be mapped, flow systems are quite stable The combination of maps can be used for many purposes