Hydrogeology in clay till. Timo KESSLER, Knud Erik KLINT, Bertel NILSSON and Poul L. BJERG

Transcription

1 Hydrogeology in clay till Timo KESSLER, Knud Erik KLINT, Bertel NILSSON and Poul L. BJERG

2 motivation Risk assessment of contaminated sites How do contaminants spread in the subsurface? Is geological heterogeneity important? Where are contaminants stored? Where do reactions occur?

3 sand lenses in clay till Clay till has extremely low hydraulic conductivity Diffusion in the matrix Fractures facilitate vertical transport Sand lenses?? How do sand lenses look in glacial tills? Which types of lenses are important for transport? How can we include sand lenses in hydrogeological models? What kind of data is required? What if no data available? Which role do sand lenses play for (reactive) transport?

4 analog site

5 analog site

6 characterization Description of single lenses Measure geometry and structures Classification Characterization of sand lenses Mapping of spatial distribution Analysis of variability Stochastic simulation

7 parameters 1. Length 2. Thickness 3. Orientation 4. Fold axis 5. Structures 6. Vertical frequency

8 Table 1: Classification of sand deposits in a typical basal till geometry

9 Table 1: Classification of sand deposits in a typical basal till shapes

10 classification Category Length l h [m] Thickness l v [m] Anisotropy l h /l v [-] Deformation Vertical spacing [m] Genetic interpretation Sample picture sand layers moderate >5.0 proglacial glacio-fluvial sedimentation sand sheets > 50 very strong glaciolacustrine sands, subglacially sheared sand bodies weak supraglacial debris flow or subglacial glacioflucial sands sand pockets strong dislocated pockets originating from larger lenses and stringers < 10 very strong strongly strained and deformed pockets

11 conclusions I How do sand lenses look in glacial tills? oval shaped and elongated lenses with horizontal tailing; strongly anisotropic and very thin due to shear deformation Which types of lenses are important to look at? thick layers are most important, but smaller lenses may create a complex channel network if they are connected

12 mapping sand lenses Define a cross-section Map the lenses Categorize the facies clay till sand lenses

13 analysis z x mean lengths - in x-direction ~80 cm; in z-direction ~7 cm proportion - in clay till typically between sand facie anisotropy - strong vertical anisotropy at ration of instationarity - certain areas within sections have higher numbers of sand lenses than others

14 stochastic simulation Transition probability (TPROGS) based on transition probabilities can deal with irregular data sets geological logs can be used model needs to be conditioned

15 stochastic simulation Multiple-point statistics (MPS) uses training images outcrops/conceptualizations are required can simulate 3D realizations

16 hydrogeological model conceptual model of the Vadsby site

17 conclusions II How can we include sand lenses in hydrogeological models? Based on mapping data sand lense distribution can be simulated and included in hydraulic models --!! Spatial resolution!! What kind of data is required? What if no data available? Data on geometry and spatial variability of sand lenses Sand lenses can be conceptualized; borehole logs can be used to condition simulations

18 importance

19 importance sand layer sand body single bodies can alter the flowpaths important reaction compartments kan be observed on logs may be treated as separate units sand sheet sand pocket sand stringer important reaction zones hydraulically only significant if connected can be accounted with stochastic model are difficult to identify on logs

20 K-values Which are the correct hydraulic conductivities? Geological layers slug tests og pumping tests (bulk K) typical values for clay till are 1* *10-8 Clay till matrix K-values in the order of 1* *10-10 Sand lenses true values are difficult to find K-values must be estimated

21 conclusions III Which role do sand lenses play for (reactive) transport? sandy layers and sand bodies can change the hydraulic regime significantly Small sand lenses may impact transport if connected network exists -- 50% of permeable facies needed; anisotropy may lower this ratio All sand lenses are important for reactive processes in the subsurface

22 end