Dept. of Earth Sciences University of the Western Cape AVAP Project Steering Committee Meeting 22 September 2004

Transcription

1 Review of Groundwater Vulnerability Assessment Methods Unsaturated Zone Dept. of Earth Sciences University of the Western Cape AVAP Project Steering Committee Meeting 22 September 2004

2 Objectives Review of current knowledge and methods of groundwater vulnerability (for unsaturated zone) Recommendations

3 Groundwater vulnerability system

4 Current knowledge Unsaturated zone Processes and factors: Intrinsic Properties of soils and geologic materials in the unsaturated zone Physical (hydraulic properties, pore size distribution, temperature, water content etc.) Chemical properties (organic matter and clay content, ph etc.) Biological properties (microbial activity etc.) Water and solute fluxes (advection is the dominant process via wetting fronts flushing solutes, molecular diffusion) Topography Depth to the water table, layering and layer thickness (length of flow path) Groundwater recharge type and rate (extent and rate of transport of contaminants) Preferential flow (short-circuiting, funnelling) Other processes (imbibition into rock matrix, capillary rise, fingering) Spatial and temporal variability Properties of subsoil often different from properties of soil Specific Major processes: 1) Solubility; 2) Volatilization; 3) Sorption; 4) Degradation Mixtures change properties of individual contaminants (solubility, sorption) Intrinsic and specific properties often overlap

5 Groundwater vulnerability assessment methods Index and overlay methods Process-based models Statistical methods

6 Criteria and review Criteria: Type of method Scale of assessment Reference location Applicable environment Cost and availability Whether it is an intrinsic or specific assessment Geochemistry involved for the process based methods Review: Description and background of the methods The required inputs The main assumptions The relevance to groundwater vulnerability assessment Case studies

7 Recommended improvements to methods Knowledge review Inclusion of chemical properties and geochemistry Solubility: Excel database Liquid and non-liquid phase for organic chemicals Precipitation and other processes for inorganic chemicals Volatilization: Excel database Sorption: Excel database and measurements Degradation: Compilation of guidelines to estimate half-life based on expected environmental factors (microbial activity, ph, temperature, water content etc.) Inclusion of physical properties of unsaturated zone (porosity, permeability, hydraulic conductivity) Multi-layer component Preferential flow (short-circuiting and funneling) and travel time Model review MIKE SHE, VLEACH, UGPF promising models Detailed familiarization and operation with models is essential in order to identify advantages and shortcomings

8 Approach 1) Inclusion of sub-ratings to improve I of DRASTIC 2) Simulation of best and worst case scenarios with suitable numerical model(s) for average conditions or polygons 3) Use of numerical model(s) in the background to generate rating for I of DRASTIC

9 Collection of baseline information for specific site Application of a water chemistry equilibrium model (PHREECQ) Attenuation of contaminants in each layer Concentrations/amounts of contaminants leaving the layer

10 Requirements Description of the categories for each factor in the form of guidelines Allocation of percentage attenuation figures to each category and factor Model calibration at case study sites Cape Flats Secunda? Midrand?

11 Tasks and time frames Task Time frame Collection, analysis, interpretation and September 2004-March 2005 documentation of field samples/data from the key study areas Testing and documenting suitable September 2004-March 2005 vulnerability assessment methods in key study areas Refinement of vulnerability assessment April 2005-March 2006 methods; Comparison between methods Contribution to final report April 2006-September 2006