Review of Groundwater Vulnerability Assessment Methods Unsaturated Zone. Dept. of Earth Sciences University of the Western Cape

Transcription

1 Review of Groundwater Vulnerability Assessment Methods Unsaturated Zone Dept. of Earth Sciences University of the Western Cape

2 Background Sililo et al. (2001) Groundwater contamination depends on: Intrinsic hydrogeological characteristics Properties of specific contaminants Physical, chemical and biological processes Groundwater pollution inventory project (IGS, CSIR and UWC) Excel database of properties of specific contaminants Urban risk assessment software Current project Hydrogeological characteristics Link between hydrogeological characteristics and contaminant properties

3 Objectives and structure of report Framework for groundwater vulnerability assessment Review of current knowledge on groundwater vulnerability (unsaturated zone) Review of methods for groundwater vulnerability (unsaturated zone)

4 Framework for groundwater vulnerability assessment Groundwater vulnerability assessment: Determination of likelihood of contaminants to reach groundwater Principal geologic and hydrologic features (NRC, 1993) Hydrogeologic framework Unsaturated zone Confining unit Aquifer properties Groundwater flow system Recharge rate Location within flow system (proximity to recharge and discharge area)

5 Groundwater vulnerability assessment process

6 Groundwater vulnerability system

7 Current knowledge Unsaturated zone Processes and factors: Hydrogeological 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 thicknesses (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 Contaminant specific Major processes: 1) Solubility; 2) Volatilization; 3) Sorption; 4) Degradation Mixtures change properties of individual contaminants (solubility, sorption) Hydrogeological and contaminant specific properties often overlap

8 Preferential flow Short-circuiting Modelling: Weighing the contribution of water and solute fluxes in micro- and macropores (e.g. SWAP model) Funnelling More difficult to model, but easier to implement control measures

9 Classification of unsaturated zone for South Africa Lack of data on unsaturated zone (subsoil) Based on South African soil classification (Sililo et al., 2001) Based on South African lithologies

10 Classification of unsaturated zone based on soils Sililo et al. (2001) discussed the dominant processes and assigned hydraulic and chemical attenuation qualitative rating to subsoil (C) horizons and materials, based on the South African soil classification.

11 Classification of unsaturated zone based on lithologies Alluvial and colluvial formation Unconsolidated sand deposits Calcrete and clay horizons Consolidated sedimentary cover Sedimentary rocks (Karoo Supergroup) Consolidated sandstone, shale, mudstone, tillite and a basal conglomeritic unit, highly fractured (TMG Supergroup) Crystalline basement Weathered material (regolith) overlying metamorphic rocks, such as granites, gneisses, meta-quartzites, and basalts with negligible primary porosity

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

13 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 models Review: Description and background of the methods The required inputs The main assumptions The relevance to groundwater vulnerability assessment Case studies

14 Discussion and conclusions Knowledge review Lack of data on physical, chemical and biological properties of the unsaturated zone (subsoil) Preferential flow Model review DRASTIC MIKE SHE, VLEACH, UGPF Detailed familiarization and operation with models is essential in order to identify advantages and shortcomings

15 Procedure 1) Identification of groundwater depths and fluctuations 2) Identification of unsaturated strata, based on soils, lithology and regolith data available 3) Groundwater recharge 4) Baseline of water chemistry (initial conditions) 5) Identification of priority contaminants 6) For stratum 1 to n: Determination/measurement of porosity, permeability and hydraulic conductivity (intrinsic properties) Preferential flow Short-circuiting Funneling and lateral flow Determination/measurement of contaminant solubility, volatilization, sorption and decay (specific properties) Multi-phase flow (e.g. non-aqueous phase liquids, NAPLs) and geochemistry processes Determination of dominant or average water and contaminant fluxes 7) Groundwater contamination and description of plume

16 Work programme Approach Time frames

17 Approach Classification of unsaturated zones (subsoils) Measurements of properties of unsaturated zone, sampled to represent the classification Hydraulic properties (permeability, porosity, water retention) Sorption of selected chemicals Improvement of methods for groundwater vulnerability assessment Comparison of methods for groundwater vulnerability assessment

18 Improvement of methods Index and overlay method DRASTIC Inclusion of multi-layer component, based on site-specific conceptual models Ranking based on measurements of hydraulic properties Permeability, porosity, water retention (equipment available at UWC) Work by University of Natal Sampling and sample preparation Inclusion of preferential flow and ranking Short-circuiting (fractures and cracks) Funnelling (lateral flow) Inclusion of chemical properties and ranking based on 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.) Familiarization and selection of suitable numerical models Example with VLEACH

19 VLEACH model Leaching rate of Simazine (by S. Maharaj) 3.00E E E E E E E E Leaching rate 2m 4m 6m 8m 10m Time (yr)

20 VLEACH model Cumulative groundwater impact of Simazine (by S. Maharaj) 2.50E E E E E E Cumulative mass (g) 9.8 2m 4m 6m 8m 10m Time (yr) Semi-steady state, generic contaminant model No half-life Detailed understanding of available models is required

21 Summary Pollution inventory Ranking of contaminants based on properties What is the fate of different contaminants at the same place? Vulnerability Transport of contaminants depends on hydrogeological set-up What is the fate of the same contaminant in different places? Integrated model

22 Options 1) Include sub-ratings to improve I of DRASTIC 2) Run numerical model(s) in the background to generate rating for I of DRASTIC 3) Run suitable numerical model(s) for best and worst case scenarios and for average conditions or polygons

23 Time frames 0-3 months Classification of unsaturated zones Development of conceptual models for case study sites Familiarization with models 3-6 months Classification of unsaturated zones Collection of samples Measurements in the laboratory Familiarization with models 6-9 months Collection of samples Measurements in the laboratory Familiarization with models 9-12 months Improvement of methods and models Compilation of report months Improvement of methods and models Comparison between methods Compilation of report