Aquifer Vulnerability Assessments and Protocols - Intermediate Zone -

Transcription

1 Aquifer Vulnerability Assessments and Protocols - Intermediate Zone - Shafick Adams and Nebo Jovanovic October 2005 Department of Earth Sciences University of the Western Cape

2 OUTLINE - Objectives - Research Approach - Preliminary Results - Problems - Future Outlook

3 OBJECTIVES Improve on the rating of I in DRASTIC Prediction of properties without extensive drilling and sampling programs Establish the effects of fractures/macropores on contaminant transport Application of simulation models to specific case studies (Cape Flats [ithemba[ labs] and Secunda [SASOL]) Develop guidelines

4 APPROACH 1. Improve on the rating of I in DRASTIC 2. Prediction of soil and contaminant properties without extensive drilling and sampling programs 3. Establish the effects of fractures/macropores on contaminant transport 4. Application of simulation models to specific case studies

5 1. Improve on the rating of I in DRASTIC 2. Prediction of soil and contaminant properties without extensive drilling and sampling programs Identified most important factors affecting vulnerability - Hydraulic properties of the unsaturated zone - Thickness of the unsaturated zone - Topography/Slope - Flow mechanism - Drainage/recharge - Travel time - Sorption capacity - Contaminant half-lifes lifes Prediction of properties without extensive drilling and sampling programs - Existing databases - Prediction tools (RETC, Rosetta, etc.)

6 1. Improve on the rating of I in DRASTIC 2. Prediction of soil and contaminant properties without extensive drilling and sampling programs Excel spreadsheet program

7 1. Improve on the rating of I in DRASTIC 2. Prediction of soil and contaminant properties without extensive drilling and sampling programs

8 CAPE FLATS AQUIFER Factors Description Comments Impact Physical Characteristics Hydraulic properties 1. Sand, minor silt, clay lenses, heterogeneous, 2. K = m/day, T = m2/day 1. Sieving analysis (UWC) 2. Available information Medium Thickness of the unsaturated zone 1-5m (average 4.6m) Some areas water table at surface High Flow mechanism Matrix, Fingering 95% matrix flow assumed Medium Recharge/drainage % of MAP (Bredenkamp et al., 1995). 2. CMB = 10% MAP mm/yr 1. Tritium profiling and CMB (Atlantis) 2. CMB (UWC CAT Site) High Sorption capacity 1. High silica, Ca sands. 2. The fraction of organic carbon of the sands ( %) is relatively low and sorption will thus be low (Sililo, 1997) Peat layers can increase Sorption capacity High Chemical characteristics Travel time Thickness = 3m; effective porosity 25-40%, K = m/day Foster and Hirata equation: Gross surcharge: <1 day Natural infiltration: > 3 years Quick estimate < 5 days High Half-lifes Contaminant specific High travel time, low sorption, thin unsaturated zone Medium to high vulnerability rating

9 APPROACH 1. Improve on the rating of I in DRASTIC 2. Prediction of properties without extensive drilling and sampling programs 3. Establish the effects of fractures/macropores on contaminant transport 4. Application of simulation models to specific case studies

10 3. Establish the effects of fractures/macropores on contaminant transport 1. Chloride solute profiling in the unsaturated zone - Identify dominant (matrix and/or preferential) flow patterns - Relatively inexpensive - Visual inspection of cores Sampling and moisture extraction equipment 2. Geology/geomorphology - Infer from borehole logs and lithologies - Not quantitative - Remote sensing/gis 3. Numerical models - Identify matrix and preferential flow characteristics - Data intensive - Calibration and sensitivity analyses

11 3. Establish the effects of fractures/macropores on contaminant transport Categorize into type of preferential flow Mole tunnels

12 APPROACH 1. Improve on the rating of I in DRASTIC 2. Prediction of properties without extensive drilling and sampling programs 3. Establish the effects of fractures/macropores on contaminant transport 4. Application of simulation models to specific case studies

13 4. Application of simulation models to specific case studies 1. VLEACH 2. MACRO can handle macropore flow in soils 3. HYDRUS 2D 4. SWMS 2D 5. SWAT 6. SWAP - preferential flow in cracking/swelling clays 7. UGPF - GIS based model (BTEX)

14 4. Application of simulation models to specific case studies 1. Data that is currently being collected through field and laboratory work: Porosity, pore size distribution Water retention Matric potential Bulk density Gravimetric and volumetric water content

15 4. Application of simulation models to specific case studies VLEACH title...cfa surface area [sq.ft] vertical thickness of a cell [ft]...1 net recharge rate [ft/year] dry bulk density [g/cm3] effective porosity [dim.less] volumetric water content [percent] organic carbon content [dim.less] contaminant conc. in recharge [mg/l]...{1 for continuous discharge ; 0 for accidental spillage CONTAMINANT INFORMATION: org.c distrib. coeff., Koc [ml/g]...{48.64 for 1,1,1-trichloroethane; 3380 for lindane} Henry's constant [dimensionless]... {0.703 for 1,1,1-trichloroethane; for lindane} water solubility [mg/l]... {1290 for 1,1,1-trichloroethane; 8 for lindane} free air diffusion coef.[sq.m/d]...0

16 4. Application of simulation models to specific case studies VLEACH 1.00E E+01 Contamination rate (g/a) 1.00E E E E E E E-14 Contamination rate (g/a) 1.00E E E E Time (a) 1.00E Time (a) 1,1,1-Trichloroethane Lindane Simulated groundwater contamination rate of 2 contaminants applied through recharge (continuous recharge). Daughter products! 1,1,1-Trichloroethane Simulated groundwater contamination rate of 2 contaminants applied to the surface at the beginning of the simulation (accidental spillage). Lindane

17 4. Application of simulation models to specific case studies 1D non-steady state model of water flow and solute transport in structured or macroporous field soils.

18 4. Application of simulation models to specific case studies 1. Data requirements: Climatic data Basic soil properties (% sand, silt, clay) Unsaturated hydraulic conductivity Solute concentrations MACRO 5.0 Can simulate non-reactive tracers, tritium and pesticides Simulates a full water balance Model structure enables quantitative evaluation of the impact of macropore flow on solute transport in structured soils.

19 4. Application of simulation models to specific case studies 1. Data requirements: Climatic data Basic soil properties (% sand, silt, clay) Unsaturated hydraulic conductivity Solute concentration MACRO 5.0 Can simulate non-reactive tracers, tritium and pesticides Simulates a full water balance Model structure enables quantitative evaluation of the impact of macropore flow on solute transport in structured soils. Preliminary model indicates that >95% of flow occurs in micropores

20 PROBLEMS SUITABLE DATA!!!!!!! Direct measurements are cumbersome and require a substantial investment in both time and money 1-D, 2-D 2 D or 3-D 3 D (increasing data requirements) Upscaling/downscaling

21 FUTURE OUTLOOK Run several models for the two study sites (contrast and compare) Refine approach Uncertainty analysis Develop a spatial database

22 THANK YOU ENKOSI DANKIE??