Hydraulic Containment, Natural Attenuation and ... · reversal of off-site contamination by groundwater drawdown, - Dewatering of the slimes dams by drawdown of phreatic surface below

Hydraulic Containment, Natural Attenuation and Phytoremediation asa Combined Remediation Strategy for an Industrial Waste Site in

South Africa

D. Duthe1, S. Lorentz2, I. Cameron-Clarke1 and A.J. Oliver3

1 Steffen, Robertson and Kirsten Consulting, Johannesburg, South Africa.

2 School of Bioresources Engineering and Environmental Hydrology,University of KwaZulu-Natal, Pietermaritzburg, South Africa.

3 Project Manager

Third International Phytotechnologies Conference, EPA, ORD, TIFSDAtlanta, GA, 20-22 April 2005

Outline

• Introduction

• Remediation strategy

• ET cover performance

• Conclusion

Duthe et al: Hydraulic Containment, Natural Attenuation and Phytoremediation..

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecR

ain,

Pot

entia

l eva

p (m

m)

Rain Potential evap.

MAP 1025 mm

MAPE 1528 mm

Alt. 90 m


Ash waste dams

Dam 3/4

Dam 2

Dam 1

Dam 6

Buried drums area

Dam 5

Aerial view of the dams and buried drums area

Dam footprint: 30haCommission: 1958Decommission: 1994+

Contaminants of concern:

TetrachloroetheneTrichloroethene1,2 Cis DichloroetheneVinyl chlorideTDS


Remediation Strategy

• Management intervention:

- Alternative water supply to the KwaMakutha village andreversal of off-site contamination by groundwaterdrawdown,

- Dewatering of the slimes dams by drawdown of phreaticsurface below the dams,

- Surface water control by installation of drainage works,

- Seepage water control by subsurface drains around thedams and in Kwamakutha.


Remediation Strategy

• Management intervention:

- Alternative water supply to the KwaMakutha village and reversal of off-site contamination by groundwater drawdown,

- Dewatering of the slimes dams by draw down of phreatic surface below the dams,

- Surface water control by installation of drainage works and

- Seepage water control by subsurface drains around the dams and in Kwamakutha.

• Proposed remediation:

- Hydraulic containment by groundwater gradient control and surface runoff control,

- Monitored Natural Attenuation and possibly enhanced attenuation by recirculation in the groundwater and

- Evapotranspiration cover. Planting of some 1600 indigenous species tolimit fluxes through the slimes and promote natural attenuation.




Source attenuation in the slimes profile

Contaminant concentrations inthe groundwater in and bellow the slimes dam

Tensiometer nest 100 - 2000 mm

Deep tensiometer nest

Automatic rain gauge

Bowen ratio evapotranspiration

Heat pulse velocity

Automatic weather stationA-pan evaporation


Pessuretransducers

Rain gauge and logger

Ceramic cupTensiometers

Monitoring of subsurface liquid fluxand water content:

Material hydraulic/physical properties: - Porosity/density - Water retention characteristic - Hydraulic conductivity characteristic

Data logger

Slimes dam surface

Bowen ratio ET

Sap flow: heatpulse velocity

DeepTensiometers

Met stationand logger

Root zone

Automaticgroundwater level recorder

Groundwater

Schematic of instrumentation for measurement of components of the slimes dam water balance


Vegetation on slimes dam 2


Shallow rooting system of the Natal Fig Grass growth on slimes dam 2

Slimes Dams Water Balance Modelling

• Since the groundwater recharge component flux cannot be measured directly, as is the case in lysimeter experiments, the observations and material properties are combined to calculate this flux.

• We also wish to evaluate the potential of the full slimes profile (15m) in absorbing peak rainfall events, retarding fluxes to the groundwater and releasing water back up to the root zone during dry periods.

• These calculations are best performed using the soil water physics inherent in the HYDRUS2D software. Once calibrated against the observations over three years, HYDRUS2D is used to predict long-term performance of a full canopy cover


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AECI Fig 0mm

Water retention characteristic

Hydraulic conductivity characteristic

HYDRUS2D inputs: hydraulic characteristics of the slimes materials


HYDRUS2D inputs: rainfall characteristics (short & long term)


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HYDRUS2D inputs: Transpiration

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Sap flow rates for Bugweed trees (2002)

Sap flow rates for Pigeonwood trees (2002)

HYDRUS2D inputs: potential evaporation and transpiration


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Observation nodes for long-term simulation

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HYDRUS-2D Finite Element Mesh Boundary conditions

Constant head ground water

Atmospheric boundary condition

Root zone

HYDRUS2D finite element configuration


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Typical comparison of measured and simulated soil water tension from which fluxes are derived


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Drainage (estimated from measurements) Store (estimated from measurements)

Components of the root zone water balance for the monitored period


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Precip Soil Evap Transpiration Drainage

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Precip Soil Evap Transpiration Drainage

Simulated root zone water balance for monitored period for Fig Tree site.

Simulated water balance for monitored period for Bare site (2000mm depth)

Long term slimes dam water balance simulation

• Long term ground water recharge fluxes are simulated by

- Correlating meteorological parameters for short term records between stations at the site and Durban International Airport,

- Simulating full vegetation cover performance using HYDRUS2D with adjusted long term inputs of rainfall and potential evaporation (28yr record),

- Extracting the discharge time series for groundwater recharge from the base of the slimes dam.

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1973 1977 1981 1985 1989 1993 1997 2001

Cumulative simulated long term recharge to groundwater: Vegetated scenario

Results of simulated fluxes to groundwater

Scope of current investigations

• Groundwater hydraulic control and Natural Attenuation assessment

- Groundwater tracer study.

- Assess natural attenuation capacity

- quantify recirculation water and mass balance

• Phytoremediation assessment

- Improving ET estimates using scintillometry

- Characterizing spatial distribution of hydraulic performance

- Evaluating capacity of slimes profile for natural attenuation

• Surface hydraulic control

- Evaluation and design of cap for buried drums area

- Design of surface runoff and seepage controls

Conclusions

• Natural attenuation has been observed in the slimes profile and groundwater

• A combined train of technologies, comprising

- hydraulic control (surface and subsurface)- phytoremediation and - managed natural and enhanced attenuation

has been defined for long term management of the site

• Phytoremediation is a valuable component in the system of remediation:

- limiting groundwater recharge to less than 0.3% of rainfall and- providing an environment for natural attenuation of VOC’s

Documents

Hydraulic Containment, Natural Attenuation and ... · reversal of off-site contamination by groundwater drawdown, - Dewatering of the slimes dams by drawdown of phreatic surface below