Monolithic WAC

David Hall†, Jan Gronow‡ and David Drury†

Golder Associates †Environment Agency ‡

Introduction

Waste Acceptance Criteria (WAC) are about to be introduced for wastes sentenced to hazardous and inert waste sites, or for stabilised non-reactive hazardous wastes sentenced to non-hazardous landfills.These criteria (agreed by the European Council) cover granular wastes only and not monolithic waste forms.

Introduction

The EC has left it to Member States to develop criteria for monolithic wastes that provide comparable environmental protection.

Monolithic

Island of Rhodes…..This monastery sits on a monolith

Essentially a solid masswhere the primary leaching mechanismis diffusion driven

Leaching

Monolithic wastes leach via diffusion.Testing usually via a tank test over a period of a few days up to 64 days.Test yields data on the rate of leaching (mg/m2

over a set time period).

The Challenge

To develop a set of criteria that matched the environmental protection standards of the granular wastes but for a different contaminant source term.The generalised scenario (hydrogeological conditions, site size and liner degradation rates) remained the same as that used for granular wastes.

Geometry

POC 1 - 10mPOC 2 - 200mPOC 3 - USZ

POC 3

POC 1 POC 2

Site 200x200x20m≈ 800,000m3

Aquifer

USZ

Engineered Barrier

Liner comprised an high density polyethylene / clay composite liner.Membrane component assumed to become non-effective at 250 years

Leachate Generation

Leachate generation solely from infiltrating rainfall.High rates of infiltration during operational period (250mm/y)Lower rates after site completion and capping (50mm/y)

Source Term Evolution

Initially attempted to use the Crank Equation but this infinite series needed the first 100,000 terms to be solved to give a stable solution and also required information that is not always readily available.Settled on the following:

1

2

1

2

tt

ttoemissionttoemission=

∑∑

Modelling Method

The modelling was conducted using an implementation of the LandSim algorithms within the performance assessment model GoldSim.This provided a highly flexible modelling environment that allows results mining and relatively easy modifications to inputs and algorithms.

Leachate Source Term

1.0e-03

1.0e-02

1.000e-01

1.0e00

1.0e01

1.0e02

1.0e03

1.0e04

1.0e00 1.0e01 1.0e02 1.0e03 1.0e04 1.0e05

(mg/

l)

Time (yr)

Waste_cell

Waste_cell.Concentration_in_Water[Cd]Waste_cell.Concentration_in_Water[Pb]Waste_cell.Concentration_in_Water[SO4]

Base of the Unsaturated Zone

1.000e-05

1.0e-04

1.0e-03

1.0e-02

1.000e-01

1.0e00

1.0e01

1.0e02

1.0e03

1.0e04

1.0e00 1.0e01 1.0e02 1.0e03 1.0e04 1.0e05

(mg/

l)

Time (yr)

Concentrations at base of unsaturated zone

Unsaturated_zone_flow10.Concentration[Cl] Unsaturated_zone_flow10.Concentration[Cd]Unsaturated_zone_flow10.Concentration[Pb]

Method

The Water Quality Standards defined for the granular WACshave been used in this study.The model is run with an initial “good guess” of the likely emission rate for each of the contaminants considered.The model is then allowed to run sufficient iterations to converge on an emission rate (for each contaminant at its respective receptor point).

Sensitivity

There were a few unknowns that needed specific attention:

Surface area for diffusion to occur (related to block size)pH dependent solubilityAvailability and depletionFilling Sequence of the landfill

Fracture Area

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Block_length [m]

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.010

0.011

0.012

0.013

0.014

0.015

Max

_con

c_1

[mg/

l]X:Block_length Y:Max_conc_1

Copper – displayingidealised relationship

Fracture Spacing

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Block_length [m]

18

20

22

24

26

28

30

32

34

36

38

Max

_con

c_1

[mg/

l]

X:Block_length Y:Max_conc_1

Sulphate - more complex response

Fracture Spacing

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Block_length [m]

1.0E-005

1.5E-005

2.0E-005

2.5E-005

3.0E-005

3.5E-005

4.0E-005

4.5E-005

5.0E-005

5.5E-005

6.0E-005

6.5E-005

7.0E-005

7.5E-005

Max

_con

c_1

[mg/

l]

X:Block_length Y:Max_conc_1

Arsenic –completelycounterintuitive response

Conclusions

WAC for monolithic wastes have been derivedSensitivity analyses showed:

Insensitive to filling sequenceMinimum fracture spacing should be 0.4mFactors such as pH dependency and availability important for the any site specific risk assessment, but not for the setting of WAC values

Acknowledgements & Disclaimer

I am indebted to my colleagues at Golder Associates for the assistance given during this projectThe views expressed in this presentation are solely those of the presenter, and not necessarily those of the Environment Agency who funded this work. Thank you for your attention.