Philip Kelsey – Earthtech Consulting Limited

Assessment of Natural Attenuation of an Unlined Area

using FEFLOW Groundwater Modelling

for Consenting of the Bonny Glen Regional Landfill Site, Manawatu

ACKNOWLEDGEMENTS: Rob Burden (Geochemistry), Alan Pattle (Peer Reviewer) and Midwest Disposals Limited

Project Background

• Refuse landfill extension, 13 44.6 hectares facility.

• 0.3 hectare unlined area in Stage 1 from previous owner.

• Propose:o Piggy-back liner over Stage 1 refuse.

o Attenuation assessment of leakage in underlying geology.

Groundwater Characterisation

Hydraulic Conductivities

• Aquifers, kaquifer = 1.9 x 10-5m/s (geometric mean of 19 tests).

• Aquitards, kaquitard = 2.8 x 10-9m/s (average triaxial data k, Fetter calcs).

• Shakespeare anisotropy 𝑘𝑎𝑞𝑢𝑖𝑓𝑒𝑟

𝑘𝑎𝑞𝑢𝑖𝑡𝑎𝑟𝑑= 6,785 strong preferential flow along bedding direction.

Water Quality

• No evidence existing landfill leakage.

• Match water quality signatures and sand aquifer layers.

• Groundwater dating (150 years) to pass through unsaturated zone.

Batch Testing

• Aquifer sorption.

Groundwater Flow Modelling

Central model area under existing landfill.

drainage

recharge

Attenuation Assessment – Mass Transport Modelling

Landfill Leakage

Existing Landfill Plus Extension

• Liner Leakage = 2.08m³/do 98% leakage from existing 0.32ha Stage 1 unlined plus 0.80ha Stage 1 to 3C clay liner areas.

o 2% leakage from HDPE/GCL design in existing Stages 3C to 7 and proposed extensions extending over 42.5ha.

• Leachate Source Concentrations:o Reducing leachate strength with time.

o Stage 1 piggy-back liner – construction prior to 2020.

Attenuation Assessment – Mass Transport Modelling

Groundwater Compliance Criteria

• Criteria adopted at western site boundary.

o Boron = 1.3g/m³ – ANZECC (80%) freshwater guideline.

o Ammonium-N = 2.3g/m³ – ANZECC (80%) freshwater guideline.

o Arsenic = 0.01g/m³ – DWS for health.

Decreasing mobility

Solutes Modelled

• Solutes most mobile in groundwater.

o Boron (critical case)

o Ammonium-N

o Arsenic

Distribution Coefficients (Kd’s) Determined

for Landfill Leachate Constituents (ml/g)

Am

mo

niu

m-N

Ars

en

ic

Bo

ron

Ch

rom

ium

Co

pp

er

Le

ad

Zin

c

Liner Loess Material 30 700 1.0 1,500 600 7,700 100

Shakespeare Gp.

Siltstone 8 70 0.7 1,500 4,000 7,700 2,300

Leachate Source Concentrations – Boron

Boron Groundwater

Compliance at Site

Boundary = 1.3g/m³

15.9 1.3g/m³

Attenuation Assessment – Mass Transport Modelling

Boron Modelling

• Peak boron at Western Boundary, t = 400 years.

• Predicted peak boron concentrations

Summary of Environmental Effects on Groundwater

• Effects on:

o Groundwater quality.

o Existing bore users.

o Surface water and springs.

Either negligible or less than minor.

o Consent Hearing – February 2015.

o Consents Issued – May 2015.

o No unexpected groundwater quality issues to 2019.

Summary

FEFLOW

• Readily incorporatedo Leachate source concentration variability (including piggy-back liner).

o Liner leakage rates for different designs.

o Attenuation settings.

o Multiple layered aquifer system.

• Model graphics and videoo Powerful visualisation tool for checking conceptual model and presenting results.