Working with Acid Sulphate Soils: A Consultant's Perspective

Working with Acid Sulphate Soils:
A Consultant’s s Perspective
John Angeloni
Senior Environmental Scientist/Geologist
ENV.Australia Pty Ltd
Level 7, 182 St George’s Tce, Perth

Presentation Outline
• ASS – Definition
• Why consider ASS
• ASS Investigations
• ASS Management
• Closure and the CS Act 2003

ASS Defined
• Soils, materials and sediments
that contain iron sulphides
which, when drained or
disturbed, react with oxygen to
form sulphuric acid
• The main form of iron sulphide
present is pyrite (FeS 2 )
• The pyrite is usually framboidal
with extremely small crystal
size and huge surface area –
hence highly reactive

Actual Acid Sulfate Soils
•AASS – (after exposure to
oxygen)
pH

Potential Acid Sulfate Soils
•PASS – (before exposure to oxygen)
pH 6.5-7.5, unoxidised iron sulfides,
still have potential to produce acid,
usually soft, sticky, gel-like mud (80%
water) but can include wet sands

Why are ASS of Concern
• If left alone, acid sulphate soils do not present a problem
• However when exposed (eg drainage or excavation) iron sulphide
react with oxygen and water to produce:-
• sulfuric acid
• iron, aluminium, arsenic, other metals and acid transported to
surrounding environment
Actual Acid Sulphate Soils
(AASS)
Watertable
Potential Acid Sulphate Soils
(PASS)

Over Regulation OR Serious Issue
• Unprecedented level of development on the Swan
Coastal Plain
• Potential effects of climate change
• Resilience of the land and groundwater to impact
• Outcomes for future generations

ASS – the Costs
• East Trinity - Queensland Government has committed >$10m over an 8 year
period, predominantly for ASS-remediation and associated repair/replacement
of corroded infrastructure
• City of Stirling - approx. $20m
• Raffles Hotel Redevelopment :
• Perth Foreshore Enhancement
– Ozone Reserve Irrigation Scheme: $600K
– Esplanade:
• Perth Arena: >$500K
• Bunbury Region: Coastal residential development, waterway/marina -
• Dunsborough Lakes – approx. $500K, just on neutralisation of a single
stockpile from previous lake
development

City of Stirling Residential Development
• Mid-December 2001: bore-irrigated
residential vegetable gardens dying in
Stirling
• Acidic and metals–contaminated
groundwater confirmed – Arsenic at up to
1000 x WHO drinking water limit
Groundwater acidity and As contamination
related to residential developments
(dewatering and disturbance of peat-based
wetlands ), succession of dry seasons and
Peat suggested as the major source of
acidity, subsequently confirmed as source
of Arsenic, other metals

WA Planning Guidelines
Example Condition:
from January 1 2004, based on ASS Risk Mapping in
coastal areas
‣Onus on developer
To prove the absence of ASS, and if ASS present to
develop management plan Prior to planning approval

ASS Risk Mapping - Perth

ASS Risk Mapping - Busselton

Aerial Photo of Site

ASS Investigations
Photo: ENV 2007
• Desktop Study: Aerial photography, ASS
Risk Mapping, soil, geology, hydrogeology
datasets
• Test pits / drill holes/groundwater bores
• Log and field test soil profile
• Sample/analyse soils and groundwater
• Prepare ASS Investigation Report and ASS
and Dewatering Management Plan
(ASSDMP)
Photo: ENV 2007 Photo: ENV 2007

ASS Investigations
TIMING
Fieldwork (generally within 4 weeks/20 days)
Laboratory analysis (generally within 3-4 weeks/say 20 days)
DEC approval of ASSDMP (nominal 45 days)
DoW issue of dewatering licence (nominal 10 days)
Photo: ENV 2007
Photo: ENV 2007

ASS Investigations-Results
• Existing or Actual Acid Sulphate Soils
(AASS)
• Potential Acid Sulphate Soils (PASS)
• Presence, extent and severity
• Is an ASSMP required

ASS Investigations - Cross Sections

ASS Investigations - Cross Sections

Management Options
1. Avoid Disturbance
2. Neutralisation Treatment
3. Dewatering Management
4. Strategic Re-burial
Photo: ENV 2007
Photo: ENV 2007
Photo: ENV 2007

Dewatering
HIGH RISK - critical component
‣ oxidation of potential ASS materials surrounding
the excavation associated with watertable
drawdown
‣ discharge/recharge of acidic groundwater
‣ discharge/recharge of groundwater with elevated
metal concentrations; TOTAL ACIDITY vs pH
‣ retention ponds for aeration, lime treatment , silt
curtains and monitoring

ASS and Dewatering Management
Aeration
Photo: , GHD 2003
Photo: ENV 2007
Photo: ENV 2007
Photo: ENV 2007
Retention/infiltration
Basins
Photo: ENV 2007
Photo: ENV 2007
Lime treatment
Photo: ENV 2007

ASS AND DEWATERING
MANAGEMENT
Inline LIME DOSING PLANT: Automatic lime
neutralisation determined by pH/EC of influent
dewatering discharge

ASS AND DEWATERING
MANAGEMENT
• Infiltration - Iron- and metal-rich deposits

Closure Reporting
• Initial Closure Report: Basis for DEC clearance of WAPC
Condition
“…management in accordance with approved ASS
management plan…”
• Post-Dewatering Closure Report – determines
Contaminated Sites classification

Contaminated Sites Act 2003
• Report of contamination – not substantiated.
• Possibly contaminated – investigation required.
• Not contaminated – unrestricted use.
• Contaminated – restricted use.
• Contaminated – remediation required.
• Remediated for restricted use.
• Decontaminated.

Aerial Photo of Site

Further Groundwater Investigation

Interpretation of potential human
health impact
• Small no. of lots classified as “CS – Restricted Use”
subject to Health Risk Assessment

Consultants Perspective
• Timing – Investigations, lab analysis, DEC review
• Too much (pre)caution: Inherent neutralising
capacity of some naturally high lime soils; natural
acidity of some soil/groundwater settings and risk of
over-liming
• CS Act 2003: overarching legislation, with a broad
definition of contamination
– Desirable to be able to restrict use of groundwater
in some areas without labelling them a
contaminated site.
– Uncertainties in DEC groundwater criteria for
classification under CS Act (eg Health impact of
pH and total acidity; metals [new guidelines
pending]

Consultants Perspective (ctd(
ctd)
– reliance on consultants to fix some knowledge gaps
– Worst case classification - remediation of an entire
wetland, extensive groundwater remediation
• Classification under CS act is based on post-work
monitoring programme, and may therefore be unknown
for a long time after the works have been conducted –
uncertainty and impact on lot prices

Consultants Perspective (ctd(
ctd)
• Costs associated with
– repairing damaged infrastructure,
– remediating disturbed ecosystems
– treating contaminated groundwater aquifers
• are much greater than any costs incurred in
preventing or minimising impacts before they occur