1

Soil and groundwater acidification on the Gnangara

Mound

Overview

• Causes and impacts of acidification• Outline recent investigation program

• Potential management issues

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What is acidification?

• Change in soil and water chemistry triggered by increased input of hydrogen ions;

• Increases solubility of metals (particularly iron and aluminium) – highly toxic to aquatic fauna, flora

• Release of arsenic, cadmium other toxicants• Loss of nutrients, organic matter, “base

cations” from soil – progressive decline of ecosystems

• Large economic impacts – damage to infrastructure, loss of fisheries, decline of agricultural productivity

H+ SO42-NO3

-

Ca+2

Mg+2

K+

Base Cation DepletionBase Cation Depletion

Al+

3

H+ SO42-NO3

-

Episodic AcidificationEpisodic Acidification

Al+

Ca+2

Mg+

2

K+

H+ SO42-NO3

-

Chronic AcidificationChronic Acidification

Al+

Ca+2

Mg+

2

K+

4

Impact of Impact of

acidification acidification

on wetland on wetland

ecosystemsecosystems

Acidification impacts on forests and woodlands – tree dieback and loss of

sensitive species

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Loss of sensitive fauna – e.g. loss of wood thrush in NE USA linked to

acidification (Cornell Uni research)

Sources of acid

• Sulfide minerals in soil – especially pyrite (VERY LARGE ACID INPUT) (Acid sulfate soils)

• Oxidation of organic matter• Agricultural land use – excessive fertiliser

use, high stocking rates, crop harvesting, timber plantations (In WA this is a major concern because of losses to agricultural productivity )

• Air pollution – sulfur and nitrogen oxides from industry, power generation, vehicle exhausts (Acid rain)

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Soils do not care about acid sources –environmental impacts are the same

To properly assess acidification risks, must consider how soils are linked to the external

environment

Conceptual modelConceptual model

AcidAcid--base balance base balance

before European before European

settlementsettlement

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Conceptual modelConceptual model

AcidAcid--base balance base balance

after European after European

settlementsettlement

Evidence that the acid-base balance is being disrupted on the

Gnangara Mound

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Investigation area – acidified wetlands shown in red, pines in green

Investigations were focussed on Bassendean Sands (grey area)

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Drilling was carried out at 18 sites –groundwater, some soil sampling

Indicators of Indicators of

groundwater groundwater

acidity at the acidity at the

water tablewater table

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Indicators of acidity Indicators of acidity

changes over the last changes over the last

2525--35 years in the area35 years in the area

Lake Gnangara

Indicators of recent acidity changes – water quality in Lake Mariginiup (Troy Cook data)

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Indicators of recent acidity changes – soil

excavation and dewatering for urban development

Ellenbrook Stage 5 Dewatering, Bore VAB 5

0

500

1000

1500

2000

2500

3000

3500

4000

01-Jan-06 02-Apr-06 02-Jul-06 01-Oct-06 31-Dec-06 01-Apr-07 01-Jul-07 30-Sep-07 30-Dec-07

Date

mg/

L as

CaC

O3

Total Acidity

Total Alkalinity

Indicators of recent acidity changes – soil

excavation and dewatering for urban development

Ellenbrook Stage 5 Dewatering - Bore VAB3

2.5

3.0

3.5

4.0

4.5

01-Aug-06 01-Nov-06 01-Feb-07 04-May-07 04-Aug-07

Date

pH

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Variation of soil, groundwater acidity with depth – site 13

Bushland Bushland -- low acidity, but subsoil low acidity, but subsoil starting to acidify?starting to acidify?

Variation of soil, groundwater acidity with depth – site 11

Rural land use, encroaching urban Rural land use, encroaching urban ––presence of clear acidification front presence of clear acidification front in groundwaterin groundwater

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Acidification front in a sandy aquifer in Denmark (source: Prof Dieke Postma)

Elev

atio

n (m

eter

s)

0 0.1 0.2 0.3 0.4 0.5

Al (mM)25

27

29

31

3335

3739

41

433 4 5 6 7

pH

GWT

Acidificationfront

mmol /100gc

0 0.2 0.4 0.6

Al

Ca

MgKNa

Elevated metal concentrations in groundwater at an acidification front in Grinsted, Denmark

(source: Prof Dieke Postma)

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Metal concentrations in groundwater at the acidification front, site 11

Cu = 65 Cu = 65 µµµµµµµµg/Lg/L

PbPb = 12 = 12 µµµµµµµµg/Lg/L

Ni = 26 Ni = 26 µµµµµµµµg/Lg/L

Zn = 82 Zn = 82 µµµµµµµµg/Lg/L

(up to 10 times (up to 10 times

background levels background levels

for the area)for the area)

Variation of soil, groundwater acidity with depth – site 18

Mirrabooka Mirrabooka borefieldborefield (Whiteman (Whiteman Park) Park) ––subsoil has acidified due to subsoil has acidified due to water table drawdown, groundwater water table drawdown, groundwater acidification front presentacidification front present

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Production bore data indicate base cations being leached from soils in this area…

0.5

1.0

1.5

2.0

01-Jan-80 31-Dec-84 01-Jan-90 01-Jan-95 02-Jan-00

Date

(Ca+

Mg)

(m

mol

e)/L

Mirrabooka bore M150

…and sulphur

Mirrabooka bore M150

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Variation of soil, groundwater acidity with depth – site 5

Pine plantation within cone of Pine plantation within cone of depression of Mirrabooka depression of Mirrabooka borefieldborefield––subsoil is acidifying, very acidic subsoil is acidifying, very acidic groundwater, deep acidification groundwater, deep acidification front due to drawdown front due to drawdown -- role of role of podzolspodzols??

Other groundwater pH profiles beneath pines

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Podzols beneath the Gnangara pine plantations (Troy Cook’s PhD work)

PodzolsPodzols form from the mobilisation of Al and Fe from upper to lower soiform from the mobilisation of Al and Fe from upper to lower soil horizonsl horizons

by organic acids leached from decomposing organic material. Orgaby organic acids leached from decomposing organic material. Organic acids mobilisenic acids mobilise

Al from the E horizon as Al from the E horizon as aluminoalumino--organic solutes and translocate to the B horizon. organic solutes and translocate to the B horizon.

AtmosphericAtmospheric--derived SOderived SO4422-- can also facilitate the transport of Al. can also facilitate the transport of Al.

…SEM evidence of poorly crystalline Al phases in podsols that may be a source of

soluble Al (Troy Cook’s PhD work)

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Increasing groundwater acidity also associated with increasing sulphate – but low levels of sulphides in sediments, low fertiliser

use. Possible atmospheric input

0.0

1.0

2.0

3.0

4.0

0 20 40 60 80 100

Total Acidity (mg/L CaCO 3)

SO

4/C

l (m

g ra

tio)

Seawater ratio

Preliminary modelling indicates that it would take about 50-100 years for acidification fronts to reach current

depths –increased air pollution in 20th C a trigger?

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Conceptual model

Acidification Acidification

appears to be appears to be

widespread on widespread on

the Swan the Swan

Coastal PlainCoastal Plain

Apparent link to Apparent link to

intense intense

groundwater groundwater

useuse

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Potential management issues –abstraction for public water supply, horticulture

• Continuing decline of water table elevation is likely to continue acid release, soil depletion – both wetland and woodland impacts likely

• Rapid rebound of the water table could mobilise stored acidity and metals and cause environmental impacts – i.e. care needed with pine clearing, vegetation thinning strategies

• Significant risk that increasing abstraction under low rainfall will cause widespread harm to both terrestrial, wetland ecosystems in the area.

Potential management issue – domestic groundwater use

• Ribbons of Blue testing of groundwater samples from land owners in NeavesRd area indicated that many private bores tested have pH <4 –potential health issue if used untreated for potable use. Also potential damage to gardens, livestock

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Potential management issues –vegetation impacts

• Reestablishing native vegetation in areas of high drawdown (Whiteman Park) may be difficult due to base cation depletion, Al toxicity

• Al toxicity is possibly a significant contributor to Banksia deaths in borefields –need to look at soil quality, not just soil moisture

• Risk of losing sensitive plant species and dependent fauna in woodlands in the area

Potential management issue – new urban development

• Soil excavation and construction dewatering could exacerbate acidity problems on Gnangara Mound –pattern observed on parts of the Jandakot Mound, Ellenbrook

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Potential management issues - wetlands

• Likely that more wetlands will acidify on the Gnangara Mound under current groundwater abstraction regime, low rainfall

• Using groundwater to maintain acidified wetlands unlikely to be sustainable- acidification front moving into superficial aquifer

Potential management issue –corrosion of infrastructure

Lake opposite Livingston shopping complex, Canning ValeLake opposite Livingston shopping complex, Canning Vale

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Potential management issue –corrosion of infrastructure