Evaluating the Risk of Potential Acid Sulfate Soils andHabitat Modification for Mosquito Control (Runneling)in Coastal Salt Marshes: Comparing Methods andManaging the Risk

GLEN P. ALSEMGEEST*

PATRICIA E. R. DALE�

Australian School of Environmental StudiesFaculty of Environmental SciencesGriffith UniversityNathan, Qld 4111, Australia

DARREN H. ALSEMGEEST

Mosquito and Pest ManagementRedland Shire CouncilCleveland, Qld 4163, Australia

ABSTRACT / Coastal environments in Australia are underdevelopment pressures. Human settlement encroaches ondisease vector salt marsh mosquito breeding areas thatare underlain by potential acid sulfate soils (PASS). Alter-

ing the hydrology by runneling solves the mosquito prob-lem but may lead to acid sulfate problems. Appropriateanalytical tools can assess the risk to the environment. Theobjective of the research was to compare three methodsof assessing PASS. The study area was a low-lying inter-tidal subtropical salt marsh that was being considered forrunneling. The results indicated that using field pH andfield peroxide pH (and the relationship between these),and also the peroxide oxidation-combined acidity andsulfate (POCAS) test, appeared to overestimate the po-tential acidity. This was because the source of acidity inthe intertidal salt marsh includes a large organic content,which is not a major environmental concern. The chro-mium-reducible sulfur test, which is not affected by or-ganic content, was found to provide the most appropriateassessment, and is recommended for use in highly or-ganic salt marshes.

Although intertidal salt marshes have importantutility and intrinsic values, they also include breedingsites for the salt marsh mosquito, Ochlerotatus vigilax(Skuse). That mosquito transmits diseases such as RossRiver virus (RRv) and Barmah Forest virus. RRv is themost common arbovirus disease in Australia, with asmany as 8,000 cases reported annually. Most of theseare in Queensland (Curran and others 1997). Controlis mandatory in Queensland and is usually carried outat the larval stage, generally by applying larvicides tobreeding sites or by physically modifying the sites. Theusual method of modification is runneling (Hulsmanand others 1989). Runneling is a minimal and simple

form of open marsh water management (Meredith andothers 1985, Hruby and Montgomery 1986). Runnelingwas developed in Australia in the mid 1980s for inter-tidal salt marshes that had not been ditched or other-wise modified, and it is used widely in Queensland,parts of New South Wales, and also Western Australia.The runnels are shallow (<0.3 m deep) spoon-shapedchannels (up to 0.9 m wide) that, as far as possible,follow natural paths of water movement and resemblechannels that already occur on some marshes. Run-neling does not drain or dewater the marsh. Runnelsenhance the natural movement of tidal water by con-necting breeding pools to the tidal source. This flusheslarvae from the marsh, increases predator access to it,and appears to have less environmental impact com-pared to larger excavations such as ditching (Dale andHulsman 1990, Dale 2001). Runneling has no majoreffect on the environment but is effective at reducingmosquito numbers (Dale and others 1993, 1996, Daleand Dale 2002).

The intertidal salt marshes are generally underlainby potential acid sulfate soil (PASS) (Saffigna and Dale1999). Runneling in such intertidal areas has the po-

KEY WORDS: Salt marsh; Habitat modification; Runneling; Riskmanagement; Organic sediments; Acid sulfate soils;Potential acid sulfate soils; Peroxide oxidation-com-bined acidity and sulfate; Chromium-reducible sulfur

Published online June 20, 2005.�Author to whom correspondence should be addressed; email:

p.dale@griffith.edu.au*Natural Resources Services, Department of Natural Resources and

Mines, Charleville, Qld 4470

Environmental Management Vol. 36, No. 1, pp. 152–161 ª 2005 Springer Science+Business Media, Inc.

DOI: 10.1007/s00267-003-0112-4

tential to disturb pyritic sediments and to create acidsulfate problems. These may include effects such as theadverse impacts on fisheries caused by acidificationfollowing ditching of salt marshes for mosquito controlnoted by Soukup and Portnoy (1986) for the north-eastern United States.

To assess and manage the risk of PASS requirestesting methods that provide reliable and appropriatedata on the abundance of reduced inorganic sulfurcompounds. These are the compounds of major con-cern in acid sulfate soil assessments due to their abilityto rapidly generate persistent acids. Other forms ofsulfur, particularly those existing as a constituent oforganic molecules and polymers within the organiclayers of the soil, are of relatively minor importancewhen assessing PASS (McElnea and Ahern 1999). InAustralia, reduced inorganic sulfur is primarily in theform of iron pyrite (FeS2), but other forms may bepresent, such as marcasite (FeS2), elemental sulfur(So), and monosulfides (e.g., Fe3S4) (Sullivan andothers 1999). A common method to assess PASS is tooxidize sulfides with concentrated (30%, 100 vol)hydrogen peroxide (Lin and Melville 1993).

Field pH testing is often used for a preliminary PASSassessment. The field pH (pHf) and field peroxide pH(pHfox) tests can provide a quick and cheap PASSassessment. Ahern and others (1998) consider threemeasures in the identification of PASS. First, a differ-ence between the pHf and the pHfox (DpHf) of at least1 unit may indicate a PASS (the higher the DpHf, themore indicative it is of the presence of PASS). Second,if the pHfox is less than 3 and the first condition ap-plies, then it strongly, but not conclusively, indicates aPASS (Ahern and others 1998). The third measure isthe strength of the reaction. Using the first two mea-sures, Saffigna and Dale (1999) found significant PASSin areas that had been modified or were proposed formodification for mosquito control (Saffigna, unpub-lished data). However, although the field tests mayindicate the presence of PASS, they do not give anyquantitative measure of the potential acidity of the soilfollowing oxidation (Hey and Ahern 1999).

The potential acidity of the soil can be quantifiedusing the peroxide oxidation-combined acidity andsulfate (POCAS) method, which assesses both the‘‘acid trail’’ and the ‘‘sulfur trail.’’ Analysis of the ‘‘acidtrail’’ involves determining the total actual acidity(TAA), the total potential acidity (TPA), the pH in 1 MKCl (pHKCl), and the pH in 1 M KCl after peroxidedigestion (pHOX). The KCl extractible sulfur (SKCl)and the extractible sulfur after peroxide digestion (SP)indicate the sulfur trail. Calculating the total sulfidicacidity (TSA = TPA – TAA), the change in pH

(DpH = pHKCl – pHOX), and the peroxide oxidizablesulfur (SPOS = SP ) SKCl) evaluates the potential togenerate acid following oxidation and determines theoxidizable sulfur content (Ahern and others 1998).Comparing the results obtained from POCAS testingwith the Action Criteria documented by Ahern andothers (1998) indicates whether the soil is likely to posean environmental risk when disturbed (i.e., contains aPASS). For soil disturbances such as those likely inrunneling operations (excavating <1,000 tonnes), theAction Criteria for coarse, medium, and fine texturedsoils are 0.03% S (18 mol H+/ton), 0.06% S (36 molH+/tonne), and 0.1% S (62 mol H+/tonne), respec-tively. Laboratory testing can indicate PASS/ASS, whenpHOX is less than 3 and/or DpH is greater than 2(Saffigna and Dale 1999, Powell and others 1996).Collecting groundwater samples and determiningCl):SO4

2) ratios using flow injection analysis also canindicate previous oxidation. The Cl):SO4

2 of naturalseawater is approximately 7.2, and if the ratio de-creases, this can indicate that an influx of sulfate viaoxidation of pyrite has occurred (Saffigna and others1996).

Common soil constituents such as organic matterand sulfate minerals can interfere with the results frommethods based on hydrogen peroxide oxidation (Sul-livan and others 1999, White and Melville 1993, Clarkand others 1996, Lin and others 1996). Ward andothers (2002) showed that such methods are not veryreliable in organic soils. The hydrogen peroxide oxi-dation methods may indicate potential problems wherethe risk is low.

As runnels are usually only shallow (<0.3 m deep)and in highly organic salt marsh sediments, measuresof the reduced inorganic sulfur content may be mis-leading if based on hydrogen peroxide oxidation.Sullivan and others (1999), in controlled laboratoryexperiments, showed that the presence of organicmatter, when using hydrogen peroxide methods, re-sulted in overestimation of the reduced sulfur contentby 0.077% per percentage organic carbon. As the Ac-tion Criterion for disturbance of <1,000 tonnes of soilranges between 0.03% S and 0.1% S (depending on thetexture of the soil), these are significant errors thatcould easily lead to wrongly identifying a soil as a PASS.This could lead to inappropriate methods, such asovertreatment of acidity or not using habitat mod-ification at all.

As an alternative to peroxide-oxidizable sulfurmethods, the chromium-reducible sulfur (CRS) meth-od can be used in PASS assessments. Sullivan andothers (1999) and Canfield and others (1986) reportedthat the CRS method is unaffected by organic matter

Assessing Potential Acid Sulfate Soils in Salt Marsh 153

and sulfates such as gypsum. The CRS method isspecific to reduced inorganic compounds, includingiron disulfides (e.g., pyrite), elemental sulfur, andacid-volatile sulfides (e.g., greigite). The CRS methodis thus an attractive tool for assessing PASS in highlyorganic salt marsh sediments. This is particularly rele-vant for shallow disturbances of the organic layer, suchas those caused by runneling.

This study has three aims. First, it compares threecommon methods used to identify PASS in shalloworganic salt marsh sediments, in an area proposed forrunneling to manage the disease vector mosquitoes.Second, it models and maps the horizontal and verticaldistribution of PASS for the study area to predict therisk of acid problems, if runneling were to be done.Third, it develops a Runneling Management Plan tominimize the PASS risks, specifically for the localmosquito control agency (Redland Shire Council), butapplicable to any similar situation.

Materials and Methods

Study Area

The study area is in Redland Shire, approximately30 km east of the state capital city of Brisbane (latitude,27oS) in coastal southeastern Queensland. RedlandShire contains areas of ASS or likely to have ASS (Fig-ure 1). The population is growing rapidly, at an aver-age annual rate of 3% (Queensland Government2000), and its proximity to RRv disease vector mosquito(Ochlerotatus vigilax (Skuse)) breeding areas raises apublic health concern. Many of the salt marsh mos-

quito breeding habitats are within the Moreton BayMarine Park, which is of high conservation status, andextra care is needed in assessing potential acid sulfateproblems.

The research site is a typical low-lying intertidal saltmarsh, requiring mosquito management. The eleva-tion of the site ranges between 0.9 and 1.4 m AustralianHeight Datum (AHD). Spring high tides greater than2.4 m AHD periodically flood the area proposed forrunneling. Marine Couch (Sporobolus virginicus L. Ku-nth) dominates the salt marsh with gray mangroves(Avicennia marina Forsk) at the tidal source and She-Oaks (Casuarina spp.) at the upland edge.

Sampling Methodology

The general sampling strategy was to conduct apreliminary assessment on 70 samples. We used thepreliminary results to select samples for more detailedassessments as explained below. Field and laboratorysamples came from seven sampling points. Five ofthese sampling points were along a proposed runnel (a104-m transect) at intervals ranging between 20.5 and33.0 m. This is in accordance with Ahern and others(1998), who recommended sampling at least every50 m. The other two sampling points were approxi-mately 300 m to the north, serving as control points inareas not subject to tidal flooding. On 21 and 25 Jan-uary 2000, we used a 100-mm jarret auger to excavateboreholes at each sampling point. We took samplesevery 0.10 m to a depth of 1.0 m, which is below thedepth of runneling.

As resource constraints made it impractical to con-duct detailed tests on all samples, we used a strategy to

Figure 1. Map showing the main study area and likely acid sulfate soil (ASS) regions within the Redland Shire. AHD, AustralianHeight Datum. (Source: Department of Natural Resources 1999.)

154 G. P. Alsemgeest and others

select subsamples for further analysis based on eitherthe likelihood of PASS or the presence of high levels oforganic material. As the stratigraphy was almost iden-tical between all sites, we included the six samples fromall horizons at one site (borehole R3). This boreholewas central to the runneling site and hence represen-tative of it. To check the representativeness, we alsoPOCAS tested 14 samples from other boreholes. Theresults indicated that the 20 samples were sufficient fordetailed analysis.

Field and Laboratory Analyses

At each obvious horizon change, we recorded thesoil texture and matrix color using the Universal SoilClassification Chart and the Munsell Soil Colour Chart,respectively. We identified and recorded any pedo-genic features indicating ASS, including jarosite andiron staining, at 0.10-m intervals in the soil profile.

Three methods were used. First, we measured thepHf and pHfox on the 70 samples (at 0.1-m intervals inthe soil profile), to assess the likelihood/presence ofPASS along the proposed runnel location, using thefield pH testing procedure from Ahern and others(1998). During pHf testing, approximately 1 g of soilwas placed in a heat-resistant tube and mixed withdeionized water at a 1:1 (soil-to-deionized water) ratio,and the mixture allowed to react for 20 min in thefield. The procedure was similar for pHfox testing, ex-cept that 30% hydrogen peroxide (adjusted to a pH of5.0) was used instead of deionized water. For both pHf

and pHfox testing, the samples reacted and digested forapproximately 20 min before measuring the pH with acalibrated field pH meter. We used the results to selectsamples for further analysis, based on their likelihoodof containing PASS.

Second, to establish a quantitative indication of thepresence of PASS, Australian Laboratory Services (anaccredited laboratory) POCAS tested the 20 samplesselected from the samples processed by the pHf andpHfox method. The standard procedure is to shake for24 hr. These included all six samples from every soilhorizon at borehole R3, as well as the 14 samples thatwere of particular concern following field pH testing.

Third, the CRS analysis was performed on the sixsamples from every horizon identified at borehole R3,as well as on six other samples that had a high organicmatter content. For comparison, detailed results arereported only for the 12 samples tested by all threemethods (at boreholes R1, R2, R3, and R4).

To minimize any oxidation occurring during stor-age and transport, all samples requiring laboratorytesting were placed in airtight resealable bags, with theair manually squeezed from the bag. To reduce bac-

terially mediated oxidation, we placed the samples inan insulated container with both dry and block ice.Australian Laboratory Services analyzed the sampleswithin 24 hr.

Modeling and Mapping Using GIS

Once the accuracy of field and analytical methodshad been assessed, we used the data to model and mapthe horizontal and vertical distribution of high-riskPASS. First, we converted field pH, POCAS, and CRSdata from Microsoft Excel into a MapInfo table. UsingVertical Mapper, we mapped the spatial variation ofeach variable (e.g., pHfox) between sampling points ateach specified depth. We interpolated across the wholearea using the natural neighbor interpolation feature.The six different depth ranges modeled and mappedwere 0.0–0.10, >0.10–0.20, >0.20–0.30, >0.30–0.40,0.50–0.60, and 0.90–1.00 m. After all the parameterswere interpolated into a raster structure, they weresuperimposed, allowing the structured query language(SQL) feature to identify high-risk areas. If soils ex-ceeded the Action Criteria of 0.1% S for STSA/SPOS andSCR, we termed this Category 1. These Category 1 areaswere of particular importance to runneling operations,as any disturbance of these soils would require a de-tailed Environmental Management Plan (EMP) (Ahernand others 1998). Category 2 and Category 3 soils pose amoderate risk and Category 4 and Category 5 soils areunlikely to contain PASS. Category 6 soils are low-risksoils, based on a negligible reduced inorganic sulfurcontent.

Results and Discussion

Soil Stratigraphy and Characteristics

The soil stratigraphy at the runnel site consisted offour main horizons (Figure 2). The relevant soil tax-onomy and Australian Soil Classification are shown inTable 1. However, of significance here is an organicsurface layer that occurs above approximately 1.3 mAHD, at depths ranging between 0.10 m and 0.15 m.The layer is characterized by its dark color, peaty odor,spongy feel, and fibrous texture. Other horizons ofinterest included the second and third soil horizonswith yellow, orange, and reddish mottles, indicatingthe presence of oxidizing conditions and the forma-tion of jarosite and iron oxides, such as hematite andgoethite. Conversely, the soils in the fourth horizonwere a dark-gray silt that showed no indication of pre-vious oxidation (i.e., no jarosite or iron oxides) andemitted a slight hydrogen sulfide odor (rotten egg gas)when disturbed. It is likely that the fourth horizon is ahigh-risk soil consisting of sulfides (the potentially

Assessing Potential Acid Sulfate Soils in Salt Marsh 155

harmful sulfur species). This is for two reasons. First,PASS is often reported as a dark-gray sticky mud (e.g.,Sammut and Lines-Kelly 2000). Second, the presenceof hydrogen sulfide (evidenced by odor) and the ab-sence of soil mottles indicated reducing conditions.

Field testing. The results of field pH assessment inwater and hydrogen peroxide (pHfox and pHf) indi-cated that PASS were likely at depths below the surfaceand at 0.15, 0.4, and 0.6 m at all boreholes at therunnel location. At higher elevations on the marsh,PASS was below the first soil horizon. At lower eleva-tions PASS was at greater depths. Overall, the severityof PASS increased with depth, with the dark-gray silt(the fourth horizon) the most indicative of PASS, withpH�s as low as 0.8 and DpH�s as high as 4.9 recorded.

Peroxide Oxidation-Combined Acidity and Sulfate(POCAS) Testing

The pHOX and DpH laboratory results indicatedPASS below elevations of 0.7 m AHD, which corre-sponds to the dark-gray silt identified in the fourth soilhorizon. However, the results from the acidity trail(STSA) and the sulfur trail (SPOS) exceeded the ActionCriteria of 0.1% S at elevations both above 1.3 m AHDand below 0.9 m AHD. This suggested that PASS ex-isted not only in the fourth soil horizon but also in the

organic surface layer. There were negative DpH�s insamples collected from high sample elevations, whichindicated the presence of organic acids. NegativeDpH�s occur when organic acids are destroyed by oxi-dation, causing the pH of the extract to rise slightly(McElnea and Ahern 1999). Comparing the acid andsulfur trails can help identify the presence of organicpeats. If the STSA is higher than the SPOS, it is likely thatorganic acids are present. The results indicated thatorganic acids were contributing to a potential overes-timation of the degree of PASS close to the surface(McElnea and Ahern 1999) at sample elevations >1.2 mAHD.

Chromium-Reducible Sulfur Testing

CRS tests were used on 12 selected samples toquantify the suspected anomalies associated with per-oxide-oxidizable sulfur methods (e.g., POCAS andfield pH testing) in organic sediments. Using the CRSmethod, the reduced inorganic sulfur levels in the first,second, and third soil horizons were all less than 0.04%

S. That is substantially less than the Action Criterion of0.1% S. The fourth soil horizon was the only one ofconcern, with reduced inorganic sulfur levels recordedabove 0.9% S (nine times the Action Criterion for finetextured soils).

Figure 2. Soil stratigraphy along proposed runnel location. Dimensions are shown in meters AHD (Australian Height Datum).

Table 1. Soil taxonomy, the Australian Soil Classification and acid sulfate soil status in the study area

Vegetation Soil Taxonomy Australian Soil Classification Acid sulfate soil status

Mangroves Sulfic Endoquept Sulfidic Intertidal Hydrosol Potential acid sulfate soil (PASS) presentSalt marsh Sulfic Endoquept Sulfidic Supratidal Hydrosol Potential acid sulfate soil (PASS) presentSalt marsh Typic Sulfaquept Sulfidic Supratidal Hydrosol Acid sulfate soil (ASS) and (PASS) present

(Source: Soil Survey Staff 1999, Isbell 1996, Fitzpatrick and others 2003)

156 G. P. Alsemgeest and others

Comparison Between Field and Analytical Methods

Table 2 shows that, of the 12 samples for which fieldpH, POCAS, and CRS testing was performed, only 1sample produced a consistent identification of PASS. APASS was positively identified at a depth of 0.9–1.0 m atborehole R3. While laboratory analysis confirmed thefourth soil horizon as a PASS, there are still inconsis-tencies in the results at shallower depths. In particular,POCAS testing in the surface soils at boreholes R1 andR2 revealed oxidizable sulfur and TSA levels to beabove the Action Criteria, while in the same samples,the CRS method found very low levels of oxidizablesulfur (Table 2).

Based on Sullivan and others (1999), the inconsis-tencies between POCAS and CRS methods suggest thatfield pH and POCAS assessments significantly overes-timate the severity of PASS due to the influence oforganic matter. Organic matter does not affect the CRSmethod (Canfield and others 1986, Morse and Corn-well 1987, Bottrell and others 1994). However, thePOCAS method may overestimate the reduced inor-ganic sulfur content by up to 0.11% S. These findingsare in accord with Sullivan and others (1999), whodocumented positive errors of up to 0.077% S perpercentage organic carbon. As the Action Criterionranges between 0.03% S and 0.10% S, these are sig-nificant errors which easily can result in the misiden-tification of PASS, the adoption of inappropriate andexpensive treatment methods, and/or the avoidance ofrunneling in favor of expensive, repetitive, and some-times controversial chemical mosquito treatments.

Recently McElnea and others (2002a, b) have pub-lished a new peroxide-based method claimed to over-come the reliability issues of Ward and others (2002).

The advantages of using the CRS method includethe following. First, it produces a high pyrite sulfurrecovery rate. Sullivan and others (1999), Canfield andothers (1986), and Newton and others (1995) reportedrecovery rates of 98.7%, 95.9%, and 94.3%, respectively.Second, the CRS method can be digested in only 1 hr,compared to a period of 2 to 7 days for hydrogenperoxide methods. Third, standard laboratory equip-ment can be used to undertake CRS tests. Finally, CRStests usually are less than half the price of POCAStesting. Therefore, although the use of SPOS (obtainedfrom POCAS methods) has gained wide acceptance byindustry, and has been approved as a standard methodwhen preparing environmental impact statements, itappears to be unsuitable and less economically viablefor use in runneling assessments in organic salt marshenvironments.

Distribution of Acid Sulfate Soil Risk and ItsManagement

Although the sample size was too small for validstatistical testing, this basic survey does meet the rec-ommendations of Ahern and others (1998) and wouldbe feasible for local authorities to implement as apractical tool.

Figure 3 illustrates the horizontal and vertical dis-tribution of PASS within the study site. Based primarilyon the results from CRS testing, the ASS assessment

Table 2. Comparison between field, POCAS, and Chromium-Reducible Sulfur results for the assessment ofPASS

BoreholeNo.

Sample Depth(m)

Field Assessment POCAS Assessment SCR assessment

pHfox DpH PASS?1 pHOX DpH TSA mol H+/t SPOS % PASS?2 SCR % PASS?3

R1 0)0.1 3.1 1.4 NO 4.6 0.4 82 0.14 YES 0.03 NO0.1)0.2 2.6 1.2 YES 4.4 0.0 44 0.02 NO 0.02 NO0.5)0.6 1.7 1.3 YES 3.9 0.0 2 0.06 NO 0.01 NO

R2 0)0.1 3.1 1.9 NO 4.7 0.5 60 0.14 YES 0.03 NO0.2)0.3 2.6 2.7 YES 4.6 0.0 6 0.04 NO 0.01 NO

R3 0)0.1 3.1 2.0 YES 5.2 )0.1 18 <0.02 NO 0.02 NO0.1)0.2 2.0 2.8 YES 4.4 )0.4 4 <0.02 NO 0.02 NO0.2)0.3 2.1 1.7 YES 4.1 )0.2 10 0.02 NO 0.03 NO0.3)0.4 2.3 1.7 YES 3.9 0.1 8 0.02 NO 0.01 NO0.5)0.6 1.8 1.7 YES 3.5 0.2 12 0.02 NO 0.01 NO0.9)1.0 1.5 2.7 YES 1.5 2.1 478 0.96 YES 0.92 YES

R4 0.2)0.3 4.9 2.0 YES 6.4 )2.4 <2 <0.02 NO 0.01 NO

1. Potential acid sulfate soils (PASS) have a soil pH less than 3 when treated with 30% hydrogen peroxide (pHfox) (Powell and Ahern 1999)

and/or a DpH greater than 2, where DpH = pHf – pHfox (Ahern and others 1998).

2. For fine textured soils (medium to heavy clays and silty clays), PASS have a TSA of 18 mol H+/t or above or a SPOS of 0.1% or above (Ahern and

others 1998).

3. For fine textured soils (medium to heavy clays and silty clays), PASS have a SCR of 0.1% or above (Ahern and others 1998).

Assessing Potential Acid Sulfate Soils in Salt Marsh 157

confirmed that generally there was no PASS in the top0.9 m, though the PASS-free zone may be as shallow as0.5 m. As the maximum runnel depth planned for thesite was 0.25 m, and the maximum runnel depth whichcan be constructed by the Redland K-3 runneling ma-chine is 0.3 m, there would be no direct interception ofPASS. However the dark-gray silt found below thedepths mentioned above was identified to have anoxidizable sulfur level 7 to 11 times greater than theAction Criterion outlined by Ahern and others (1998).Figure 3 shows that, as greater depths are reached, thePASS risk becomes greater. This has implications forother forms of habitat modification that might involvedeeper excavation.

Although runnel depths are usually confined to thesurface horizon, where the risk of PASS is low, there isstill a need for risk management. The AS/NZS 4360(1999) process has been adopted here to develop aRMP (Figure 4). The plan is to manage the effects ofrunneling operations in Redland Shire (Redland ShireCouncil 1998). The plan developed evaluation criteriato assess the risks associated with proposals for run-neling projects. According to the Queensland StatePlanning Policy on Acid Sulfate Soils, treatment of ASS

may not be needed, depending on the potential acidityplus existing acidity and the volume of material to beextracted (Queensland Government 2002).

The potential ASS risks associated with runnelinginclude on-site and off-site risks and risks that impacton the Redland Shire Council, which is responsible formanaging mosquitoes. The on-site risks include thedirect interception and subsequent oxidation of PASSand a potentially large area of contamination due tothe dispersal of spoil over the site. The off-site risksinclude the release of contaminants into the sur-rounding waterways, such as sulfuric acid and metals,principally Al and Fe. Finally, the risks to the councilinclude risk of litigation under the EnvironmentalProtection Act 1994, breach in permit agreementsunder the Fisheries Act 1994, loss of support for run-neling by the Department of Primary Industries(Fisheries), and community dissatisfaction that mayjeopardize runneling. To manage these ASS risks, it isimportant to conduct a risk analysis and evaluationand, if necessary, implement appropriate treatmentmethods to minimize the risks to acceptable levels. It isdifficult to predict the risks because of very limitedprevious ASS research relating to shallow disturbances,

Figure 3. Spatial distribution of acid sulfate soil risk areas at varying depths. CRS, chromium reducible sulphur; POCAS,peroxide oxidation-combined acidity and sulfate; TSA, total sulfidic acidity.

158 G. P. Alsemgeest and others

such as runneling (Saffigna and Dale 1999). However,the consequences would depend heavily on many site-specific factors, including the degree of reducedinorganic sulfur disturbed, the rate at which the con-taminants disperse, and the environmental sensitivityof the site. As all runneling operations occur below 5m AHD in salt marsh sediments prone to PASS, thepossibility of disturbing these soils warrants consider-ation. The likelihood of off-site risks and risks to thecouncil are largely unknown at this stage. However,risk treatment is required when an identified risk isfound to be unacceptable when compared to the riskevaluation criteria.

During the initial period of uncertainty, the pre-cautionary principle should be adopted. In Australiathis principle is embedded in national environmentallegislation. This is ‘‘that lack of full scientific certaintyshould not be used as a reason for postponing a mea-sure to prevent degradation of the environment where

there are threats of serious or irreversible environ-mental damage’’ (Section 391 (2), Environment Pro-tection and Biodiversity Conservation Act, 1999).However, if there is such a risk, a project may proceedsubject to all reasonable and practicable measuresbeing taken to minimize that risk.

There are various generic risk treatment options,such as avoiding or reducing the risk, reducing theconsequences, containing the risk, or transferring therisk. These can be applied to runneling operations(AS/NZS 4360 1999). Treating the on-site risks willreduce all risks. Thus preventing the direct intercep-tion of PASS and drainage of the site will also manageboth the off-site risks and the risks to Redland ShireCouncil. The Risk Management Plan shown in Figure 4is proposed for all future runneling operations con-ducted by the Redland Shire Council. The method isrelevant to other agencies contemplating runneling formosquito management in similar environments.

RUNNELLING UNDER CONSIDERATION

Does Field testing (pHf –pHfox ) indicate PASS?

If YES, then perform CRS test

Action Criteria exceeded?

If YES then seek expert advice

Runnelling possible, subject to conditions?

If NO, then use alternative mosquito control

If YES, then • develop management plan • apply for permit

• proceed in compliance with permit

Runnel RunnelDo not runnel

If NO, then proceed to plan

runnelling, including permit

requirements

If NO, then proceed to plan

runnelling, including permit

requirements

Figure 4. Risk Management Plan proposed for runneling operations in areas of likely PASS.

Assessing Potential Acid Sulfate Soils in Salt Marsh 159

The RMP recommends field pH testing for pro-posed runnelling operations. Although field pH testingwas found to be misleading in some samples, it may stillbe an effective and practical method to initially assesssalt marsh sediments. As field pH testing alone appearsto give an overestimate, no further action is necessary ifit does not indicate PASS. However, if field testing re-veals the likely presence of PASS, then laboratoryanalysis is recommended. Since peroxide-oxidationmethods, such as POCAS testing, may give misleadingreduced inorganic sulfur concentrations in organicsediments, the CRS test is preferred. The managementplan is expected to be economically feasible for run-neling operators. Field pH testing is quick and simpleto implement, laboratory analysis may not be required,and, if required, the CRS method is approximately halfthe price of POCAS testing. If the field and laboratoryresults confirm the presence of PASS, it is recom-mended that the risks be avoided by implementingchemical treatments or transferred by seeking advicefrom an expert.

It is particularly important that the risks, the effec-tiveness of the risk management plan, strategies, andthe management system that is set up to control itsimplementation are closely monitored and reviewed.Monitoring and review should focus on ensuring thatchanging circumstances do not increase the acid sul-fate risks associated with runneling.

Conclusions

The research has shown that field pH testing andthe POCAS test can be misleading in intertidal organicsoils. In particular, the POCAS test can overestimatethe potential acid sulfate problem. The CRS test ap-pears to be a more reliable indicator. For habitatmodification for mosquito control, it is important tonot overestimate the risk, as this may lead to overre-liance on chemical treatment when simple and mini-mal habitat modification would be acceptable. Themanagement plan proposed is a practical tool to guidedecision-making for those involved in relatively minorhabitat modification such as runneling. It acknowl-edges the importance of properly assessing the risk andmanaging it in accordance with the precautionaryprinciple that is established in legislation.

AcknowledgmentsThe provision of funding from the Redland Shire

Council and Griffith University is gratefully acknowl-edged. Ian Phillips (Faculty of Environmental Sciences,

Griffith University) and Kylie Hey (Queensland AcidSulfate Soils Investigation Team, Department of Natu-ral Resources) are acknowledged for their expertiseand advice during the development of this study. Wethank Col Ahern (Queensland Department of NaturalResources) for his helpful comments on an earlymanuscript and recognize Scott Dunsdon (Mosquitoand Pest Management, Redland Shire Council) for hisassistance in undertaking field activities. We also thankthe manuscript reviewers for their helpful and con-structive comments and Bernie Powell (Department ofNatural Resources and Mines, Queensland) for assis-tance in revising the paper.

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