bio fuel ash in a road construction: impact on soil solution chemistry

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Waste Management 26 (2006) 599–613

Bio fuel ash in a road construction: Impact on soil solution chemistry

R.T. Thurdin a,*, P.A.W. van Hees b, D. Bylund a, U.S. Lundstrom a

a Department of Natural Sciences, Mid Sweden University, SE-851 70 Sundsvall, Swedenb Orebro University, Department of Science, SE-701 82 Orebro, Sweden

Accepted 30 June 2005Available online 4 October 2005

Abstract

Limited natural resources and landfill space, as well as increasing amounts of ash produced from incineration of bio fuel and muni-cipal solid waste, have created a demand for useful applications of ash, of which road construction is one application. Along nationalroad 90, situated about 20 km west of Solleftea in the middle of Sweden, an experiment road was constructed with a 40 cm bio fuel ashlayer. The environmental impact of the ash layer was evaluated from soil solutions obtained by centrifugation of soil samples taken onfour occasions during 2001–2003. Soil samples were taken in the ash layer, below the ash layer at two depths in the road and in the ditch.In the soil solutions, pH, conductivity, dissolved organic carbon (DOC) and the total concentration of cations (metals) and anions weredetermined. Two years after the application of the ash layers in the test road, the concentrations in the ash layer of K, SO4, Zn, and Hghad increased significantly while the concentration of Se, Mo and Cd had decreased significantly. Below the ash layer in the road aninitial increase of pH was observed and the concentrations of K, SO4, Se, Mo and Cd increased significantly, while the concentrationsof Cu and Hg decreased significantly in the road and also in the ditch. Cd was the element showing a potential risk of contamination ofthe groundwater. The concentrations of Ca in the ash layer indicated an ongoing hardening, which is important for the leaching rate andthe strength of the road construction.� 2005 Published by Elsevier Ltd.

1. Introduction

The population in the world is growing and becomingincreasingly concentrated to rural areas and cities. Thesehave to be supported by resource fluxes of food, energy,building materials, etc., demanding transportation, whichconsumes energy and emits pollutants. This developmentincreases the importance of recycling resources within therural areas to lower costs for transportation and reducethe amount of waste to be disposed.

For sustainable heat and electricity production, bio fuelswill in the future be used to a higher degree. This will gen-erate larger amounts of ash as a waste or by-product. Also,an increased incineration of municipal solid waste will in-crease the amounts of ash. In Sweden combustible wasteswere not allowed to be landfilled after January 1, 2005,

0956-053X/$ - see front matter � 2005 Published by Elsevier Ltd.

doi:10.1016/j.wasman.2005.06.018

* Corresponding author. Tel.: +46 070 521 70 56; fax: +46 060 61 61 07.E-mail address: [email protected] (R.T. Thurdin).

which will generate incineration to a higher degree. The an-nual amounts of ash produced in Sweden were estimated byBjurstrom (2002) to be 1 million tons. If the ash is recycledfor different purposes such as road construction or cementproduction, it could save both natural resources and energy.

1.1. Legislations and regulations affecting the use of ash

There are different ways to use the ash. The SwedishBoard of Forestry (SBF) has prescribed that wood ashshould be applied on forest soil as a way of recycling nutri-ents at whole-tree harvest. SBF recommends that up to3000 kg ha�1 of stabilized ash can be spread in forests(Skogsvardsstyrelsen, 2001).

Another alternative is to use ashes in construction. InSweden roughly 70 million tons of ballast materials wereused in the construction industry during 2001, and of theseless than 10% were recycled materials (Arell, 2003). Con-crete waste from the construction industry, blast furnace

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600 R.T. Thurdin et al. / Waste Management 26 (2006) 599–613

material from the steel industry, mining residuals, ashesand some sludges were potential materials for recyclingand use in the construction industry (Arell, 2003).

In Sweden, there are ongoing investigations on the techni-cal and environmental properties of ash for use as road con-struction material, but so far the development of genericregulations have not been developed for ashes or for recycledmaterials in general. In Sweden, the available guidelines forashes are the same as for waste. Waste is classified as inert,non-hazardous or hazardous according to the criteria inthe EU-directive on Landfill of Waste as of decision 2003/33/EG. In practice though, it has been impossible to applythis classification of inert waste as a way to get a generalacceptance for recycling of ash in Sweden (Wilhelmssonand Paijkull, 2003). Other guidelines available are those forcontaminated soil (Swedish EPA, 1997). Although no genericguidelines are available, there are examples where ash hasbeen used for road construction and for the purpose of fillingout water areas to build land. A site where ash is to be appliedhas to be tried by the environmental authorities to receive anenvironmental permit according to the Swedish Environ-mental Protection Law (SFS 1998:808), which includes theabove mentioned regulations. This can be difficult andexpensive especially when no reference projects are available.In practice, this holds back the development of large-scaleuse of secondary materials. A market for secondary materi-als needs environmental quality demands that are wellknown to authorities, producers and potential end-users.

The objective of this work was to evaluate the environ-mental impact on soil chemistry when using bio fuel ash ina road construction by studying the soil solution concen-trations.

1.2. Composition and properties of ash

The composition and properties of ash depend on thetype and origin of the fuel and also on the combustiontechnique and flue gas cleaning system used. The variationsof quality at one combustion plant and between differentplants can be quite comprehensive. During the combustionprocess, the inorganic constituents of the fuel are decom-posed, dehydrated and volatilized. Due to volatilization,trace elements (e.g., Zn, Cd, and Hg) are accumulated inthe flue gas cleaning system where the fly ash is collected.Bottom ash contains glassy amorphous particles, whichhave lower solubility compared to fly ash, but also havelower hardening capacity.

Wood ash chemistry has been investigated by Steenariand Lindqvist (1997) and Steenari et al. (1999b). Thesuggested major reactions in wood ash all take place in thewater phase. Calcium oxide (CaO) reacts with water formingcalcium hydroxide (Ca(OH)2), referred to as portlandite.

CaOþH2O) CaðOHÞ2 ð1Þ

Ca(OH)2 then reacts with carbon dioxide forming calcite(CaCO3), which precipitates on the ash particle surfacesand pores.

CaðOHÞ2 þ CO2 ) CaCO3 þH2O ð2ÞWhen calcium sulphate (CaSO4) and calcium aluminiumoxides (Ca3Al2O6) are present together at pH > 10.5, themineral ettringite ðCa6Al2ðOHÞ12ðSO4Þ3 � 26H2OÞ can beformed. At lower pH, gypsum ðCaSO4 � 2H2OÞ is formed.

pH > 10.5

3CaSO4 þ Ca3Al2O6 þ 32H2O

) Ca6Al2ðOHÞ12ðSO4Þ3 � 26H2O ð3Þ

pH < 10.5

CaSO4 þ 2H2O) CaSO4 � 2H2O ðgypsumÞ ð4Þ

The hardening capacity of ash is at its maximum when itcomes out fresh from the incinerator. As reactions takeplace in contact with the atmosphere and moisture, thehardening capacity is lost. Dry storage of ash in silos ispossible but the ash then becomes compact and loses its flu-idity, which complicates the handling of the ash. Dry flyash also causes dust problems. Steenari et al. (1999a) andTegner et al. (1997) found that in ash piles, the carbonationof CaO forming CaCO3 occurs on the surface creating anair tight crust which prevents carbon dioxide (CO2) fromentering further and reacting with Ca(OH)2.

1.3. Leaching characteristics for ashes

There are a number of leaching tests developed for reg-ulatory purposes, environmental impact assessments, scien-tific studies and daily practice in waste management (vander Sloot, 1996; van der Sloot et al., 1996). The influenceof pH on the leaching characteristics from ashes has beenstudied (van der Sloot, 1996, 2001). The leachability ofCl is rather independent of pH (van der Sloot, 1996). Cl it-self also can have an impact on the leaching of metals by itsability to form complexes (van der Sloot, 1996). K can beregarded as easily soluble over the whole pH range(0–14), and Steenari et al. (1999a) showed that the accumu-lated release of K from wood ash was about 80% of the to-tal content in de-ionized water and about 70% in a 0.005 MH2SO4 solution at a liquid/solid ratio of 10. The pH depen-dency of several metals (Cd, Cr, Cu, Pb and Zn) in ashesgenerally shows a pH or pH-interval at which the leachedconcentration is at a minimum. At pH levels below andabove this minimum the concentrations increase. For Pbthis minimum occurs at a pH around 9, Zn at around 10while for Cd the minimum can be found at a pH higherthan 12 (van der Sloot, 2001). The pH dependency canbe altered by complexation as is the case for Cu. The leach-ing of Mo is not so pH dependent above pH 9. At a pH be-low around 8, the concentration decreases with pH.

During the hardening of the ash, the leaching of Ca de-creases as CaCO3 is formed (Steenari et al., 1999b). Theformation of CaCO3 in the ash is one reaction occurringas CO2 in the atmosphere is dissolved in the pore water, re-ferred to as the carbonation process. During the carbon-ation process pH is neutralized and additional sorption

R.T. Thurdin et al. / Waste Management 26 (2006) 599–613 601

sites are formed on the ash matrix, which can reduce theleaching of trace elements (Meima et al., 2002). High con-centrations of DOC (dissolved organic carbon) can formcomplexes with trace elements and increase the leaching(van der Sloot et al., 1996; Meima et al., 2002). Hardeningof an ash and agglomeration to larger particles reduces theinitial leaching of trace elements as they are retained in theash matrix. In the long time-scale, however, pH will be low-ered and trace elements released (Steenari et al., 1999b).

1.4. Field experiments using ash in road construction

A number of authors have investigated the possible useof fly ash and other by-products for use in road construc-tion (Azizian et al., 2003; Carling and Hjalmarsson, 1998;Churchill and Amirkhanian, 1999; Deschamps, 1998;Ferreira et al., 2002; Lahtinen, 2001; Mulder, 1996a; Nuneset al., 1996; Scheetz and Earle, 1998; Schreurs et al., 2000;Zhou et al., 2000). Road construction materials must with-stand the traffic loads. Ash stabilized soil can increase thestrength of a road construction (Mulder, 1996b; Zhouet al., 2000). Lahtinen (2001) has investigated the use of dif-ferent fly ashes in combination with pulp industry fibresludge, blast furnace slag and stainless steel slag for theconstruction of low-volume roads; the advantages of con-structions using fly ash were, among others, a longer life-time and less material consumption saving natural gravelsand crushed stone.

1.5. Soil solution sampling

Soil solution can be defined as the aqueous liquid phasein soil whose composition is influenced by flows of matterand energy between it and its surroundings and by thegravitational field of the Earth (Sposito, 1989). The compo-sition of the soil solution is influenced by interactions withsoil minerals, plant roots, fungi and micro-organisms andtherefore can give information about these processes (Kel-ler and Domergue, 1996; Ranger et al., 2001; Strobel,2001). Soil solutions are obtained on-site by the installationof lysimeters, or by centrifugation or extraction of soil sam-ples. Centrifugation as a method to obtain soil solution hasbeen described by Davis and Davis (1963); Wyndham andBath (1976), and by Giesler and Lundstrom (1990, 1993).

ROAD 90 SOLLEFTEÅ →

→← →←

Road

CELL 1

5 % Cement

30 m : :

CELL 2

2,5 % Cement 2,5 % Merit 5000

30 m : :

Ditch : : : ::Sample positions with t

Fig. 1. Experim

Centrifugation makes sampling of soil solution in relativelydry soils possible. A disadvantage is that rather small sam-ple volumes can be obtained.

2. Site and methods

2.1. Site description

The experiment road is situated close to Forsmo, north-west of Solleftea, along the National road 90 in the middleof Sweden at 63�16 033 0 0N, 17�12 052 0 0E (RT 90 coordinatesystem). The total length of the experiment road was130 m, divided into four different sections here referred toas cells (Fig. 1). The three cells containing ash mixtureswere adjacent to each other, and then there was a 10 m dis-tance to the cell without ash, hereafter called control cell.Under the road construction the �in-situ� soil consisted ofsand overlying silt in all cells. The old road was asphaltcovered and had a superstructure consisting of naturalgravel in a 0.9 m layer. The reconstructed road was asphaltcovered, with a superstructure consisting of 0.5 m crushedrock (new material) and beneath it 0.4 m of ash mixture. Inthe control cell only crushed rock was used in the super-structure. By mistake ash-mixture was first placed in thecontrol cell. The ash-mixture was removed within 2 h fromthe control cell and was not exposed to rain but the cell wasmost likely contaminated anyway.

2.2. Ash mixtures

In the experiment road, ash mixtures were used consist-ing of 95% (dry w/w) bio fuel ash and of 5% of differentactivators or hardening agents. All of the activators con-tained CaO, known to be important for the hardening ofash. In cell 1, 5% w/w of Portland cement was added. Incell 2, 2.5% w/w Portland cement and 2.5% w/w of Merrit5000 were added. Merit 5000 is a commercial ballast prod-uct of the company Merox (Sweden) made from blast fur-nace slag from the steel industry. In cell 3, 2.5% w/wPortland cement and 2.5% CaO were added. The bio fuel partof the ash mixtures contained equal amounts (dry w/w) oftwo different bio fuel ashes: (1) fresh fly ash (filter ash) fromfluidized bed combustion (CFB) at a paper mill and (2) ashfrom a roster boiler stored on a landfill for 6 years; both

→←

→←→←

CELL 3

2,5 % Cement 2,5 % Lime

30 m : :

~ 10 m

(Control cell)

30 m

: : :

: : : : :wo boreholes on each.

ental road.


bottom and fly ash (filter ash) in unspecified amounts orig-inating from wood and peat based bio fuels, stored in 4 mthick layers and exposed to rain and atmosphere. Thewarm fresh fly ash had to be mixed with 10–20% waterfor practical reasons because of the dust. A large part ofthis water evaporated during the transport. The fly ashwas about 48 h old when the final ash mixtures were pro-duced. The landfill ash had hardened and had a dry weightof around 45% and a pH of around 8. The fly ash, there-fore, provided the major hardening capacity for the finalmixture together with the hardening agents, i.e., Portlandcement, slaked lime and Merit 5000.

2.3. Sampling

The sampling program was designed to measure thetransport of elements from the ash by analysing the porewater in the ash (ash layer solution) and at two depths inthe soil (soil solution) below the ash and at two depths inthe ditch (Fig. 2). This meant sampling in the unsaturatedpart of the soil where no groundwater was available. Thechoice of sampling soil solutions was made as soil solutionalso reflects small changes of the soil chemistry.

The samplings were carried out on four different occa-sions during a period of 2 years. The first occasion was inJune 2001, which was about 2 weeks before the old roadwas reconstructed with ash. The other occasions were Octo-ber 2001, June 2002, and June 2003. Samples were taken onone side of the road and in one of the ditches at three posi-tions in the control cell and at two positions in each of thethree ash cells (Fig. 1). On every new sample occasion thepositions of the bore holes were moved about 0.5 m fromthe old ones. The bore holes were backfilled with the samematerial and the pavement was mended. In June 2001 sam-ples were taken from two depths in the road and in theditch. On the sample occasions after the road reconstruc-tion, samples were also taken in the ash layer in the road.In the road, the sampling depths were 0.5–0.9 m (ash layer),0.9–1.2 m (depth 1) and 1.2–1.5 m (depth 2) from the road

Fig. 2. A cross-section of the experimental road show

surface. In the ditch, the sampling depths were 0.4–0.7 m(depth 1) and at 0.7–1.0 m (depth 2) from the ditch bottomsurface. The sampling depths in the ditch were placed some-what deeper than in the road to avoid that the material sam-pled was dug out during the construction. The depthinterval of 30 cm was chosen to get enough soil sample fromtwo bore holes. The equipment used to sample the cores wasa motorized hydraulic drill rig using a 1-m long steel corecatcher or an earth borer. The soil cores and the samplesfrom the earth borer were put into cylindrical soil samplecups, made of PVC (polyvinylchloride) with a length of72 mm and an inner diameter of 46 mm giving a volumeof about 120 ml. At each sampling position in the roadand in the ditch, two holes were drilled. From each hole,samples were taken from the ash layer and from depth 1and depth 2 in the road and in the ditch. Each samplingdepth was 30 cm. From each hole, three sample cups persampling depth could be collected making a total of sixfrom both holes. These six sample cups were centrifugedand the soil solution obtained was poured into the samebeaker making a composite sample. In total, the numberof composite samples at each sampling depth was the sameas the number of sampling positions.

2.4. Sample preparation and analytical methods

The centrifugation drainage technique described byGiesler and Lundstrom (1993) was used to obtain soil solu-tions from the soil samples. This method does not add anywater to the soil samples; it only extracts the water in thesoil. The sample cups are placed in centrifuge cups andput in the centrifuge six at a time. The centrifuge cups con-sist of two parts. The upper part contains the soil with smallholes in its bottom and the lower part collects the soil solu-tion during centrifugation. A Beckman J2-HS centrifugewith a JA-14 rotor (Spinco division, Palo Alto, Ca, USA)was used. The soil samples were stored at 4 �C and centri-fuged within 36 h after sampling at a speed of 14,000 rpm(�7600 kPa) for 30 min. The centrifugates were before the

ing sampling positions and depths of bore holes.

Table 1Major chemical composition of ash mixtures

% w/wa Ash mixtures RV 90

Ash mix. cell 1 Ash mix. cell 2 Ash mix. cell 3

SiO2 21.9 26.3 26.8Al2O3 9.89 11.0 10.5CaO 26.2 23.5 23.5Fe2O3 9.35 8.74 9.28K2O 1.99 2.34 2.28MgO 1.62 2.15 1.86MnO 0.779 0.679 0.651Na2O 0.589 0.777 0.711P2O5 1.90 1.73 1.87S 0.66 0.59 0.45Cl 0.1 0.2 0.2LOIb 22.0 19.3 18.9TCc 12.3 11.3 11.3TOCc 11.4 9.7 7.9

a By dry weight.b LOI, loss on ignition at 550 �C.c TC, total carbon; TOC, total organic carbon.


different analyses filtered through a hydrophilic 0.45 lm fil-ter (Millex HV, Millipore) and were then stored at 4 �C insample tubes made of polypropylene, caps included, (pH,conductivity, DOC and Ion Chromatography) or polysty-rene with polyethene caps (ICP-MS). Samples for the deter-mination of cations and metals were acidified withconcentrated HNO3 (Fluka Trace Select >69.5%). Immedi-ately after centrifugation, pH and conductivity were deter-mined using a Beckman U32 pH Meter, Beckmanelectrode, 1 M KCl/saturated AgCl (Beckman InstrumentsInc., Fullerton, CA, USA) and a Mettler Toledo MC 226,InLab 730 electrode (Mettler-Toledo GmbH, Schwerzen-bach, Switzerland). Dissolved organic carbon (DOC) wasdetermined within 48 h using a Shimadzu 5000 TOC-instru-ment (Shimadzu, Osaka, Japan). Anion concentrations forCl, NO3, PO4 and SO4 were determined using ion chroma-tography (IC, Dionex DX-120 with a Dionex IonPacAS9-HC 4-mm (10–32) column). The total concentrationsof cations and metals were determined using ICP-MS(inductively coupled plasma mass spectrometry) (ICP-MS,VG PQ Excel, Thermoelemental, Winsford, England).Analysis of the chemical composition of the fresh ash mix-tures, taken from the mixing plant, was made at AnalyticaAB laboratories (Lulea, Sweden). Digestion of the sampleswas made in HNO3 (Boron) or a HNO3, HCl and HF (Cd,Cu, Co, Hg, Ni, Pb, and S) or in a LiBO2-smelt (accordingto ASTM D3682) (Na, Si, Al, Ca, Fe, K, Mg, Mn and P).ICP-AES (ICP-atomic emission spectroscopy) was usedfor all major and trace elements except for Pb where ICP-QMS (ICP-quadropole mass spectrometry) was used andHg where AFS (atomic fluorescens spectroscopy) was used.Total carbon content (TC), total organic carbon (TOC) andloss on ignition (LOI) were also determined. XRF (X-rayfluorescence) was used to determine the total Cl content.

To examine any formation of crystalline minerals, X-raypowder diffraction (XRD, Siemens D5000) measurementswere performed on ash mixture samples from all sampleoccasions. The XRD measurements were performed atthe Chalmers University of Technology.

2.5. Statistical methods

Analysis of variance (ANOVA) and regression analysiswere made using the software MINITAB (Minitab Inc.,PA, USA). The temporal variations evaluated by ANOVAand regression analysis were made at statistical significancelevels of p < 0.01, p < 0.05 and p < 0.10 and n = 6. The AN-OVA was performed with concentration as a function of thedifferent sample occasions (one-way ANOVA). There werefour sample occasions giving four levels in the ANOVA.

3. Results and discussion

3.1. Control cell

Initially, the soil solution concentrations in the ash cellsand the control cell were supposed to be compared and any

differences would show the impact of the ash mixtures. Inthe samples taken in June 2001, before the ash was applied,the control cell showed higher soil solution concentrationscompared to the ash cells. During the construction of theroad, ash was also accidentally put in the control andalthough it was removed within 2 h contamination wasprobably caused. These circumstances made the controlcell unfeasible and the results were therefore not used forcomparison with the ash cells.

3.2. Total chemical composition of ash mixtures

The chemical compositions of the major elements in theash mixtures are presented in Table 1 as oxides. The mostabundant elements were SiO2, CaO, Al2O3 and Fe2O3,which made up around 68–69% (w/w) of the total amount.Although the total composition of the ashes shows largevariations, the concentrations of these elements seemedhigh compared to many other wood ashes investigated(Tegner et al., 1997; Steenari and Lindqvist, 1997; Steenariet al., 1999). Loss on ignition was around 20% (w/w). Thisis much higher than what Steenari et al. (1999) have foundfor wood and peat ash, which were well below 10% (w/w)of LOI. As CaO and Al2O3 were present together withSO4 and pH was well above 10.5, ettringite should havebeen able to form. The relatively high content of Ca wasrelated to the different activators added to the ash mixtures.

The trace element content is presented in Table 2 whereit has been compared to the Swedish guideline values forcontaminated soil (Swedish EPA, 1997). The concentra-tions of Zn and Cd were the only ones exceeding the guide-line values. Zn exceeded all the different guideline valuesbeing more than twice as high as the guideline value for soilwhere the land use would not be limited. The Cd concen-trations were below the guideline value for soil with limitedland use where no groundwater has to be protected but

Table 2Trace element content in ash-mixtures with reference values for soil concentrations with respect to land use (Swedish EPA, 1997)

mg/kga Ash mixtures RV 90 Ref 1 (KM)a Ref 2 (MKM-GV)b Ref 3 (MKM)c

Ash mix. cell 1 Ash mix. cell 2 Ash mix. cell 3

Cr 64.2 63.9 69.7 120 250 250Cu 80.8 87.9 95.3 100 200 200Co 12.9 17.4 16.5 30 60 250Ni 42.1 52.2 58.4 35 150 200Zn 868 771 703 350 700 700Se na na na No ref No ref No refMo 6.31 6.81 8.66 No ref No ref No refCd 2.66 1.95 2.07 0.4 1 12Hg 0.47 0.33 0.45 1 5 7Pb 27.0 23.9 28.4 80 300 300

a Guideline values for soil concentrations not limiting the land use (school, playground etc.) (Swedish EPA, 1997).b Guideline values for soil concentrations limiting the land use and demanding groundwater protection (offices, industries, roads etc.) (Swedish EPA,

1997).c Guideline values for soil concentrations limiting the land use demanding no groundwater protection (offices, industries, roads etc.) (Swedish EPA,

1997).


much higher than the guideline value for not limited landuse.

XRD analyses of the ash mixtures were performed onsamples taken in June 2001, from the mixing plant, andfrom the sampling occasions in June 2002 and June 2003.The mineralogical composition could not be determinedas no crystalline compounds were found and any mineralsformed must have been amorphous.

3.3. Temporal variations in soil solution concentrations

3.3.1. Statistical evaluation

No significant differences were found between the differ-ent ash cells and therefore they were evaluated together.

The temporal variations of pH, DOC, K, Ca, SO4, Cl,Cu, Zn, Se, Mo, Cd, Hg and Pb were evaluated using

Table 3Summary of results for the temporal variations and the regressions for concen

Parameters Road Ditch

Ash Depth 1

pH X99a

DOC X95 X99c

NaMg X99, (�)99

AlK X99, (+)99 X90, (+)90

CaCl X99

SO4 X99, (+)95 X99, (+)99

Cu X99, (�)99

Zn X99, (+)99

Se X90, (�)95

Mo (�)90 (+)90

Cd X99, (�)99 (+)90

Hg (+)90 X90, (�)95

Pb X99

a X indicates statistically significant temporal variations in the concentrationb � and + indicates decreasing or and increasing statistically significant trenc Confidence level (90%, 95% or 99%).

ANOVA and regression analysis, and were summarized inTable 3. The ANOVA showed if there was significant vari-ation in the concentrations. It could not tell us whether theconcentration had increased or decreased significantly andtherefore regression analysis was performed.

3.3.2. Calcium and pH as indicators of the ash hardening

processesThe pH of the ash layer was above 12 on all sample

occasions (Fig. 3(a)). The decrease in pH in the ash layerswas not significant. The pH of the ash layers will eventuallydecrease as atmospheric CO2 dissolves in the pore water ofthe ash and as soluble hydroxides of alkali and alkalineearth metals are leached. Steenari et al. (1999a) found thatthe concentration of Ca in hardened wood ash leachate(H2O, L/S 16) was less than 10% of the concentration in

tration versus time

Depth 2 Depth 1 Depth 2

X99, (�)90 X99

X99 X99

(+)90b

(+)90

X90, (�)95 X95

X99, (+)99

(+)90 X90

(+)95 X99, (+)95 X90, (+)95

X90, (+)95 X99, (�)95

X95, (�)99 X99, (�)99 (�)95

X99 X95, (+)95

(+)95

X90, (+)95

X99, (+)99 X99, (+)95 X95

X99, (�)99 X90, (�)95 X95, (�)95

X95 X95

.ds with time for the concentration change.

pH

6

7

8

9

10

11

12

13

May-01 Aug-01 Nov-01 Feb-02 Jun-02 Sep-02 Dec-02 Apr-03 Jul-03

Ash

Road depth 1

Road depth 2

Ditch depth 1

Ditch depth 2

Ca (mg/l)

0

20

40

60

80

100

120

140


Ash

Road depth 1

Road depth 2

Ditch depth 1

Ditch depth 2

a

b

Fig. 3. Road and ditch sample trend lines for (a) mean pH values (n = 6) and (b) mean total Ca concentrations (n = 6) with standard error bars.


fresh fly ash and that the concentrations of Ca decreasedwith time during hardening. Steenari et al. (1999a) alsofound that the leachate concentrations in hardened asheswere between 44 and 64 mg l�1 and in non-hardened ashes730–940 mg l�1. In the ash layer solutions of the experi-ment road, the total Ca concentrations (Fig. 3(b)) werelower than 40 mg l�1, indicating that Ca had formed lowsoluble CaCO3 (calcite). The accessibility of atmosphericCO2 in the ash layers below the asphalt road surface wasnot measured but could be regarded as low as only theash on the road sides was open to the atmosphere. Theashes must then have been in contact with enough atmo-spheric CO2 to be hardened during storage on the landfill,the production of the ash mixtures, the transport to theroad and the construction of the road. The high pH andthe fact that calcium and aluminum oxides were present

in the ash mixtures should have made the formation ofettringite (Eq. (3)) possible but the results of XRD analysison samples from the ash layers from all sample occasionscould not verify this. Also, the formation of CaCO3 couldnot be verified. This could have been due to the formationof amorphous of ettringite and CaCO3, which cannot bemeasured by XRD. However, the content of inorganic car-bon indicates that CaCO3 may have formed (Table 1).

At depths 1 and 2 below the ash layer, the pH washigher at the first sampling occasion after reconstructionof the road, but then declined (Fig. 3(a)).

3.3.3. DOC (dissolved organic carbon)

The DOC concentration showed significant temporalvariations but no significant increase or decrease in con-centration (Table 3). The ash layer solutions and the soil

Table 4Reference values

Parameter Reference/guideline concentrations (lg l�1)

Groundwatera Drinking waterb Inert waste leachatec

SO4 No ref 100 1500Cl 4 100 460Cu No ref 200 600Zn 50 No ref 1200Se No ref 10 40Mo No ref No ref 200Cd 0.06 5 0.02Hg 0.001 1 2Pb 0.1 10 150

a Swedish groundwater median values, fluvio-glacial deposits northernSwedish coast (Swedish EPA, 2000).

b Concentration limits drinking water (Swedish National FoodAdministration, 2001).

c Guideline values for inert waste determined by upflow percolatingleaching test prEN 14405 (L/S = 0.1) (Criteria for waste to landfill: 2003/33/EG, according to article 16i, App. II in EU-directive 1999/31/EG).

Table 5Effect-related classification concentrations for groundwater (SwedishEPA, 2000)

Parameter(lg l�1)

Very low Low Moderate High Very high

Zn 65 5–20 20–300 300–1000 >1000Cd 60.05 0.05–0.1 0.1–1 1–5 >5Pb 60.2 0.2–1 1–3 3–10 >10


solutions in the ditch showed the highest concentrations inJune 2002 and thereafter decreased while the concentrationin the road (depth 1) seemed to increase from June 2002(Fig. 4). The increase in the ditch might have been causedby the physical disturbance of the soil during the construc-tion work, while the later increase in the road might indicatethat the ash released DOC, which was transported to thesoil layer where the impact was larger at depth 1 than atdepth 2. Johansson (2003) found that a higher amount ofLOI in ashes increased the leaching of DOC and that thisleaching increased during aging of ashes. However, therelatively high concentration of organic material in theash-mixtures compared to other ashes has not resulted inexceptional concentrations of DOC compared to soil andcreek water in general. This is of importance as high DOCconcentrations can lead to high concentrations of organi-cally bound trace elements (van der Sloot et al., 1996;Meima et al., 2002).

3.3.4. Reference values

Beside the temporal variations and linear regressions, thetotal mean (n = 6) concentrations in the sample solutionsfrom the ash layer and the soil were compared with severalreference values (Tables 4 and 5). To have a reference onnatural background levels, Swedish median values ingroundwater were used (Swedish EPA, 2000). Swedishdrinking water standards were used as a reference withthe risk of contamination of water wells in mind (SwedishNational Food Administration, 2001). The EU criteria forinert wastes have guideline values from leaching testsaccording to pr EN 14405 (Council decision: Criteria forwaste to landfill: 2003/33/EG). For Zn, Cd and Pb the ef-fect-related concentrations for groundwater based on effectson humans and animals were available. The concentrationsfor SO4, Cl, Cu, Zn, Se, Mo, Cd, Hg and Pb and relevantreference values are presented in Figs. 6–14.

DOC

0

10

20

30

40

50

60

70

80

90

May-01 Aug-01 Nov-01 Feb-02 Jun-0

Ash

Road depth 1

Road depth 2

Ditch depth 1

Ditch depth 2

Fig. 4. Road and ditch sample trend curves for the mean to

3.3.5. Cation and anion concentrations

The effect of leaching from the ash was most evident forK (Fig. 5), SO4 (Fig. 6) and Cl (Fig. 7). The concentrationsof K and SO4 increased significantly (Table 3) in the ashlayer and at depth 1 and depth 2 in the road. The K con-centrations were still increasing in the ash layers solutionsand had increased in the soil solutions in June 2003. TheSO4 concentrations were all below the inert waste criteria

(mg/l)

2 Sep-02 Dec-02 Apr-03 Jul-03

tal DOC concentration (n = 6) with standard error bars.

K (mg/l)

0.1

1.0

10.0

100.0

1000.0

10000.0


AshRoad depth 1

Road depth 2

Ditch depth 1

Ditch depth 2

log

Fig. 5. Road and ditch sample trend curves for the mean total K concentration (n = 6) with standard error bars.

SO4 (mg/l)

1.0

10.0

100.0log

1000.0

10000.0


Ash

Road depth 1

Road depth 2

Ditch depth 1

Ditch depth 2

L/S 0.1 Inert Waste1500 mgl-1

Drinking water limit100 mgl-1

Fig. 6. Road and ditch sample trend curves for the mean total SO4 concentration (n = 6) with standard error bars and reference values.


(1500 mg l�1) but exceeded the drinking water limit(100 mg l�1) in the ash layer and at depth 1 and depth 2in the road in June 2003. Cl concentrations increased atdepth 2 in the road and at both depths in the ditch. In June2002 and June 2003 the inert waste criterion (460 mg l�1)was exceeded in the ash and the drinking water limit wasexceeded (100 mg l�1) at depth 1 and depth 2 in the road.All samples had concentrations above the groundwatermedian value (4 mg l�1).

3.3.6. Solubility, mobility and potential release of metals

The highest concentrations of Cu (Fig. 8) were found inJune 2001, before the ash layers were applied. The concen-trations were below the limit for not drinkable water(200 lg l�1) and the criteria for inert waste (600 lg l�1).The soil solution concentration of Cu at depth 1 and depth

2 in the road and in the ditch decreased significantly (Table3). The decrease in Cu concentration in the ash layer couldbe due to carbonation, precipitation together with DOC orsorption to Al-minerals (Meima et al., 2002).

The concentration of Zn in the ash layer solutions(Fig. 9) was in October 2001 about the same as in the soilsolutions at depth 1 and depth 2 in the road and the ditch,and then started to increase significantly (p < 0.01) (Table3). In June 2003, the ash layer solutions concentration ofZn reached around 700 lg l�1, classifying it as high accord-ing to the effect-related concentrations, but was still belowthe inert waste criteria (1200 lg l�1). The concentration ofZn in the soil solutions reached its highest levels in June2002, classifying it as moderate, and then fell back in June2003 to concentrations close to the groundwater medianvalue (50 lg l�1). Zn concentrations increased in the ash

Cl (mg/l)

1

10

100

1000

May-01

log

Aug-01 Nov-01 Feb-02 Jun-02 Sep-02 Dec-02 Apr-03 Jul-03

Ash

Road depth 1

Road depth 2

Ditch depth 1

Ditch depth 2

Drinking water limit100 mgl-1

L/S 0.1 Inert Waste 460 mgl-1

Groundwater median 4 mgl-1

Fig. 7. Road and ditch sample trend curves for the mean total Cl concentration (n = 6) with standard error bars and reference values.

Cu (µg/l)

10

100log

1000


Ash

Road depth 1

Road depth 2

Ditch depth 1

Ditch depth 2

Drinking water limit 200 µgl-1

L/S 0.1 Inert waste criteria 600 µgl-1

Fig. 8. Road and ditch sample trend curves for the mean total Cu concentration (n = 6) with standard error bars and reference values.

Zn (µg/l)

10.0

100.0

log

1000.0

10000.0


Ash

Road depth 1

Road depth 2

Ditch depth 1

Ditch depth 2

High effect300-1000 µgl-1

Moderate effect20-300 µgl-1 Groundwater median

50 µgl-1

L/S 0.1 Inert Waste1200 µgl-1

Very high effect> 1000 µgl-1

Fig. 9. Road and ditch sample trend curves for the mean total Zn concentration (n = 6) with standard error bars and reference values.



layer and at depth 1 in the ditch. DOC can increase the re-lease of Zn by complexation (van der Sloot et al., 1996) andat depth 1 and 2 in the road the concentrations of Zn andDOC showed a similar variation in concentration.

For Se (Fig. 10), the ash layer solutions had concentra-tions above the reference values on all occasions. Thedrinking water limit (10 lg l�1) and inert waste criteria(40 lg l�1) for Se were exceeded at depth 1 in October 2001.

The concentration of Mo (Fig. 11) in the ash layer solu-tions was higher than the inert waste criteria (200 lg l�1).Data from the ditch are lacking but indicate a peak inOctober 2001. The release of Mo from ashes is pH depen-dent and is likely to decrease if the pH in the ash layer de-creases to below 8. This might not occur at all. Anincreasing number of sorption sites, due to carbonationin the ash can reduce Mo concentrations (Meima et al.,2002), but this will probably occur slowly as CO2 can be ex-pected to enter the ash layer at a slow rate.

Se (µg/l)

0.1

1.0

10.0log

100.0

1000.0

May-01 Aug-01 Nov-01 Feb-02 Jun-02 Sep

Fig. 10. Road and ditch sample trend curves for the mean total Se co

Mo (µg/l)

0.1

1.0

10.0

log

100.0

1000.0

10000.0


Fig. 11. Road and ditch sample trend curves for the mean total Mo co

The Cd concentrations (Fig. 12) in October 2001 andJune 2002 were much higher in the ash layer compared tothe inert waste criteria (0.02 lg l�1) and the drinking waterlimit (not shown, 5 lg l�1) and also classified as very highshowing that Cd is a critical element for use of recycledash. The groundwater median value (0.06 lg l�1) was ex-ceeded on all occasions except for in June 2001. The Cdconcentrations in the ash layer solutions decreased signifi-cantly (p < 0.01) (Table 3) but were in June 2003 still highaccording to the effect-related classification. The overallconcentration increase in the soil solutions at depth 1 anddepth 2 in the road and in the ditch occurred between June2001 and October 2001. In June 2003 the concentrationswere moderate in the road and moderate to low in theditch. This show the high mobility of Cd and its effect onthe soil solutions below the road, and possible effects onthe groundwater downstream. Cd has an affinity forCaCO3 surfaces where co-precipitation or solid-solution

-02 Dec-02 Apr-03 Jul-03

Ash

Road depth1

Road depth2

Ditch depth 1

Ditch depth 2

L/S0.1 Inert Waste40 µgl-1

L/S0.1 Drinking water10 µgl-1

ncentration (n = 6) with standard error bars and reference values.

-02 Dec-02 Apr-03 Jul-03

Ash

Road depth 1

Road depth 2

Ditch depth 1

Ditch depth 2



Cd (µg/l)

0.00

0.01

0.10

1.00

10.00log

100.00

1000.00

10000.00

100000.00


Ash

Road depth 1

Road depth 2

Ditch depth 1

Ditch depth 2

Very High effect > 5 µgl-1

High effect 1-5 µgl-1

Moderate effect 0.1-1 µgl-1

Groundwater median 0.06 µgl-1


Fig. 12. Road and ditch sample trend curves for the mean total Cd concentration (n = 6) with standard error bars and reference values.


formation could limit the Cd concentrations (Comans andMiddelburg, 1987). On the other hand, higher concentra-tions of Cl� and DOC can mobilize Cd from the waste(van der Sloot, 1996; van der Sloot et al., 1996). A studymade on the effect of de-icing salts on roads (Norrstromand Jacks, 1998) showed that higher salt concentrations in-creased the mobility of heavy metals due to dispersion ofcolloids and ion exchange. Extraction with calcium chlo-ride has also been shown to increase the leachability ofZn and Cd due to complexation and competition for sorp-tion sites on the soil (van der Sloot et al., 1996). The use ofde-icing salts and the chloride content of the ash itselfcould therefore increase the leaching of trace elements fromthe ash layers.

The concentration of Hg (Fig. 13) in the ash layer solu-tions was higher than all the reference values. The concen-tration in the ash layer solutions increased significantly(p < 0.1). At depth 1 and depth 2 in the road and in theditch, the concentrations decreased (Table 3) and were

Hg (µg/l)

0.000

0.001

0.010

0.100log

1.000

10.000

100.000


Fig. 13. Road and ditch sample trend curves for the mean total Hg co

higher than the groundwater median value (0.001 lg l�1)but lower than the drinking water limit (1 lg l�1, notshown) and the inert waste criteria (2 lg l�1). The watersolubility of elemental Hg is rather high and several species(HgCl2(aq), Hg(OH)2(aq)) are aqueous and not pH depen-dent with respect to solubility (Stumm and Morgan, 1996).The volatility of Hg0(aq) is also high (KH = 1.2·10�1)implicating that the ash layer can volatilize Hg to theatmosphere.

Significant temporal variations (Table 3) for Pb (Fig. 14)were found in the ash layer solutions and in the soil solu-tions at depth 2 in the road and at depth 1 in the ditch.The Pb concentrations reached maximum values in June2002 and then decreased. Overall, the variations in concen-tration between the samples were quite large. In June 2001and until June 2002, most samples were high according tothe effect-related classification (Table 5). All solutions fromthe ash layer and soil layers were below the drinking waterlimit (10 lg l�1) and the inert waste criteria (not shown in

-02 Dec-02 Apr-03 Jul-03

Ash

Road depth 1

Road depth 2

Ditch depth 1

Ditch depth 2

Groundwater median0.001 µgl-1


Drinking water 1 µgl-1


Pb (µg/l)

0

2

4

6

8

10

12

14


Ash

Road depth 1

Road depth 2

Ditch depth 1

Ditch depth 2

Groundwater median0.1 µgl-1

Moderate effect 1-3 µgl-1

Low effect 0.2-1 µgl-1

High effect 3-10 µgl-1

Drinking water 10 µgl-1

Very high effect > 10 µgl-1

Fig. 14. Road and ditch sample trend curves for the mean total Pb concentration (n = 6) with standard error bars and reference values.


Fig. 14, 150 lg l�1) but exceeded the groundwater medianvalue (0.1 lg l�1) (Table 4). In June 2003, the concentra-tions were low or moderate, except for at depth 1 in theroad were the concentration was high. Norrstrom andJacks (1998) found that Pb was bound to the soil close toroads and in this study the relatively high concentrationsof Pb in the ditch showed that Pb had been accumulatedin the soil over the years. The release of Pb in the ash ismainly controlled by pH (van der Sloot, 1996), but DOCalso has a potentially large effect on leaching of Pb (vander Sloot et al., 1996; Meima et al., 2002). We found thatPb concentrations in the ash layer and DOC had a similarvariation.

The potential release of elements from the ash layer togroundwater is determined by the release of elements inthe ash layer, the water transport and also by reactions likeadsorption, complexation, pH change, etc. taking place inthe soil below. One possible interpretation of the resultswith respect to transport of elements was made, based onthe results of regression analysis. When the concentrationsin the ash layer solutions of an element showed a significantincrease and this increase also was significant in the soilsolutions, typically with some delay in time, this was inter-preted as a transport of that element, e.g., K, SO4, and Cl.For Se, Mo and Cd, the decreasing concentration in the ashlayer solutions and the increasing concentration in the soilsolutions were also interpreted as a transport of these ele-ments. For Cu, Zn and Hg the concentrations in the ashlayer solutions showed an increase but this was not re-flected in the soil solutions and the interpretation was thatno transport had occurred from the ash layer.

The concentrations of the different metals in the soilsolutions can be transported to the groundwater by perco-lating precipitation. The percolation or leaching of materi-als in a road construction have been considered by Carlingand Hjalmarsson (1998) and was assumed to take placeonly in the material on the edges of the road construction.

Reid et al. (2001) have assumed that 10% of the precipita-tion percolates through the paved surface entering the roadconstruction. The assumption made by Reid et al. (2001) isa worst case assumption in comparison to the assumptionmade by Carling and Hjalmarsson (1998). The percolationthrough the shoulders of the road is probably a betterassumption during the first years after the construction ofthe road when the paved surface is intact. With time therewill be small and large cracks in the paved surface due towear, frost heave, etc. and precipitated water will also per-colate through the road construction and not only on theshoulders. The percolation through the road will be con-centrated to where the cracks in the paved surface are situ-ated but might, due to the low permeability of the ashlayer, have some time to spread horizontally by diffusion.

The percolation through the road will to a large extentdetermine the total transport of elements from the ashlayer. The precipitation in the area of the test road is about700 mm per year making 10% 70 mm or 70 l m�2. Thehighest concentrations in the ash layers exceed the refer-ence values for drinking water for SO4, Cl, Se, Cd andHg and the inert waste criteria was exceeded by Cl, Se,Mo, Cd and Hg. The concentrations will be diluted whenthey reach the groundwater surface by groundwater up-stream and from the sides. The concentrations will be fur-ther diluted when the groundwater reaches an outflow area,e.g., a lake. The Swedish EPA (1997) has set generic guide-line values for transport of substances from contaminatedsoil, assuming that soil pore water concentrations are di-luted 15 times by the groundwater. If the highest concen-trations in the ash layers were diluted 15 times, Cd wouldbe the only element exceeding the drinking water limitand the inert waste criteria. Cd would also have a very higheffect according to the effect-related classification concen-trations for groundwater. This showed the high mobilityof Cd and its possible transport to groundwater. Still, theconcentrations in the soil solutions showed that Cd was


not yet transported to groundwater at such highconcentrations.

4. Conclusions

The results from the investigations showed that the pHin the ash layer solutions was strongly alkaline (pH 12–13) on all sample occasions but slowly decreased.

The low concentrations of Ca in solutions of the ashlayer indicated an ongoing hardening with respect to theformation of CaCO3.

K, SO4, Cl, Se, Mo and Cd were leached from the ashlayer. Concentrations of Zn and Hg still increased in theash layer after 2 years, pointing out the potential risk of re-lease of these elements to the underlying soil in the future.

Cd and Zn showed the highest potential for contamina-tion of the groundwater but so far the assumed expecteddilution of the soil solutions as they enter the groundwatersurface will keep concentrations below or close to ground-water median values.

Although Zn and Cd showed high concentrations in theash layer and the total concentrations were higher thanunlimited land use guideline values, the use of ash in infra-structure constructions, especially in urban areas, couldsave natural resources and energy and thereby reduceanthropogenic pollution and be environmentally bettercompared to storage in landfills.

Acknowledgments

This work was funded by the Knowledge Foundation,MVM konsult AB, Mid Sweden University, the European

Regional Development Fund and the Swedish National Road

Administration (SNRA) and this is greatly acknowledged.

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