ORIGINAL ARTICLE

Variation of geochemical risk associated with the useof ophiolitic washing mud as refilling materialin a basalt quarry of the Northern Apennine (Italy)

M. Voltaggio Æ M. Spadoni

Received: 19 December 2006 / Accepted: 16 January 2007 / Published online: 17 February 2007� Springer-Verlag 2007

Abstract Ophiolitic sequences in Northern Apen-

nines are usually exploited as source of raw material

for civil engineering works. Grinding procedures of

basalts imply the production of dusts with relatively

high concentration of PHES. This paper studied the

increase of geochemical risk when washing mud pro-

duced at Sasso di Castro quarry site (Tuscany) is re-

used as rock keeper in a near dismissed quarry and

highlighted geochemical fractionation produced on the

base of different mineral hardness. Co, Cr, Ni and V

concentration measured in washing mud were higher

than the limits fixed by the Italian law but compatible

with background values. The mobility of these four

elements during future weathering processes were

estimated by considering the element transfer coeffi-

cients and assuming weathered rocks and soils as two

different natural analogues of the future state of

washing mud. The future concentration was estimated

by considering the average lifetime of mineral grains

calculated through their dissolution rate, molar volume

and grain diameter. The variations of geochemical

concentrations were used to estimate the percentage

increase of the geochemical risk at the displacement

place. After 50 years the associated geochemical risk is

still considerably lower than the probability to be

damaged by a single landslide event.

Keywords Geochemical risk � PHES � Environmental

geochemistry � Washing mud � Ophiolites

Introduction

Ophiolitic rocks, representing remnants of the Middle

Jurassic–Early Cretaceous lithosphere of the Ligurian

Tethys, widely outcrop in Italy along the Northern

Apennine mountain belt in Tuscany, Liguria, and

Emilia Romagna regions. Presently, ophiolites are

mined in this area in about 70 different quarry sites

where they also undergo specific grinding processes for

the production of inert gravels destined to civil engi-

neering works. As a consequence high amounts of dust

and fine material can be potentially released in the

surrounding environment. To minimize the impact on

the health of both workers and environment, grinding

is generally associated with in situ washing of gravels

for the separation of dust and its removal as washing

mud.

Though the main risk associated with this mud is due

to the possible release of asbestos mineral fibres, the

relatively minor ‘‘geochemical’’ risk associated with

high concentration of potentially hazardous elements

and species (PHES) is also worthwhile to be investi-

gated.

Furthermore, the environmental conservation and

recovery of the study area is also of particular interest

for its high ecological value in terms of biodiversity,

presence of endemic flora and typical landscape.

Within this perspective it has to be considered as the

rehabilitation of degraded sites requires the use of

refilling materials that should be as much geochemi-

cally similar as possible to local soils and rocks. In the

case of ophiolites, the use of asbestos-free washing

mud for this purpose could also represent an eco-

nomically advantageous solution for their definitive

storage that is worthwhile to be carefully evaluated.

M. Voltaggio � M. Spadoni (&)Consiglio Nazionale delle Ricerche,Istituto di Geologia Ambientale e Geoingegneria,Via Bolognola 7, Rome 00138, Italye-mail: massimo.spadoni@igag.cnr.it

123

Environ Geol (2007) 53:417–432

DOI 10.1007/s00254-007-0657-2

Environmental mobility of Potentially Hazardous

Elements and Species (PHES) is strongly influenced by

geochemical environment in the soil, especially with

respect to redox conditions and leaching intensity.

Sometimes natural processes of mineral fractionation

and consequently of PHES accumulation have been

ascribed to merely physical processes associated with

mechanical erosion and differential transport of rock

and soil particles (e.g. Horowitz and Elrick 1987;

Muller et al. 2001; Wang et al. 2003; Tsai et al. 2003).

The geochemical background is usually calculated by

analyzing soil samples or stream and overbank sedi-

ments, the latter having the property of averaging

elemental concentrations on wide areas like catchment

basins. The influence of differently sized rock particles

in the determination of geochemical background val-

ues reserves particular interest, being also implicit in

the conventional definition of soils, (USDA 1993;

ISSS-ISRIC-FAO 1998) and of active stream sedi-

ments respectively as <2 mm sized aggregates and

<150 lm sized particles (Darnley et al. 1995; Salminen

et al. 1998).

The aim of this paper is to provide the geochemical

characterization of washing mud produced in the Sasso

di Castro (SC) quarry site, in Tuscany (Fig. 1a), in

order to assess possible environmental hazards linked

to their use as refilling material inside the near dis-

missed ophiolitic quarry site of Monte di Beni (MB,

Fig. 1b).

Case history

Massive basalts are extracted from an ophiolitic se-

quence at SC and crushed on site to produce inert

materials for the Bologna-Firenze tract of the new high

speed lines of Italian railways (Consorzio CAVET

1995). The crushing process, leading to the production

of differently sized gravels, caused the accumulation of

residual silty mud as by-product of gravel washing. A

significant share of washing mud was displaced in the

near and nowadays abandoned quarry of MB where, in

the past, an ophiolitic sequence very similar to the SC

one has been intensely mined (Fig. 1c).

However, displacement of washing mud needs

accurate geochemical investigation in order not to

transgress the Italian law that fix concentration limits

for the concentration of PHES in soils (Gazzetta Uf-

ficiale 1999). The departure from these limits is only

allowed in case the natural background concentrations

are higher than the fixed values (Gazzetta Ufficiale

2006).

Ophiolitic sequences outcrop at SC and MB

(Fig. 1c) in form of huge olistoliths dipped within the

Chaotic Complex (mainly limestone fragments in a

shaly matrix) of the external Liguride units (Late

Cretaceous, Cremonini and Elmi 1971). The lithologi-

cal sequence includes massive basalts, cut by frequent

basaltic dikes and intruded by plagiogranitic bodies,

overlained by pillow basalts and pillow basalt breccia

(Bocchi et al. 1976; Calanchi et al. 1987). Red bedded

cherts and Calpionella limestones outcrop at the top of

the sequence. At the contact between plagiogranites

and basalts, thermometamorphic processes led to the

formation of trondhjemite with xenoliths of lower

amphibolite facies (Calanchi et al. 1987). Mineralogy

of unaltered basalts consists of plagioclase (50–60%),

augitic clinopyroxene (30–40%), olivine (around 5%)

and accessory Fe–Ti oxides and apatite.

At SC quarry site massive basalt blocks are pres-

ently detached with explosive blasts and then grinded

into differently sized gravels through the passage into a

series of steel mills. After crushing, gravels are washed

with water for the removal of the residual fine fraction

(dust). The latter is finally separated from water by

subsequent filtration and pressing.

On the contrary, MB quarry is now dismissed and

the whole area has been included in a protected

natural site for its peculiar botanic and fauna value

(Site of European Community Interest). After the

occurrence of a huge landslide at MB, triggered by

the instability of the quarry front, the local adminis-

tration authorized the displacement of the washing

mud produced at SC to be used as rock keeper for

dissipating the energy of falling blocks. This decision

caused a civil protest and legal actions, still not de-

fined, for the high concentration of some PHES

(namely V, Co, Cr and Ni) that were measured in the

washing mud exceeding the limits fixed by the Italian

law. In this paper the legitimacy of the decision taken

by the local administration was not discussed or

judged. However a specific site evaluation on the in-

crease of geochemical risk connected to such a use of

washing mud was provided and framed in the wider

context of a possible correct procedure for estimating

the potential environmental impact of residual mining

products.

Percentage increase of the geochemical risk

In absence of an universally accepted definition of

‘‘geochemical hazard’’ and ‘‘geochemical risk’’, in this

paper we derived their operative definition from

UNESCO (1972).

Geochemical risk was therefore assumed as the

product of: (1) geochemical hazard, (2) vulnerability

and (3) value of elements at risk;

418 Environ Geol (2007) 53:417–432

123

• geochemical hazard is the probability that a given

amount of chemical element in an environmental

matrix (e.g. soil, water, air, etc.) causes health

damage to man by ingestion, inhalation, dermal

contact or other indirect pathways along an expo-

sure time equal to the average human life;

• vulnerability is the fraction of people exposed to the

geochemical hazard with respect to the entire

people living in a given area;

• value of elements at risk is the number of individuals

living in the area where the geochemical risk is

computed.

The geochemical risk for a given area is therefore

given by the number of damaged individuals over a

human lifetime.

The evaluation of geochemical hazard requires the

knowledge of the bioaccessibility (B) of chemical ele-

ments in soil, defined as that ‘‘amount of contaminant

that is soluble, due to simulated in vitro gastric func-

tions, and has the potential to cross the intestinal wall’’

(Stewart et al. 2003). Bioaccessibility depends mainly

on three factors: elemental concentrations (C), effi-

ciency of release (r), which, in turn, is linked to the

dissolution rate of mineral phases, and soil ingestion

rate (i), accordingly to the equation:

B ¼ KCri; ð1Þ

where K is a biological constant.

The identification of geochemical hazard also re-

quires a careful consideration of epidemiologic studies

and the use of models of interaction between chemical

elements and humans that are still not well assessed for

most of the PHES. In the case these data are not

available, the percentage increase of the geochemical

hazard and, consequently, of the geochemical risk can

at least been estimated by assuming that no threshold

Fig. 1 a Picture of Sasso diCastro quarry front; b Montedi Beni landslide and area ofrefilling; c Sketch of theophiolitic sequence at Sassodi Castro (from Calanchiet al. 1987, modified)

Environ Geol (2007) 53:417–432 419

123

effect exists in the relation between bioaccessibility

and geochemical hazard and that for low values of

bioaccessibility the relationship between these two

quantities is linear. Accordingly to these assumptions

the percentage increase of geochemical risk (IGR%)

consequent to a variation of the soil condition is:

IGR% ¼CiariaiiaCibribiib

� 1

� �100; ð2Þ

where subscript i stays for a generic hazardous element

and subscripts b and a stay for soil conditions respec-

tively before and after modification.

It is evident that, in case of soils, ingestion rate and

efficiency of release may play a more important role

than concentration itself in modifying the geochemical

risk of a site.

In the case of this study, we calculated the per-

centage increase of the geochemical risk since the

estimation of the real value of geochemical hazard in

MB area is not possible for the lack of specific and

accurate epidemiologic studies. We also considered the

future possible geochemical evolution of the washing

mud displaced at MB, including this way, an aspect

normally neglected in site specific hazard analysis.

Materials and methods

Sampling

The statistical significance of sampling procedure was

granted by collecting three sub-samples at each sam-

pling site, accordingly with a triangle scheme, that were

subsequently merged together. The overall weight of

each composite sample was not less than 2 kg. Six

different sample categories and a total of 31 samples

were collected:

• Washing mud (code: L, 10 samples). Silty mud from

SC quarry, transitorily accumulated at SC (7L) or

displaced inside MB inactive quarry area (16L1,

16L2, 16L4, 17L1, 17L2, 18L1, 18L2, 18L3), were

sampled at different places and depths (up to 1 m).

One sample of mud deriving from an heavy

weathered sector of the SC quarry front was also

collected (8L). Samples were successively dried at

<40�C before gently disaggregating particles using

an agate mortar.

• Soils (code: S, 7 samples). Residual soils developed

on gently sloping topographic surfaces made of

basalt talus and finer colluvial sediments were

sampled at SC (35S1, 35S2, 40S2) and MB (29S,

30S1, 30S2, 31S2) quarries. The two top horizons (A

and B) were homogeneously sampled by digging 40/

60 cm minipits. After drying at <40�C, soil aggre-

gates were gently crushed in an agate mortar and

successively sieved at <2 mm, accordingly to the soil

international standard procedure (e.g. EPA 1996).

• Rock, fresh (code: R, 8 samples) and altered (code:

P, one sample). Samples of basalt rocks with

different degrees of alteration were taken from

the quarry fronts at SC (1R, 3R, 10R, 12R, 15P) and

MB (19R, 26R, 28R). A sample of basalt breccias

(14R) was also collected in SC quarry site. Rocks

were roughly crushed in the lab for size reduction

using a heavy-metal free steel mill. Subsequently

some representative samples were further ground in

an agate mortar to obtain particles <75 lm.

• Gravels (code: G, 2 samples). Gravels from quar-

ried basalts at SC (5G, 6G), consisting in medium

and fine sized fragments of rocks crushed on-site

were sampled and treated as the previous samples

of parent rock.

• Talus (code: TT, 3 samples). Talus, mainly com-

posed of basalt gravels and blocks, were sampled in

the areas neighbouring SC (39TT), and MB (21TT,

23TT), quarries. Sample preparation was the same

as for the rock samples.

Analytical procedures

A first set of analysis was executed to measure the

concentration of major and minor chemical elements in

the different sample categories to assess their short and

long term environmentally available fraction. Further

analysis were aimed to identify the mineralogical

composition of samples.

In particular the following analyses were carried out:

4-acid strong digestion and analysis by ICP-MS

A 0.25 g sample split was heated in a 1:1:1 HNO3–

HClO4–HF solution up to fuming and taken to dryness.

The residue was successively dissolved in HCl. Con-

centrations of 41 elements (Ag, Al, As, Au, Ba, Be, Bi,

Ca, Cd, Ce, Cu, Co, Cr, Hf, K, La, Li, Mg, Mn, Mo, Na,

Nb, Ni, Fe, P, Pb, Sb, Sn, Rb, S, Sc, Sr, Ta, Th, Ti, U, V,

W, Y, Zn, Zr) were measured by Inductively Coupled

Plasma-Mass Spectrometry.

Tessier sequential leaching

Tessier sequential extraction procedure (Tessier et al.

1979) adapted by Campanella et al. (1995) was carried

out in order to selectively dissolve specific phases in a

particular order and hence to determine quantitatively

420 Environ Geol (2007) 53:417–432

123

the speciation and availability of Co, Cr, Ni, V. This

procedure consist of five steps:

• Step 1 (exchangeable fraction + bound to carbon-

ate fraction): 5 g sample was extracted with 90 ml

of NH4Ac and adjusted to pH 5 with HOAc.

• Step 2 (metals bound to Fe–Mn oxides fraction):

45 ml of NH2�OH�HCl and HOAc in 25% (v/v)

were added to the residue from step 1 for 24 h and

the solution was brought to 100 ml after centrifu-

gation.

• Step 3 (metals weakly bound to organic matter): the

residue from step 2 was leached with 25 ml HCl

0.1 M for 24 h and the solution was brought to

100 ml after centrifugation.

• Step 4 (metals bound to organic matter): the

residue from step 3 was leached with 25 ml NaOH

0.5 M for 48 h and the solution was brought to

50 ml after centrifugation.

• Step 5 (metals bound to sulphides): the residue

from step 4 was leached with 25 ml HNO3 8 M for

3 h at 80�C and the solution was brought to 50 ml

after centrifugation.

Electron probe microanalysis (EPMA)

Thin sections of rocks, soils and mud were analysed for

the major elements, in order to identify the most

abundant minerals, and for Co, Cr, Ni and V.

X-ray diffractometry (XRD)

Some fine ground powder from 12 samples of soils,

mud and rocks were analyzed using XRD technique

for the qualitative determination of the different min-

eralogical phase content.

Results and discussion

Mineralogical composition

Mineralogical modal analyses of the SC-MB ophiolites

have been previously given by Bocchi et al. (1976) and

Calanchi et al. (1987). These authors found a rather

constant abundance of Ab90–An10 plagioclase (50–

60%) and variable amounts of augitic pyroxene (12–

40%), olivine (5%), clinochlore (5–14%) and accessory

Fe–Ti oxides with pargasite (up to 10%). However

these estimates suffer of poor accuracy because the SC

and MB basalts appear altered under a static, hydro-

thermal ocean-floor type metamorphism resulting in

green-schists and lower amphibolite facies. In these

facies, primary plagioclase is replaced by a variable

mineral assemblage consisting of albite, clinochlore,

prehnite, and muscovite. Furthermore primary augitic

pyroxene is often replaced by clinochlore, epidote and

by a number of Fe–Ti oxides (ilmenite, rutile, magne-

tite). Finally primary ilmenite is replaced by titanite.

Similarly to most of gabbroic rocks outcropping in

the ophiolites of Northern Apennine, accessory parg-

asite is contained in interstices between plagioclase and

clinopyroxene and as rims around interstitial Fe–Ti-

oxides (Tribuzio et al. 2000). Pyrite and calcite are also

occasionally present. Ophiolites mineralogical vari-

ability is often accompanied by high variability in their

grade of porphiricity and by their unhomogeneous

structure frequently interested by veinlets and dykes.

The complexity of the mineralogical composition,

including primary phases (plagioclase and augitic

pyroxene), metamorphic phases (clinochlore and tita-

nite) and neo-formed clay minerals due to weathering

(kaolinite), is made evident from the data in Table 1,

where the whole mineral suite identified by XRD and

by electronic microprobe analyses is displayed.

A reliable modal-based comparison between

ophiolitic rocks, washing mud and local soils is pre-

cluded for the different representativeness of their thin

sections, much more indicative of the average compo-

sitions in the case of washing mud and soils than in the

case of rocks. Furthermore the mineral assemblage

consisting of a high number of mineralogical phases

(more than 20), many of which are solid solution terms,

makes complex the use of the refinement Rietvield

method (Kleeberg 2004). Therefore, the comparison

between sample types can be better done calculating

the grade of similarity of mineralogical composition

based on the recording of specific minerals in thin

sections (20 casual grains for each thin section) or

identified through electron microprobe microanalyses

and by XRD analysis of powdered samples (Table 1).

Mineralogical similarity analysis

The grade of mineralogical similarity is given in terms

of the Jaccard’s similarity coefficient (JSC) that is

based on the presence–absence relationship of mineral

phases shared between members of to two different

groups (Real and Vargas 1996) accordingly to the fol-

lowing equation:

JSC ¼ j

aþ b� jð Þ ; ð3Þ

where j is the number of mineral phases common to

both members and a and b are the number of species

Environ Geol (2007) 53:417–432 421

123

found on the member a and b, respectively. The coef-

ficient ranges from 1 to 0, where 1 indicating complete

similarity.

The mean of JSC (MJSC) calculated for rocks,

washing mud and soils showed that mud and rocks

have a grade of mineralogical similarity (MJSC = 0.60)

notably higher than rock and soil (MJSC = 0.38) or

mud and soil (MJSC = 0.38). The main differences

between mud/rocks and soils can be ascribed to the

transformation of primary oxides like ilmenite and

magnetite into goethite and to the transformation of

primary pyrite into parabutlerite [Fe(SO4)(OH)2 -

H2O]. Absence of calcite in the acid environment of

soils (measured values of pH around 5.5) are due to its

dissolution. Furthermore, phyllosilicates (muscovite,

sauconite, prehnite, saponite) are clearly more abun-

dant in soils than in rocks. The marked absence of

titanite in soils is expected, as this Ti-mineral displays

the highest dissolution rate during weathering (Velbel

1999) among the common orthosilicates, after olivine.

Discriminant analysis of analytical data from 4-acid

digestion

Geochemical similarities and differences are better

highlighted when the observed mineralogy is discussed

in the light of concentration data of major and trace

elements. On this purpose discriminant analysis is a

powerful statistical tool to differentiate among popu-

lation groups using a reduced number of variables in

order to simplify the interpretation of geochemical

underlying processes. Discriminant analysis was ap-

plied to the concentration data obtained from 29

samples treated with 4-acid digestion and belonging to

six different sample groups, namely soils, washing mud

(fresh), gravel, rocks, talus and altered rock (Table 2).

Since discriminant analysis gives reliable results only

when applied to normal distributed population, the

one-sample Kolmogorov–Smirnov test was prelimi-

narily run to compare the observed cumulative distri-

bution function of the variables to the normal

theoretical distribution. Ag, Bi, Be, Cd, P, Pb, Sn and

U did not pass the K–S test at 0.01 probability level and

were removed from the further statistical elaboration.

Major elements with the highest discriminant capabil-

ity were identified through the driven leave-one-out

software procedure (SPSS 1999). The elements were

preferentially selected among those particularly abun-

dant in clinochlore, augite, plagioclase and Ti–Fe

oxides, since they are the most common minerals of

the studied rock as already discussed above. The use

of only four variables, namely Na, Ca, Ti and Fe,

gave a rather good discriminant score of 79.3%. TheTa

ble

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422 Environ Geol (2007) 53:417–432

123

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0.3

Environ Geol (2007) 53:417–432 423

123

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(%)

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(mg

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(mg

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(%)

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(mg

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(mg

/k

g)

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(mg

/k

g)

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(mg

/k

g)

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(mg

/k

g)

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(mg

/k

g)

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(mg

/k

g)

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(mg

/k

g)

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(mg

/k

g)

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(mg

/k

g)

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(mg

/k

g)

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2

(mg

/k

g)

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Gro

up

s:1

mu

ds,

2so

ils,

3ro

cks,

4g

rav

els

,5

alt

ere

dro

cks,

6ta

lus

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tect

ion

lim

its:

10

.1p

pm

;2

1p

pm

;3

0.0

01

%;

40

.01

%;

50

.1%

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he

de

tect

ion

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its

are

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sed

on

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fid

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sta

nd

ard

de

via

tio

ns)

424 Environ Geol (2007) 53:417–432

123

repetition of the same leave-one-out driven procedure,

extended to minor elements, suggested to add Mo and

Ba, obtaining six variables as the best choice to mini-

mize the number of variables and maximize the dis-

criminant score, reaching 96.6% of properly classified

samples (29/30).

The two main canonical discriminant functions F1

and F2, whose coefficients are reported in Fig. 2, can

be interpreted by considering the scatterplots Fe versus

Ba, Mo versus Ti and Ca versus Na (Fig. 3) which

suggest different contributions of physical and chemi-

cal processes to the geochemical fractionation.

Geochemical fractionation: mechanical versus

chemical processes

Soils and washing mud, despite the results of the

mineralogical similarity grade analysis, are the sample

groups with the highest chemical similarity, considering

the Fe versus Ba scatterplot (Fig. 3a), being on average

more Ba and Fe-enriched than talus, gravels and par-

ent rocks. Ba-enrichment in the fine washing mud

comes from Ba abundance in phyllosilicates (musco-

vite, clinochlore) that are easily crumbled during

crushing for their low mechanical resistance. These

minerals are preserved in soils for their relative high

stability during weathering. Fe-enrichment in washing

muds can be ascribed to main two factors: the prefer-

ential partition of clinochlore in fine fraction during

crushing, and the primary dispersion of Fe–Ti oxides

(magnetite and ilmenite) and Fe-sulphide (pyrite) as

small interstitial accessory minerals disseminated in the

ophiolitic matrix, similarly to what has been recorded

Fig. 2 Scatterplot of the first two discriminant function used inthe discriminant analysis to classify cases into groups

Fig. 3 Scatterplots among the six variables used in the discrimi-nant analysis: a Fe versus Ba; b Mo versus Ti; c Ca versus Na

Environ Geol (2007) 53:417–432 425

123

in most of the ophiolites of the Northern Apennine

(Tribuzio et al. 1999). The dispersion of these phases

support their mechanical release during crushing and,

consequently, their higher concentration in washing

mud. The persistence of Fe in soils is imputable to the

high stability of clinochlore during weathering and

pedogenesis processes that, acting over naturally

occurring incoherent sediments, led to the alteration

of magnetite into insoluble Fe(III)-oxyhydroxides

(goethite). Discriminant function 1 is dominated by

Fe and shows increasing values in the sequence

rocks < soils < washing mud, suggesting its link with

mainly mechanical processes acting on mineral phases

of different hardness. The importance of mechanical

processes are evident in the Mo versus Ti scatterplot

(Fig. 3b) showing how parent rocks are Ti-enriched

with respect to washing mud and soils. This is a con-

sequence of the decreasing abundance of titanite and

Ti-oxides in the sequence rocks > washing mud > soils

that can be explained by its high hardness and resis-

tance to crushing. The decrease of primary Ti-bearing

minerals in soils is also consistent with studies on the

relative mineral stabilities in saprolites derived from

weathering of meta-basalts and meta–gabbros (Sch-

roeder et al. 2000) where mineral dissolution rates

increase in the sequence goethite < quartz < phyllosi-

licates < Fe–Ti oxides < plagioclase. In the same scat-

terplot, washing mud show higher Mo concentration

than to soils and rocks that can be explained by the

relative abundance of Mo-bearing pyrite and its pref-

erential partition in fine fraction. These observations

and the relatively high positive value of Mo-coefficient

in the discriminant function 1 confirms that this

function reflects mainly mechanical effects in the

geochemical fractionation processes.

The Ca versus Na scatterplot (Fig. 3c) confirms the

mineralogical similarity grade analysis, highlighting the

preferential hydrolysis of Ca-plagioclase (anorthite)

with consequent heavy geochemical differentiation of

soils from rocks and mud. The high coefficient of Na

and Ca in the discriminant function 2 suggests that this

function reflects essentially chemical weathering pro-

cesses. The higher Ca/Na ratio in washing mud in

comparison with the rocks can be explained with the

relatively higher abundance of pargasite (the ratio of

its modal abundance in washing mud versus parent

rocks being approximately 2:1). This mineral is more

easily disgregated and has a Ca/Na ratio higher than

the average of the parent rocks.

Data discussed above give a first clear indication

that technological process of rock crushing can pro-

duce statistically significant geochemical fractionation,

whose extent can be compared to that produced in

natural weathering by a combination of mechanical

and chemical processes. From this point of view the

first two statistical function of our discriminant analysis

can be considered as two chemo-functions describing

respectively mechanical (function 1) and chemical

(function 2) processes, respectively identified by highly

positive and highly negative values.

A focus on the PHES belonging to the first

transition series metals

The variability trend of the first transition series metals

(TE) in the different sample groups (Fig. 4), highlights

enrichment of Cr and Ni and depletion of Sc, Ti and V

in washing mud and soils with respect to their parent

rock, while Mn, Fe, Co and Cu concentrations are

similar. The same diagram shows an analogue variation

of TE in talus with respect to soils and washing mud.

TE concentrations in washing mud are intermediate

between parent rocks and soils while the altered rock

sample shows constant TE enrichment with respect to

unweathered rocks.

TE show preference for solid over liquid phases

according to their OSPE (octahedral site preference

energy) values, following a predictable order Ni >

Co > Cu for bivalent ions and Cr > V > Ti for triva-

lent ions (White 2005). During weathering these ele-

ments should display a relative increase in Ni and Cr as

actually highlighted by their higher abundance in the

sample of altered rock (sample P15).

The relatively high Cr and Ni concentrations in soils

in spite of the absence of clinopyroxenes, a phase en-

riched in octahedral sites, is a consequence of the

abundance of oxidized phases and organic matter

Fig. 4 Plot of the averaged concentration of transition elementsin the different sample categories standardized by the averageconcentrations in the basalt rocks

426 Environ Geol (2007) 53:417–432

123

which act as sinks for these elements. On the contrary

the higher concentration of Cr and Ni in washing mud

than in parent rocks is a consequence of crushing

process. These evidences must be carefully considered

since most of TE are PHES and their concentration

limits in soils have been regulated by the Italian law

(Gazzetta Ufficiale 1999). In the SC washing mud tol-

erance limits were passed for residential and, in some

cases, industrial use of the soils relatively to Co, Cr, Ni

and V (limits for residential/industrial use: Co = 20/

250; Cr = 150/800; Ni = 120/500; V = 90/250). For this

reason in the following we specifically focused on the

mineralogical allocation and environmental behaviour

of these four elements whose descriptive statistics for

each sample group are reported in Table 3.

Electron microprobe analyses identified, in our

samples, augite, ilmenite, titanite, clinochlore, magne-

tite and plagioclase as the minerals with the highest

concentration of Ni, V, Co and Cr (Fig. 5).

Ni is particularly concentrated in magnetite while Cr

appears increasingly concentrated in the sequence:

augite < magnetite < ilmenite. As discussed above, the

dissemination of these oxides in the rock matrix caused

their higher concentration in washing mud than in

parent rocks. On the contrary V is more abundant in

rocks because of its high concentration in titanite. Fi-

nally, Co concentration is quite uniform both in the six

mineral phases and in the different sample groups.

As a direct consequence of these mineral assem-

blages, Cr concentration in washing mud at SC is

higher than the Italian tolerance limit of 200 mg/kg for

residential areas, while this limit has been passed only

for two out of eight rock samples. Ni concentrations

are also relatively high in washing mud where they

overpass the residential limits, while only in one rock

sample was recorded a value higher than 120 mg/kg. V

and Co overpass the residential limits both in rocks and

in washing mud without showing significant difference

of concentration among the two sample groups. Cr and

Ni show average concentrations higher in altered rock

than in fresh rock due to OSPE effects and higher in

soils than in parent rocks due to formation of sink

phases. Finally V displays the lowest concentration in

soils likely due to weathering of titanite and Ti-augite.

Background values

Accordingly to the Italian laws, an altered soil where a

PHES exceed the fixed limits needs remediation if its

concentration in PHES overpasses the concentration of

local natural background. This statement can be for-

malized saying that remediation is needed when the

‘‘enrichment factor’’ (Liu et al. 2006) or ‘‘pollution

index’’ (Subramanian et al. 1998) are >1. The natural

background can be calculated from the average PHES

concentrations in local natural unpolluted soils. In this

study, the four considered elements show average

concentrations and standard deviations in local natural

soils equal to: V = 176 ± 46 ppm, Cr = 243 ± 134 ppm,

Ni = 170 ± 122 ppm, Co = 41 ± 11 ppm. After com-

paring these values and their standard deviations with

the mean values of concentration in washing mud

(V = 198 ± 21 ppm, Cr = 208 ± 14 ppm, Ni = 158 ±

16 ppm, Co = 44 ± 5 ppm) it is clear that no remedi-

ation action should be required. However, in spite of

the close similarity with the natural background, the

Table 3 Descriptive statistics for Co, Cr, Ni and V concentra-tions in washing mud, soils and rocks

N Min Max Mean SD Var

CoW. mud 9 39.0 55.0 43.4 5.3 27.8Soils 6 25.0 49.0 41.3 10.9 117.9Rocks 8 13.5 39.0 32.7 8.1 65.5

CrW. mud 9 186.1 225.7 208.9 13.7 188.8Soils 6 112.2 423.7 242.9 134.6 18129.0Rocks 8 40.5 203.9 137.5 51.9 2689.3

NiW. mud 9 134.8 178.9 158.7 15.0 224.4Soils 6 63.8 338.3 170.0 122.2 14921.2Rocks 8 19.9 126.8 73.6 29.2 849.9

VW. mud 9 173.0 228.5 197.9 21.6 468.4Soils 6 113.0 232.5 175.4 46.6 2174.4Rocks 8 129.5 285.0 244.7 52.9 2799.6

Fig. 5 Average concentration of V, Cr, Co and Ni in theminerals where they are mostly abundant

Environ Geol (2007) 53:417–432 427

123

reutilization of washing mud at MB quarry site could

increase the geochemical hazard, since bioaccessibility

depends also on soil ingestion rate and release effi-

ciency.

Realise efficiency: discussion of Tessier sequential

leaching

Seven samples of washing mud, one sample of mud

coming from crushing of deeply weathered rocks and

four samples of soils from SC and MB were subjected

to Tessier modified sequential leaching (Tessier et al.

1979; Campanella et al. 1995) and examined for Co, Ni,

V, and Cr. Washing mud appeared as a rather homo-

geneous medium with respect to the soils, the latter

showing a far more pronounced geochemical variabil-

ity when considering altogether the five steps of the

analysis (Fig. 6).

Figure 6 highlights how Cr and Ni are extracted at a

similar rate and in a rather low amount (always below

15%) from washing mud and soils. It is interesting to

note that the sample of altered washing mud released

these elements at an extent comparable to that of fresh

washing mud. On the other hand, Co and V proved to

be more extractable from soils (up to 42% for Co and

21% for V) as a consequence of the association of Co

and V with insoluble organic complexes after the

complete alteration of the primary Co e V bearing

minerals (e.g. titanite). Generally, the exchangeable

(step 1) and acid soluble (step 3) fractions of heavy

metals are considered potentially bioavailable, while

the reducible (step 2) and oxidizable (step 4 and 5)

fractions are relatively stable. Therefore, to evaluate

possible release changes, we should compare the total

percentage extracted during the steps 1 and 3 from soils

and fresh washing mud. The results for soils are: 2.3%

(Ni), 5.9% (Co), 2.7% (V), 1.8% (Cr). The results for

washing mud differ slightly: 2.7% (Ni), 3.7% (Co),

2.6% (V), 2.0% (Cr) with the only exception of the

significantly lower Co-extractability. Therefore no sig-

nificant changes of bioaccessibility can be highlighted

when considering concentration and release efficiency.

Fig. 6 Fractions of Co, Cr, Niand V extracted during thefive steps of the sequentialleaching for nine samples ofmuds (filled circle) and soils(circle)

428 Environ Geol (2007) 53:417–432

123

Soil ingestion rate and bioaccessibility

Soil ingestion rate is closely influenced by grain size

and has a direct influence on bioaccessibility. Soil

ingestion depends primarily on the skin adherence.

Driver et al. (1989) found statistical increases in skin

adherence with decreasing of particle size. Unsieved

soils showed average adherence of about 0.6 mg/cm2

while average adherence for particle size lower than

150 lm (silty mud) is about 1.4 mg/cm2. Given the silty

nature of the washing mud at SC, a positive percentage

increase in skin adherence at the disposal site in MB

can be estimated around 130%.

Percentage increase of geochemical risk at MB:

present and future

From Eq. 2 we can estimate the percentage increase of

geochemical risk at MB after the displacement of the

washing mud for V (+149 ± 65%), Cr (+140 ± 132%),

Ni (+150 ± 177%) and Co (+54 ± 43%). It is clear

how, considering the associated errors, the geochemi-

cal risk is significantly increased in the case of Vana-

dium, mainly due to reduced grain size, while no

significant estimation can be done for the other ele-

ments due to the relatively high associated errors.

Since washing mud has not undergone significant

geochemical alteration yet, higher concentrations of

environmentally available Co, V, Ni and Cr have to be

expected in the next years, after prolonged weathering.

An estimation of the future release of PHES can be

provided considering data from in situ weathered rocks

and soils as two different natural analogues of the fu-

ture state of washing mud. The percent mass changes

of these elements during weathering can be estimated

using the dimensionless element transfer coefficient t

(Anderson et al. 2002) given by:

tj;w ¼Cj;wCi;p

Cj;pCi;w� 1

� �100; ð4Þ

where Cj,w(p) is the concentrations of a mobile element

j in weathered (w) or in parent (p) rock and Ci,w(p) is

the concentrations of a reference element i assumed

immobile in the same materials. Following Anderson

et al. (2002) we used Zr as reference immobile ele-

ment. Values of tj,w for the considered elements in al-

tered rocks are respectively equal to 233% (Ni), 144%

(Cr), 47% (V) and 41% (Co). The soil tj,w values are

rather similar: 203% (Ni), 133% (Cr), –6% (V), and

69% (Co). This convergence of transfer coefficient

suggests the existence of common geochemical pro-

cesses leading to the in situ formation of altered rocks

and soils. The mass increase of the considered ele-

ments, and especially of Ni and Cr, can be attributed

mainly to the magnetite–goethite transformation since

goethite in soils is particularly abundant in these two

elements (Quantin et al. 2002; Godgul and Sahu 1995).

A realistic forecast about the future concentrations of

all the four elements in altered washing mud can be

produced by: (a) using average element transfer coef-

ficient for soils and altered rocks and (b) supposing

that the Ci,w/Ci,p ratio relative to Zr in the transfor-

mation fresh mud-altered mud is close to the average

between the values measured for the transformation

parent rocks-soils and parent rocks-altered rocks.

When making these two assumptions, the predicted

concentration after a lag of time comparable to that of

Table 4 Dissolution rateparameters for magnetite,ilmenite and augite (fromPalandri and Kharaka 2004)

Mineralspecies

Molar volumecm3/mol

Log racid Logmol/cm2 s–1

D, avg graindiam. cm

n pH R, mol/cm2s–1

Magnetite 44.0 –12.59 0.01 0.28 5.5 10–14.13

Ilmenite 31.7 –12.35 0.01 0.42 5.5 10–14.66

Augite 66.8 –10.82 0.01 0.70 5.5 10–14.67

Fig. 7 Variation of the concentration of Co, Cr, Ni and V inwashing mud over the time. Black points are placed on Cr and Nicurves when they reach the concentration limits fixed by the law.Co and V curves already starts from concentration values higherthan the fixed limits

Environ Geol (2007) 53:417–432 429

123

soil formation or in situ weathering is 628 ppm for Ni,

621 ppm for Cr, 85 ppm for Co and 313 ppm for V.

We suppose that variation of concentration (X) over

time follows an exponential law:

Xt ¼ X0 þ XF �X0ð Þ 1� e�kt� �� �

; ð5Þ

where subscripts 0 and F point out the initial and final

situation and k is the transformation constant of the

fresh mud to weathered mud. According to this equa-

tion when t = 0, Xt = X0 and when t = infinite Xt = XF.

Mineral grain persistency in a soil, expressed in

terms of average lifetime (T), control the transforma-

tion constant value between fresh and altered washing

mud and can be predicted by the mineral dissolution

rate (r), its molar volume (Vm) and its grain diameter

(d) accordingly to the following equation (Kowalewski

and Rimstidt 2003; Jurinski and Rimstidt 2001):

T ¼ d

2rVm: ð6Þ

The dissolution rate of a mineral species in a specific

soil pH (equal to 5.5 in our case) can be estimated

starting from its dissolution rate at pH = 0 (racid)

accordingly to the following equation; Palandri and

Kharaka 2004):

Log rð Þ ¼ Log racidð Þ � npH soilð Þ; ð7Þ

where n is the reaction order with respect to H+.

Values of racid and n for magnetite, ilmenite, augite,

the mineral phases more relevant for Ni, Cr, V and Co

in the studied mud, are given in (Palandri and Kharaka

2004) and the parameters of interest for these three

mineral are given in Table 4. The application of Eqs. 6

and 7 to calculate lifetimes of the above mentioned

mineral species gave 556 years for magnetite,

2,239 years for ilmenite and 1,087 years for augite.

Unfortunately no accurate dissolution rate value is

reported for titanite in scientific literature. Since

accordingly to the Goldich series titanite is more

resistant to weathering than olivine, we can estimate its

lifetime as >366 years which is the calculate lifetime

for fayalite in the assumed conditions of soil pH.

The decay constants of these phases are therefore: k(1/T) = 0.0018a–1 for magnetite, k = 0.00044a–1 for

ilmenite, k = 0.000911a–1 for augite, k < 0.0027a–1 for

titanite. Given the uncertainty on the titanite data, we

took the value measured for magnetite, k = 0.0018a–1,

as conservative value for the transformation constant

of the washing mud. This value is really precautionary

as magnetite, during its transformation in goethite, is

predicted to develop a protective surface layer made of

goethite in function of the stoichiometric coefficients in

the reaction magnetite–goethite and their respective

molar volumes (for the calculations see the function

given in Velbel 1993). This is consistent with the fact

that V-enrichment process needs the presence of goe-

thite, as also highlighted by the high correlation coef-

ficient between Fe and V in soil samples.

Figure 7 highlights how over a period ranging be-

tween 0 and 50 years all the metals reach the back-

ground value assumed as the average concentration in

local soils. This time lag can be considered as largely

conservative also considering the major uncertainty

concerning the future value of pH in the weathered

mud which we have assumed to vary from the present

value of 7.2 to that measured in soils equal to 5.5. On

the contrary if we assume narrower variation the

average life time of the minerals will be notably in-

creased. Calculated lifetime for magnetite at pH close

to neutrality, for example, increase up to 112,000 years.

If we recalculate the percentage increase of geo-

chemical risk at a time = 70 years, in spite of the ex-

pected higher concentrations in altered washing mud,

the percentage increase of geochemical risk still keeps

significant only for V (+164 ± 69%), the other values

being affected by high uncertainty: +189 ± 159% for

Cr, +232 ± 238% for Ni and +76 ± 47% for Co.

Independently from the significance of these percent-

age increases, a wider discussion should be made

about the actual increase of geochemical risk ex-

pressed in number of damaged individuals. Unfortu-

nately, data on health effect of soil ingestion at usual

background concentrations are known only for nickel.

A single dose of nickel from soil ingestion equal to

0.009 mg/kg of body weight can produce contact der-

matitis (Calabrese et al. 1997). This is a non lethal

toxic dose whereas the lethal one is of 570 mg/kg of

body weight. When considering a standard weight of

10 kg for a child with an age of 2–4 years frequenting

MB quarry and an average soil ingestion of

50 ± 25 mg soil/day, the annual probability for him to

be affected by contact-dermatitis before and after a

time = 70 years is equal to 0.003 and 0.01%, respec-

tively. However the annual probability to be damaged

by a landslide event occurring at the MB site during

the same lag of time is at least equal to the 0.1%. It is

clear that if the engineering reutilization of the

washing mud at MB succeeds to reduce the geological

hazard even only of the 10%, the future increase of

geochemical risk regarding Ni-soil ingestion might be

accepted. In addition, it must be considered as the

altered washing mud can act as a sink for all the four

considered PHES. This is a further advantage for the

environment because the mud act over the time as

430 Environ Geol (2007) 53:417–432

123

filters for infiltrating waters reducing health risks due

to drinkable use of local ground water.

Conclusions

Washing mud originated as residuals of the techno-

logical process of basalt crushing have peculiar geo-

chemical features that must be investigated in order to

address their possible utilization in civil engineering.

However their environmental behaviour has also to be

interpreted by the light of their alteration over time

with consequent modification in the release rate of

PHES.

This study highlighted the many environmental

problems associated with the use as refilling material of

washing mud produced in an ophiolitic rock quarry of

the northern Apennine (Tuscany, Italy). Here washing

mud showed geochemical fractionation due to

mechanical processes acting on minerals of low hard-

ness as well as on primary finely disseminated mineral

phases, leading to their enrichment in V, Ni, Cr and

Co, up to values higher than parent rock but com-

pletely framed within the local natural background.

Geochemical fractionation on possible natural ana-

logues of future altered washing mud, like soils and

altered rocks, showed positive mass enrichment of V,

Ni, Cr and Co, mainly due to chemical weathering,

with consequent formation of both mineral and organic

phases acting as sinks for these elements. A simulation

based on the decay time of magnetite, ilmenite, augite

and titanite at pH 5.5, equal to the present value in

local soils, showed a clear trend to increase the con-

centration of V, Ni, Cr and Co in washing mud over the

time. In particular Ni concentration would reach the

background values within 50 years. Though this evo-

lutionary trend cause an increase of the absolute geo-

chemical risk due to soil ingestion, the usage of

washing mud has not to be precluded in case it reduces

significantly the risk associated with the landslide at

Monte di Beni quarry site.

Acknowledgments We sincerely acknowledge Giuseppe Cav-arretta for his scientific support and Maria Esposito, GiampaoloMarruzzo, Bruno Passariello, Luciano Passeri, Marcello Serra-cino, Elena Spaziani and Emanuela Tempesta for their activitiesof analytical determination in the geochemical laboratories.

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