Impaginato Atti Il Quaternario Italian Journal of Quaternary Sciences 1155(1), 2002, 111-120 SSOOIILL GGEEOOCCHHEEMMIISSTTRRYY AANNDD PPEEDDOOLLOOGGIICCAALL PPRROOCCEESSSSEESS.. TTHHEE CCAASSEE SSTTUUDDYY OOFF TTHHEE QQUUAATTEERRNNAARRYY SSOOIILLSS OOFF TTHHEE MMOONNTTAAGGNNOOLLAA SSEENNEESSEE ((CCEENNTTRRAALL IITTAALLYY)) EE..AA..CC.. CCoossttaannttiinnii11,, MM.. AAnnggeelloonnee22,, RR.. NNaappoollii11,, 1Istituto Sperimentale per lo Studio e la Difesa del Suolo, Mi.P.A., P.za D’Azeglio, 30, 50121 Firenze 2ENEA, Tein-Chim, C.R. Casaccia, C.P. 2400, 00100 Roma A.D. ABSTRACT The role of soil as an environmental filter is widely acknowledged, although not fully understood in all the processes involved. Unfortunately in the monitoring of some environmental parameters indicative of soil quality, such as heavy metals, we can observe a general tendency to simplify the issue. In fact, data refer only to a part of soil, i.e. the uppermost part, or the plow layer, while characte- ristics and processes which occur in the lower parts of the profile are neglected. On the other hand many soils, like Paleosols, which are quite widespread in Italy, have a very thick and complex profile, in which a significant elemental concentration can take place as a result of natural, pedological processes. This stresses the importance of in-depth investigation when the object of laboratory analysis is to provide advice for specific land uses. Aim of this work was to study the role played by soil forming processes in addressing element behaviour in some soils of the Montagnola Senese territory. Results of this work show an accumulation of many elements with respect to parent material. However this trend was not uniform in all cases, pointing out that their re-distribution in soil horizons can be related to different pedogenetic processes. The accumulation of some elements in soils can be to some extent related to organic matter content, pH and cation exchange capacity, but mainly in the upper horizons, while clay richness seems to play a more important role in determining the element concentration in all soil horizons: correlation coefficients with high level of significance have been found between clay and Ti, K and Cr, but also Fe, Zn and Pb are corre- lated with clay content, with the exception of those horizons, which are affected by element redistribution caused by oxidative-reductive processes. Several elements show a time dependent concentration process. Ti, K, Na and Mn seem to increase through time from the Holocene, to the Upper and Middle Pleistocene; Cr, Pb and Zn, similarly to Fe, from Holocene up to the Lower Pleistocene. The accumulation process proceeds along with clay neo-genesis and illuviation, but it can be affected by clay impoverishment, due to ferrolysis, together with the element mobilisation produced by reducing conditions. If clay impoverishment is characteristic of eluvial horizons and bleached streaks of fragipan and glossic horizons, mobilisation of Fe, Zn and Pb is manifested in the reduced parts of almost all the horizons with bad drainage. RIASSUNTO L’importanza del suolo come ‘filtro ambientale’ è generalmente riconosciuta, sebbene non ancora pienamente compresa in tutti i pro- cessi che ne sono coinvolti. Nel monitoraggio di alcuni parametri ambientali indicativi la qualità del suolo, come ad esempio i metalli pesanti, si tende in genere a semplificare le cose, riferendosi solo ad una parte del suolo, quella più superficiale, o strato lavorato, mentre le caratteristiche degli orizzonti sottostanti ed i processi che avvengono nella parte profonda del profilo non sono considerati. D’altra parte molti suoli, in particolare i paleosuoli, piuttosto diffusi in Italia, sono caratterizzati da un profilo molto profondo e complesso, nel quale si può verificare un significativo incremento di elementi per cause naturali, legate a processi pedologici. Scopo di questo lavoro è quello di studiare il ruolo svolto dai processi pedogenetici nell’indirizzare l’accumulo di elementi nei suoli della Montagnola Senese. I risultati del lavoro evidenziano il verificarsi di un accumulo di elementi rispetto al materiale parentale; tuttavia questa tendenza non si è realizzata allo stesso modo per tutti i suoli. Ciò suggerisce che la ridistribuzione degli elementi negli orizzonti pedologici può essere messa in relazione con differenti processi pedogenetici. L’accumulo di alcuni elementi nei suoli è stato correlato al contenuto in sostan- za organica, al pH e alla capacità di scambio cationico, soprattutto nei primi orizzonti, mentre un ruolo maggiore è svolto dall’argilla nel determinare il contenuto in elementi di tutti gli altri orizzonti. Coefficienti di correlazione altamente significativi sono stati riscontrati tra argilla e Ti, K e Cr ma anche con Fe, Zn e Pb. Tuttavia questa evidenza non si manifesta negli orizzonti caratterizzati da una ridistribu- zione degli elementi provocata da processi di ossidoriduzione. Molti elementi evidenziano un processo di accumulo dipendente dal tempo. Ti, K, Na e Mn mostrano di aumentare col tempo passando dall’Olocene al Pleistocene superiore e medio; Cr, Pb e Zn, analogamente al ferro, dall’Olocene fino al Pleistocene inferiore. Il processo di accumulo procede di regola assieme alla neogenesi ed accumulo di argilla, ma può essere influenzato da altri due pro- cessi: l’impoverimento di argilla, attivo negli orizzonti eluviali, glossici e nei fragipan, e la mobilizzazione degli elementi, che avviene in tutti gli orizzonti che presentano condizioni riducenti. Key words: trace and heavy metals, soil geochemistry, Paleosols, Siena, Italy Parole chiave: metalli pesanti ed in traccia, geochimica dei suoli, paleosuoli, Siena, Italia. 11.. IINNTTRROODDUUCCTTIIOONN Knowledge of the concentration of trace elements in soil, especially heavy metals, is of utmost environ- mental relevance when the purpose is to determine the pollutant rate related to anthropogenic influences. In the last decade several western countries have carried out many research programs to establish valid background levels to be used as reference points to discriminate contamination. Unfortunately in our country a systematic and organic national program to assess trace element distribution and background values in soil is far from being completed. As a direct consequence we can wit- ness the great difficulties encountered by national and regional legislation to establish valid and useful referen- ce points to assess contamination. However, little is known about the thresholds which can cause damage to the soil-plant ecosystem and the limits can vary widely according to the different countries' approach to this problem (Adriano et al., 1995; Tab. 1). Besides, recent studies have demonstrated the irreversible effects upon the soil microbial ecosystem of soil metal concentrations having values which are well below European and Italians limits (Brookes, 2001). Element content in soils may vary according to agricultural practices, but, to a great extent natural causes also play a role. The influen- ce of parent material, for instance, is well-known (Fergusson, 1990; Angelone & Bini, 1992); nevertheless the contribution of long-lasting natural processes, like those occurring in paleosols, has not been well-establi- shed yet. Moreover, legislation related to soil quality and pollution generally tends to simplify the issue, in particu- lar it refers to only a part of the soil, that is the uppermo- st part, or the plow layer, while characteristics and pro- cesses of the lower parts of the profile are often neglec- ted. This simplification does not take into account the possibility that the upper part of the soil can be thinned, or even removed by some agricultural practices or because of soil erosion. Another simplification concerns soil horizons often considered to be homogeneous, whereas element accumulation in soils, and particularly in paleosols, can occur only in parts of some horizons. This can be of great relevance in interpreting analytical results for practical purposes, not only those related to the environment, but also to the agricultural uses of soils (Costantini, 1999), and should steer the soil sampling. In Mediterranean countries, and particularly in Italy, Paleosols are quite widespread and a signi- ficant elemental concentration can occur in some parts as a result of paleopedological pro- cesses. These processes are usually accompanied by a more or less pronounced soil redde- ning or the formation of nodules, but exceptions are frequent. In this work, the elemental distribution in some soils of the Montagnola Senese territory has been studied and put in relation to main soil properties (pH, clay, OM, CEC), major elements (Fe, Al), estimated soil age, soil morphology (genetic horizons, redoximorphic features) with the aim of studying the role played by soil forming processes in addressing element behaviour. 22.. MMAATTEERRIIAALLSS AANNDD MMEETTHHOODDSS 22..11 SSooiill aannaallyyssiiss The soils of the Montagnola Senese territory have been previously studied by Costantini et al. (1996) to explain the genesis of fragipan and other close-packed horizons. More than 50 profiles of the area have been described and analysed. A dozen of them, developed on acid metamorphic rocks and on mainly siliceous collu- vial and alluvial deposits, were also studied for their ele- ment composition. Soil description followed the Soil Survey Staff methodology (1993), routine analysis was in compliance with the Italian official methods (SISS, 1985). Plinthite nodules were submitted to the test of Wood and Perkins (1976), with immersion of samples for two hours in water, to check the persistence of aggregation. The counting of nodules and pseudomorphic nodules in the sands was made on 26 horizons pertaining to seven selected soils, utilising the optical microscope at diffe- rent magnifications and considering 300 grains from each sand class; percentage values of nodules were then referred to percentage of fine earth, without taking into account possible differences in specific gravity. Geochemical analysis was performed on 67 soil horizons belonging to 12 representative profiles. Soil samples were dried beforehand at 40°C, ground and sieved through a 2 mm Teflon coated sieve. Homogenised sub-samples were ground with an agate ball mill to obtain a fraction < 0.1 mm. Approximately 500 mg of homogenised sub-sample were weighed in a Teflon bomb. Metal extraction was carried out adding a mixture 5 ml of Aqua Regia (5 ml) and ultra pure HF (2 ml). Saturated H3BO3 was successively added to buffer the excess of HF. Major and trace element analysis was carried out by Perkin Elmer 5100 AAS at flame (K, Na, Mn); and with an AAS equipped with a Zeeman back- ground corrector for Cd, Cu, Cr, Pb and Zn. ICP was used to analyse Ca, Al, Mg, Fe and Ti. All the analytical procedures were tested beforehand with a data quality control programme using international soils CRMs, sam- ples duplicate and reagent blanks. 112 Total soil metal concentration, mg kg-1 soil European Union Year CCdd CCuu CCrr PPbb ZZnn Community 1986 1-3 50-140 100-150 50-300 150-300 France 1988 2 100 150 100 300 Germany 1992 1.5 60 100 100 200 United Kingdom 1989 3 135 400 300 300 Italy 1992 1.5 100 (a) 100 300 (a) = Bartlett test for soil oxidation capacity <1 Table1 - Maximum concentrations of metals allowed in agricultural soils treated with sewage sludge (after Adriano et al., 1995). Concentrazioni massime di metalli permesse nei suoli agricoli trattati con fanghi di depurazio- ne (da Adriano et al., 1995). E.A.C. Costantini, M. Angelone & R. Napoli 33.. RREESSUULLTTSS AANNDD DDIISSCCUUSSSSIIOONN 33..11.. GGeenneerraall oouuttlliinneess ooff tthhee aarreeaa The study area is a small ridge located in Central Tuscany, covering just under 20 km2. It is made up of several hills, with dominant heights ranging from 400 to 500 meters and a maximum of 671 meters a.s.l. The area underwent intense geomorphological evolution during the Pliocene and Quaternary, with alternating periods of erosion and stability. The rising of the ridge led to the erosion of the slopes, but several surfaces remained stable (e.g. karst depressions) or were stable over a long period (e.g. colluvial areas). Four main litho- logical units could be distinguished: i) acid metamorphic rocks, consisting of chloritic and sericitic fine-grained schist, jasper, quartzose micro and macro conglomera- te and violet schist breccias (Mesozoic); ii) calcareous rocks, composed of flint limestone, marble, dolomite and cavernous limestone Mesozoic in age, but partially reworked by the Miocene sea; iii) mainly calcareous or iv) mainly siliceous colluvial and alluvial deposits, the mineralogy of which derives from the mixing of the above mentioned rocks. Climatic data for the area were obtained from Simignano (SI) and Siena. At Simignano (43°18' Lat. N; 419 m a.s.l., 8 km west of Siena) the average annual rainfall is 1019 mm, with maximum in October (119.8 mm) and minimum in July (36.4 mm). In Siena (43°19' Lat. N; 348 m a.s.l.) the average annual temperature is 13.2°C, the warmest month is July (22.1°), the coldest is January (5.8°). The soil moisture regime, evaluated by the Newhall Computation (Newhall, 1972) is "udic" with a water holding capacity of 200, 100 and 50 mm, whereas the soil temperature regime, according to Soil Taxonomy (Soil Survey Staff, 1999) is "mesic" (8