Introduction

Most of the investigations related to the problem of burials focus on determining such components as ion content in the saturation and aeration zones ((Pacheco et al. 1991; Knight and Dent 1995), enteric bacteria (Dent and Knight 1998; Matos 2001), amino acids (Żychowski et al. 2002) and organic compounds (Żychowski 2006; Spongberg and Becks 2000a) in groundwater, adipocere (Forbes et al. 2004) and elements present in the subsoil (Spongberg and Becks 2000b; Mączyńska et al. 2006). Bones of the buried are another subject of investigation. As time goes by, bones lose calcium and phosphorus, the two major components, and undergo mineral alterations. For instance, bone may be mineralized by chemical compounds occurring in the vicinity of the burial site: silicon dioxide–amorphous silica to quartz, oxyhydroxides of iron and manganese, or even calcium carbonate–calcite (Pawlikowski et al. 2007). Organic acids formed during the decomposition of corpses dissolve the silicate minerals of the grave-containing sediments with simultaneous formation of phosphoric acid. For this reason, newly formed minerals, e.g., microcrystals of secondary quartz, may accompany crystallization of phosphorus-containing compounds. Simultaneously, the fibrous structure of the bone collagen disappears and is replaced by shred-like, irregular organic forms (Pawlikowski et al. 2007). Such processes are proof of the chemical alterations that take place within archeological sites (Pawlikowski et al. 2000). Studies on decomposition mechanisms are only in their initial stages. For instance, the decomposition of starch grains in soils (Haslam 2004) provides end products, which may be used in the identification of both damaging and preserving factors of the process.

Elements contained in decomposition products also make it possible to identify the material that has decomposed (Trąbka 2003) or the places of specific ceremonies or handicraft activities (e.g., painting (Parnell et al. 2002). Higgins and Burns (1975) suggest the connection of P with Fe, Ca, Mg and Al during the decomposition process of corpses. Also, other scientists have confirmed these results (Dent et al. 2004).

My studies have to either prove or disprove the idea that new compounds could be formed out of decomposition products and the components of the natural environment. The emergence of such new compounds and the condition of human corpses were documented by photographs of microscopic images under suitable magnifications of up to 60 and 120 times. Because of cultural concerns, the ground samples were collected primarily from the vicinity of graves. Wherever the owner of the land consented, boreholes were drilled to collect samples. The material subject to analysis did not concern the superficial layer of the substrate. The ground at the sampling sites was dug and subjected to serious deformation.

The aim of the investigations

Investigations were carried out to identify processes and mineral products (P-bearing ones, Fe compounds and clay minerals) in diversified subsoil sediments of nine mass graves and their surroundings, as well as the relations between the development of sediments within the mass graves and the scale of migration of the products of corpse decomposition. Determining the state of preservation of the decomposition products in the mass graves from World War 1 and 2 was an additional aim of the research. The investigation concentrated also on the secondary minerals and their types formed during corpse decomposition. Bone microfragments found in sediments adjacent to the graves (none of the graves was directly penetrated) were also studied with a focus on their alterations.

Methods

Samples were collected from various depths in vertical profiles of nine mass graves or in their immediate vicinity, and represented the sediments overlaying the grave, the sediments from the burial horizon and, when possible, the sediments underlying the grave (studied downslope). The sampling sites were selected considering the development of the mass grave sediments.

Samples from five sites (graves and their surroundings) were collected in such a way that the rock texture was preserved. After digging a pit of about 2-m deep, small horizontal openings were made and a plastic box sampler 4 cm high was carefully inserted upward into the roof of each opening. The following sites were sampled while applying this method:

  • Bochnia, below of a grave from WW1, on the slope, in a line of graves; depths: 0.40–0.44 m, 0.62–0.66 m, 1.38–1.42 m and 1.6–1.64 m;

  • Niepołomice, 8 m to the west of a grave from WW2; depths: 0.3–0.34 m, 0.66–0.70 m, 1.22–1.26 m and 1.88–1.92 m;

  • Dukla, on a gentle slope, 4 m down from a mass grave from WW2; depths: 0.56–0.60 m, 0.74–0.78 m and 1.64–1.68 m;

  • Barwinek, 2 m down from a mass grave from WW2, situated on a gentle slope; depths: 0.26–0.30 m, 0.86–0.90 m and 1.30–1.34 m;

  • Nowy Sącz, at a Jewish cemetery, close to buildings, near a mass grave from WW2; depths: 0.36–0.40 m, 0.86–0.90 m and 1.26–1.30 m.

Therefore, a total of 17 samples were collected from these sites. The depth of the sample collection resulted from differences in vertical profiles. The respective owners of land where the graves were situated gave permissions to dig near some graves.

Additional samples were also taken from other mass graves to collect material from those horizons of the profiles, where the sediments showed definite changes of color, in comparison with the color dominating the profile. It was deemed that the coloration in the ground was indicative of the presence of studied compounds, i.e. iron compounds, and that these components differentiated the sampled sediments. The presence of decomposition products was also expected in this material. Materials of this type were taken from mass graves from WW2 in Niepołomice, Zbylitowska Góra, Dukla and Barwinek and from a mass grave from WW1 in Błonie. Additional samples were collected, depending on the type of sediments, using an auger drill with a sampler. This manner of collecting 14 samples was applied to graves on land where the landowners consented to drilling. The samples were obtained from mass graves from WW2 in:

  • Niepołomice, at depths 0.5 m, 1.1 m and 1.3 m;

  • Zbylitowska Góra, at depths 0.5 m and 1.2 m;

and mass graves from WW1 in:

  • Rajbrot, at depths 0.6 m, 1.6 m and 2.2 m;

  • Błonie, at depths 0.6 m, 1.0 m and 1.6 m;

  • Brzesko, at depths 0.5 m, 1.4 m and 1.9 m.

The material collected was dried and hardened in an epoxy resin. Next, standard thin sections were cut with a diamond saw and polished. Mineralogical and petrographical observations were carried out with the use of optical polarizing microscopes (universal Olympus BX-51 and Polmi A, the latter with an attachment for taking photomicrographs at magnifications of up to 120×). The latter microscope was also used in observations of grain mounts applying side illumination (magnifications up to 60×). Using a digital, computer-controlled camera Olympus DP-12 with the analysis program and equipped with a CCD sensor with a resolution of 3.34 MP, the author observed mineralized zones of sediments at magnifications of up to 30×.

Observations were also made on samples covered with carbon using a scanning electron microscope (JEOL 5200).

Characteristics of sampling sites

Mineralogical and petrographical observations of sediments were made on samples collected from nine mass graves dating back to World Wars 1 and 2 (Fig. 1). It has been assessed that between 310 and 10,000 people (Table 1) had been buried without coffins in graves. The sites studied were situated in various physical–geographical units (Table 2). These sites were selected to take into account different substrates of extreme permeability. Both sandy and loamy sediments were included, so that difficulties associated with the determination of diversity of permeability of the substrate with depth were avoided. The fluctuations in the water table levels were established based on the observation of the area and interviews with local residents. Those located within the Sandomierz Basin include: a Pleistocene sandy terrace of the Vistula River in Niepołomice; an escarpment of a high, Pleistocene gravel terrace of the Dunajec River in Zbylitowska Góra modeled by a landslide; and a sandy, slightly inclined top surface of the Szczepanów elevation in Brzesko.

Fig. 1
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Location of the mass graves studied in SE Poland

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Table 1 Characteristics of the burials
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Table 2 Characteristics of the mass grave sites: geographical location, landforms, subsoil, land use
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The foreland of the Carpathians was represented by two sites with mass graves from WW1, i.e., Błonie near Tarnów and the cemetery (with mass grave from WW1) in Bochnia. The grave in Błonie lies in the upper part of a gently inclined slope of the foreland escarpment. The other mass grave (Bochnia site) is situated in the upper part of the loamy front of the foreland escarpment with an inclination of 6–8% and surrounded by graves from a parish cemetery.

The Carpathian Foothills is represented by two mass graves: of soldiers of WW1 in Rajbrot (Wiśnicz Foothill) and WW2 Soviet soldiers from the period of the Dukla Pass battle in 1945 (Jasło Foothill). The grave in Rajbrot is situated at an elevation, at the woodland border and dug in a sandy ground with an admixture of larger fragments of flysch sandstones from the Silesian unit of the Carpathians. The grave in Dukla lies in the middle part of a gently inclined slope, and dug in loams and clays close to other graves from WW1 and of a parish cemetery. The geographical environment of low mountains is represented by a mass grave of civilians, dug in loams and clays in Barwinek in the Low Beskid Mountains. The grave lies on a forested, flattened area in the lower part of the gently inclined slope. Investigations were also carried out in the Sądecka Basin at a cemetery containing a mass burial from the period of WW2. The site is situated on an arable, silty terrace close to the confluence of the Kamienica River and the Dunajec River in Nowy Sącz.

Two mass graves from WW1 are adjacent to other burial places: a parish cemetery in Bochnia and Jewish cemetery in Brzesko. The other two were dug outside official burial areas: in the woodland border in Rajbrot and in Błonie. Three other mass graves from WW2 were situated in woodlands outside cemeteries in Niepołomice, Zbylitowska Góra and Barwinek, whereas two others––in Nowy Sącz and Dukla––were situated close to other graves. Trees were planted after digging the latter two mass graves dating from WW1, according to the local custom. However, in the majority of the mass grave sites, only the tree trunks have remained, whereas older trees can be found in Rajbrot and Brzesko.

Results

Investigations pertaining to the occurrence of phosphorus, as well as the co-occurrence of iron compounds and clay minerals in the vicinities of the nine mass graves studied, have shown a range of certain regularities.

The iron compounds that accumulate on sand grains (mainly on quartz and feldspars) contain anthropogenic phases, for instance calcium phosphates, accompanied by calcium carbonates, e.g. in Dukla, Bochnia, Niepołomice and Nowy Sącz. Sandy environments favor the oxidation of organic matter and formation of various organic acids, including carboxylic, fatty, sulfonic and amino ones, followed by a drop of pH. These processes led to macerating or dissolving bone hydroxyapatite, which is indicated by the presence of P in the sediments of the grave sites (Pawlikowski 1995). The author has ascertained the absorption of anthropogenic phosphorus in mineral, poorly crystalline phases in Nowy Sącz and amorphous forms in Niepołomice (Fig. 2). Calcium phosphates were absorbed on well-crystalline iron oxyhydroxides and on clay minerals that rim grains of various minerals, for instance in Niepołomice and Rajbrot. Where iron hardpans (orsteins) formed, part of the (OOH)3− groups of lepidocrocite and goethite were replaced by PO4 3− anions and formed the amorphous and hydrated mineral substances identified by the author.

Fig. 2
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A zone of sand cemented with iron compounds enriched in phosphorus. Niepołomice, close to the mass grave from WW2, at a depth of 1.22–1.26 m below the surface. Polarizing microscope, crossed polaroids, magnification ×120

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If decomposition takes place in a water-saturated environment, phosphides––one of the products of decomposition––hydrolyze and phosphorus is released as PH3 and P2H4, with a simultaneous reduction of Fe3+ to Fe2+ (Żychowski 2009). Under these conditions, crystalline minerals containing Al and Fe change into non-crystalline mineral substances with better sorption properties. Therefore, a higher amount of iron and clay minerals in sandy sediments favors in situ preservation of decomposition products, for instance in Brzesko and Zbylitowska Góra (Fig. 3). The aforementioned significant features of sediments were observed in the burial environments studied (Table 3). The results obtained indicate certain interdependences between the development of grave site sediments and the presence of mineral iron compounds, clay minerals and anthropogenic phosphorus (Table 4), as summarized below:

Fig. 3
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Quartz grains (white) with clay minerals and traces of Fe–Mn compounds. Brzesko, above the mass grave from WW1, at a depth of 0.50 m. Polarizing microscope, side illumination, one polaroid, magnification ×60

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Fig. 4
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Quartz grains with coatings of iron compounds that contain traces of phosphorus. Błonie, mass grave from WW1, at a depth of 1.60 m. SEM

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Fig. 5
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Grains of quartz and aggregates of clay minerals in yellow sands with fine fragments of sandstone. Rajbrot, downslope of the mass grave from WW1, at a depth of 2.16–2.20 m, collected on 10 May 2006. SEM

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Fig. 6
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A section across a microcavern in beige clays intercalated with bluish-white clays filled with secondary substances. Barwinek, close to the mass grave from WW2, at a depth of 1.30–1.34 m. SEM

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Table 3 Petrographical characteristic of the mass grave sediments
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Table 4 Forms of occurrence of iron compounds, clay minerals and phosphorus in the mass grave sediments
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  • the amount of iron compounds enriched in anthropogenic P is higher in the more sandy horizons of the sediments, e.g., in Rajbrot;

  • iron compounds in loams occur most often at the contacts of layers with different permeability, in hollows formed by rotten roots and in minor fractures, e.g., in Barwinek;

  • phosphorus occurs in traceable amounts in sediments rich in clay minerals.

These regularities are most probably associated with the limited migration of water in such sediments, for instance in Bochnia, Dukla and Barwinek.

The iron compounds identified in the burial sites studied have the following features, in that they:

  • often represent substances with a poorly ordered structure, e.g., in Niepołomice, Dukla, Błonie and Zbylitowska Góra;

  • form microconcretions, e.g., in Rajbrot; and

  • migrate easily in groundwater, e.g., in Niepołomice.

On the other hand, in more compact sediments with low permeability, containing a substantial admixture of clay minerals, the products of decomposition of human corpses usually do not migrate, e.g., in Dukla and Barwinek. Consequently, decomposition takes place in a reducing environment while carbon with its derivatives condense within burial sites as a result of oxygen being removed from organic matter. The organic slimes formed in such a process show elevated amounts of sulfur, nitrogen, etc. This type of decomposition leads to carbonification and conservation of tissue.

The results obtained show the significant dependence between the development of the sediments and the mechanism of P binding by mineral substances (Table 5). In considering other significant features of the environment (Tables 3, 4), certain regularities have been detected, as described below.

Table 5 Preservation of plant fragments and human tissue in the mass grave sediments
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  1. 1.

    Sandy sediments, poorly cemented with clay minerals, localized on flat elevations, undergoing sporadic oscillations in the groundwater table and containing detrital grains with clay-ferruginous rims, represent an environment favoring the preservation of decomposition products, concentrated in the form of aggregates. Almost all such aggregates contain admixtures of anthropogenic phosphorus, e.g., in Rajbrot and Brzesko.

  2. 2.

    Sandy sediments characterized by frequent oscillations in the groundwater table represent an environment of fast decomposing and leaching decomposition products in the form of ions, including Ca2+, CO3 2−, PO4 3−, OH−, F− and Cl−, which are removed from the burial site, e.g., in Niepołomice (Żychowski et al. 2006a). Only on the surfaces of a few grains can the colmatage of decomposition products enriched in amorphous phosphates be seen (Żychowski et al. 2006b). Sporadically occurring clay minerals are absorption sites of decomposition products. Additionally, in the burial sites, e.g., in Błonie and Niepołomice, preserved tissue fragments can be found. The soil enriched in such organic substances shows lower absorption capacity than pure mineral sediments.

  3. 3.

    In the silty sediments containing an admixture of grains of other sizes, larger precipitates of phosphate substances are covered with trace amounts of secondary iron compounds. Sporadic oscillations of the groundwater table within the burial horizon favor crystallization of the phosphate substances, e.g., in Nowy Sącz.

  4. 4.

    In the alluvial gravels containing an admixture of clay minerals situated on a landslide-disturbed front of a high terrace of the Dunajec River in Zbylitowska Góra, mineralization with iron compounds is poor and these substances show poor structural ordering. This fact can be explained by the considerable variety in water conditions found at this site (swellings of mountainous rivers). These sediments do not contain secondary precipitates of anthropogenic phosphorus either. In turn, a higher concentration of phosphorus has been found in sediments with weak permeability that occur above the burial site in Zbylitowska Góra.

The sediments with a substantial content of clay minerals of the illite group, situated on scattered elevations and gently inclined slopes, are in some places richer with hydrated, secondary iron oxide–hydroxides that adsorb anthropogenic phosphorus. These compounds are mostly concentrated on streaks of organic substances, along borders between silt and clay sediments, on bone surfaces, in hollows formed by rotting roots, in fine fractures and sporadically among grains of detrital material, e.g., in Bochnia, Barwinek and Dukla.

The author has also found at the burial places various samples of plant tissue (Table 5) that were difficult to identify:

  • fragments of mineralized tissue, e.g., in Niepołomice;

  • organic shreds on quartz grains, e.g., in Rajbrot;

  • rotted root fragments, e.g., in Dukla;

  • plant microshreds, e.g., in Brzesko;

  • black organic fragments, e.g., in Bochnia.

A small part of organic matter must have been mixed with sediments during the digging of mass graves. The sediments studied often reveal veins or streaks of organic substances, e.g., in Barwinek and Bochnia. The same refers to alluvia, in which a thin zone of organic precipitates may be observed, e.g., in Nowy Sącz.

Microscopic observations point to significant differences in the preservation of human tissue within sediments between the graves from WW1 and WW2. In the graves from WW1, soft tissue microfragments were observed (size from several to 15-or-so micrometers), but a more detailed determination of their origin has not been possible. Also, bones occur there occasionally as fine, poorly preserved fragments, revealing secondary mineralization of their surfaces with iron oxyhydroxides (Figs. 7, 8). These small fragments have been washed down into the sediments enclosing mass graves and are proof of a migration of decomposition products outside the grave site. Single fragments of spongy bone found in sediments have been altered, and a part of these have even turned isotropic. The bone tissue fragments observed under the microscope are mineralized to various degrees with secondary iron compounds, e.g., in Brzesko and Zbylitowska Góra. These bones have also revealed signs of penetration by aluminosilicates from the enclosing sediment. Such a situation was described by other researchers who found silicates, carbonates and manganese in the bones tested (Pawlikowski et al. 2007).

Fig. 7
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A fragment of spongy bone mineralized with iron oxyhydroxides, occurring in a dark gray clay with precipitates of iron compounds. Bochnia, close to the contemporary burials, downslope of the mass grave from WW1, at a depth of 0.62–0.66 m. Polarizing microscope, crossed polaroids, magnification ×120

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Fig. 8
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A bone microfragment with signs of secondary mineralization. Bochnia, close to the contemporary burials, downslope of the mass grave from WW1, at a depth of 1.38–1.42 m. SEM

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The sediments enclosing grave sites from WW2 contain fragments of soft tissue, the amount of which varies depending on the location of the burial. Most of these fragments were observed in Zbylitowska Góra in alluvial sediments covered with impermeable clays. The tissue there is considerably altered and partly mineralized by secondary Fe compounds. The bones in Nowy Sącz are better preserved. Fragments of bone tissue observed under the microscope show only signs of incipient mineralization. However, in the spongy bones from Dukla, the places among osseous trabelculas have been filled with clay sediment. In Niepołomice, bone microfragments were also covered with other, microscopically unidentifiable products of these alterations. Fragments of altered cortex bone have been found close to the last mentioned grave.

Bone fragments have not been found in samples from a trench dug in the vicinity of the mass grave in Barwinek. This grave site is situated in clay-rich sediments (Table 3).

In eight of nine sampling sites, i.e., in Niepołomice, Rajbrot, Bochnia, Dukla, Nowy Sącz, Zbylitowska Góra and Brzesko, ion exchange has probably taken place between solutions from the grave subsoil and the bone matter, mineralized as a result into amorphous compounds, mostly iron oxyhydroxides.

During the sampling campaign, the author did not collect larger fragments of bones that were easily spotted. These were left where they lay in order to respect the dead and the grave sites. It is probably a reason that no secondary minerals, calcite among others, were observed precipitating on the outer or inner walls of such bones and in bone fractures. The same refers to the recrystallization of microcrystalline, biogenic hydroxyapatite into authigenic crystalline apatite.

Summary

Mineralogical and petrographical investigations were carried out in the vicinities of mass graves from WW1 and WW2. Mass graves were dug in sediments differing considerably in their development and represented silts, silty clays, silty and sandy clays, sandy silts and fine gravels. The grave sites also differ in their water conditions. In some mass graves, the groundwater table reaches periodically the burial horizon, e.g., in Niepołomice and Zbylitowska Góra, while in others it is situated below the burial horizon, e.g., in Rajbrot and Brzesko, or shows temporary increases, e.g. in Nowy Sącz. in Błonie, percolating water has flooded the mass grave situated in permeable sediments at the front of the foreland escarpment. In turn, a higher moisture content has been recorded from time to time in the remaining grave sites, i.e., in Dukla, Barwinek and Bochnia.

Psammitic and psephitic sediments (sands and gravels) with minor amounts of calcium carbonate, clay minerals and iron compounds favor the leaching of decomposition products from the immediate surroundings of mass graves. The distance of this transportation depends on the slope inclination, rain intensity and oscillation in the groundwater table. The presence of calcium carbonate, clay minerals and iron compounds results in precipitation and/or sorption of the decomposition products. Precipitation is also controlled by the pH of sediments and water. Changes in water conditions and pH values, characteristic of a temperate climate, lead to the formation of amorphous and, less frequently, weakly crystalline secondary phases, containing decomposition products. These new phases represent most often various forms of iron phosphates, e.g., ferric FePO4 or weakly soluble ferrous Fe3(PO4)2 varieties, if the respective ions (H2PO4 −, Fe3+ or Fe2+) are present in the environment.

The sorption of most phosphorus in the form of the PO4 3− ion takes place on secondary iron oxyhydroxides, as those amorphous compounds are positively charged. If left in the aeration zone above the stagnating water table, their internal structure tends to reorder. These compounds become less stable if the subsoil has been permeated with water, when the redox potential decreases and Fe3+ ions are reduced to Fe2+ ions, where various forms of P are released. Thus, in subsoil consisting of sands or gravels, absorption depends on an admixture of iron compounds and clay minerals. In contrast, circulation of water is limited in sediments rich in clay minerals and, in such cases in the immediate vicinity of burials, phosphorus occurs only in traces, being mostly bound in the immediate burial location. Migration of decomposition products in clay sediment is possible if the sediment has at least one of the following features that form water channels: fractures, intercalations of coarser-grained intercalations, openings resulting from rotten plant roots or tunnels bored by animals.

Mineralogical and petrographical investigations indicate that the processes of decomposition within mass graves followed by the leaching of decomposition products out of these sites depend first of all on the character of sediments in which the burial took place and on water conditions affected by landforms. The time that elapsed since the burial is significant in the assessment of how far decomposition processes have advanced and this refers to the preservation of both soft tissue and bones. Microfragments of human soft tissue are present in the sediments of the graves from WW2, e.g., in Niepołomice and Zbylitowska Góra. The presence of plant tissue in the form of small veins, shreds, thin zones of precipitation, etc., is widespread in most of the graves studied.

Discussion

Between the pH values 6.6 and 8.1, i.e., within the so-called recrystallization window, hydroxyapatite dissolves and recrystallizes again. Additionally, Berna et al. (Berna et al. 2004) suggest that below pH 7.5, the original bone apatite may be fully replaced by authigenic apatite. If pH values decrease below 7, the rapid dissolution of original or recrystallized hydroxyapatite starts. The presence of high amounts of calcium carbonates in sediments may slow down bone maceration, particularly in the environment of carbonate rocks, and result in secondary mineralization (Pawlikowski and Niedźwiedzki 2002). However, the sediments studied often show increased acidification around bones due to the presence of carbonic acid and organic acids, as well as methane. Acidification is explained by the formation of CO2 when bone collagen oxidizes, followed by the reaction of the gas with water to form H2CO3, which then dissociates. In the water environment, at pH below 6.6, bone apatite is dissolved and both P and Ca are gradually leached from the burial site as Ca2+ and PO4 3− ions (Żychowski et al. 2006a). A part of these ions in the first stage of the process, i.e., on leaving osseous trabeculas, crystallize in the marginal part of the bone being altered as secondary carbonates and phosphates, among which calcium phosphates prevail (Pawlikowski and Niedźwiedzki 2002). The variability of water conditions and pH of subsoil solutions were observed in the burial sites even over a 1-year time span, and it is characteristic of the climate of Poland. Therefore, no secondary minerals have been found as a result of petrification.

Bones are stable in alkaline environments at pH values above 8.1. Such conditions were found in sandy dunes in a semi-dry steppe (Bsk according to Koppena) in the south of Australia, where calcium and hydrogen carbonate ions are dominant. As a result, secondary calcite forms in the bones found there (Pate and Hutton 1988; Pawlikowski et al. 2007). Also, hydroxyapatite transforms into vivianite-hydrated ferrous phosphate––in a reducing environment, in the presence of decomposing human remains and iron compounds, at a relatively low pH and Eh (Maritan and Mazzoli 2004). Another phosphate, authigenic mitridatite Ca2Fe3 3+(PO4)3O2·3H2O, forms in waterlogged sediments in the presence of higher amounts of Ca and at higher pH and Eh values, as was observed, e.g., north of the Turkana Lake in SW Ethiopia (Rogers and Brown 1979). The issue of ion exchange has been noted by many bone researchers (Hassan et al. 1977; Parker and Toots 1980; Pate and Brown 1985; Sillen 1988). The conditions prevailing in the graves studied vary in the course of each year, which does not favor the emergence of crystalline forms of minerals.

There is an exchange of chemical substances among the minerals of the bones and some components of the surrounding sediment (Waldron et al. 1979; Nelson and Saucer 1984; Pawlikowski et al. 2007). Mechanical disintegration enhances penetration of the bones by water and accelerates chemical changes. Better soluble components are dissolved and their elements leached, whereas the weakly soluble compounds, e.g., of Fe, Al, Ti and Mn, are left behind. The ions present in soil solutions also reacts with major bone minerals that include dicalcium phosphate dihydrate (DCPD), octacalcium phosphate (OCP), amorphous calcium phosphate (ACP), tricalcium phosphate (TCP) and hydroxyapatite (HAP), of which hydroxyapatite is the most stable (Neuman 1980). The essential elements of human bones are thus phosphorus and calcium (Posner 1985) and both are leached out in the process of demineralization. On the contrary, the surfaces of hollows resulting from chemical or physical weathering (e.g. leaching or fracturing, respectively) can be mineralized with silica and iron or manganese compounds, depending on the environment (Pawlikowski et al. 2007).

My studies indicate that the form of sediments significantly affects the decomposition processes. An environment of sands and gravels with a small amount of clay minerals and Fe compounds favors the leaching of decomposition products out of burial sites. Obviously, the migration is stronger if the site is situated on a slope, within a zone of oscillating groundwater table. The sorption of phosphorus takes place if clay minerals and Fe compounds are present in sands and gravels, and migration is slowed down by fine-grained sediments (silts and pelites). Stabilization of the water table results in structural ordering of decomposition products present in the aeration zone: they crystallize and, therefore, are left within the burial site. Migration is possible only within the burial site if sediments of various types are mixed and loosened, but it almost comes to a halt if the clays surrounding the burial site are homogenous, without fractures and open channels formed by rotting roots. Undisturbed subsoil forms a barrier blocking the movement of decomposition products. The products of decomposition could be gradually released depending on human decisions. Too rapid a release of such products can, however, bring about a degradation of the environment. The cemeteries should not be sited in the areas where moisture levels are too variable, e.g., on periodically flooded areas or those with a too high groundwater table.

Conclusions

The formation and the presence of decomposition products of human remnants in the subsoil of mass graves depend mainly on the character of sediments, in which the graves were dug. The author has also established that the mechanism of binding phosphorus, the essential, characteristic element of mass burials affecting the natural environment, is significantly related to the grave subsoil and water conditions within the subsoil horizon. Water exerts a particular role, e.g., via hydrolysis. For this reason, the prolonged presence of a high groundwater table near the burial places, as well as its oscillations resulting in the periodical flooding of corpses are essential. It has been found that stabilization of decomposition products in situ is related to the presence of iron compounds and clay minerals, particularly in finer-grained, aleuritic–pelitic sediments.

The presence of secondary minerals and amorphous substances within the environment of mass graves suggests the migration of many ions. Further investigations will center on detailed identification of those ions and new chemical compounds, and also on establishing quantitative relations among them in various environments where mass graves are located. Extending the investigations to other regions with diversified geological and climatic conditions also seems necessary, as well as completing the list of possible environment types of mass graves. Distribution and migration of heavy elements derived from old weaponry, characteristic of some mass graves from WW2, is another problem put forward in this paper.