Between the eruption of Somma-Vesuvius in AD 79 and today a relative rise of the groundwater table can be determined for the Sarno River plain. Consequently, in many parts of the inner plain, although terrestrial before AD 79, the pre-AD 79 paleosol has come under the influence of groundwater. To calculate this approximate rise of the groundwater table since AD 79 and to confine the zone where the primal terrestrial pre-AD 79 paleosol is recently influenced by groundwater dynamics, present-day groundwater data were combined with the pre-AD 79 topography (Vogel and Märker, 2010 [14] ; Vogel et al., 2011 [15] ).

At first, the depth of the present-day mean annual groundwater table was modelled from more than 5600 groundwater observation points (by courtesy of the Autorità di Bacino del Sarno). Thereafter, this groundwater table was subtracted from the pre-AD 79 digital elevation model (DEM) of the Sarno River plain to determine the area where the groundwater table recently lies above the pre-AD 79 soil surface. Vogel et al., 2011 [15] , modelled the pre-AD 79 floodplain or wetlands related to the paleo-Sarno River which are characterised by fluvial/palustrine deposits before AD 79. In general, wetlands show a water table that stands at or near the ter- rain surface for a sufficient period of the year to allow the development of palustrine deposits (Brinson, 1993 [19] ). Consequently, the reconstructed paleofloodplain can be considered as the area where, before AD 79, the mean groundwater table was at or near the pre-AD 79 paleosurface (Figure 1).

From these two groundwater levels (at the pre-AD 79 surface, today and before AD 79) the mean “altitude above channel network” index (AACN; SAGA terrain analysis module; 0.06% channel network density) was deduced from the pre-AD 79 DEM. The AACN calculates the vertical distance to the pre-AD 79 Sarno River and thus directly reflects the groundwater table. Finally, by subtracting the arithmetic means of the two indices the approximate rise of the groundwater table since AD 79 was determined.

At the archeological excavation of Villa Regina (Boscoreale) a soil hydrological monitoring system was in- stalled within the pre-AD 79 paleosol in natural stratification. It aims at determining if the pre-AD 79 paleosol is subject to soil water flow from the overlying volcanic deposits either vertically by percolation or laterally by interflow. The soil moisture and the soil liquid phase were determined in-situ by means of frequency domain re- flectometer (FDR) and tension lysimeters (suction cups), respectively. Tension lysimeters were used since this methodology predominantly samples the mobile fraction of soil water that moves through inter-aggregate pores or preferential flow channels and thus reflects the chemical transport within the stratigraphy (Wolt, 1994 [7] ). Due to the depth of the pre-AD 79 paleosol of about 8 m a vertical installation from the present-day surface was not feasible. Consequently, the installation was implemented laterally inside the natural undisturbed stratifica- tion of the side walls of the archaeological excavation of Villa Regina (Figure 2).

Altogether four sampling systems were installed, two within the lower section of the AD 79 pumice fallout layer and two within the pre-AD 79 paleosol underneath. The composition of the soil liquid phase may not di- rectly refer to specific chemical processes active in the pre-AD 79 paleosol. Nevertheless, the net effect of recent

transport of nutrients into the paleosol is manifested in the concentration of substances in the soil water extract (Wolt, 1994 [7] ). Consequently, the main objective of the given sampling design is to determine if a soil water flow to the pre-AD 79 paleosol occurs. Moreover, we provide data of the general trends in the soil liquid phase chemistry, i.e. the occurrence of certain ions in the lysimeter solution at the point of sampling (Grossmann and Moss, 1994 [20] ; de Vries and Leeters, 1994 [21] ; Wolt, 1994 [7] ; Manderscheid and Matzner, 1995 [22] ).

Due to the remarkable depth of burial and the absence of interactions with present-day plant roots the compo- sition of the soil liquid phase within the pre-AD 79 paleosol will be less heterogeneous compared to the rooting zone of the modern soil. This may also be favoured by the relative chemical homogeneity of the AD 79 pumice fallout layer overlying the pre-AD 79 paleosol. Consequently, four sampling systems are expected to yield qua- litative data on the soil liquid phase chemistry of the sampling area.

In the paleosol the suction cups were installed in slurry of the paleosol material to ensure an optimal contact between the instrument and the surrounding soil and a minimum of disturbance. Because of the coarseness of the AD 79 pumice lapilli layer above, the suction cups could not be installed in its own material. Hence, silica slurry of fine sand was used having a low adsorption capacity to minimise the influence of the slurry material on the soil liquid phase.

To extract the soil liquid phase a transient tension (vacuum) of 550 mbar (pF 2.7) was established biweekly to the suction cups by evacuation with an external pump. Due to a slight decrease of tension during the two weeks between evacuations an applied tension of 550 mbar enables the sampling of the soil liquid phase near field ca- pacity. Even though the gradient generated by the suction cups will not exclusively act on macropores but to some extent also on smaller pores (Grossmann and Udluft, 1991 [23] ) the applied tension predominantly sam- ples the mobile soil water fraction. The approximate pore diameter that is drained at a certain soil water potential can be calculated by means of the law of capillary rise or the Young-Laplace equation:

where r is the radius of the capillary tube, the surface tension of water, the contact angle, h the water potential, p_w the density of water and g the gravitational acceleration (Scheffer and Schachtschabel, 2010 [24] ). Hence, applying a tension of 550 mbar and assuming complete wettability (= 0) of the substrate the suction cups extract water from soil pores of a minimum diameter of 5.4 µm which corresponds to intermediate sized pores.

Through the introduction of oxygen into the soil during installation of the suction cups the biological activity of the paleosol may be stimulated causing a mobilization of nutrients. This can influence the composition of the soil liquid phase directly after installation (Grossmann, 1988 [25] ). For this reason, the obtained lysimeter solu- tion was rejected for the first months after installation. From the installation to the first soil water sampling the system had about three months to re-equilibrate.

The suction cups were installed 1.5 m inside the stratigraphy to prevent them from being influenced by water entering the soil profile from the open excavation side of Villa Regina. Furthermore, the installation was carried out on the upslope part of the excavation assuring that rain water directly entering the excavation is diverging. Additionally, the entire installation was covered by a roof over its whole length (Figure 2).

Suction cups were used that collect the solution in their interior. That has the advantage that the extracted wa- ter stays cool and protected from sunlight until the sampling takes place, minimising chemical transformations within the lysimeter solution. Due to its specific adsorption characteristics the filter material of the suction cup may have an influence on the composition of the sample. Consequently, the investigation of a certain analyte requires a particular filter material. As this study focusses on dissolved inorganic substances polyamide filters were used (Litaor, 1988 [26] ; Grossmann, 1988 [25] ; DVWK, 1990 [27] ; Haberhauer, 1997 [28] ; Spangenberg et al., 1997 [29] ; Tischner et al., 1998 [30] ).

The composition of the soil liquid phase was monitored over a period of 20 months to cover the variability throughout the year. The autumn and winter period was studied twice. This period is believed to be most rele- vant because the precipitation maximum and relatively low evapotranspiration between October and March is most likely to cause soil water flow down to the pre-AD 79 paleosol. The soil liquid phase in the suction cups was controlled biweekly, sampled and analysed for: 1) pH value, 2) major cations (Ca²⁺, Mg²⁺, K⁺, Na⁺) and anions (, , , Cl^?) because of their importance for soil fertility and nutrient conditions and 3) Al³⁺ because of its relevance in terms of the acidity status and plant physiology (Al-toxicity) (Hendershot and Courchesne, 1991 [31] ; Wolt, 1994 [7] ; Fillion et al., 1999 [32] ; Derome, 2002 [33] ; Roman et al., 2002 [34] ; Strahm et al., 2005 [35] ).

Since the composition and amount of the soil liquid phase sampled by the tension lysimeters varies with soil water content, additionally, three soil moisture sensors (SM200) where installed in the paleosol measuring the volumetric water content (θ) every six hours. The SM200 soil moisture sensor is a frequency domain reflecto- meter (FDR) measuring θ indirectly by determining the permittivity of the soil (Delta-T Devices Ltd., 2006 [36] ).

The soil liquid phase of the vadose zone strongly depends on water deriving from precipitation. To get a rough estimate of how the precipitation regime interacts with the soil moisture of the pre-AD 79 paleosol and its soil water chemistry throughout the year, meteorological data were collected (Campania Region). It may not be possible to relate a single precipitation event to the soil water sampled in the paleosol at a depth of 8 m. Never- theless, especially the time between the first precipitations in autumn and the first response in the paleosol’s soil moisture as well as the first soil water sampling will be of particular interest. Moreover, the first soil water flow after the dry summer season is expected to have a strong influence on the composition of the soil liquid phase in terms of higher element concentrations that are leaching into the pre-AD 79 paleosol (Arthur and Fahey, 1993 [37] ).

At Scafati, Pompeii and Boscoreale modern soils and associated pre-AD 79 paleosols were identified in four stratigraphies (DAI16, DAI18, VdM, VdR). They were described in the field and the soil solid phase was sam- pled for the subsequent soil physical and chemical laboratory analyses.

Following generally Retallack, 1998 [2] , 2001 [3] and French, 2003 [4] , as well as in particular Foss, 1988 [9] , Foss et al., 2002 [10] and Inoue et al., 2009 [11] , we hypothesise that post-burial soil changes in the pre-AD 79 paleosols may above all be associated with nutrient transport from the overlying strata and decomposition of soil organic matter (SOM). Hence, to determine post-burial soil developments, the soil solid phase of both the pre- AD 79 paleosols and the associated modern soils were analysed and compared. Thereby, the modern soils were taken as a reference for an unburied volcanic soil.

To assess the effects of soil water flow and associated nutrient transport on the mineral soil properties the fol- lowing analyses were carried out: 1) soil pH measured in 0.01 M CaCl₂ (DIN ISO 10390 [38] ); 2) effective cation exchange capacity (CEC_eff) and the amount of exchangeable cations (K, Na, Ca, Mg, Mn, Al and Fe) by sequential extraction with unbuffered ammonium chloride; and 3) total inorganic carbon (TIC) by elementary analysis using the dry combustion reference method (DIN EN 13137 [39] ).

To study the post-burial decomposition of soil organic matter in the paleosols total organic carbon (TOC) was determined by elementary analysis using the dry combustion reference method (DIN EN 13137 [39] ) as well as total nitrogen using the modified method of Kjeldahl (DIN ISO 11261 [40] ).

As mentioned earlier Foss, 1988 [9] , stated that the environment for soil development before and after AD 79 was similar resulting in a relative similarity between the paleosols and the modern soils around Pompeii. Hence, for this comparison we presuppose similar climatic conditions before AD 79 and today. This is indicated by Sa- dori and Narcisi, 2001 [41] , from lacustrine sediments in Sicily as well as from Buccheri et al., 2002 [42] , from sea core data of the Salerno Gulf (southern Tyrrhenian Sea). However, similar vegetational conditions at the in- vestigated sites have to be hypothesized as they are more difficult to determine. Isotope analysis of δ13C and δ15N of the pre-AD 79 paleosols and the modern soils revealed a predominance of C₃ vegetation before AD 79 and today (Oelmann and Ruppenthal, 2012, written communication). Moreover, the comparison between the pre- AD 79 paleosols and the modern soils was only carried out at the same topographic location of each sampling point. This eliminates the influence of the relief as a soil forming factor in explaining soil variations.

The particular chemical composition of the modern soils and the pre-AD 79 paleosols may differ due to a slightly different geology of the parent volcanic material as well as due to anthropogenic input in terms of fertil- izers or waste materials. This may falsify the comparison of the two soils. To characterise the chemical compo- sition of the soils, total amounts of Na, Ca, K, Mg, Mn, Al and Fe were determined by element analysis using aqua regia (nitrochloric acid) and AAS /ICP-AES. To eliminate the effect of a different geology of the soil sub- strate and anthropogenic input ratios were introduced correcting a particular measured element concentration by its total concentration (see Equations (2) and (3)).

The CEC ratio allows for a comparison of the nutrient status of the paleosols and the modern soils even if both soils exhibit different total element concentrations. A consistently higher CEC ratio in the paleosols may for instance indicate nutrient input via leaching. As CEC is the sum of exchangeable cations the CEC ratio was calculated by dividing the cation exchange capacity by the sum of the total concentrations of K, Na, Ca and Mg (Equation (2)).

The nutrient ratio is used for the comparison of a particular nutrient cation, hence dividing the amount of an exchangeable cation (K_exch, Na_exch, Ca_exch, Mg_exch) by the total amount of the respective element (K_tot, Na_tot, Ca_tot, Mg_tot) (Equation (3)). Thus, the nutrient ratio helps to identify which nutrients are predominantly leaching from the volcanic deposits into the paleosols.

By means of division by the total element concentrations, the CEC ratio and the nutrient ratios also eliminate methodical defects of the CEC method with respect to CaCO₃ contents > 10 g∙kg⁻¹ which may cause dissolution of CaCO₃ and increased values of CEC and exchangeable Ca²⁺ (Dohrmann and Kaufhold, 2009 [43] ).

For the comparison between the modern soils and the pre-AD 79 Roman paleosols only topsoils were consid- ered as in general they are characterised by high SOM and nutrient turnover rates as well as by major soil chemical reactions due to plant roots and microorganisms. Finally, considering the results of both the soil liquid and soil solid phase analysis major differences between the unburied modern soils and the buried paleosols are utilised to gain valuable insights into potential post-burial soil developments of the pre-AD 79 paleosols.

The combination of the present-day groundwater table with the pre-AD 79 topography reveals that a large area of the inner Sarno River plain is affected by a groundwater table lying at or above the pre-AD 79 paleosurface (Figure 3). Altogether this comprises the area near the river network which amounts to around 75 km² or 37% of the entire Sarno River plain. However, this includes an area of about 25 km² or 32 % where the pre-AD 79 layer was originally (before burial) characterised as a terrestrial paleosol. Consequently, there must have been a considerable rise of the groundwater table from AD 79 to the present day. This post-AD 79 groundwater rise was determined to approximately 1.8 m.

Tables 3 and 4 show the results of the soil solid phase analysis for the modern soils and the associated pre-AD 79 paleosols. Only when a soil parameter shows the same trend in all four pairs of soils, we assumed this to be the result of post-burial effects.

The modern soils are well penetrated by roots and soil fauna and generally appear loosely structured. In con- trast, the paleosols have a massive structure with little recognizable macro porosity. The mean bulk density of the paleosols is 1.4 g/cm³ and that of the modern soils is 1.1 g/cm³. The Munsell soil colour (on moist samples) of the modern soils is very dark gray and that of the paleosols is dark grayish brown. The pH values of the mod- ern soils are between 5.8 and 7.4 and of the paleosols between 7.1 and 8.7. Furthermore, in contrast to the mod- ern soils, in the paleosols the pH value is not consistently increasing with increasing depth. The CEC ratios (Equation (2)) of the paleosols are higher compared to the modern soils. This coincides with higher nutrient ra- tios (Equation (3)) for Ca and K. The modern soils have low to moderate soil organic matter (SOM) contents whereas in the pre-AD 79 paleosols it is very low to low. This corresponds to amounts of total organic carbon (TOC) of 8 to 16.3 g∙kg⁻¹ for the modern soils and 3.1 to 8.9 g∙kg⁻¹ for the paleosols. Accordingly, nitrogen ranges between 0.5 and 2.6 in the modern soils and between 0.1 and 1.7 in the paleosols.

As mentioned above, after AD 79 the groundwater table increased by about 1.8 m. To some extent, this can be an indirect result of the AD 79 eruption. The burial of the Sarno River plain is believed to have left a bare land- scape free of vegetation and covered with unconsolidated volcanic material. Consequently, due to these unstable conditions especially the surrounding steep mountain slopes were vulnerable to severe erosion processes during intense rainfall events causing debris flows and landslides (Cinque et al., 2000 [45] ; Cinque and Robustelli, 2009 [46] ). Even though the AD 79 pumice lapilli layer show a very high hydraulic conductivity, the above PDC deposits consist of fine tephra that, together with intense rainfall, can generate infiltration excess overland flow and thus erosion. Erosion and the destruction of the vegetation cover cause a reduction of the water reten- tion capacity which, in turn, results in increased runoff and an increasing groundwater table in the adjacent river plain (Vogel and Märker, 2012 [18] ). The latter may also be caused by the accelerated deforestation and the clearance of the natural vegetation since the Middle Ages (around 700 BP, see Schneider, 1985 [47] ; Buccheri et al., 2002 [41] ; Russo Ermolli and di Pasquale, 2002 [48] ).

Considering this groundwater rise, the Sarno River plain must be divided into two different zones of potential post-burial soil developments:

1) the lower areas of the inner Sarno River plain where after AD 79 the originally terrestrial pre-AD 79 pa- leosol has come under the influence of a rising groundwater table; and

2) the higher areas of the Sarno River plain where the paleosol is still part of the vadose zone and more likely influenced by vertical or lateral soil water flow.

This demonstrates that the mechanism of post-burial soil developments being active in the pre-AD 79 paleo- sols is by no means uniform for the entire Sarno River plain but strongly depends on the paleotopography and the paleolandscape position.

Vogel and Märker, 2012 [18] , studied the effect of the post-burial groundwater rise on the originally terres- trial pre-AD 79 paleosols. Under the present-day influence of groundwater dynamics the paleosols are character- ised by: 1) higher amounts of organic carbon and nitrogen; 2) a darker Munsell colour of the buried topsoil; 3) lower amounts of calcium carbonate; 4) increased sulphate concentrations together with a decreased pH value; and 5) redoximorphic features in the overlying AD 79 pumice fallout layer.

From the relative stratigraphic position of the pre-AD 79 paleosols and the modern soils and by comparison with the eruption history of Somma-Vesuvius an estimation of their approximate soil age was made. With respect to the paleosols, the total soil age has to be subdivided into two main soil development periods, i.e. the duration of pre-burial and post-burial soil development. The parent volcanic material at Boscoreale (VR) and Pompeii (VdM) can most likely be ascribed to the Avellino eruption or the following AP1/AP2 eruption (3450 - 3000 BP; Andronico and Cioni, 2002 [63] and references therein). At Scafati (VdR) the pre-AD 79 paleosol developed from the later AP eruption products of Somma-Vesuvius (AP3 - AP6) which were dated between 2.710 ± 60 BP (Rolandi et al., 1998 [64] ) and 217 BC (Stothers and Rampino, 1983 [65] ; Rolandi et al., 1998 [64] ). Conse- quently, the pre-burial age of the pre-AD 79 paleosols, i.e. the time they developed upon the ancient land surface, ranges between 1050 and 1500 years for Boscoreale and Pompeii and between 300 and 800 years for Scafati. In contrast, the post-burial soil age of all pre-AD 79 paleosols is approximately 1900 years. Thus, they existed for a much longer period of time under buried than under unburied conditions. This fact demonstrates the necessity to critically evaluate the possibility of post-burial soil developments in the paleosols before drawing conclusions on their ancient soil characteristics. Considering both, pre-burial and post-burial duration, the total soil age of the pre-AD 79 paleosols can be estimated between 2200 and 3400 years. According to its tephrostratigraphic position the age of the modern soils is between 1300 and 1900 years as they are underlain by the pyroclastic fallout of a medieval eruption of Somma-Vesuvius (DAI16, VdM, VdR) and reworked ash layers of the final stage of the AD 79 eruption (DAI18).

The results of the soil solid phase analysis show distinct characteristics that distinguish the topsoils of the buried pre-AD 79 paleosols from the unburied modern soils at the same topographic location. The paleosols have a pH value that is more than one unit higher compared to the modern soils. This corresponds with the ob- servations of Foss, 1988 [9] , Foss et al., 2002 [10] and Inoue et al., 2009 [11] . The lower pH values of the mod- ern soils most likely result from:

1) Active accumulation and decomposition of soil organic matter (SOM) producing organic acids;

2) Nutrient uptake and proton release of plant roots;

3) Acid deposition from the atmosphere (acid rain); and

4) Leaching of basic cations from the upper horizons by percolating soil water.

In contrast, the results of the soil liquid phase study verify that the pre-AD 79 paleosols are rather subject to post-burial nutrient input by leaching from the overlying deposits which increased their pH value. This confirms the earlier mentioned assumption of Foss, 1988 [9] , Foss et al., 2002 [10] and Inoue et al., 2009 [11] . However, as described above this process only takes place during the rainy winter season when mobile soil water is pre- sent. Furthermore, in the paleosols the pH decreasing influence of plants and active accumulation of SOM is missing.

It was determined that the pH of the paleosols often does not consistently increase with depth as can be seen for most of the modern unburied soils. After burial, the former A-horizons of the paleosols are subject to ion accumulations from the overlying strata. Hence, the boundaries between zones of leaching and accumulation can gradually blur (Holliday, 2004 [66] ).

The post-burial nutrient accumulation in the paleosols also results in higher CEC ratios compared to the mod- ern soils. With regard to the nutrient ratios only Ca and K are clearly enriched in the paleosols. Hence, Ca and K seem to be the dominant base cations to be transported through the profile which is congruent with the results from the soil water analysis. This only partly corresponds to Foss, 1988 [9] and Foss et al., 2002 [10] , who pre- sumed that especially Ca²⁺ and Mg²⁺ leach from the overlying volcanic sediments and recharge the surface ho- rizons of the paleosols. Due to the neutral to slightly alkaline pH value of the pre-AD 79 paleosols the surface charge of the soil colloids is expected to be predominantly negative. Thus, adsorption of anions to the exchange sites of the paleosols is negligible. Hence, they are rather leached out of the soil profile.

The soil organic matter (SOM) contents are low to moderate for the modern soils and very low to low for the paleosols. Thus, the amount of organic carbon (TOC) is approximately 7 g∙kg^?1 lower in the paleosols. Macro- scopically, this agrees with a Munsell soil colour (on moist samples) having higher value and chroma, i.e. very dark gray compared to dark grayish brown. According to TOC, the paleosols also contain lower amounts of ni- trogen than the modern soils. The decreased concentrations of TOC and N in the paleosols may be due to post- burial decomposition of SOM. With burial in AD 79 the accumulation of SOM within the paleosols has stopped. As the AD 79 pumice fallout layer covering the pre-AD 79 paleosols is well aerated no air exclusion took place. Hence, microbial activity to gradually decompose SOM is not stopped even though it may have slowed down. This is supported by Crowther et al., 1996 [1] , who studied post-burial change in a humic rendzina soil in Eng- land. They state that decomposition reduced the amounts of TOC by about 29 % in only 32 years after burial. Only in extremely anaerobic conditions SOM may be preserved from decomposition after burial (see Dimbleby, 1984 [67] ). Hence, Crowther et al., 1996 [1] , conclude that the present-day concentrations of TOC in the buried soils are not reflecting their original SOM status.

In the Sarno River plain, a second reason for reduced amounts of TOC in the paleosols may be due to interac- tion with hot volcaniclastic fallout during the AD 79 eruption. To some extent, heating of the pre-AD 79 paleo- sols during contact with the AD 79 pumice fallout may have caused sublimation of SOM. Thomas and Sparks, 1992 [68] , modelled the process of tephra cooling during fallout from eruption columns. They found that the heat loss of clasts decreases with increasing grain size and decreasing fall height. Whereas smaller clasts are deposited cold, they conclude that larger clasts of Plinian fallout can retain enough heat to pose hazards to life and property by igniting fires.

Finally, soil structure and bulk density of the modern soils and the pre-AD 79 paleosols are used as an indica- tor of soil compaction. Already from the macroscopic comparison of the two soils it is evident that the pre-AD 79 paleosols show a massive soil structure with little recognizable secondary porosity, i.e. structure-dependent porosity. This may result from: 1) the superimposed load of 3.3 (VdR) to 8 m (VR) of overlying post-AD 79 deposits; 2) the absence of root penetration; and 3) the reduction of active soil biota such as earthworms. How- ever, as shown by the soil moisture and hydraulic conductivity measurements, no waterlogging occurred in the paleosols as the texture-dependent primary porosity of the sandy loam still permits soil water movement. In contrast to the paleosols, the unburied modern soils are well penetrated by roots and soil fauna and generally appear more loosely structured. This is in accordance with a slightly increased mean bulk density of 1.4 g/cm³ for the pre-AD 79 paleosols compared to 1.1 g/cm³ for the modern soils.