(2) The distribution characteristics of elements in the cultural soil of Changsha ancient city.

1. It is proved that the ancient well silt represents the soil of the ancient city of Changsha more than 2000 years ago.

The black mud of ancient wells in Han Dynasty should be the soil taken from the vicinity of ancient wells more than 2,000 years ago or the domestic garbage of Changsha ancient city. Because the ancient wellhead is buried deep underground 10m, the influence of supergene action can be avoided to some extent; Judging from the reticular laterite in Mawangdui, 5 kilometers away, the found body has been preserved for more than 265,438+000 years without decay, so it can be considered that the influence of groundwater is very small under certain conditions. From the ratio of characteristic elements of ancient well sludge to surrounding raw soil (Table 6- 12), it is similar and seems to have a source relationship, that is, the sludge from ancient well landfill should come from nearby, which can represent the soil environment of Changsha, an ancient city more than 2000 years ago. In addition, judging from the composition and preservation of the soil buried in the ancient well, sawdust and straw mat filler, it seems that it is not intended to be "properly preserved", like being abandoned in the well (in fact, Qiu Donglian of Changsha Institute of Archaeology once said to the author: It is indeed abandoned).

Table 6- 10 Table of Trace Elements in Cultural Layer of Typical Sites in Changsha Area

sequential

Note: The unit of Au is 10-9, other trace elements are 10-6, TOC is 10-2, and magnetic susceptibility is 10-6.

Table 6- 1 1 Comparison of Trace Elements Contents between Ancient Well of Han Dynasty and Red Soil on Both Sides of Pinghetang Site in Changsha

Note: gold content unit is 10-9, other trace elements are 10-6, TOC and TC are 10-2, and magnetic susceptibility is 10-6.

Table 6- 12 Content Ratio of Elements in Ancient Well of Han Dynasty and Its Surrounding Soil in Changsha

In Table 6- 12, the ratio of element content in the cultural layer soil has little change with the surrounding native soil, which is close to that in the basin, but quite different from that in other basins, which is a manifestation of regional differences in geochemical composition.

2. The soil environmental conditions in Changsha in different historical periods reflected by cultural layers.

Based on the systematic comparative study of the average values of soils of multiple cultural layers in different locations in the ancient city of Changsha, and the endogenous soil (and nearby background soil) and the raw soil and topsoil in Changsha research site (Table 6- 13), it is found that the content of heavy metal elements in the soil of cultural layers is much higher than the average values of endogenous soil or raw soil and topsoil in the site as standard values. According to the enrichment index (enrichment index = content value/standard value, the standard value is the average value of two reticulated red soils in Pinghetang site, and those with index > 2 are regarded as light pollution), the order is as follows:

Western Zhou culture: characterized by high Cd( 16. 15), P( 1 1.47), Mn(8.95), Ag(4.52) and Hg(2.02).

Chinese culture: S( 12.56), Hg( 1 1.74), Au(7.3), N(5.54), P(5. 18), Cd(3.76).

Wei-Jin culture: Gao Mn( 16.04), Hg( 12.72), Ag( 10), P(8.89), Cd(4.47), Au(3.27) and Cu(2.4).

Tang Wenhua: mercury (40.82), gold (30.72), phosphorus (18. 18), silver (14.62), manganese (12.99), cadmium (9.8/.

Northern Song Culture: Mercury (150), Gold (96.2), Silver (55.65), Manganese (20.85), Tin (19.0 1), Phosphorus (15.37).

Southern Song culture: gold (85.38), silver (49. 15), mercury (39.74), tin (14), phosphorus (13.23) and manganese (13.06).

Ming culture: including Ag(40.43), Hg(32.74), P(20.63), Au( 17.5 1), Cd( 12.6 1) and Sn(7.85).

At the turn of Ming and Qing dynasties: Ag( 16.96), Hg( 12.9 1), p (1.59), Au(8.86), N(6.94), sn.

Qing culture: including Ag(5 1.86), Au(44.58), Hg(26.02), P( 15.75), CD (1 1), and Sn.

It can be seen that the soil of a specific cultural layer in Changsha was rich in heavy metals as early as the Western Zhou Dynasty, that is to say, since the "advanced human civilization" (such as smelting), there has been "pollution". The "Ancient City of Qin and Han Dynasties" left a rich cultural heritage for Changsha, but it also left traces of human impact on the environment, which generally became more and more serious with the historical process.

The accumulation trend of heavy metals in soil has obvious differences in different historical dynasties. These differences are as follows: in some areas, the cultural level has been greatly improved. For example, in the profile of the Fire Palace, the content of heavy metals in the Tang Dynasty, the Northern Song Dynasty and the Southern Song Dynasty increased abnormally, even several times higher than that in modern urban soil. The main reason for this situation may be that the Tang Dynasty, the Northern Song Dynasty and the Southern Song Dynasty were relatively developed in Changsha history, which caused heavy metal pollution in urban soil.

The results show that these polluted soils not only contain high levels of heavy metals and organic pollutants, but also have high organic carbon, total carbon and magnetic susceptibility in all cultural layers, especially in the Ming Dynasty, the magnetic susceptibility, organic carbon and total carbon were 78. 18, 28.22 and 24.93 times of the background value, respectively, and showed a synchronous upward and downward trend.

The soils contaminated by heavy metals in these deeply buried historical and cultural layers generally do not cause harm to the environment, but with the development and construction of cities, these hidden contaminated soils will be abandoned in the suburbs, and the pollutants in them will pollute groundwater through infiltration; Dissolved and absorbed by crops, endangering human health; The attached dust will also pollute the air on inhalable particles and destroy the ecosystem.

Table 6- 13 Soil Enrichment Index (Content/Standard Value) in Different Historical Periods in Changsha