## 4 Results
### Surface and Subsurface Soils  
Soil active acidity varied greatly along the sampling sites, and even within the soil profile (Table 1). The lowest pH(H2O) values (1.68-1.84) were registered in the yellow zone adjacent to the sulphide piles. Soil core 6 (transect north) of the ochre zone showed a remarkable increase in pH(H2O) with depth, changing from 3.63 to 6.29 within 30 cm. In the ochre zone the pH(KCl) was by about 0.6 units lower than the pH(H2O), which is indicative of some salt-replaceable acidity. Another potential pool of soil acidity arises from the eventual oxidation of pyrite. Pyrite occurs in most soil core samples of the yellow and white zones. It is relatively abundant in the topsoil of the yellow zone and in certain samples from the subsurface levels. In the ochre zone, pyrite is also present as a minor component. Under SEM examination, the crystals of pyrite usually exhibit dissolution pits on surfaces. Jarosite was found in soil samples with pH values ranging between 1.6 and 4.3. It is the most abundant and widespread of the AMD minerals in the soil around the waste disposal area, and responsible for the distinctive yellow colour of this proximal zone. The tidal channels are filled with jarosite-rich (over 70%) mud. Jarosite also occurs as a subordinate mineral in the white zone, whereas it is lacking in the ochre one. In most samples, the XRD pattern (Figure 4b) fits well with the standard pattern of natrojarosite, which is another jarosite-group mineral with Na instead of K. The SEM-EDS analysis confirmed that they were indeed composed of up to 7% Na2O. Fine aggregates of natrojarosite seen under SEM exhibit pseudocubic crystals less than 1 \(\upmu\)m in size (Figure 5b). Quartz, mica, kaolinite, and feldspars were found in all samples in varying amounts. These silicates are dominant in the deeper core samples of the distal zones (Figure 4c), as they are primary components of the native soil. In addition, a variety of resistant accessory minerals such as amphibole, barite, monazite, and zircon were detected by SEM-EDS. Iron oxides, notably hematite, occur mainly in the most distal zone, giving rise to the reddish-ochre colour of the soil. Halite is present in the sampling sites closest to the salt marsh, that is, in the white and ochre zones, where the tidal influence is strongest. Major and trace element concentrations in soil samples are listed in Tables 2 and 3. The soil geochemistry is largely dominated by iron (up to 51.76% Fe2O3) and sulphur (up to 49.03% S), which is compatible with the mineral composition of the acid sulphate soils. There is a good positive correlation between Fe2O3 and total sulphur (R2 = 0.73) because of the presence of pyrite and/or jarosite in many samples, except in the soils of the ochre zone where Fe is mainly in the form of oxides. Alumina is a major constituent of the soil samples from the distal zone (up to 19.11% Al2O3), reflecting the abundance of clay minerals (mica and kaolinite); hence Al2O3 appears strongly inversely correlated with Fe2O3 (R2 = -0.83). All of the other major elements appear in lower concentrations. The highest K2O content (5.74-6.48%) is observed in the tidal channel, where jarosite is the dominant phase. In the ochre zone, K2O is well correlated (R2 = 0.78) with Al2O3 due to the occurrence of dioctahedral mica. The relatively high content of Na (up to 4.53% Na2O) found in the yellow zone is linked to natrojarosite, whereas in the white and ochre zones (up to 3.45% Na2O) it is mostly related to halite. While Mg is more abundant in the ochre zone (up to 2.16% MgO), Ca is present in all samples at concentrations less than 1% CaO, its distribution being controlled largely by gypsum. In the transect south (Table 2), the highest concentrations of PTEs were measured in the surface soil adjacent to the sulphide heaps, reaching values (in mg kg\({}^{-1}\)) up to 9838 Pb, 1538 As, 1486 Zn, 705 Cu, 225 Sb, 83 Co, 13 Tl, and 4.6 Cd.