3.1. Lithologic Reconstruction
A lithological column was performed after correlating with the relevant references of nearby areas and collecting physicochemical data of the sediment samples, e.g., visual observations of the color, particle size distribution, electrical conductivity, and S content (Figure 2
). The lithostratigraphic reconstruction and landform evolution in the Kanto Plain have been studied by different parties [29
], and research there is ongoing. For this research, the sediments of different depositional environments were identified and described, along with the chemical characteristics. Marine sediment thickness was confirmed from the EC value, S content, the presence of marine shell fragments, and correlation with other references [30
]. Marine sediment is separated from nonmarine sediment by 3 m of transitional sediment (Figure 2
). The upper part consists of nonmarine sediment; it includes 2–3 m of organic matter-rich, alluvial soil, underlain by about 1.5 m of peat and organic-rich clayey silt. From a depth of 6.2 to 7.8 m, tuff mixed clays are distributed over fluvial sediment.
Fluvial sandy sediments occur from about 7.8 to 16 m in depth. They consist of yellowish-brown, medium to fine sand, which is distinct from the 3 m thick, bluish gray, medium to coarse sand and 1 m thick, light gray, fine sand. A yellowish-brown color is an indication of an oxidizing environment for sand, which is clearly distinct from the reduced, bluish-gray sand.
This thick, fluvial sandy layer can be considered as the shallow aquifer for this area. Clayey silt is present from a depth of 16 to 20 m; it could be described as transitional sediment after confirming its physicochemical data from Figure 3
and with reference [30
]. Marine sediment is distributed from 20 to 27 m.
The marine sediment consisted of light gray-colored, silty clay with fragments of marine shells. At a depth of 22 m, the content of the marine shell fragment was relatively higher than in the other part. The sediment color was bluish gray from a depth of 24 to 27 m, which is an indication of more reductive condition than here than elsewhere.
3.2. Physical and Chemical Characteristics of Soil and Sediment Leachate
The pH, EC, ORP, and OM content (%) of sediment leachate were determined. The pH ranged from a minimum of 6.37 to a maximum of 8.04. The marine sediment showed a relatively lower pH than the nonmarine sediment. In the marine sediment, the pH ranged from 6.55 to 6.95. In the fluvial sediment, the pH ranged from 6.65 to 8.04. The pH was 6.65 in peat, which was relatively lower than that of the upper and lower alluvial sediment layers. The marine sediments showed several times higher EC values than the nonmarine fluvial, tuffaceous, and alluvial clays. The EC was relatively higher than that in other part in topsoil up to a depth of 3 m. Though it is difficult to get the ORP under laboratory conditions, an attempt was made to measure it from the leachate. Marine sediment showed a relatively lower ORP value (less than 200 mV) than nonmarine sediment.
Soil Eh (ORP) less than 300 mV is generally considered to be anaerobic [45
]. As the measurement was performed on the leachate, it does not actually represent the soil/sediment ORP. The value of the ORP of the upper fluvial sediment was relatively higher (299–307 mV). The color of these samples was yellowish-brown, which is also an indication of relatively oxidized conditions compared to the other samples. The OM content was very low in the fluvial sediments. Upper alluvial and peat showed relatively higher percentages, i.e., 21% and 67%, respectively. Tuffaceous clay at a depth of 6.2 m showed a relatively higher value, i.e., 31%, whereas the range in the fluvial sediment was 1–2%. In the marine and transitional sediment, the OM content showed a range of 3–8%. However, sediment at depths of 25 m and 26 m showed relatively higher values, i.e., 23% and 16%, respectively.
3.3. Total Content of As and other Elements in Vertical Profile
The total concentration of As, Pb, Ni, Fe, Mn, and S in soil and sediment of the vertical profile is plotted in Figure 3
. The concentration range of As in the vertical profile was far below that of the general soil standard limit (150 mg/kg) [46
]. The concertation range indicates natural origin, although it is relatively higher than the world soil average content of As, i.e., 6.83 mg/kg [45
]. In the transitional sediment, the As content was 26 mg/kg at a depth of 17 m, which was higher than any other layer in the vertical profile. In the marine sediment, the range of As concentration was 7 to 17 mg/kg. It has been reported that As in lowland sites often exists in sediment, as it is adsorbed on the surface of clay minerals, iron oxyhydroxide, or arsenopyrite (FeAsS) as an impurity in framboidal pyrite [47
The distribution of Pb is similar throughout the vertical profile, except in alluvial clay. The concentration of Pb was 69 mg/kg at 3 m, which was much higher than that of the other part in the profile. The lowest value in the marine sediment was 10 mg/kg at a depth of 22 m, where the marine shell fragment content was relatively high. The world average soil content of Pb is 27 mg/kg [45
]. The average concentration in the vertical profile did not exceed the world average, except in the surface alluvial clay at a depth of 3.2–3.4 m [45
]. The Pb content in the surface soil, alluvial clay, peat, and tuffaceous clay was higher than in any other part, i.e., 117 to 158 mg/kg. The highest concentration in the profile was 158 mg/kg in the tuffaceous clay at 6.2–6.4 m. The occurrence of As and Pb in sediment in lowland sites occurs due to the adsorption on iron oxyhydroxide and clay minerals [48
]. The total Ni content in the vertical profile exceeded world soil average concentrations, i.e., 29 mg/kg, as well as the average agricultural soil concentration of Japan [45
The concentrations of Mn and Fe were high at a depth of 8 to 12 m. The color of the sediment at this depth is brownish gray to yellowish brown. It is different from other portions of the vertical profile, which were assumed to be oxidized. In the marine sediment, Mn ranged from 248 mg/kg to 728 mg/kg. The content of Mn was 395 mg/kg in peat, which is below the world average value of 488 mg/kg [45
]. The content of Mn was 1255–1386 mg/kg at a depth of 2–3 m in the surface soil and sediment, which is higher than the world average value.
The distribution of Fe in the vertical profile ranged from 3 to 7%. In the fluvial sediment, Fe content was 4 to 7%, whereas in the marine and transitional sediment, it was about 4%. At a depth of 22.2–22.4 m, the Fe content was about 3%, which was the lowest value in the profile.
The S content was 0.16 to 0.72% and 0.41 to 0.57% in the marine and transitional sediment, respectively. At a depth of 22–27 m, the content was relatively lower than the upper part of the marine and transitional sediment. In the fluvial sediment, the range was from 0.4 to 0.6%, which was much lower than the other part in the vertical profile. Generally, the S content in the terrestrial sediment was lower than 0.3%. Terrestrial sediment can be differentiated from marine sediment when it displays a range of S mass % of 0.3–3% [49
]. However, the S content was 0.76% in the peat, which was higher than any other nonmarine deposit.
3.4. Chemical Speciation and Potential Mobility of As and other Elements (Pb, Ni, Fe & Mn) in Marine and Nonmarine Sediment
In this study, four inorganic chemical speciations were determined using a four-step sequential extraction analysis. The residual concentration (residual, sulfide, and organic matter bound) was calculated by subtracting all four inorganic speciations from the total content and calculating the percentage with potential mobile fractions (Figure 4
and Figure 5
). The percentages of all four steps is shown in Figure 4
and Figure 5
. The residual and oxidizable bound percentages were more than 97–99% for As in the marine sediment. In the marine sediment, the speciation followed a trend, i.e., Fe–Mn oxide bound > water soluble > carbonate bound > ion exchangeable bound. However, in some parts, e.g., at a depth of 21 m, the trend was Fe–Mn oxide bound > carbonate bound > water soluble > ion exchangeable bound. On the other hand, in fluvial (nonmarine) sediment, the trend was Fe–Mn oxide bound > carbonate bound > ion exchangeable > water soluble. Reducible (Fe–Mn oxide) bound seemed to be the most dominant fraction after the oxidizable and residual fractions for As; this is in agreement with other studies, as the reductive dissolution or desorption of iron oxide is the main mechanism for the leaching of As in nonmarine/terrestrial environments [51
Potential mobile fractions were up to 5% at a depth of 11 m; in contrast, they were less than 1 to 3% of the total content in other parts. Fe–Mn oxide bound was the main dominating speciation between the potential mobile fractions in peat, alluvial clay, and tuffaceous clay. These were less than 2% of the total concentration. The potential mobile fractions in the fluvial environment showed maxima of 375.6 µg/L at a depth of 11.2–11.4 m, 297. 49 µg/L in tuffaceous clay at 7.8–8 m, and 34.42 µg/L in peat, which was relatively far lower than elsewhere. The highest concentration was 427.52 µg/L in the transitional environment at 17.2–17.4 m, and 234.7 µg/L in the marine environment at 21.2–21.4 m.
The potential mobile fraction of Pb was mainly Fe–Mn oxide bound in the fluvial and transitional sediment. The percentage was less than 1% of the total concentration. This indicates a lower possibility of Pb leaching from sediment, as 99% of the total content comprised residual and oxidizable fractions. On the other hand, the potential mobile fractions of Pb in marine sediment were negligible, i.e., less than 0.1%. So, the leaching possibilities of Pb from marine sediment are negligible under natural conditions.
The trend of the potential mobile fractions of Ni was different to those of As and Pb, i.e., carbonate bound ≥ Fe–Mn oxide bound > water soluble > ion exchangeable bound in the marine, transitional, and lower part of the fluvial sediment. The potential mobile fractions were less than 1% in all of the vertical profile. The percentage of the potential mobile fractions was relatively higher (0.5%) at a depth of 22 m in the marine sediment and 14 m in the fluvial sediment compared to other parts of the vertical profile. Residual and organic matter bound fractions comprised more than 99% of the total content, which indicates a low probability of leaching from sediment.
For Fe, the fractions in the core sediment followed the trend Fe–Mn oxide bound > carbonate bound, whereas water soluble and ion exchangeable fractions were unavailable from the potential mobile fractions. Residual and organic matter bound were the main parts, i.e., more than 99% of the total concentration. The higher amount of Fe in the residual and organic matter bound reflects its strong immobile bonds in crystalline structures (e.g., magnetite, goethite, and hematite) [54
]. Moreover, the speciation trend implies very poor leaching behavior of both marine and nonmarine sediment under various environments (neutral, oxidizable or reducible, etc.). The Fe–Mn oxide bound percentage was relatively high (up to 1%) in the upper part of the core, where alluvial, tuffaceous clay and peat were distributed up to 8 m from the top (Figure 5
The speciation trend of Mn was different from that of Fe in the peat; the carbonate bound was the dominating fraction after the residual fractions. The trend was Fe–Mn oxide bound ≥ carbonate bound in the marine sediment; in contrast, water solubility and ion exchanges are negligible among the potential mobile fractions. In the fluvial sediment for Mn, ion exchange was the dominant speciation; this indicates easy leaching of Mn into the environment; this trend reflects the possible mobility and leaching behavior in soil or groundwater under acidic or reducible environments. The potential mobile fractions were lower in percentage (~1%) for the fluvial sediment compared to the marine, upper alluvial, peat, and tuffaceous clay (1–2%). The presence of carbonate bound percentage in the alluvial, peat, lower fluvial, marine, and transitional sediment implies the possibility of some Mn leaching under low pH values.
3.6. Chemical Properties of Pore Water
The pore water concentrations of As and other elements are plotted in Figure 7
. The concentration of As exceeded the permissible limit of 10 µg/L [55
] for leachate and drinking water in some samples. A concentration was 29.79 µg/L was observed in the nonmarine tuffaceous clay at a depth of 7.8–8 m, which was about 3 times the permissible limit. In fluvial sediment, which was just below the tuffaceous clay, the concentrations were 23.99 µg/L and 15.95 µg/L at a depth of 8.2 and 9.2 m, respectively. The concentration exceeded the permissible limit.
Sediments at this depth might be influenced by leaching from tuffaceous clay. The sediment pore water concentration above the tuffaceous clay also exceeded the permissible limit. This might have influenced the elevated value at a depth of 11.2 m. The pore water concentrations of all other nonmarine sediment were below the permissible limits. However, the As concentration of the pore water was 39.44 µg/L at a depth of 17.2 m in the transitional sediment, i.e., about 4 times higher than the permissible limit. At this depth, the Fe concentration was also relatively higher than in other parts. Therefore, it is possible that Fe acts as scavenger for As leaching in transitional sediment, although this must be cross checked.
The concentrations of As in the pore water of the marine sediment were 53.62, 25.59, 54.5, and 41.85 µg/L at depths of 21, 24, 25, and 26 m, respectively. These concentrations were several times higher than the permissible limit. However, there was no clear relationship between the Fe and Mn concentrations for the elevated concentration of As. In this case, sulfate concentrations or other factors, such as pH or EC might play an important role in As leaching from marine sediment [48
]. The Pb concentrations in the pore water for all sediments were much lower than the permissible limit (10 µg/L). Therefore, the leaching behavior of Pb and As was different, according to their chemical characteristics. The Ni, Fe, and Mn concentrations in pore water were not so high, and no fixed permissible limits have been set for Fe and Mn in primary drinking water. However, in the tuffaceous clay and transitional sediment at depths of 6.2–6.4 m and 17.2–19.4 m showed higher concentrations than the permissible limit for Fe (300 µg/L) for secondary drinking water. The Mn concentration in the pore water showed several times higher values in fluvial, transitional, upper 2 m of marine, and tuffaceous clay than the permissible limits for secondary drinking water (50 µg/L) [56
A Pearson correlation was observed among As and other metals in pore water in terms of the total content. Other physicochemical parameters were measured in the sediment of the fluvial, transitional, and marine depositional environments (Table 2
, Table 3
and Table 4
). The number of observation points for peat, tuffaceous clay, and alluvial sediment in the nonmarine depositional environment were very few; as a result, they were not considered for Pearson correlation. Another set of correlations was measured between the pore water of As and other metals with potential mobile fractions of different depositional environments (Table 5
, Table 6
and Table 7
As content in the pore water showed a significant positive correlation with total As content in the fluvial and transitional sediment. However, these didn’t exhibit any correlation in the marine sediment (Table 2
, Table 3
and Table 4
). Moreover, a significant (p
< 0.05) positive correlation was observed with the total Fe, Mn, and pH of the leachate (R2
= 0.71, R2
= 0.76 and R2
= 0.76) and the OM % in the fluvial environment (p
< 0.01). This result indicates that the leaching of As in the terrestrial fluvial environment depends on Fe, Mn, and organic matter content. The main binding of potential mobile fractions (Figure 3
) in the fluvial sediments was also Fe–Mn oxide bound; this suggests that the leaching of As occurred in the fluvial sediment due to the change of the oxidation-reduction environment of amorphous binding with Fe and Mn oxide. The organic matter content influenced the environmental conditions. This result shows good agreement with other research, especially concerning fluvial environments such as the Bengal delta [57
]. In the transitional and marine environment, the As in the pore water did not exhibit any correlation with Fe and Mn. However, the As in the pore water showed a positive correlation with total S content (R2
= 0.95) and ORP of leachate (R2
= 0.94) in the transitional environment. In contrast, it showed a positive correlation with pH (R2
= 0.63) and significant positive (p
< 0.05) correlation with the Ni concentration in the pore water (R2
= 0.75) in the marine environment. Moreover, the total As content showed significant (p
< 0.01) positive correlation with the EC of leachate in the marine environment (Table 4
The As content in the pore water exhibited a significant (p < 0.05) positive correlation with potential mobile fractions in the fluvial and transitional environment. The As in the pore water also showed a significant (p < 0.01) positive correlation with the potential mobile fractions of Mn (R2 = 0.9) in the fluvial environment. However, there was no significant correlation between the pore water and the potential mobile fractions of As in the marine environment. The potential mobile fractions of As showed a significant (p < 0.05) positive correlation with the Mn content in the pore water and a positive correlation with the total S and pore water Fe concentration in the transitional environment.
In the transitional sediment, it seems that the leaching of As was controlled by S content and ORP. S reduction, along with As enrichment, may play important roles in leaching from the transitional sediment to environments such as pore water. In contrast, Mn plays the role of the scavenger in the fluvial and transitional environment for As. It is difficult to understand the leaching behavior of As in the marine environment, as it is different from that in the nonmarine fluvial and transitional environment. The potential mobile fractions of As had a significant (p
< 0.01) positive correlation with the fraction of Pb in the marine environment. However, this research showed that pH and EC had important effects on the leaching of As from the marine sediment (Table 6
and Table 7
). There was a significant (p
< 0.01) positive correlation between the total Pb, Ni, Fe, and Mn in the marine sediment, as most of the parts were strongly bound with residual and organic matter (Figure 3
and Figure 4
Pb had a significant positive correlation with Ni in the fluvial and transitional sediment and did not show a good relationship with total Fe and Mn content. The potential mobile fractions of Pb and Ni showed a positive correlation with the pore water concentration (R2 = 0.97 and R2 = 0.92, respectively) in the transitional sediment. Pb also had a positive correlation with the total content in the marine environment.
Pb in the pore water showed a significant (p < 0.05) positive correlation (R2 = 0.87) with the OM content (%) in marine environment. There were only a few data in the transitional sediment which affected the significance level of the correlation. Therefore, it seems that the reductive dissolution of Fe–Mn oxide bound of Pb is the main process associated with its leaching in the fluvial and transitional environments. However, a significant positive correlation with OM content (%) and total content of Pb in the marine environment implies a tendency for a low level of leaching in the pore water.
Ni in the pore water had a positive correlation without any significance with total Pb, Ni, OM content (%), and the potential mobile fractions of Ni in the transitional sediment. Ni showed a significant (p
< 0.05) positive correlation with the potential mobile fractions of Mn, pH of the leachate, pore water As, and Pb; it exhibited a significant (p
< 0.01) correlation (R2
= 0.96) with pore water OM content (%) in the marine environment (Table 6
and Table 7
This implies that the leaching of As, Ni, and Pb in the marine environment is controlled by pH and OM content (%), where organic matter plays an important role in lowering pH or reducing conditions. It is also evident that a medium degree of mobility of Ni, Mn, and As occurs under acidic or reducing environments with variable potentials [45