Spatial variability assessment of La and Nd concentrations in coastal China soils following 1000 years of land reclamation

Purpose

Rare earth elements (REE) play a prominent role in the understanding of geochemical processes. Lanthanum (La) and neodymium (Nd) are considered as two representative examples of REE. The objectives of this study were to investigate their concentrations and spatial variability in coastal soils and to assess the effect of long-term reclamation. The coastal plain of Cixi City (China) has undergone reclamation for over 1000 years, providing an ideal place to perform such research.

Materials and methods

In total, 329 soils samples were collected by means of a nested sampling methodology, combined with grid and transect techniques. The total contents, as well as their diethylenetriamine penta-acetic acid (DTPA) extractable concentrations, were measured by an inductively coupled plasma-mass spectrometry (ICP-MS). Statistics and geostatistics tools were used to analyze their spatial variability. Combined variogram models were developed to quantitatively describe their spatial structure. Variance components were estimated to assess the proportion of their total variance captured by reclamation history and different sampling stages.

Results and discussion

Both the La and Nd concentrations showed great variability in space. Reclamation activity accounted for 42–84% of their total variance. There were significant differences in their contents between reclaimed zones (P < 0.05). They exhibited moderate spatial dependence, and the discontinuity in space generally occurred around sea dikes, as indicated by the spherical components of the La and Nd concentration variogram models. A gradually increasing tendency of elemental contents with reclamation time was observed, which was also reflected in the linear component of the variogram models. However, in the areas covered by the 3rd to the 10th zones, intensive application of phosphorus fertilizer, land consolidation, or household appliance factories led to large fluctuations of elemental contents being superimposed on the relatively homogeneous content distributions, thus causing the observed periodic features of their variogram models. Additionally, a 100-m sampling distance was found to be appropriate for future REE investigations.

Conclusions

Geostatistical modeling showed a great potential to analyze quantitatively the spatial variability of La and Nd and to assess human forces on soil genesis. The investigation of the spatial distribution of REE identified the prominent role played by the geogenic component. By means of spatial analysis, a time-series of elemental contents on a millennium scale was also reconstructed. Accumulation of their concentrations with time, as well as pollution, occurred in the surface soils. Reclamation history was the most important factor controlling La and Nd spatiotemporal variability.