## 4 Discussion
in the trade activity between the former Soviet Union and the United States was also noted in the 1970s, which reflects the development of industry in the former Soviet Union. Significant changes in industry and agriculture in the former Soviet Union during the 1970s also had a dramatic impact on the earth surface of the basin. The correlation coefficients (Table 3) indicate that the significant enrichment in PTEs is related to the significant enhancement in human activities after the 1970s. The sedimentary record of Ebinur Lake in neighboring China suggests an enhancement in human activities since the early 1960s [60], while the sedimentary records of Bosten Lake [62] and Chaiwopu Lake [63] indicate an enhancement in human activities since the 1950s. This reflects spatial differences in background anthropogenic activities between different watersheds. Through this research, we discovered the relationship among the intensity of human activities and the accumulation of PTEs in the sediment core from North Aral Sea, but the specific source of PTEs was not determined. For this, the results of this research may need to be integrated with isotope tracing studies [64; 65; 66; 67], as well as source apportionment models [68; 69; 70; 71] combined with the emission inventory for PTEs in the Aral Sea basin. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Parameters** & **V** & **Cr** & **Co** & **Ni** & **Cu** & **Zn** & **As** & **Cd** & **Pb** & **LF** \\ \hline V & 1 & 0.998 & 0.997 & 0.993 & 0.983 & 0.993 & 0.961 & 0.960 & 0.990 & 0.987 \\ Cr & - & 1 & 0.998 & 0.996 & 0.989 & 0.996 & 0.966 & 0.970 & 0.995 & 0.989 \\ Co & - & - & 1 & 0.998 & 0.991 & 0.998 & 0.973 & 0.975 & 0.995 & 0.990 \\ Ni & - & - & - & 1 & 0.997 & 1.000 & 0.972 & 0.983 & 0.999 & 0.989 \\ Cu & - & - & - & - & 1 & 0.997 & 0.965 & 0.990 & 0.998 & 0.984 \\ Zn & - & - & - & - & - & 1 & 0.969 & 0.981 & 0.999 & 0.989 \\ As & - & - & - & - & - & - & 1 & 0.967 & 0.969 & 0.962 \\ Cd & - & - & - & - & - & - & - & 1 & 0.986 & 0.972 \\ Pb & - & - & - & - & - & - & - & - & 1 & 0.987 \\ LF & - & - & - & - & - & - & - & - & - & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Pearson correlation coefficients (r) among the low-frequency magnetic susceptibility (LF) and geo-accumulation index (I\({}_{\text{geo}}\)) of PTEs (n = 30, \(p<0.001\)). Figure 5: Comparison between sedimentological records and historical records of human activities. The data of magnetic susceptibility were from the sedimentary record of North Aral Sea in this study. The trade data between the former Soviet Union and the United States (million dollars) were from the unpublished source ([PERSON]. East-West Trade and Péente. 28 February, 2005: [[http://www.mv.helsinki.fi/home/miklossy/luennot/Aunesluoma.ppt](http://www.mv.helsinki.fi/home/miklossy/luennot/Aunesluoma.ppt)]([http://www.mv.helsinki.fi/home/miklossy/luennot/Aunesluoma.ppt](http://www.mv.helsinki.fi/home/miklossy/luennot/Aunesluoma.ppt))). The data about surface area of irrigated Land (million hectares) in Aral Sea basin were from the reference [61]. The PERI inferred the polluted degree of PTEs in the Lake sediments of North Aral Sea (Table S1). The results showed that \(\rm E_{r}\) values for the entire sediment core follow the ascending order of \(\rm V<Cr\)\(<\)\(\rm Zn<Co<Pb<Ni<Cu<As<Cd\). With the exception of Cd, the ecological risk of individual elements was lower than 40, suggested the ecological risks for PTEs were low. Before 1970, the average ecological risk of Cd was 29.7, lower than 40. However, after 1970, the average Cd ecological risk index increased to 114.5, suggesting the Cd constitutes a considerable risk to the ecosystem of North Aral Sea (Figure 6). The RI values in the sediment core were all lower than 150 before 1970 AD, indicating that the integrated ecological risk of PTEs was at a low risk level. Since 1970, however, it is worth noting that the RI values have exceeded 150, which represents moderate risk. In particular, at a depth of 7 cm (ca.