From 1970 to the end of the 1990s, fairly constant patterns of Zn
and Cu concentrations are observed in MNL2; in contrast, moderate, but still increasing, Pb concentration patterns and 206Pb/207Pb
ratios are seen (Fig. 2). Roughly similar patterns are observed for
MNL3 (Fig. 2). There are three possible reasons for the reduction
in heavy metal accumulations during this period, in spite of the population increase (Fig. 3): (1) the effect of the 1977 legal framework
for environmental protection, Presidential Decree PD1151,
(Global Environmental Forum, 1997); (2) decreases in industrial
output caused by economic crises (Fig. 3) such as the Second Oil
Crisis in 1979 and the Asian Financial Crisis of 1997, and/or political
problems; and (3) a dilution effect due to increasing sedimentation
rates. However, the pattern of economic depression is
clearly different from that observed for heavy metal concentrations
in the core samples (Fig. 3). Moreover, the observed patterns of
heavy metal concentrations and isotope ratios for MNL2 (Fig. 2)
are similar to those for MNL1, where sedimentation rates have
been constant over the last 100 years, although resolution for
MNL1 is not as good as it is for MNL2. These observations strongly
support the idea that establishment of the Presidential Decree
PD1151 was to some extent effective in reducing pollution. Moreover,
the Presidential Decree PD1151 did not ban the use of leaded
gasoline, which is a major source of Pb pollution (Callender,
2005). This could be the reason why only Pb pollution continued
to increase over the time. Consequently, it is assumed that a partial
decline in pollution was achieved from the 1970s to the end of the
1990s.