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Ganl. Bull. Singapore 70 (Suppl. 1) 2018 
Paleudult that forms on highly weathered Bukit Timah Granite (Chia et al., 1991; 
Fauziah et al., 1997). We cannot find a description of the Tengah Series, which in Nee 
Soon is associated with the freshwater swamp in the centre of the lower catchment, 
where mineral sediments and organic material have accumulated over time (Fig. 2a). 
Based on sampling this material to 2m depth, we do not believe it is a peat soil, which 
was reported in the past (Taylor et al., 2001), because the organic matter content is less 
than the 65% threshold defined by the FAO (Andriesse, 1988). The highest percentage 
of organic material we measured was about 40-50%. We therefore recommend 
referring to the Nee Soon freshwater swamp forest as an organic-rich wetland, not a 
peat swamp. Further, the Tengah Series appears to be a depositional material consisting 
of substantial clay with small layers of sand and varying contents of organic matter. 
Ives (1977) demarcated a “high ground” zone within the Rengam soils in Nee 
Soon (Fig. 2a). In agreement, we find the upper and lower portion of the catchment 
to have different soil geochemical signatures. The differences are present in the 
mineralogy of the soil in two pits in the upper and lower catchment (Fig. 2a). For 
example, the soil in the upper-catchment pit (at 250 cm depth) is composed largely 
of Quartz (79%), followed by Kaolin (17%) and small amounts of Gibbsite (3%) and 
Goethite (1%). The soil in the lower pit (at 250 cm depth) has much more Gibbsite 
(42%), and comparatively less Quartz (33%). The Kaolin content is similar (18%). 
Goethite is slightly higher (5%) and a small amount of Illite is present (1%). This 
mineralogy is typical of a highly weathered residual soil. 
Soil in the upper pit is slightly more acidic (all horizons in the 2 m profile): pH 
(determined in water) ranges from 3.4 to 4.2 versus 4.3 to 4.6 in the lower pit. Soil 
organic carbon in the 20-30 cm A horizons ranges from 2 to 5% and 2 to 3% for the 
upper versus lower pits, respectively, and discussed further by Rahman (2016). The 
texture of the B horizon in the upper pit is a sandy clay and sandy clay loam, whereas 
the B horizon in the lower pit is mostly clay (upper 1 m) and sand clay loam (lower 1 
m). The upper pit has Si0 2 concentrations of 55-69 ppm within the profile, whereas 
the ranges of concentrations in the lower pit profile varies between the upper (58-65 
ppm) and lower (35-52 ppm) 1 m halves. The lower pit soil also contains more Fe 2 0 3 
(4-10%) and Ti0 2 (0.40-0.5%) than the upper pit (2-3% and 0.1-0.2%, respectively). 
The geochemical zonation in the catchment soil is apparent in the spatial 
distribution of several elements in 227 surface and 30 subsurface samples. Several 
elements have significantly higher concentrations in the lower catchment, compared 
with the upper catchment (Mann-Whitney U-test; a = 0.05): As, Ba, Cr, Cu, Fe, Mn, 
Pb, Sr, Ti, V, and Zn (data not shown). Subsurface samples tend to corroborate the 
geochemical patterns found in the surface samples, indicating that enrichment in 
the forested lower catchment is natural. The enrichment in some heavy metals gives 
the impression of contamination in the lower catchment (shown for Cr in Fig. 3a). 
However, we believe the enric hm ent is natural, reflecting a zonation in the underlying 
granite bedrock, or some topographically controlled hydro-geomorphological process 
affecting soil chemistry occurring over very long time scales (i.e., not anthropogenic). 
The higher concentration of Fe 2 0 3 and Ti0 2 in the lower soil pit provides corroborating 
evidence that the enrichment of some associated metals is natural (assuming that oxide 
