40 
Ganl. Bull. Singapore 70 (Suppl. 1) 2018 
these results we conclude that polluted areas do not currently pose a threat to the 
freshwater aquatic ecosystem, but this issue should be monitored and some of the 
polluted areas may require remediation. 
Stream hydrology 
The drainage system in the 4.8 km 2 Nee Soon catchment includes a third order stream 
that flows into the Lower Seletar River (Fig. 2b). Comparison with old maps and field 
observations shows that the stream has been altered drastically in the lower part of the 
catchment. For example, the main channel was straightened above where it joins the 
outflow from the spillway of the Upper Seletar Reservoir (Fig. 2b; Lower 3 section). 
Because the stream system has been so greatly altered, Murphy (1997) divided it into 
hydrological “operational units” (Fig. 2b), rather than dividing the drainage network it 
into meaningful hydrological sub-catchments (discussed below). 
Streams draining from the upper catchment typically have slope gradients less 
than or equal to about 5° and coarse sandy or sandy-loam streambeds. Streams on low- 
gradient terrain accumulate dense mats of organic material. Where the streams reach 
the swamp, they are shallow and narrow, and they move across the swamp, as evidenced 
in cores by thin layers of channel sand separated by organic-rich clay. Streams in the 
upper catchment tend to maintain flow year round, indicating a significant groundwater 
contribution that returns to the surface via springs. 
The water balance of the Nee Soon catchment is primarily driven by rainfall 
associated with two monsoon seasons, the Northeast monsoon (October to early 
January) and the Southwest monsoon (late March to May), yet rainfall is typically 
plentiful even in the inter-monsoon seasons—except in occasional droughts that 
are, for example, associated with phenomena such as El Nino (Ziegler et al., 2014). 
Mean annual rainfall at Nee Soon is 2330 mm; monthly means range from 159 to 
288 mm (http://www.weather.gov.sg/climate-climate-of-singapore/). In the simulation 
of the water balance for the catchment, modelled evapotranspiration (1100 mm) and 
stream runoff (2200 mm) represented 33% and 66% respectively of the total rainfall 
input (Sun et al., 2018; Liong Shui-Yui, Tropical Marine Science Institute, personal 
communication). 
We find that the main stream draining the upper part of the catchment has a mean 
pH value of 6.29 ± 0.38 (n = 27). Two tributary streams to the main channel have very 
similar mean values of 6.05 ± 0.27 and 6.03 + 0.44. The springs in the upper part of 
the catchment are the most acidic, with values ranging from 4.49 ± 0.18 to 4.88 ± 0.33. 
Other surface water and ground water samples have pH values less than 6.0, including 
the swamp in the mid part of the catchment (5.69 ± 0.48). We believe stream pH in 
Singapore is low because of both natural (low buffering related to acidic granite) and 
anthropogenic (influenced through acid rain) phenomena. During the last two years of 
the study, we measured rainfall pH values ranging from 3.41 to 6.35 (collected at the 
National University of Singapore). Our initial analysis of rainfall pH does not reveal 
a strong relationship between low pH and indicators of acid rainfall, S0 4 2 and N0 3 , 
