Spatial and temporal variation in tap water quality across Phnom Penh, Cambodia

Phnom Penh, Cambodia is a city that underwent massive turmoil
in the late 1900s. Under the Khmer Rouge regime, the city's
population dropped from over 1 million to below 50,000 and nearly all
municipal infrastructure was gutted. After decades of zero
improvement, the Phnom Penh Water Supply Authority
miraculously reconstructed a robust water network that is now
viewed worldwide as an example for developing countries like
Cambodia. Today the city is growing rapidly, and with an
increased anthropogenic footprint around Phnom Penh and the
Mekong river, clean water is more crucial than ever. Additionally,
the four-way confluence of the Mekong and Tonle Sap River
creates a hydrologic setting that varies massively month to
month, which may impact the city's water quality. Few external
studies have been done on this system, and almost no data exists
on water quality at the tap. In this study we assess the quality of
Phnom Penh's water at the point of delivery (i.e. the tap). We use
stable isotopes (δ2H and δ18O), major ions, nutrients, and trace
metals to identify spatial and temporal trends across five
sampling campaigns between June and October of 2024. Samples
were taken from 38 unique taps for a total of 109 samples that
represent most regions of Phnom Penh. We found that isotopic
data reflects seasonal shifts in flow direction at the confluence of
the Mekong and Tonle Sap rivers, and spatial variation allows us
to characterize network residence time and variations in water source
between different areas of the city. The quality of water, which is
generally high upon leaving the city's water treatment plants,
remained high quality at the tap in most samples, with common
contaminants typically falling within WHO guidelines. A number of
locations showed very high concentrations of trace metals,
including lead, which we link to water corrosivity (i.e., Cl-SO4 mass
ratio). Notably, we find Phnom Penh tap water is strongly
corrosive, which highlights significant risk of metal contamination
in areas of the city or buildings with metal piping/plumbing
fixtures.