Nepal Drinking Water Quality Standard (NDWQS): Parameter Reference Table

The National Drinking Water Quality Standard (NDWQS) is Nepal's official benchmark for safe drinking water. It was promulgated in the Nepali year 2062 Bikram Sambat, corresponding to 2005 AD, and is issued under the authority of the Water Resources Act 2049 (1992). The standard was prepared through the then Ministry of Physical Planning and Works and is today administered in the drinking-water sector by the Department of Water Supply and Sewerage Management (DWSSM) under the Ministry of Urban Development.

The NDWQS defines the maximum permissible concentrations for a fixed list of physical, chemical and microbiological parameters. Any water supply agency, municipality, community water-users' committee or bottled-water producer supplying water for human consumption is expected to deliver water that conforms to these limits. Alongside the standard, the government issued the 'Implementation Directives for National Drinking Water Quality Standards, 2062', which describe monitoring frequency, sampling and the institutional mechanism for enforcing the values.

Because the official standard exists mainly as a Nepali-language government notice and a scanned PDF, it can be hard for students, laboratory technicians, NGO field staff and ordinary consumers to read. This page reproduces the full parameter list in plain English, converts nothing, and places each Nepali limit next to the World Health Organization (WHO) guideline value so readers can see where Nepal follows WHO and where it is deliberately more lenient.

Every NDWQS value is a maximum concentration or a permissible range, not a target. If a sample result is at or below the listed number, the parameter passes; if it is above, the water fails for that parameter. Concentrations of dissolved substances are given in milligrams per litre (mg/L), which is numerically the same as parts per million for dilute water. Turbidity is measured in Nephelometric Turbidity Units (NTU), colour in True Colour Units (TCU), electrical conductivity in microsiemens per centimetre, and bacteria as Most Probable Number per 100 millilitres (MPN/100 mL).

A distinctive feature of the NDWQS is the parenthetical 'acceptable when no alternative source is available' value. For a few aesthetic parameters, the standard lists a strict everyday limit and, in brackets, a higher figure that may be tolerated where communities genuinely have no better water source. For example, turbidity is 5 NTU but up to 10 NTU may be accepted, colour is 5 TCU with up to 15 TCU tolerated, and iron is 0.3 mg/L with up to 3 mg/L tolerated in the absence of an alternative. These relaxations apply only to characteristics that affect taste, appearance or acceptability, never to toxic parameters such as arsenic, lead or cyanide.

Two values in the standard are ranges rather than single ceilings. pH must fall between 6.5 and 8.5, and residual (free) chlorine at the point of consumption should be between 0.1 and 0.2 mg/L to keep the water disinfected without an objectionable taste. Fluoride is also expressed as a band, 0.5 to 1.5 mg/L, because a small amount protects teeth while excess causes fluorosis.

The physical parameters describe how water looks, tastes and behaves rather than a specific poison. Turbidity, the cloudiness caused by suspended particles, must not exceed 5 NTU (with 10 NTU tolerated where no alternative exists); low turbidity matters because particles shelter microbes and reduce the effectiveness of chlorination. Colour is capped at 5 TCU (up to 15 TCU when unavoidable), and taste and odour must be 'not objectionable'.

The standard limits Total Dissolved Solids (TDS) to 1000 mg/L. TDS is the combined weight of all dissolved minerals and salts; water above this figure often tastes salty or bitter and can indicate contamination, although moderate TDS is harmless and even desirable. This is the figure people mean when they search for the 'safe water TDS standard in Nepal'. Related to it, electrical conductivity, an easy field proxy for dissolved ions, is limited to 1500 microsiemens per centimetre.

pH is required to lie between 6.5 and 8.5. Water that is too acidic corrodes pipes and can leach metals such as lead and copper, while very alkaline water tastes soapy and reduces chlorine efficiency. These physical limits broadly match WHO's acceptability guidance, which sets 5 NTU as a treatment target for effective disinfection and treats TDS, colour and pH as aesthetic rather than strictly health-based concerns.

The chemical parameters carry the greatest health weight because several are toxic even at low doses. Arsenic, the contaminant most searched for in the context of Nepal's Terai groundwater, has an NDWQS maximum of 0.05 mg/L. This is the single most important difference from WHO: the WHO provisional guideline value for arsenic is 0.01 mg/L, five times stricter. Nepal retained the older 0.05 mg/L figure because meeting 0.01 mg/L across millions of tube wells was not immediately feasible, so many programmes treat 0.05 mg/L as the enforceable limit while flagging wells above 0.01 mg/L for priority action.

Fluoride is set at 0.5 to 1.5 mg/L, matching the WHO upper guideline of 1.5 mg/L; below this range teeth get little protection and above it dental and skeletal fluorosis appear. Nitrate is limited to 50 mg/L (as nitrate), identical to WHO and important because high nitrate from sewage or fertiliser can cause 'blue-baby syndrome' (methaemoglobinaemia) in infants. Iron (0.3 mg/L, up to 3 mg/L where no alternative exists) and manganese (0.2 mg/L) are largely aesthetic, staining laundry and giving a metallic taste.

For the clearly toxic heavy metals, the NDWQS aligns closely with WHO: cadmium 0.003 mg/L (equal to WHO), chromium 0.05 mg/L (equal to WHO's provisional value), lead 0.01 mg/L (equal to WHO), mercury 0.001 mg/L (stricter than WHO), and cyanide 0.07 mg/L. Other chemicals include ammonia 1.5 mg/L, chloride 250 mg/L, sulphate 250 mg/L, copper 1 mg/L (WHO allows 2 mg/L), total hardness 500 mg/L as calcium carbonate, calcium 200 mg/L, zinc 3 mg/L, aluminium 0.2 mg/L, and residual chlorine 0.1 to 0.2 mg/L.

Microbiological safety is the most immediate health concern in Nepal, where diarrhoeal disease is common. The NDWQS is strict here and allows no relaxation. Escherichia coli (E. coli), the definitive indicator of recent faecal contamination, must be zero (0 MPN per 100 mL). Any detectable E. coli means the water is not safe to drink without further treatment such as boiling, chlorination, filtration or solar disinfection.

Total coliform bacteria, a broader indicator group, must also essentially be absent. The standard requires 0 MPN per 100 mL, with the practical monitoring expectation that at least 95 percent of samples taken over a monitoring period contain no coliforms. This '95 percent of samples' phrasing recognises that occasional single positives can occur in a large distribution network, but persistent or widespread coliform presence signals a system fault.

Because microbiological quality can change within hours, the Implementation Directives call for these bacterial parameters to be tested frequently, typically monthly, and far more often than most chemical parameters, which change slowly. For household users the key takeaway is simple: clear-looking water can still contain E. coli, so appearance alone never confirms safety.

Nepal's standard is modelled on the WHO Guidelines for Drinking-water Quality, currently in their fourth edition, but it is not identical. For most heavy metals and for nitrate and fluoride the two are the same, giving Nepali consumers WHO-equivalent protection. The headline divergence is arsenic, where Nepal's 0.05 mg/L is five times more permissive than WHO's 0.01 mg/L; this reflects the enormous cost of retrofitting arsenic removal across Terai tube wells rather than any belief that 0.05 mg/L is fully safe.

It is worth understanding the status of the WHO figures used for comparison. WHO describes several values as 'provisional', including arsenic (0.01 mg/L) and chromium (0.05 mg/L), meaning they are constrained by measurement or treatment limits rather than by health data alone. WHO also treats iron, zinc, chloride, sulphate, TDS, colour and aluminium as aesthetic parameters without formal health-based guideline values, so the Nepali limits for those substances are best read as acceptability thresholds, not danger levels.

In short, passing the NDWQS means water is legally safe in Nepal, but for the most vulnerable users, especially infants and pregnant women drinking arsenic-affected groundwater, aiming for the tighter WHO arsenic value of 0.01 mg/L offers extra protection. Consumers should also remember that a standard is only as good as its testing; a well or tap that has never been analysed has an unknown, not a safe, status.