SDG 6.3.2. Ambient Water Quality

Indicator 6.3.2: Proportion of bodies of water with good ambient water quality

Unlike Indicator 6.3.1, which focuses on assessing wastewater inputs, this indicator focuses on actual environmental water quality, to support Target 6.3’s call: “By 2030, improve water quality …” Indicator 6.3.2 is thus meant to assess which water bodies have “good ambient water quality” - a task that is complicated by the large number of potential water-quality parameters to be monitored (each with different implications for ecological and human health), and by the difficulty of determining what levels are acceptable for each parameter (i.e., what “good” means).

The UN approach to this conundrum has been to suggest which parameters should be monitored and reported, but let each country set their own thresholds for “good” water quality. The parameters of interest recommended by the UN for Level 1 (basic) monitoring are the following (with my own comments):

  • Dissolved oxygen (DO): This is a good choice because it is relatively easy to measure and interpret and is critical to aquatic organisms. 
  • pH: This is easy to measure, and could potentially capture serious pollution issues, such as discharge of alkaline tannery wastewaters or highly-acidic industrial wastes.
  • Conductivity: This is easy to measure, and could potentially identify high-salinity discharges. But water bodies vary naturally in their conductivity, so the only way to have a threshold value would be to have separate categories for freshwater, brackish water, and saltwater. 
  • Nutrients (N and P): These are somewhat harder to measure. They can provide useful information on trophic status, although given that DO is being measured, their added value is less clear. More importantly, it is notoriously hard to set thresholds of acceptable concentrations for nutrients, given the large range in natural nutrient loading and the differential sensitivity of different ecosystems to eutrophication problems. On the other hand, increasing awareness of the health effects of NO3- in drinking water (and the ubiquity of NO3- as an agricultural pollutant) justify a focus on that form of nitrogen in particular.

Obvious gaps in these recommended parameters include some measure of microbial contamination (e.g., E. coli), some measure of toxicity, and some form of direct monitoring of aquatic organisms (biomonitoring). The UN recommends these as options for Level 2 monitoring as countries are able.

target values for water-quality parameters

Acceptable levels of water-quality parameters vary dramatically by country. Source: UN report.

The threshold issue is even more confounding, as exemplified by this figure, which shows the widely-varying target values reported by different countries for the five parameters discussed above. For nitrogen, for example, levels considered acceptable by different countries range from <0.1 mg/L to 10 mg/L. This suggests that reported results, expressed as the fraction of water bodies meeting thresholds by country, are fairly meaningless.

We are still near the beginning of the process of standing up this indicator, and no doubt future reports will be more successful. But significant barriers stand in the way of a global water-quality monitoring network. Besides the complex issues of parameters and thresholds, perhaps the most important barrier to successful global assessment of water quality is that monitoring requires resources, and the countries with the most need for monitoring often have the fewest resources. Aid agencies and NGOs would often prefer to fund an intervention rather than a monitoring project. Yet a basic water-quality monitoring program is a real necessity in order to make management decisions, including where to spend cleanup funds. In addition, developing a water-quality monitoring program can help improve a country’s technical capacity, both in personnel and in lab facilities.

map of global water quality

Country-reported values for Indicator 6.3.2. Red: 0-20%; orange: 20-40%; yellow: 40-60%; green: 60-80%; blue: 80-100%. Source: UN Water.

This map shows current results from monitoring of Indicator 6.3.2, expressed as the percentage of water bodies that are meeting country-specific ambient water-quality targets - although, as suggested above, the results are difficult to interpret given the large variation in those targets from country to country.

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