Understanding water quality

Water quality improvement is the main purpose of constructed wetlands. But what exactly does good water quality mean?

A simple definition of water quality is easy: water quality refers to the biochemical properties of a given water. Improving water quality is the purpose of constructed wetlands – in the eyes of their designers and builders. 

Where the picture becomes complicated is in judging this water quality improvement. Which properties of water are most crucial? How do we judge the values of these properties as good or poor, adequate or sub-standard? I would argue there’s no final judgement possible on whether the constructed wetlands have improved water quality, only situated knowledges, reflecting different water uses and priorities.

We could start with local understandings of water quality, as I understand them after a series of interviews in both study villages. What emerges are judgments made primarily through sensory perception. Taste, smell and sight are key for judging good from poor water quality. However, these judgements are also mediated through the technologies/objects of cooking and water storage.  And the impact of water upon human and non-human bodies is also important: health impacts, impact upon crops, and the linking of mosquitoes and dirty water. Finally, knowledge of what is in the water (eg. sewage) feeds into water quality judgements, as well as the results from sporadic water quality testing. These water quality judgements are shared through complex social networks. As a result water quality knowledge is uncertain, and this uncertainty is recognised. 

Approaching water quality through literature, I’ve developed the framework in the table below.

… for people… for a more-than-human world
Harm based stand-ard Presence of pathogens, heavy metals and other toxic substances. Non-toxic levels of nutrients, heavy metals and other toxic substances. Temperature. 
Use based stand-ard

Requirements for irrigation reuse, drinking water supply downriver.Adequate quality and flow patterns for life-processes.

Harm based standards apply to any water that is being ‘wasted’ i.e. released back ‘into the environment’. They consider the impact that this water would have on people and other beings that might encounter this water as it continues to circulate. In the use-based case, good water quality indicates that the water is suitable for a particular purpose. In this case, the specific purpose intended for the water will determine how water quality is judged. For example, this approach covers drinking water standards, irrigation water, and water for aquaculture. Thinking through these two categories shows that they are not necessarily distinct. However, use-based standards pay more attention to the future of water.

The second axis is whether water quality standards focus on human use/impact only, or if they are responsible to a broader ecological community. For example, E. coli is a water quality parameter of concern because it indicates that fecal bacteria are being transmitted through water flows, this is a public health concern. On the other hand, biological oxygen demand (BOD) indicates how much organic matter is in water, and so how much oxygen would be depleted from the water as this organic matter is decomposed. This loss of oxygen has cascading ecological impacts. 

However, standards are not the only way that a scientific judgement of water quality improvement is made. Within my case study locations, efficiency is also a crucial discourse, and one that aligns with the scientific literature on constructed wetlands.

Whether judging standards or efficiency, water quality is measured by a whole range of technical equipment and standards: BOD bottles, Colilert trays, ion sensitive electrodes, Oxitop meters, Ion spectrometers, portable handheld meters, UV light box, American Public Health Association standard methods. These techniques have histories of development that link them to particular places. For example, the incubation time of the biological oxygen demand test (5 days), was decided in the early days of water quality testing based on the maximum length of English rivers. After five days, water in an English river will have reached the sea, and so no longer be a concern. This example suggests that results must be treated carefully to be relevant to the wastewaterscapes at my site. My approach here is strongly influenced by the work of scholars in STS (science and technology studies).

This introduction to water quality demonstrates why the definition of benefits is an important part of the research task. The complicated processes of meaning-making around a benefit must be folded into understanding how a benefit is produced.