Water quality sites and sondes
The Environment Agency is the environmental regulator for England. It has a wide range of responsibilities, from managing flood risk, to regulating waste, to supervising fisheries and navigation.
One of its key responsibilities is regulating water quality of freshwater resources (rivers, lakes, canals, ponds and groundwater).
There is a legally binding national target for 77% of rivers to be in good ecological health by 2027.
Measuring progress against these goals, investigating pollution incidents or checking compliance with discharge permits are some of the reasons the Environment Agency takes water quality spot samples. All the samples are recorded in the Water quality data archive.
However, spot samples only show part of the picture. Underwater water quality sensors called 'sondes' can provide readings every 15 minutes. The data is recorded in the Hydrology data explorer.
Phosphorus is a naturally occurring element with the chemical symbol P and essential for life. Natural phosphorus levels in rivers are low, so it is a limiting growth element for plants and algae.
However, raised levels of phosphorus remove this limit, and leads to an overgrowth of aquatic plants and algae. These plants produce oxygen during the day but consume oxygen after dark. The death and breakdown of these organisms by bacteria also consumes oxygen. This lowering of dissolved oxygen makes it harder for invertebrates and fish to survive, and has a knock-on effect up the food chain. This process is called 'eutrophication'.
The Environment Agency estimates that in England, between 60% and 70% of phosphorus in rivers comes from sewage treatment works. Agriculture is the second biggest input, at 25% (Environment Agency, 2019).
Elemental phosphorus dissolves very easily, so it mostly exists in the environment as orthophosphate (PO43-), often called just 'phosphate' or 'reactive phosphate'.
These are the upper thresholds used by the Environment Agency for most of the Thames basin:
| High | < 0.036 |
| Good | 0.036–0.069 |
| Moderate | 0.069–0.173 |
| Poor | 0.173–1.003 |
| Bad | > 1.003 |
Note these thresholds are based on levels of 'phosphorus equivalent' (PO4 as P).
Orthophosphate is heavier than the phosphorus component in it, because of the four extra oxygen atoms. To convert phosphorus equivalent to orthophosphate, use the following formula:
Orthophosphate (PO4) = Phosphorus equivalent (PO4 -P) × 3.066
Nitrogen is a naturally occurring element with the chemical symbol N2. In its gas form, it makes up 80% of the Earth's atmosphere. In other forms it's one of the most important nutrients for plant growth.
Nitrate (NO3-) is one of the ions of nitrogen, which dissolves readily in water. Nitrate is very stable, and nitrogen can persist in this form for years, leaching through soil and into rivers and groundwater.
As with phosphorus, some nitrate is important for plant and algal growth, but too much can lead to an overgrowth of plants and algae, leading to a drop in oxygen levels and knock-on effects on invertebrates, fish and other aquatic life (eutrophication).
The Environment Agency estimates that in England, agriculture is the dominant source of nitrate in water (about 70% of total inputs), with sewage effluent a secondary contributor (25-30%) (Environment Agency 2021).
55% of England is designated as being in a Nitrate Vulnerable Zone due to high levels of nitrate in rivers, groundwater or eutrophication of lakes, estuaries and reservoirs.
Nitrate concentrations are regulated in drinking water but there are no laws in the UK governing nitrate concentrations in rivers. However, in Ireland the threshold for good ecological status is 8mg/l of nitrate (1.8mg/l nitrogen equivalent).
As with phosphorus, there is a difference between 'nitrogen equivalent' (NO3 as N) and nitrate, due to the additional oxygen atoms in nitrate. The formula to convert the two is:
Nitrate (NO3) = nitrogen equivalent (NO3-N) × 4.42
Nitrite (NO2-) is another ion of nitrogen. Unlike nitrate, it is directly toxic to aquatic life. However, concentrations of nitrite are typically quite low (below 0.5 mg/l) as it rapidly gets converted to the more stable nitrate. Higher concentrations of nitrite indicate a more recent pollution event.
Ammonia (NH3) is a chemical which is highly toxic to aquatic life, lowering invertebrate and fish growth and reproductive success, and killing fish in higher concentrations. It readily dissolves in water, where it exists as ammonia (NH3), or as the ammonium ion with an extra hydrogen atom, which gives it a positive charge (NH4+).
These two types of 'ammoniacal nitrogen' are in equilibrium in water, with the proportions shifting depending on pH (how acid or alkaline the water is) and the temperature of the water.
That's why there are slightly different ecological thresholds for ammonia depending on the alkalinity of the river. The Thames basin tends to have a very high alkalinity (greater than 200mg/l of CaCO3), and so mostly has these upper thresholds for ammonia:
| High | < 0.3 |
| Good | 0.3–0.6 |
| Moderate | 0.6–1.1 |
| Poor | 1.1–2.5 |
| Bad | > 2.5 |
It can be present in water naturally, but the majority of ammonia comes from sewage discharges (treated and untreated), industrial effluent (it's a common chemical in industrial processes) and fertilisers.
Dissolved Oxygen (DO) is one of the most important indicators of river water quality since it is necessary for all aquatic life to survive.
The concentration is reduced by the respiration of living organisms and replenished by photosynthesis of aquatic plants, algae and re-aeration from the atmosphere (running water dissolves more oxygen than slow-moving or still water).
DO levels tend to follow a daily and seasonal pattern. The normal cycle in rivers and lakes shows an increase in DO during the day and a drop in DO at night.
Values over 100% can occur when photosynthesis from plants and algae produces so much oxygen that the water becomes supersaturated. This happens in spring, summer and autumn.
Waste discharges high in nutrients and organic matter, such as untreated sewage, slurry or milk, can cause DO to drop in a watercourse due to increased microbial activity (respiration). This occurs during the breakdown of organic matter.
Dissolved oxygen can be measured as a percentage (%) or in mg/l.
These are the lower thresholds for dissolved oxygen in most of the Thames basin:
| High | > 70 |
| Good | 60–70 |
| Moderate | 54–60 |
| Poor | 45–54 |
| Bad | < 45 |
Biochemical oxygen demand (BOD) measures the amount of oxygen consumed by microorganisms in decomposing organic matter in stream water. BOD directly affects the amount of dissolved oxygen in rivers and streams: the greater the BOD, the more rapidly oxygen is depleted in the stream.
These are the upper thresholds for BOD in most of the Thames basin:
| High | < 4 |
| Good | 4–5 |
| Moderate | 5–6.5 |
| Poor | 6.5–9 |
| Bad | > 9 |