This is not a nitrate standard …

Much of my professional life takes place in the collision zone between ecology and bureaucracy.   These make uneasy partners: ecologists like to think of themselves as Lone Rangers riding out to put the world to rights rather than as small cogs in big administrative machines, but the reality is that environmental regulators need both “carrots” and “sticks”, and wielding the latter makes them part of the criminal justice system, with all the responsibilities – and paperwork – that that implies.

I’ve spent quite a lot of time over the past few years working on developing standards for nutrients in freshwaters.   Roughly speaking, I have been helping to define the freshwater equivalent of the 30 miles per hour speed limit.   Speed limits work partly because everyone understands the dangers of driving too fast in urban areas, and partly because we know that there is a good chance of being caught by a speed camera if we drive too fast.   And so it is (or should be) for pollutants: the lower dashed line on the graph of phosphorus in the Ouseburn in the previous post is the “30 mph” limit based on an understanding of how phosphorus interacts with freshwater ecology.   There is a lot I could write about how these values are derived (a subject for another day) but that, in a nutshell, is what we are trying to achieve.

When my students are analysing the data from the Ouseburn, they find standards for ammonia, BOD and phosphorus relatively easily via the UK TAG website but they come to me each year wondering why they cannot find equivalent values for nitrate.  The UK TAG document says “we consider the general understanding of this [nitrogen] to be insufficient at present for it to be used as a basis for setting standards or conditions.”  This was disingenuous in the extreme because I know that DEFRA has been extremely reluctant to set standards for nitrogen as this would focus attention on agricultural pollution which is both much harder to manage and would incur the ire of the farming lobby.   A few years ago, Nigel Willby and I calculated the nitrogen concentrations that would support good status in UK rivers as a by-product of a project to revise phosphorus standards.  We had the data we needed and it struck us that no-one would turn down our “buy one, get one free” initiative.   Not so.  Our figures for nitrogen were quietly excised from our report using the very good argument that it was going to be a hard enough job to argue the case for tighter phosphorus standards without confusing the issue with nitrogen too.  Since then, little has happened, as far as I know, to push nitrogen up the regulatory agenda.

The table below shows these values simply to indicate the values of oxidised nitrogen (which is mostly as nitrate rather than nitrite) that are associated with different levels of ecological status in UK rivers.  They have no regulatory significance, but should give us a rough idea of how much nitrogen is “too much” when we are trying to understand the ecology of a river.

Predicted Total Oxidised Nitrogen (nitrate-N + nitrite-N)  concentrations associated with EQR values modelled for two altitudes (20 and 200 m asl) and four alkalinities (10, 50, 100 and 200 mg L-1 CaCO3).   Boundaries are normalised at 0.8 (high/good), 0.6 (good/moderate), 0.4 (moderate/poor) and 0.2 (poor/bad)); 0.7 and 0.9, therefore, represent conditions at the middle of their respective status classes.


Applying these values to the Ouseburn, a lowland, hard water river, we see that most of the values lie below this threshold except for those from Woolsington, the headwater stretches which are surrounded by agricultural land where there is likely to be extensive use of artificial fertilisers.   The open circles on the right of the graph are values collected by my students each autumn, which may explain why they are lower than the annual mean values that the closed circles to the left represent.  But note, too, the high values in the Airport tributary in the early 1990s.   These occur at about the same time as the high ammonium concentrations that I discussed in “Part of the problem?”.   The ammonium that the airport released into the stream has a nitrogen atom bound to four hydrogen atoms using strong bonds.   Some microorganisms are able to break these bonds and use the energy that is released to drive their own cells.   This is what we see in Airport tributary where the high nitrate is the result of a two-step process that breaks down the ammonium first to nitrite and then to nitrate.


Trends in concentrations of nitrogen as nitrate in the Ouseburn over time.  Woolsington is upstream of the airport, Airport tributary (Abbotswood Burn) receives runoff from Newcastle Airport and Jesmond Dene is about 10 km downstream from the airport.  Closed symbols are annual means of data collected by the National Rivers Authority and Environment Agency; open symbols are means of data collected and analysed by Newcastle University Geography students in October (once also in February) of each yearThe dashed line is the modelled concentration above which the river is unlikely to support “good ecological status.

The final graph shows concentrations of nitrogen as ammonium and nitrate at the three sites we’ve been examining, along with the (official) standard for nitrogen as ammonium plus my unofficial guide value for healthy nitrate-nitrogen concentrations.  This gives us four quadrants, with bottom left representing situations where both forms of nitrogen are at concentrations that should not significantly impair ecology.   This is the case at the two sites downstream from the airport from the mid-1990s onwards, once ammonia concentrations were under control.

The top right quadrant, by contrast, has just a small number of values from Airport tributary from the early 1990s, when high ammonia was rapidly oxidised to yield high nitrate-nitrogen concentrations too.   A further cluster of sites, mostly from Airport Tributary and Jesmond Dene, also have high ammonium concentrations, although nitrate-nitrogen concentrations are below the threshold.  Finally, at the top left, we have values from Woolsington where concentrations of nitrogen as ammonium are low but nitrate-nitrogen concentrations may be a problem, due to agriculture.

For a river that is less than 20 kilometres from source to mouth, there is a lot happening in the Ouseburn, which makes it ideal for students to become acquainted with the complexities of environmental regulation.   You can read the history of pollution control in the graphs of data too: from intercepting toxic point sources in the 1980s to more general concerns about overflowing storm sewers in the 2000s and, now, more interest in diffuse nutrient pollution from farmland in the headwaters.   That, too, is a good lesson for undergraduates: pollution, itself, is not an unambiguous concept and its definition has evolved as our understanding increases.  One lesson that we can draw from this episode about nitrate standards is that the scientific argument about sensible levels of any pollutant can quickly become obscured by politics and vested interests.  That can never be a good thing.


Relationship between nitrogen as ammonium and as nitrate in the Ouseburn between 1989 and 2016.  The diagonal line has slope = 1 and the horizontal and vertical dashed lines indicate the position of the maximum concentrations that are likely to support good ecological status.   There is no differentiation between Environment Agency data and data collected by Newcastle University students on this graph.


One thought on “This is not a nitrate standard …

  1. Pingback: More diatoms from the roof of the world – microscopesandmonsters

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