Trends in nitrate concentrations in UK groundwater

Groundwater quality varies with time over various scales, from daily to seasonally and annually.

Statistical methods can be applied to long-term datasets to estimate trends, to assess seasonality and, based on those findings, to make predictions.

Variations by month

Figure 1. Seasonal variation of nitrate concentrations by month for a borehole in the Chalk aquifer.

All groundwater nitrate time-series data held by BGS in 2006 were analysed using a semi-automated method. About 33 per cent of datasets showed seasonal behaviour. This was visualised by plotting all data for each month on the same graph.

Seasonal series usually have higher concentrations in the spring months during periods of maximum groundwater levels.

Mechanisms causing this are being investigated — see Nitrate fluctuations in groundwater

Observed trend in nitrate concentrations

Figure 2. Nitrate concentrations show a rising trend.

Rising trends in nitrate concentrations have been observed at many groundwater abstraction points.

We undertook a project to see if these trends were occurring across the UK. The figure shows the same data as above plotted against time.

The average nitrate concentration from all the data was about 38 mg/l in 2001.

Three different statistical methods were used to assess the trend over time and only where these agreed was the result used.

This showed for all the data that the average trend was an increase of 0.34 mg/l per year. In the important Chalk and Permo-Triassic sandstone aquifers the average was about 0.4 mg/l.

Predicting future nitrate concentrations

For the Chalk and Permo-Triassic sandstone aquifers assuming the observed trends continue, by 2015 the median concentration will have increased to 44 mg/l.

Where other site-specific information is available which is better characterised, such as water levels, predictions of both trend and seasonal peaks can be made. In the example groundwater levels are used as a surrogate for recharge seasons in a simple model. This allows the capture of a variety of local and undefined factors. We can see that predicted peak concentrations will exceed the 50 mg/l drinking water limit in all successive years with the mean concentration also exceeding the limit after 2012.

Figure 3. Predicted nitrate concentrations (grey buffered region of the plot) from 2003 to 2015 using long-term water levels to provide average seasonal variations.

Publications

This work was reported in Stuart, et al. 2007. Screening for long-term trends in groundwater nitrate monitoring data. Quarterly Journal of Engineering Geology and Hydrogeology, 40. 361–376.