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
All groundwater nitrate time-series data held by BGS in 2006 was analysed using a semi-automated method. About 33 per cent of datasets showed seasonal behaviour. This was visualised by plotting all the 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.
Observed trends in nitrate concentrations
Rising trends in nitrate concentrations have been observed at many groundwater abstraction points. A BGS project investigated whether these trends were occurring across the UK.
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 the result was used only when all the methods agreed. This showed that, for all the data, 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 and assuming the observed trends continue, the median concentration should have increased to 44 mg/l by 2015.
Where other site-specific information is available that is better characterised, such as water levels, predictions of both trend and seasonal peaks can be made. Groundwater levels can be used as a surrogate for recharge seasons in a simple model, which allows the capture of a variety of local and undefined factors. The 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.
Seasonal variation of nitrate concentrations by month for a borehole in the Chalk aquifer. BGS © UKRI.
Nitrate concentrations show a rising trend. BGS © UKRI.
Further reading
Stuart, M E, Chilton, P J, Kinniburgh, D G, and Cooper, D M. 2007. Screening for long-term trends in groundwater nitrate monitoring data. Quarterly Journal of Engineering Geology and Hydrogeology, Vol. 40, 361–376. DOI: https://doi.org/10.1144/1470-9236/07-040
Contact
Please contact Dan Lapworth for further information