Nitrate trends in groundwater

Stuart, M.E.; Kinniburgh, D.G.. 2005 Nitrate trends in groundwater. Nottingham, UK, British Geological Survey, 74pp. (IR/05/137) (Unpublished)

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Abstract/Summary

Some 450 groundwater nitrate datasets from England were examined and, where the data were suitable, trends determined. Although the datasets were not randomly selected from all possible boreholes in England, they represent a fairly broad cross-section of such boreholes. They covered a wide range of aquifers. Many of these were from working public supply wells, and the selection may therefore exclude high nitrate sources which have already been taken out of supply. Datasets from observation wells were also included, but many of these had much less available data. More than one third of the datasets were rejected for being too short (span of less than 5 years or fewer than 20 observations), too irregular or too variable. Time series in which there were obviously highly nonlinear trends were also excluded. Trends were determined by linear regression. Tests were included for the lack of linearity, the presence of outliers, for seasonality and for possible breaks in the trend including reversals of trend. After exclusion of data where trend fitting was unsatisfactory, 309 datasets were finally selected from 191 different sites. For multi-borehole sites, median values were used to obtain the summary statistics. For these 191 sites groundwater nitrate concentrations were found to be rising at an average of 0.34 mg NO3 L-1 year-1. Average trends were greatest in the Lincolnshire Limestone aquifer (0.96 mg NO3 L-1 year-1) and lowest in the Magnesian Limestone aquifer (0.18 mg NO3 L-1 year-1). Average trends for the Chalk and Triassic sandstone aquifers were 0.38 mg NO3 L-1 year-1 and 0.44 mg NO3 L-1 year-1, respectively. An assessment of seasonality in nitrate concentrations was also made by including a term for the month of sampling in the regression model. Significant (p<0.05) seasonality was found in about one third of the series tested. This showed higher concentrations during the winter months. Breaks in a linear trend were detected by fitting a piecewise linear regression to the data with automatic detection of the break point. 21% of the time series analysed showed a significant improvement in the overall fit when such a break was included. 10.5% of these indicated an increase in trend with time and 10.5% a decrease. The best-fitting model was used to estimate the nitrate concentration on 1 January 2000 and January 2015 for all sites. For 2000, this showed that nitrate concentrations in the major aquifers on this date were broadly similar apart from the Magnesian Limestone. The highest concentrations were in the Oolitic limestone (50 mg NO3 L-1) and the lowest in the Magnesian Limestone (8.2 mg NO3 L-1). The Chalk and the Triassic sandstone had average concentrations of 42 mg NO3 L-1 and 46.3 mg NO3 L-1 respectively. The average of all sites was 37.8 mg NO3 L-1. The highest nitrate concentrations were found in areas around the Wash, the Chalk of south Yorkshire/East Anglia, and the Permo-Triassic Sandstone of Yorkshire/Nottinghamshire. By 2015 the average concentration will have increased to 43.6 mg NO3 L-1. The highest concentrations are predicted to be in the Lower Greensand (58.8 mg NO3 L-1) and the lowest in the Magnesian Limestone (12.3 mg NO3 L-1). The Chalk and the Triassic sandstone will have average concentrations of 50.5 mg NO3 L-1 and 52.6 mg NO3 L-1 respectively. In 2000, 34% of sites exceeded the 50 mg/L. It is estimated that if present trends continue, 41% of groundwater sources could exceed the 50 mg/L standard by 2015.

Item Type:	Publication - Report
Programmes:	BGS Programmes > Groundwater Management
Funders/Sponsors:	British Geological Survey, UKWIR, Defra
Additional Information. Not used in RCUK Gateway to Research.:	This report has been internally reviewed, but not externally peer-reviewed. Report made open in October 2023.
Date made live:	12 Oct 2023 12:21 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/536076

Item Type:

Publication - Report

Programmes:

BGS Programmes > Groundwater Management

Funders/Sponsors:

British Geological Survey, UKWIR, Defra

Additional Information. Not used in RCUK Gateway to Research.:

This report has been internally reviewed, but not externally peer-reviewed. Report made open in October 2023.

Date made live:

12 Oct 2023 12:21 +0 (UTC)

URI:

https://nora.nerc.ac.uk/id/eprint/536076