Screening for long-term trends in groundwater nitrate monitoring data

Abstract

A large body of UK groundwater nitrate data has been analysed by linear regression in order to define past trends and estimate future concentrations. Robust regression was used. The datasets showed too many irregularities to justify more traditional time series approaches such as ARIMA-type methods. Tests were included for lack of linearity, outliers, seasonality and a break in the trend (by piecewise linear regression). Of the series analysed 21% showed a significant improvement in the overall fit when a break was included. Half of these indicated an increase in trend with time. Significant seasonality was found in about one third of the series, with the largest nitrate concentrations usually found during winter months. Inclusion of nearby water level data as an additional explanatory variable successfully accounted for much of this seasonality. Based on 309 datasets from 191 distinct sites, nitrate concentrations were found to be rising at an average of 0.34 mg NO 3 L-1 a-1. In 2000, 34% of the sites analysed exceeded the 50 mg L-1 EU drinking water standard. If present trends continue, 41% could exceed the standard by 2015. We explored an alternative to the previously-proposed WFD aggregation approach for estimating trends in whole groundwater bodies (the 'Grath' approach, spatially average then find the trend). We first determined the trends for individual boreholes and then spatially averaged these. This approach preserves information about the spatial distribution of trends within the water body and is less sensitive to 'missing data'.

A review of the impact of climate change on future nitrate concentrations in groundwater of the UK

“…Nitrate is already widespread in the aquifers of the UK often at elevated concentrations (Rivett et al, 2007 (Stuart et al, 2007). Under current climate conditions and agricultural practices concentrations are predicted to continue to rise in the UK Chalk aquifer (Wang et al, in press).…”

“…It is the most widespread groundwater quality problem facing the UK water industry and environmental regulators, as it is the single biggest cause of groundwater body status failure under the WFD (UKWIR, 2004). Nitrate concentrations are predicted to continue to rise in many places over the next decade (Stuart et al, 2007). If present trends continue, many groundwater sources could exceed the drinking water standard by 2015, as indicated by trend analysis reported in River Basin Management Plans.…”

Macronutrient status of UK groundwater: Nitrogen, phosphorus and organic carbon

“…The data also indicate lower concentrations across area which are underlain by poorly productive strata. Stuart et al (2007) assessed data from the major aquifers in England and calculated predicted concentrations for the 1 st January 2000 using a statistical technique to model long-term trends (Table 2). …”

Assessing and Aggregating Trends in Groundwater Quality

“…Therefore, trend detection becomes more efficient when the aforementioned spatial and temporal variability are reduced by taking into account the physical and chemical temporal characteristics of the body of groundwater, including flow conditions, recharge rates and percolation times (GWD, Annex IV, (2(a)(iii) ). Several statistical techniques, modelling techniques and combinations of both are available for trend analysis and some of the promising techniques have been tested in the TREND2 work package and at UK abstraction sites, including age dating and transferfunction approaches (Visser et al 2008, Stuart et al 2007). …”

“…However, solute concentrations in samples of discharging groundwater from a single abstraction borehole or spring also depend on numerous 'local' or site factors such as borehole depths and open or screened interval, depths and lengths of groundwater flow paths, possible groundwater quality stratification in the aquifer and changes (at various timescales) in groundwater levels, directions of flow and pumping regime. These sources of variation are often superimposed on one another in a time series of an individual parameter, such as nitrate or chloride, at a drinking water abstraction site and their resolution can be a challenging task (Stuart et al, 2007). …”