View source for OR/16/033 The UK groundwater nitrate legacy

===Integrating with the saturated zone at the catchment scale===
Wang et al. (2013)<ref name="Wang 2013">WANG, L, BUTCHER, A S, STUART, M E, GOODDY, D C, and BLOOMFIELD, J P. 2013. The nitrate time bomb: a numerical way to investigate nitrate storage and lag time in the unsaturated zone. ''Environmental Geochemistry and Health'', Vol.&nbsp;35, 667–681.</ref> developed an integrated modelling method to simulate the nitrate transport in both the unsaturated and the saturated zones at the catchment scale. Three BGS numerical models, the NTB model&nbsp;—&nbsp;described above, GISGroundwater&nbsp;—&nbsp;a groundwater flow model (Wang et al., 2010<ref name="Wang 2010">WANG, L, MANSOUR, M, and A H. 2010. Description of the GIS based regional groundwater flow model. ''British Geological Survey  Internal Report'', IR/10/070.</ref>) and N–FM a nitrate transport model for the saturated zone developed for this work, were integrated to verify and support each other to provide information on nitrate lag time in the groundwater system at a catchment scale. The Eden Valley, which has thick Permo-Triassic Sandstone unsaturated zones and a nitrate groundwater pollution problem, was selected as a case study area.

''Modelling water levels''<br>
The unsaturated zone (USZ) thicknesses used in the NTB model are too coarse for a catchment scale study. Therefore, a simple and easy-to-use groundwater flow model, GISGroundwater, was used to simulate the long-term average steady-state groundwater levels (GWLs) for the area to derive high spatial resolution of the thicknesses of the Permo-Triassic sandstone USZs in the Eden Valley. GISGroundwater is a seamless GIS 2-dimensional numerical finite difference groundwater flow model (Wang et al., 2010<ref name="Wang 2010"></ref>).

The Penrith and St Bees Sandstone formations were simplified as a single layer aquifer with a distribution of hydraulic conductivity values. The modelling extent was defined by a (100&nbsp;m&nbsp;×&nbsp;100&nbsp;m) GIS layer. A GIS layer containing distributed K values was entered into the model; river nodes and river stages entered were derived from a Centre for Ecology and Hydrology (CEH) river system dataset and a digital elevation model) dataset from CEH; groundwater abstraction data were also entered into the model using a GIS layer.

An average groundwater recharge of 1&nbsp;mm day<sup>-1</sup> was used in the groundwater flow model which was calibrated by comparing the simulated long-term average GWLs with observed ones in 39 boreholes. Figure&nbsp;2.10 shows that the modelled and observed GWLs correlate indicating that the steady-state groundwater flow model for the study area was well calibrated. The K values for modelling the groundwater flow in the Penrith and St Bees Sandstones were 3.5&nbsp;m day<sup>-1</sup> and 0.6&nbsp;m day<sup>-1</sup> respectively. The distributed Permo-Triassic Sandstone USZ thickness map for the area was then derived by subtracting the modelled long-term average GWLs from the DSM dataset.

[[Image:OR16033fig2.10.jpg|thumb|center|350px|  '''Figure 2.10'''&nbsp;&nbsp;&nbsp;&nbsp;Comparison of observed and simulated water levels.        ]]

In the study area, the modelled thickness of the Permo-Triassic sandstone USZs is greatest, 183&nbsp;m in the northwest of the Eden Valley, and reduces to 0&nbsp;m (i.e. GWLs are the same elevation as the river stages) along the River Eden and its tributaries. Notably SPZs generally have a thicker USZ than other parts of the study area. The nitrate travel time in the Permo-Triassic sandstone USZs correlating with the USZ thickness, ranges from 0 to 61 years with a mean value of 12 years; strip areas along streams have short travel times (0–1 year) due to thin USZs, whilst mountainous areas in the east and west of the Eden Valley have longer nitrate travel times.

''Modelling nitrate dilution in the saturated zone''<br>
N–F M&nbsp;—&nbsp;a GIS nitrate transport model for the saturated zone was developed to simulate yearly nitrate concentration at a borehole by considering the process of nitrate leaching from the bottom of the soil zone, the nitrate movement in the USZ and dilution in the saturated zone. The simulated pumped nitrate concentration in boreholes was compared with observed values to validate the numerical modelling parameters, such as the nitrate transport velocity in the USZ, the thickness of the USZ, and the aquifer hydraulic conductivity values used for deriving the thickness of the USZ.

Figure 2.11 shows the conceptual model of N–FM. The dilution process was simplified by assuming that nitrate arriving at a borehole dilutes in water pumped out of the borehole, and the groundwater flow within a groundwater Source Protection Zone (SPZ), reaches a steady-state, i.e., the long-term recharge volume within a SPZ equals water pumped out of the borehole in the SPZ. Not all leached nitrate reaches the abstraction borehole due to attenuation in the USZ and the saturated zone. Nitrate concentration may be lowered due to denitrification and absorption in USZs; nitrate in the saturated zones will be absorbed by small pores or transported outside of SPZ due to the diffusion and dispersion processes. Therefore a nitrate attenuation coefficient (''NAC'') was introduced into this model. With this conceptual model, the depth of the saturated zone, and the thickness of active groundwater zone can be ignored, and the nitrate dispersion and diffusion processes can be simplified in simulating yearly nitrate concentration at a borehole in the SPZ.

[[Image:OR16033fig2.11.jpg|thumb|center|400px|  '''Figure 2.11'''&nbsp;&nbsp;&nbsp;&nbsp;Conceptual cross-section for simulating nitrate transport and dilution in groundwater in the N–FM model.    ]]

The modelling showed that the peak nitrate loading around 1983 has affected most of the study area (Figure&nbsp;2.12). For areas around the SPZs of Bowscar, Beacon Edge, Low Plains, Nord Vue, Dale Springs, Gamblesby, Bankwood Springs, and Cliburn, the peak nitrate loading will arrive at the water table in the next 34 years (Figure&nbsp;2.12). Statistical analysis shows that 8.7% of the Penrith Sandstone and 7.3% of the St Bees Sandstone have not been affected by peak nitrate.

[[Image:OR16033fig2.12.jpg|thumb|center|350px|  '''Figure 2.12'''&nbsp;&nbsp;&nbsp;&nbsp;Map of Eden valley catchment showing travel time of the 1983 peak nitrate concentration to abstraction points.    ]]

Distributed maps were produced for nitrate concentration at the water table for each year between 1925 and 2040. The results show that the average nitrate concentration at the water table across the study area has reached its peak and will decrease over the next 30 years (Figure&nbsp;2.13). Some unaffected areas with thicker USZs around Beacon Edge, Fairhills, Bowscar, Nord Vue, Low Plains, Gamblesby, and Bankwood Springs, will be affected by peak nitrate loadings between 2020 and 2030, and then retain a high nitrate concentration (172&nbsp;mg&nbsp;NO<sub>3</sub> l<sup>-1</sup>) (before any groundwater dilution) around 2040.

[[Image:OR16033fig2.13.jpg|thumb|center|350px|  '''Figure 2.13'''&nbsp;&nbsp;&nbsp;&nbsp;Predicted decrease of nitrate loadings at the water table from 2010 to 2040.    ]]

The results show that:

* The model provides predictions of nitrate concentrations at public supply boreholes;
* catchment scale groundwater modelling simulated water levels and derived unsaturated zone thickness;
* the NTB nitrate application function and unsaturated zone velocities were applicable;
* measured water levels are needed to calibrate the flow model;
* SPZs are needed to calculate nitrate dilution in the saturated zone.