Editing OR/16/036 Development of the BGS unsaturated zone model

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==Total unsaturated zone nitrate storage==     
 
==Total unsaturated zone nitrate storage==     
 
===Introduction===
 
===Introduction===
Management of legacy nitrate in groundwater in England and Wales both at national and local levels requires an understanding of the storage of nitrate in the unsaturated zone. Work by Wang et al. (2012b)<ref name="Wang 2012b"></ref> has identified the peak year for nitrate to arrive at the water table. Parris (1998)<ref name="Parris 1998">PARRIS, K. 1998. Agricultural nutrient balances as agri-environmental indicators: an OECD perspective. ''Environmental Pollution'', Vol.&nbsp;102, 219–225.</ref> and Worrall et al. (2009)<ref name="Worrall 2009">WORRALL, F, BURT, T, HOWDEN, N, and WHELAN, M. 2009. Fluvial flux of nitrogen from Great Britain 1974–2005 in the context of the terrestrial nitrogen budget of Great Britain. ''Global Biogeochemical Cycles'', Vol.&nbsp;23.</ref> show that Great Britain is a net sink of reactive nitrogen, with potentially 300 kt N stored in groundwater. However, estimations of the total mass of nitrate in the unsaturated zone have not been undertaken to date. This section details an approach that has estimated the total mass of nitrate in the unsaturated zone of England and Wales.
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Management of legacy nitrate in groundwater in England and Wales both at national and local levels requires an understanding of the storage of nitrate in the unsaturated zone. Work by Wang et al. (2012b)<ref name="Wang 2012b">WANG, L, STUART, M E, BLOOMFIELD, J P, BUTCHER, A S, GOODDY, D C, MCKENZIE, A A, LEWIS, M A, and WILLIAMS, A T. 2012b. Prediction of the arrival of peak nitrate concentrations at the water table at the regional scale in Great Britain. ''Hydrological Processes'', Vol.&nbsp;26, 226–239.</ref> has identified the peak year for nitrate to arrive at the water table. Parris (1998)<ref name="Parris 1998">PARRIS, K. 1998. Agricultural nutrient balances as agri-environmental indicators: an OECD perspective. ''Environmental Pollution'', Vol.&nbsp;102, 219–225.</ref> and Worrall et al. (2009)<ref name="Worrall 2009">WORRALL, F, BURT, T, HOWDEN, N, and WHELAN, M. 2009. Fluvial flux of nitrogen from Great Britain 1974–2005 in the context of the terrestrial nitrogen budget of Great Britain. ''Global Biogeochemical Cycles'', Vol.&nbsp;23.</ref> show that Great Britain is a net sink of reactive nitrogen, with potentially 300 kt N stored in groundwater. However, estimations of the total mass of nitrate in the unsaturated zone have not been undertaken to date. This section details an approach that has estimated the total mass of nitrate in the unsaturated zone of England and Wales.
  
 
===Methodology===
 
===Methodology===
In this high level national scale quantification of nitrate storage in the unsaturated zone it was deemed suitable to select high and moderate productivity aquifers based on BGS 1:625&nbsp;000 scale hydrogeological mapping. Areas overlain by low permeability superficial deposits (Griffiths et al., 2011<ref name="Griffiths 2011">GRIFFITHS, K J, MACDONALD, A M, ROBINS, N S, MERRITT, J, BOOTH, S J, JOHNSON, D, and MCCONVEY, P J. 2011. Improving the characterisation of Quaternary deposits for groundwater vulnerability assessments using maps of recharge and attenuation potential. ''Quarterly Journal of Engineering Geology and Hydrogeology'', Vol.&nbsp;44, 49–61.</ref>) were excluded from the analysis. The time and spatially variable NIF derived from NEAP-N and the BGS NIF as discussed in section 0 was used as a nitrate input. Point source discharges of nitrate, such as slurry heaps or septic tanks, have previously been estimated as contributing <1% of the total nitrate flux to groundwater (Sutton et al., 2011<ref name="Sutton 2011">SUTTON, M A, HOWARD, C M, ERISMAN, J W, BILLEN, G, BLEEKER, A, GRENNFELT, P, VAN GRINSVEN, H, and GRIZZETTI, B. 2011. ''The European nitrogen assessment: sources, effects and policy perspectives''. (Cambridge University Press.)</ref>) and have not been considered in this study. It is possible that these could be important for some areas or aquifers. Transport of nitrate through the unsaturated zone on a 1&nbsp;km scale was derived using the approach of Wang et al. (2012) (Wang et al, 2012a<ref name="Wang 2012a">WANG, L, BARKWITH, A, JACKSON, C, and ELLIS, M. 2012a. SLiM: an improved soil moisture balance method to simulate runoff and potential groundwater recharge processes using spatio-temporal weather and catchment characteristics. The 12th UK CARE Annual General Meeting. Bristol, UK.</ref>, 2012b<ref name="Wang 2012b"></ref>). The total mass of nitrate in the unsaturated zone was derived for each year (1925 to 2050) for each 1&nbsp;km grid cell and summed across the study area by aquifer. For any year, ''t'', the total nitrate in unsaturated zone, ''N<sub>USZ</sub>'' for a given grid cell with an unsaturated travel time, ''TT<sub>USZ</sub>'' and a time-variant nitrate input function, ''NIF'', can be calculated as:
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In this high level national scale quantification of nitrate storage in the unsaturated zone it was deemed suitable to select high and moderate productivity aquifers based on BGS 1:625&nbsp;000 scale hydrogeological mapping. Areas overlain by low permeability superficial deposits (Griffiths et al., 2011<ref name="Griffiths 2011">GRIFFITHS, K J, MACDONALD, A M, ROBINS, N S, MERRITT, J, BOOTH, S J, JOHNSON, D, and MCCONVEY, P J. 2011. Improving the characterisation of Quaternary deposits for groundwater vulnerability assessments using maps of recharge and attenuation potential. ''Quarterly Journal of Engineering Geology and Hydrogeology'', Vol.&nbsp;44, 49–61.</ref>) were excluded from the analysis. The time and spatially variable NIF derived from NEAP-N and the BGS NIF as discussed in section 0 was used as a nitrate input. Point source discharges of nitrate, such as slurry heaps or septic tanks, have previously been estimated as contributing <1% of the total nitrate flux to groundwater (Sutton et al., 2011<ref name="Sutton 2011">SUTTON, M A, HOWARD, C M, ERISMAN, J W, BILLEN, G, BLEEKER, A, GRENNFELT, P, VAN GRINSVEN, H, and GRIZZETTI, B. 2011. ''The European nitrogen assessment: sources, effects and policy perspectives''. (Cambridge University Press.)</ref>) and have not been considered in this study. It is possible that these could be important for some areas or aquifers. Transport of nitrate through the unsaturated zone on a 1&nbsp;km scale was derived using the approach of Wang et al. (2012) (Wang et al, 2012a<ref name="Wang 2012a">WANG, L, BARKWITH, A, JACKSON, C, and ELLIS, M. 2012a. SLiM: an improved soil moisture balance method to simulate runoff and potential groundwater recharge processes using spatio-temporal weather and catchment characteristics. The 12th UK CARE Annual General Meeting. Bristol, UK.</ref>, 2012b<ref name="Wang 2012b">WANG, L, STUART, M E, BLOOMFIELD, J P, BUTCHER, A S, GOODDY, D C, MCKENZIE, A A, LEWIS, M A, and WILLIAMS, A T. 2012b. Prediction of the arrival of peak nitrate concentrations at the water table at the regional scale in Great Britain. ''Hydrological Processes'', Vol.&nbsp;26, 226–239.</ref>). The total mass of nitrate in the unsaturated zone was derived for each year (1925 to 2050) for each 1&nbsp;km grid cell and summed across the study area by aquifer. For any year, ''t'', the total nitrate in unsaturated zone, ''N<sub>USZ</sub>'' for a given grid cell with an unsaturated travel time, ''TT<sub>USZ</sub>'' and a time-variant nitrate input function, ''NIF'', can be calculated as:
  
 
[[Image:OR16036equation7.jpg|frameless|center|175px| ]]
 
[[Image:OR16036equation7.jpg|frameless|center|175px| ]]
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The increase in unsaturated zone nitrate storage is dominated by the Chalk, with 70% of the total mass in 2015. Increases are also observed in other aquifers such as the Permo-Triassic Sandstones (4% total mass in 2015), Oolitic Limestones (3% total mass) and numerous other locally important formations (23%). The Chalk, Permo-Trias and Oolites have peak mass years of 2015, 1991 and 1992 respectively. The year in which the total peak mass of unsaturated zone nitrate for England and Wales occurs is significantly affected by the majority of mass being in the Chalk.
 
The increase in unsaturated zone nitrate storage is dominated by the Chalk, with 70% of the total mass in 2015. Increases are also observed in other aquifers such as the Permo-Triassic Sandstones (4% total mass in 2015), Oolitic Limestones (3% total mass) and numerous other locally important formations (23%). The Chalk, Permo-Trias and Oolites have peak mass years of 2015, 1991 and 1992 respectively. The year in which the total peak mass of unsaturated zone nitrate for England and Wales occurs is significantly affected by the majority of mass being in the Chalk.
  
The Chalk dominates the increase in unsaturated zone storage because of its large outcrop area, extensive agricultural land use (87%) and thick unsaturated zones (Wang et al., 2012b<ref name="Wang 2012b"></ref>). Thick unsaturated zones results in long travel times and consequently a large increase in nitrate storage. Figure 2.22 shows the spatial distribution of nitrate stored in the unsaturated zone in 1960 and 2015. Increases in nitrate storage in the chalk of southern and north east England can be observed. Increases are particularly large in interfluve areas where travel times are long due to thick unsaturated zones.
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The Chalk dominates the increase in unsaturated zone storage because of its large outcrop area, extensive agricultural land use (87%) and thick unsaturated zones (Wang et al., 2012b<ref name="Wang 2012b">WANG, L, STUART, M E, BLOOMFIELD, J P, BUTCHER, A S, GOODDY, D C, MCKENZIE, A A, LEWIS, M A, and WILLIAMS, A T. 2012b. Prediction of the arrival of peak nitrate concentrations at the water table at the regional scale in Great Britain. ''Hydrological Processes'', Vol.&nbsp;26, 226–239.</ref>). Thick unsaturated zones results in long travel times and consequently a large increase in nitrate storage. Figure 2.22 shows the spatial distribution of nitrate stored in the unsaturated zone in 1960 and 2015. Increases in nitrate storage in the chalk of southern and north east England can be observed. Increases are particularly large in interfluve areas where travel times are long due to thick unsaturated zones.
  
 
[[Image:OR16036fig2.22.jpg|thumb|center|500px|  '''Figure 2.22'''&nbsp;&nbsp;&nbsp;&nbsp;Change in unsaturated zone nitrate storage for 1925–2050 for moderate and highly productive aquifers.    ]]
 
[[Image:OR16036fig2.22.jpg|thumb|center|500px|  '''Figure 2.22'''&nbsp;&nbsp;&nbsp;&nbsp;Change in unsaturated zone nitrate storage for 1925–2050 for moderate and highly productive aquifers.    ]]
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The estimated peak nitrate mass of 1400&nbsp;kt&nbsp;N is substantially greater than previous first approximations of 300&nbsp;kt&nbsp;N (Worrall et al., 2009<ref name="Worrall 2009">WORRALL, F, BURT, T, HOWDEN, N, and WHELAN, M. 2009. Fluvial flux of nitrogen from Great Britain 1974–2005 in the context of the terrestrial nitrogen budget of Great Britain. ''Global Biogeochemical Cycles'', Vol.&nbsp;23.</ref>). However, in general this study corroborates with previous work suggesting that the subsurface is a significant store of reactive nitrogen. Whilst the total nitrate storage in the unsaturated zone is now decreasing, travel times in the saturated zone can be considerable (Wang et al., 2016<ref name="Wang 2016">WANG, L, STUART, M E, LEWIS, M A, WARD, R S, SKIRVIN, D, NADEN, P S, and COLLINS, A L. 2016. The changing trend in nitrate concentrations in the major aquifers due to historical nitrate loading from agricultural land in England and Wales. ''Science of the Total Environment'', Vol.&nbsp;542, 694–705.</ref>) and consequently the peak saturated zone mass may not have occurred yet. Further research is required to assess how this storage compares with other postulated terrestrial stores such as in-stream N retention and terrestrial N uptake in land not in production.
 
The estimated peak nitrate mass of 1400&nbsp;kt&nbsp;N is substantially greater than previous first approximations of 300&nbsp;kt&nbsp;N (Worrall et al., 2009<ref name="Worrall 2009">WORRALL, F, BURT, T, HOWDEN, N, and WHELAN, M. 2009. Fluvial flux of nitrogen from Great Britain 1974–2005 in the context of the terrestrial nitrogen budget of Great Britain. ''Global Biogeochemical Cycles'', Vol.&nbsp;23.</ref>). However, in general this study corroborates with previous work suggesting that the subsurface is a significant store of reactive nitrogen. Whilst the total nitrate storage in the unsaturated zone is now decreasing, travel times in the saturated zone can be considerable (Wang et al., 2016<ref name="Wang 2016">WANG, L, STUART, M E, LEWIS, M A, WARD, R S, SKIRVIN, D, NADEN, P S, and COLLINS, A L. 2016. The changing trend in nitrate concentrations in the major aquifers due to historical nitrate loading from agricultural land in England and Wales. ''Science of the Total Environment'', Vol.&nbsp;542, 694–705.</ref>) and consequently the peak saturated zone mass may not have occurred yet. Further research is required to assess how this storage compares with other postulated terrestrial stores such as in-stream N retention and terrestrial N uptake in land not in production.
  
The approach adopted in this analysis and that of Wang et al. (2012b)<ref name="Wang 2012b"></ref> is likely to be beneficial for the targeting of catchment management activities at national and regional scales. For example, Figure 2.22 illustrates that legacy nitrate in the unsaturated zone at a national scale is dominated by the Chalk. Figure 2.23 shows that within the Chalk, there a substantial historical mass of nitrate in the unsaturated zone of southern England, particularly in interfluve areas where travel times are long. Consequently, environmental managers should take into account this mass when considering the implementation of catchment mitigation measures in attempts to improve groundwater and surface water quality. This could also be important when setting environmental objectives (such as for the WFD status assessment) which involve a simple assessment of water quality metrics, e.g. measured concentrations and associated statistics, and to demonstrate their achievement.
+
The approach adopted in this analysis and that of Wang et al. (2012b)<ref name="Wang 2012b">WANG, L, STUART, M E, BLOOMFIELD, J P, BUTCHER, A S, GOODDY, D C, MCKENZIE, A A, LEWIS, M A, and WILLIAMS, A T. 2012b. Prediction of the arrival of peak nitrate concentrations at the water table at the regional
 +
scale in Great Britain. ''Hydrological Processes'', Vol.&nbsp;26, 226–239.</ref> is likely to be beneficial for the targeting of catchment management activities at national and regional scales. For example, Figure 2.22 illustrates that legacy nitrate in the unsaturated zone at a national scale is dominated by the Chalk. Figure 2.23 shows that within the Chalk, there a substantial historical mass of nitrate in the unsaturated zone of southern England, particularly in interfluve areas where travel times are long. Consequently, environmental managers should take into account this mass when considering the implementation of catchment mitigation measures in attempts to improve groundwater and surface water quality. This could also be important when setting environmental objectives (such as for the WFD status assessment) which involve a simple assessment of water quality metrics, e.g. measured concentrations and associated statistics, and to demonstrate their achievement.
  
 
[[Image:OR16036fig2.23.jpg|thumb|center|600px|  '''Figure 2.23'''&nbsp;&nbsp;&nbsp;&nbsp;Spatial distribution of total unsaturated zone nitrate mass (as kg N) in England and Wales in: a) 1960 and b) 2015.    ]]
 
[[Image:OR16036fig2.23.jpg|thumb|center|600px|  '''Figure 2.23'''&nbsp;&nbsp;&nbsp;&nbsp;Spatial distribution of total unsaturated zone nitrate mass (as kg N) in England and Wales in: a) 1960 and b) 2015.    ]]

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