OR/18/011 Construction of a UK denitrification map

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Stuart, M E. 2018. Review of denitrification potential in groundwater of England. British Geological Survey Internal Report, OR/18/011.

Approach[edit]

It is proposed to use a ‘lines of evidence’ based approach. This will develop a series of GIS layers that will be overlain to provide the three main lines.

Hydrological settings favouring development of low redox zones[edit]

This line of evidence will draw on the material set out in UK aquifers. Literature data are predominantly related to the confined zones of the three principal aquifers. The distance of the redox boundary from outcrop has been established for these aquifers. This approach will be extended to the confined zones of other principal and secondary aquifers where this can potentially be corroborated by monitoring data.

Other areas that need to be considered are areas where there are significant low permeability or high organic carbon superficial deposits and areas of floodplain deposits that may confine underlying shallow groundwater

Data which need to be collected are:

  • Aquifer subcrop – This will be derived from the BGS 3D model. This can provide depths for the top of the main formations, e.g. Chalk Group, Lower Greensand Group, Corallian Group, Great and Inferior Oolite Groups, Lias, Sherwood Sandstone Group, Zechstein Group, Coal Measures, Dinantian Limestone, Devonian. The total aquifer depth will be defined as 400 m below the surface to retain consistency with other approaches.
  • Superficial deposits:
  • Low permeability deposits classified using the approach of Griffiths et al. (2011)[1]. This approach will need to take account of both lithology and thickness
  • Organic deposits will include peat
  • Alluvium will be used as a simple surrogate for the presence of a hyporheic zone

Groundwater monitoring data indicating potential denitrification[edit]

In the first instance, values from the Environment Agency strategic monitoring network will be used. This will include oxidised nitrogen species, TON or NO3-, NO2- and NH4+. It will also include parameters which could be indicative of suitable reducing conditions, DO, DOC, Mn2+, Fe2+, SO42-. These will also be used to attempt to delineate areas where widespread denitrification is unlikely. Indicator values for the presence of denitrification will be drawn from Processes, measurements and indicators and UK aquifers, mainly from Table 2.4.

Data which need to be collected are:

  • Environment Agency strategic monitoring data for TON or NO3-, NO2-, NH4+. DO, DOC, Mn2+, Fe2+, SO42-, including designated aquifer, borehole depth and confined/unconfined.

Mass balance approaches[edit]

These will identify areas where inputs of N do not appear to be reflected in current groundwater concentrations. Particularly where N is absent Inputs could include N applications to arable land and urban areas which will be drawn from data used in the NVZ delineation process, and also aerial deposition. Aerial deposition is represented within the NEAP-N model.

  • nitrate loading concentrations related to the NVZ designation process
  • estimate of nitrate in recharge — The Baseline series reports make an estimation for each area
  • Figure 5.1    Map component layers.
  • Figure 5.2    Overlay of data layers.

References[edit]

  1. GRIFFITHS, K, MACDONALD, A, ROBINS, N, MERRITT, J, BOOTH, S, JOHNSON, D, and MCCONVEY, P. 2011. Improving the characterization of Quaternary deposits for groundwater vulnerability assessments using maps of recharge and attenuation potential. Quarterly Journal of Engineering Geology and Hydrogeology, Vol. 44, 49–61.