OR/18/011 Introduction

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

Background

An understanding of the fate of nitrate in groundwater is vital for managing risks associated with nitrate pollution, and to safeguard groundwater supplies and groundwater-dependent surface waters Rivett et al. (2008). In recognition of the risks and observed impacts, the European Commission introduced a directive to control the inputs of agricultural sources of nitrate to groundwater and surface water through the Nitrates Directive and the more recent Water Framework Directive. One of the main measures to control nitrate is the designation of nitrate vulnerable zones (NVZs). Recent review of the designation of NVZs in England has highlighted that in some locations measured groundwater nitrate concentrations were not as high as expected based on the model used. A possible reason for this is that the nitrate is being transformed through denitrification.

Aims and approach

To develop a tiered approach to assessing denitrification potential across groundwater in England (and Wales) using literature and existing groundwater and other relevant data. It is proposed to use three main lines of evidence:

1. Literature data on documented low redox zones indicating denitrification. These are predominantly in the confined zones of major aquifers. Sources will include papers and other published material, such as the BGS/Environment Agency Baseline reports and the Environment Agency’s groundwater quality reports.
2. Groundwater monitoring data. In the first instance, values from the Environment Agency strategic monitoring network will be used. This will include nitrogen species, TON or NO₃^-, NO₂^- and NH₄⁺. It will also include parameters that could be indicative of suitable reducing conditions, dissolved O₂, dissolved organic C, Mn²⁺, Fe²⁺, SO₄^2- and Eh. These will also be used to attempt to delineate areas where widespread denitrification is unlikely.
3. Mass balance approaches. These will identify areas where inputs of N do not appear to be reflected in current groundwater concentrations. Inputs could include N applications to arable land and urban areas, and aerial deposition.

Each of these lines of evidence is likely to provide a partial picture of denitrification potential. A tiered approach will be developed to combine these into a useful format together with applicability and limitations for each type of evidence. The results will be related to the geological setting for both bedrock and superficial deposits.

Structure

This report addresses the first line of evidence. Processes, measurements and indicators provides a high-level review of denitrification processes, their measurement and parameters that could be used as potential indicators to be taken forwards to the evaluation of monitoring data. Typical settings reviews typical settings from the international literature and UK aquifers reviews relevant data and estimates from the UK related to denitrification; this will include both published literature and Environment Agency and BGS reports. This will provide conceptual type settings in England where denitrification is likely to occur. Construction of a UK denitrification map provides and approach to data requirements and construction of the map.