Atmospheric deposition at groundwater dependent wetlands phase 2 : nutrient source apportionment case studies from England and Wales

Groundwater dependent terrestrial ecosystems (GWDTEs) face multiple pressures from both atmospheric and terrestrial sources, resulting in the loss of protected habitats and biodiversity. One of the most critical issues facing GWDTEs in England and Wales is anthropogenic pollution from nutrients. Anthropogenic nutrients can originate from a wide range of sources including industry and agriculture, and can be transmitted via multiple pathways including; surface waters, catchment runoff, groundwater, and atmospheric deposition. These multiple pathways pose a problem for environmental regulators and managers. In order to reduce nutrient damage to wetlands, environmental regulators must first have the tools to identify the dominant sources and pathways (source attribution) of nutrients. Environmental regulators need cost effective tools to identify the most common source of nutrients in order to implement effective measures to reduce pressures. However there are a lack of source apportionment studies for GWDTEs, and no framework by which to assess multiple sources of nitrogen. This report aims to bridge that gap by considering both atmospheric and terrestrial sources of nitrogen in one study. Three GWDTEs were studied all characterised during previous Water Framework Directive investigations; Wybunbury Moss, Newbald Becksies and Cors Bodeilio. Each site benefited from existing monitoring data and an evidenced based conceptual model, significantly reducing costs to this project. Field data collection included; inorganic chemistry of groundwater, surface water and rainfall, nitrogen and oxygen isotopes and CFC /SF6 and NH3 /NO2 diffusion tubes deployed to quantify atmospheric dry gaseous deposition. Desk based analysis included; modeled atmospheric source apportionment from www.APIS.ac.uk, catchment nutrient modelling using the ‘Farmscoper’ tool and calculation and comparison of nutrient fluxes against site relevant critical loads from both modeled and measured atmospheric deposition data. We found that;  Modelled atmospheric deposition data (www.APIS.ac.uk) was broadly comparable to our monthly on-site data collected at the three GWDTEs, but individual sites showed differing variability in ammonia concentrations compared with the national data. Modeled data provides a reliable way to quickly assess atmospheric loading at GWDTEs for national scale assessments, however site specific assessments should undertake their own measurements of ammonia concentrations.  Detailed on site assessments of the pressure from atmospheric deposition to individual habitats are possible using National Vegetation Classification (NVC) mapping combined with Critical Load thresholds and modelled atmospheric deposition. Together these can provide a high resolution picture at site scale, provided vegetation mapping is available  Open access modelling tool FarmScoper (ADAS) was successfully applied, however in both examples the modelling shows that even with land use changes the reduction in terrestrial nitrate would not be significant enough to meet the proposed groundwater ‘threshold’ values for nitrate.