Groundwater nitrogen composition and transformation within a moorland catchment, mid-Wales

Abstract

The importance of upland groundwater systems in providing a medium for nitrogen transformations and processes along flow paths is investigated within the Afon Gwy moorland catchment, Plynlimon, mid-Wales. Dissolved organic nitrogen (DON) was found to be the most abundant form of dissolved nitrogen (N) in most soils and groundwaters, accounting for between 47 and 72% of total dissolved nitrogen in shallow groundwater samples and up to 80% in deeper groundwaters. A significant correlation between dissolved organic carbon (DOC) and DON in the shallow groundwater system suggests that during the autumn and winter months the flux of DON from the soil zone may be regulated by the wetting up and release of soil organic matter. DON needs to be taken into account when considering catchment N balances otherwise N export from upland catchments could be significantly underestimated. DON may also be an important source of bio-available N in surface waters and marine systems fed by upland catchments. A conceptual model of N processes is proposed based on a detailed study along a transect of nested boreholes and soil suction samplers within the interfluve zone. Shallow groundwater N speciation reflects the soil N speciation implying a rapid transport mechanism and good connectivity between the soil and groundwater systems. Median dissolved inorganic N concentrations were an order of magnitude lower within the soil zone (10-25 mg/L-N) than in the shallow groundwaters (100-750 mg/L-N).Given the rapid hydrostatic response of the groundwater level within the soil zone, the shallow groundwater system is both a source and sink for dissolved N. Results from dissolved N2O, N2/Ar ratios and dissolved N chemistry suggests that microbial N transformations (denitrification) may play an important role in controlling the spatial variation in soil and groundwater N speciation. Reducing conditions within the groundwater and saturated soils of the wet-flush zones on the lower hillslopes, a result of relatively impermeable drift deposits, are also important in controlling N speciation and transformation processes.

Macronutrient status of UK groundwater: Nitrogen, phosphorus and organic carbon

“…DON can comprise a significant proportion of the total N load in some circumstances, up to 90% in forested glacial outwash in Canada with concentrations increased by anthropogenic impacts (Kroeger et al, 2006). Lapworth et al (2008b) also found DON was the most abundant form of dissolved N in both soils and groundwaters in a moorland catchment, accounting for between 47 and 72% in shallow groundwater samples and up to 80% in deeper groundwaters.…”

“…For sites in upland mid Wales, Lapworth et al (2008b) found the range of DOC in groundwater ranging from 0.4 mg/L to 10.6 mg/L with a median of 2.1 mg/L. All samples were below 4.3 mg/L except one.…”

“…The inorganic species are largely dependent on the redox condition of the water with nitrate being stable under oxidising conditions and ammonia and nitrite being metastable under reducing conditions. DON has not commonly been analysed for in groundwater, but may form a significant component, certainly in the Chalk (Peach et al, 2006) and in upland catchments (Lapworth et al, 2008b).…”

“…These processes are not easily distinguishable based on nitrate concentrations and lithology alone, a range of hydrochemical and isotope techniques have been used effectively to constrain denitrification processes (e.g. Feast et al, 1998;Gooddy and Darling, 2009;Lapworth et al, 2008b). Alternative urban sources may, however, still be significant and explain some elevated occurrences; for example, the Birmingham aquifer in the West Midlands, underlying the UK's second city, is classified in the maximum concentration band.…”

Understanding Phosphorus Mobility and Bioavailability in the Hyporheic Zone of a Chalk Stream

“…chemical and microbiologically mediated nutrient and weathering transformations on particle surfaces (Boulton et al 1998;Gooseff et al 2002;Greenwald et al 2008). Understanding these processes is important and may vary depending on the hydrology, land use, ecological sensitivity and management of a particular system: to determine the sustainable limits of abstraction (Cook et al 2003) in a water-sensitive system, to understand the hydrological function of the riparian zone (Fernald et al 2001;) and/or to assess the function of the riparian zone in regulating biogeochemical processes (Findlay et al 1993;Jones et al 1995;Hill 1996;Jarvie et al 2006a;Rassam et al 2006;Lapworth et al 2008).…”