Groundwater nitrogen composition and transformation within a moorland catchment, mid-Wales.
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. Groundwater 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 soilwater N speciation implying a rapid transport mechanism and good connectivity between the soil and groundwater systems. Median nitrate concentrations were an order of magnitude lower within the soil zone (<5-31 microg/L) than in the shallow groundwaters (86-746 microg/L). 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 N(2)O, N(2)/Ar ratios and dissolved N chemistry suggests that microbial N transformations (denitrification and nitrification) 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.
Lapworth
D J
DJ
British Geological Survey, Maclean Building, Crowmarsh Gifford, Wallingford, Oxon, OX10 8BB, UK. djla@bgs.ac.uk
Shand
P
P
Abesser
C
C
Darling
W G
WG
Haria
A H
AH
Evans
C D
CD
Reynolds
B
B
D002244:Carbon / Q000032:analysis; D004784:Environmental Monitoring; D055593:Geological Phenomena; D005844:Geology; D007501:Iron / Q000032:analysis; D008962:Models, Theoretical; D009566:Nitrates / Q000032:analysis; D009573:Nitrites / Q000032:analysis; D009584:Nitrogen / Q000032:analysis*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000032:analysis; D010944:Plants; D000644:Quaternary Ammonium Compounds / Q000032:analysis; D011891:Rain; D012987:Soil / Q000032:analysis; D014852:Wales; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000032:analysis*; D014881:Water Supply / Q000032:analysis*
D009566:Nitrates; D009573:Nitrites; D000644:Quaternary Ammonium Compounds; D012987:Soil; D014874:Water Pollutants, Chemical; D002244:Carbon; D007501:Iron; D009584:Nitrogen; D010100:Oxygen