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.

Penetration of herbicides to groundwater in an unconfined chalk aquifer following normal soil applications.

The persistence and penetration of the herbicides isoproturon and chlorotoluron in an unconfined chalk aquifer has been monitored over a 4-year period through soil sampling, shallow coring and groundwater monitoring. Chlorotoluron was applied on plots as a marker compound, having never been used previously on that, or surrounding fields. The fieldsite had a 5 degree slope with soil depths of 0.5 to 1.5 m and a water table between 20 and 5 m from the soil surface. Where the water table was deepest (9-20 m below surface (mbs)) little or no positive herbicide detections were made. However, where the water table was at only 4-5 mbs, a regular pesticide signal of around 0.1 microg/l for isoproturon and chlorotoluron could be distinguished. Over the winter recharge period automatic borehole samplers revealed a series of short-lived peaks of isoproturon and chlorotoluron reaching up to 0.8 microg/l. This is consistent with a preferential flow mechanism operating at this particular part of the field. Such peaks were occurring over 2 years after the last application of these compounds. Shallow coring failed to uncover any significant pesticide pulse moving through the deep unsaturated zone matrix at the fieldsite.

Impact of the New Zealand flatworm (Artioposthia triangulata) on soil structure and hydrology in the UK.

Through north west Europe, concern has been growing in recent years about the predatory nature of Artioposthia triangulata on indigenous earthworm species. In this study, the consequence of earthworm depletion by A. triangulata on soil structure and related hydrological processes is examined. Field measurements compare differences in saturated hydraulic conductivity between sites overrun by A. triangulata and neighbouring unaffected sites. Laboratory experiments were conducted on soil cores containing worms only, worms and A. triangulata, and a control with no worms or A. Triangulata. Differences in the water release characteristic and bulk density between treatments was studied. It is concluded that in the short term, infestation by flatworms and consequent depletion of earthworms will increase infiltration. However, as macropores degenerate or are removed over time, an increased risk of surface run off may result in increased pollution and flood hazards, whilst reduced drainage and subsequent waterlogging may reduce agricultural productivity in certain soils. More research into understanding the processes is required.