Source strengths, transport pathways and delivery mechanisms of nutrients, suspended solids and coliforms within a small agricultural headwater catchment

Abstract

Analysis of water samples and accompanying flow data collected (on ~100 occasions) from well defined land drain outlets located in a small catchment in NE Scotland were made over a five year period. The complex relationship between individual sources that can exist even within a small (200 ha) agriculturally managed headwater catchment was clearly evident. On average ~60% of the measured flow from the catchment outlet was accounted for, with ~50% originating from field drains and 10% from the farmyard. Certain field drains stopped flowing during the summer. Flow from the farmyard was continuous, and because livestock were present all year round also represented a renewable source of potential contaminants. The majority of nitrate and suspended sediment originated directly from field drainage. The variability in nitrate concentration between individual field drains was large and probably reflected differences in soil drainage properties. Farmyard drainage contributed a large proportion of the ammonium, phosphate and Faecal Indicator Organisms (FIO) measured as a flux from the catchment. On numerous sampling occasions the combined flux from individual sources was greater than the corresponding loss measured at the catchment outlet. This was attributed to result from the temporary storage/retention mechanisms (sedimentation, transformation or biological uptake/exchange) that can operate within the stream channel. Despite many fields being grazed and/or receiving regular applications of slurry/manure, the majority ~60% of the total flux of FIO still originated from the 'farmyard', with significant contributions from the field drains only occurring during the autumn. The presence of field drinkers and secure well maintained fencing denying cattle access to the open drainage channel (often a recommended best management practice) may well have contributed to this observation. Benefits to water quality that might arise from riparian management, such as buffer strips in this particular situation may be limited due to the dominant contribution originating from land drains and farmyard.