Pathways of contaminant transfers to water from an artificially-drained soil under intensive grazing by dairy cows

There is limited quantitative understanding of the pathways and temporal patterns involved in the transfer of nitrogen (N), phosphorus (P), faecal bacteria and sediment from grazed temperate pastoral land to water. Because these transfers have the potential to significantly impair water quality, many catchment and community groups want to identify farming systems and management strategies that minimise the risks that farming activities pose for their water quality goals. Here we document the concentrations and loads of N, P, sediment and the faecal indicator bacteria E. coli (E. coli) measured in mole-pipe drainage (3-year monitoring period) and surface runoff (2-year monitoring period) collected from hydrologically-isolated plots grazed by dairy cattle in southern New Zealand. We also evaluate the effectiveness of restricted autumn grazing management as a strategy that could potentially reduce these losses. Monitoring indicated that 62 and 25% of estimated surplus rainfall was captured as mole-pipe drainage and surface runoff, respectively. Mole-pipe drainage was the most important pathway of N loss, accounting for c. 80% of the total soluble N discharged in the combined flows from mole-pipe drainage and surface runoff. In contrast, surface runoff proved to be an important pathway for P and sediment movement, and contributed 68% of the calculated flux of the faecal bacteria E. coli. Temporal patterns of loss suggest that the management of urinary N deposited in the months preceding winter drainage events is key for reducing potential N losses in drainage. A restricted autumn grazing strategy reduced losses of dissolved N in subsurface drainage by 43% on average, although statistically significant reductions were only observed for 2 of the 3 measurement years. Temporal patterns of P, sediment and E. coli fluxes suggest that management strategies which can reduce surface runoff generated during spring on these poorly-drained soil types may be the most effective approach for reducing the impacts of these contaminants on surface water quality. ã 2015 Elsevier B.V. All rights reserved.