Contaminant losses in overland flow from dairy farm laneways in southern New Zealand

Runoff from dairy farms can be an important source of stream pollutants such as nitrogen (N), phosphorus (P), faecal bacteria and sediment. Although much recent progress has been made in understanding the role of grazed pastures as sources of these pollutants, relatively little is known about the role of non-pasture areas such as stock lanes. Here we document the concentrations and loads of N, P, sediment and Escherichia coli (E. coli) in overland flow from 7 laneways sites located on 4 dairy farms in an intensively farmed catchment in southern New Zealand. Monitoring over a 2-year period indicated that on average 23% of incident rainfall was captured as overland flow discharging from the sites. Overland flow was very enriched in nutrients and faecal bacteria, with mean concentrations similar to those found in farm dairy effluent. Concentrations were observed to be higher at sampling sites close to the milking parlour, presumably due to the greater frequency of excreta depositions in these more highly trafficked lengths of the farm laneway network. Although assessments of laneway contributions to catchment scale discharges of stream pollutants suggested this source made a relatively minor contribution when expressed on an annualised basis, laneways were predicted to be a potentially important source of stream pollutants during summer when stream flows are relatively low and temperatures are warmer. These findings show the potential impacts that laneway overland flow may have on streams if poorly-sited and/or poorly designed lanes discharge directly to streams.

► We measured pollutant losses in overland flow from dairy farm lanes.
► This flow was very enriched in nutrients and faecal bacteria.
► Extrapolated to a catchment scale, pollutants yields were however relatively low.
► Findings show the potential impact that poorly sited lanes may have on water quality.