Contaminant Losses in Overland Flow from Cattle, Deer and Sheep Dung

Abstract

The loss of contaminants (phosphorus, nitrogen, sediment and faecal bacteria) in overland flow represents a risk to surface water quality. This study examined the loss of contaminants in overland flow after application of cattle, deer and sheep dung at an equivalent stocking rate (15 su ha−1) to field plots. After 1, 3, 7, 12, 19 and 30 days three replicate intact samples of the control and each dung treatment were excavated using a cutting blade (100 cm long × 20 cm wide × 10 cm deep), placed into boxes and simulated rainfall used to generate flow. Data indicated that P fractions, NH4+-N, suspended sediment (SS) and E. coli concentrations generally decreased with time since application due to chemical and physical breakdown of dung. The exceptions were E. coli from sheep dung (relatively constant at 5.25 log10 cfu 100mL−1) and NO3−-N for all dung (constant between 0.5 and 1.0 mg NO3−-N L−1). Integrating data with time and expressing this relative to current limits for lowland surface water quality gave an estimate of the potential risk posed by dung released into overland flow during a grazing period of 30 d. This showed that the greatest risk was for E. coli > dissolved reactive P > total P > NH4+-N, while NO3−-N and SS risk was negligible. However, since most risk was accounted for by the first few days after deposition when dung was wet, risk could be minimised by not grazing on areas likely to produce overland flow (e.g., wet soils).