Effect of chemical amendments to dairy soiled water and time between application and rainfall on phosphorus and sediment losses in runoff.

Dairy soiled water (DSW) is a dilute, low nutrient effluent produced on Irish dairy farms through the regular washing down of milking parlours and holding areas. In Ireland, there is no closed period for the land application of DSW except where heavy rain is forecast within 48 h. Chemical amendments have the potential to decrease phosphorus (P) and suspended sediment (SS) loss from DSW applied to land. This study examined the impact of three time intervals (12, 24 and 48 h) between DSW application and rainfall and five treatments (control, unamended DSW, and DSW amended with lime, alum or ferric chloride (FeCl(2))) on P and sediment losses from an intact grassland soil in runoff boxes. Rainfall was simulated at 10.5 ± 1 mm h(-1). Phosphorus concentrations (1-1.6 mg L(-1)) in runoff from DSW application, while not quantitative measures of P loss to surface waters in the field, indicated the importance of incidental P losses and that the current 48 h restriction in Ireland is prudent. Unamended DSW application increased P loss by, on average, 71%, largely due to an increase in particulate phosphorus (PP) loss. All three amendments were effective in decreasing P and SS losses in runoff and, apart from the SS results for lime, were significantly different (p<0.05) to the control at at least one time point. Lime (a 64% reduction in total phosphorus (TP) in comparison with DSW only) was less effective than alum or FeCl(2), likely due to the lower solubility of CaCO(3) in water. Chemical amendment showed potential to decrease P losses from land application of DSW, but the efficacy of such amendments would need to be assessed in field trials and a cost-benefit analysis conducted to further examine whether they could be practically implemented on farms.