Dairy cow deposits on farm roadways are a potential source of contaminants entering streams. Phosphorus (P), suspended sediment (SS) and Escherichia coli (E. coli) loads in 18 runoff events over 12 mo from two-halves of a section of dairy farm roadway that spilt into an adjacent P-impacted stream were measured. The runoff from one half was untreated while the other half was directed through a filter of steel melter slag [termed aluminium chlorohydrate (ACH)-altered slag] sprayed with 1% ACH solution to improve P sorption capacity. An uncertainty analysis was conducted to ascertain potential loads of P lost from roadways considering variation in deposit weight, number and P content. Over the monitoring period, the total load decreased P (92%), SS (98%) and E. coli (76%) from the ACH-altered slag roadway compared to the control. However, uncertainty analysis showed that the amount of dung-P deposited on the roadway could be 10-fold greater.