Leaching of from stony soils after effluent application.

Irrigation of dairy shed effluent (DSE) onto land is an integral part of New Zealand's farming practice. The use of inappropriate soils can result in contamination of ground waters with microbes and nutrients. A gap in our knowledge is the ability of stony soils to safely treat DSE. Replicates of four stony soils were collected from the Canterbury region of New Zealand as intact soil lysimeters 460 mm in diameter and up to 750 mm deep. The soils had either stones to the surface or 300 to 600 mm fines over stones. To determine breakthrough characteristics, a pulse of DSE (25 mm depth) spiked with bromide (2000 mg L) was applied to the soil cores followed by continuous artificial rainfall, for one pore volume, at 5 mm h. Leachate aliquots were analyzed for , bromide, and NH-N. The lysimeters were then subjected to hoof pugging using a mechanical hoof, and the leaching characteristics of the soil were determined again. breakthrough curves revealed that the potential for to leach through the soils was high for Selwyn very stony soil and low for other soils analyzed. After pugging, leaching of increased in Mackenzie soil with stones to the surface. For most other soil cores, concentrations in soil leachates were low. In soils where stones are close to the surface, especially where the soil matrix is sandy, we anticipate that shallow groundwater is vulnerable to microbial contamination under some land management practices.