To determine the pollution hazard associated with the long-term storage of cow slurry, two boreholes were drilled to a depth of nearly 35 m in the unsaturated zone of the Upper Chalk beneath an unlined, earth-banked lagoon. Chalk porewater was extracted by centrifugation from successive 0.45 m length core sections and their chemical and biological composition determined. Porewaters from the first borehole, which was sited in the deepest part of the lagoon, were discoloured and showed the highest concentrations of bicarbonate (HCO3), dissolved organic carbon (TOC), ammonium-nitrogen (NH4-N) and organic phosphorus (Po) in the first 6 m directly beneath the base of the lagoon. Below this depth, element concentrations decreased more sharply and amounts of nitrate-nitrogen (NO3-N) increased. Porewaters from the second borehole, which was sited at the edge of the lagoon, were almost colourless and showed less elevated concentrations of determinants compared to the first borehole with the exception of NO3-N. However, large increases in TOC, NH4-N and Po were observed at 29 m in the second borehole indicating that the borehole had intercepted slurry which had migrated rapidly through the chalk profile by preferential flow along fissures in the Chalk. There was visible evidence of slurry contamination on fissure faces of chalk cores extracted from both boreholes. Microbial activity was detected only on fissure faces and not in the porewaters of either borehole. However microbially mediated reactions were important in terms of the chemical transformations (organic carbon oxidation, nitrification, nitrate reduction) taking place beneath the lagoon.