BEHAVIOR AND IMPACT OF COW SLURRY BENEATH A STORAGE LAGOON - II - CHEMICAL-COMPOSITION OF CHALK POREWATER AFTER 18 YEARS

To determine the pollution hazard associated with the long-term storag e 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-b anked lagoon. Chalk porewater was extracted by centrifugation from suc cessive 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 t he highest concentrations of bicarbonate (HCO3), dissolved organic car bon (TOC), ammonium-nitrogen (NH4-N) and organic phosphorus (P-o) in t he 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, whic h was sited at the edge of the lagoon, were almost colourless and show ed less elevated concentrations of determinants compared to the first borehole with the exception of NO3-N. However, large increases in TOC, NH4-N and P-o were observed at 29 m in the second borehole indicating that the borehole had intercepted slurry which had migrated rapidly t hrough the chalk profile by preferential how along fissures in the Cha lk. There was visible evidence of slurry contamination on fissure face s of chalk teres extracted from both boreholes. Microbial activity was detected only on fissure faces and not in the porewaters of either bo rehole. However microbially mediated reactions were important in terms of the chemical transformations (organic carbon oxidation, nitrificat ion, nitrate reduction) taking place beneath the lagoon.