BEHAVIOR AND IMPACT OF COW SLURRY BENEATH A STORAGE LAGOON - 1 - GROUNDWATER CONTAMINATION 1975-1982

The chemical and biological composition of groundwater sampled from a 76 m deep borehole situated 80 m from an unlined, earth-banked lagoon excavated in Upper Chalk and continuously used to store liquid cow man ure was regularly monitored from August 1975 to January 1982 for signs of pollution. The lagoon became self-sealing 3 months after the start of filling in 1975, but was further enlarged and re-filled in autumn 1976, and emptied by dragline in September 1978. and 1980. No signific ant changes in groundwater composition were detected in the period Aug ust 1975 to December 1978. In January 1979, significant numbers of Esc herichia coli and a large, rapid increase in concentrations of ammoniu m-nitrogen (NH4-N), phosphorus (P), potassium (K), sodium (Na), chlori de (Cl) and sulphate (SO4) were detected in the groundwater. Nutrient concentrations fluctuated at elevated levels throughout 1979 but decre ased during 1980. Additional bacterial and chemical (NH4-N, P and K) c ontamination was recorded in January 1981 and subsequent months. The c ontamination was considered to have occurred as a result of fissure fl ow through the unsaturated zone of the Upper Chalk following repeated disruption of the self-sealing layer during emptying of the lagoon by dragline in September 1978 and 1980. The results indicate that unlined , earth-banked slurry storage lagoons need to be carefully managed to avoid biological and chemical pollution of Chalk aquifers.