Hydrogeologic study of the upper water-bearing zone in the vicinity of the Palos Verdes landfill

The upper water-bearing zone near the Palos Verdes Landfill has been affected by a variety of natural and artificial influences. The natural influences include fluvial and mass wasting processes. The natural influences have produced variations in the properties of the unconsolidated materials. The artificial activities include mineral mining, commercial and residential land development, landfilling, and impacts from associated manmade subsurface structures. These activities have altered the physical and chemical nature of the site's hydrogeologic regime. The site's ground-water characteristics are derived from a complex orchestration of geology, hydrogeology, and aqueous geochemistry. The stratigraphy consists of five primary units: (1) bedrock, (2) alluvium, (3) mine tailings, (4) engineered fill, and (5) slide material. Much of the upper weathered portion of the bedrock was removed during mining and landfill operations. The impact of these operations is evident in the heterogeneity of the hydrogeologic and Qeochemical regime across the site. The quality of the water being discharged through the site as underflow, has been influenced by landfill operations; organic and inorganic water quality has been degraded. Vinyl chloride has been detected in water samples collected from the site at concentrations up to 5,400 parts per billion. The water quality impact is related to (1) residual organic contaminants present before the installation of the cement-bentonite barrier, (2) contaminant transfer from fugitive landfill gas migration, (3) liquid migration from the landfill, and (4) sources not associated with the landfill. The inorganic water-quality data for wells M-01 B and M-028 compares well with data from the Portuguese Bend Landslide; thereby indicating that the inorganic water quality in these two wells may not have been affected by landfill operations. Recommendations are made to overcome limitations in the available database. These recommendations include geophysical techniques, constant discharge pumping tests, and naturally occurring stable isotope analysis. The incorporation of these recommendations into future work at the site is intended to augment the existing database and provide a means of quantifying the extent of landfill impact on local water quality.