The current groundwater-monitoring program includes monitoring wells placed around the active areas of the Simi Valley Landfill & Recycling Center. The locations of the wells are principally selected to intercept groundwater that flows beneath the landfill. The depth of the monitoring wells has been selected to monitor the uppermost groundwater. Monitoring wells are sampled and inspected on a regular basis in accordance with state and federal guidelines.
The site is underlain by alternating sandstone, claystone, and clayey siltstone beds of the Sespe Formation, and alluvial deposits of sand and gravel in the axis of the main canyon. The bedding of the Sespe Formation generally strikes approximately northeast-southwest and dip to the northwest between 25 and 35 degrees. Groundwater has been encountered in monitoring wells within the Sespe Formation at depths ranging from about 10 to 180 feet below grade.