Effects of Precipitation on Landfill Ground Water Monitoring Data

By Arnie Sugar, LHG

Presented at the 4th Symposium on the Hydrogeology of Washington State
Tacoma, Washington, April 8-10, 2003

Abstract

Ground water monitoring data over a 12 year period was evaluated at a landfill in Pierce County. The landfill was closed under the Minimum Functional Standards for Solid Waste Handling (Chapter 173-304 WAC) in 1991. Closure included a 60 mil membrane, drainage layers, topsoil, surface water control, and passive gas venting. Ground water quality samples were collected from four years before closure to eight years after closure, and were analyzed for volatile organic compounds, four metals (arsenic, manganese, iron, and zinc) chloride, sulfate, total organic carbon, total coliform bacteria, pH, and specific conductivity.

Post-closure ground water monitoring data indicated a general decrease in concentrations of most indicator parameters after closure, then a marked increase four to five years after closure. Long term (1-5 years) trends in analyte concentrations correlated to precipitation trends, with soluble and mobile parameter concentrations exhibiting the greatest apparent sensitivity to precipitation. This data suggests that long term precipitation trends may strongly influence ground water monitoring results, and therefore must be considered during performance and regulatory compliance evaluation.