USE OF ISOTOPE ANALYSES TO HELP EXPLAIN THE GEOCHEMISTRY OF GROUNDWATER CONTAINING ANOMALOUS SULFATE CONCENTRATIONS NEAR A MUNICIPAL LANDFILL IN AN AREA OF GEOTHERMAL ACTIVITY

Relatively large concentrations of sulfate were detected in monitoring wells at a municipal landfill in northern California. The region is also home to geothermal activity including hot water springs and vapor dominated geysers. The geothermal activity is commercially tapped for electric power generation within 25 to 30 km of the landfill. The main question was whether the sulfate detected in some of the down gradient monitoring wells originated from the landfill refuse or if the sulfate was from naturally occurring sources in the area. The concentration of sulfate in the monitoring wells ranged from approximately 1 to nearly 2000 mg/L. The largest concentrations were observed in the down gradient wells. Samples were collected from an up gradient well, two down gradient wells and the landfill leachate. Several isotope analyses, including δ³⁴S, δ¹³C, δ¹⁸O, δD, ³H, and ¹⁴C, were completed to help determine the source of sulfate in the monitoring wells and whether leachate may have impacted the down gradient wells. The δ³⁴S values, which ranged from -5 ‰ to +9 ‰, suggested the source of sulfate was naturally occurring, probably caused by oxidation of pyrite in the surrounding sediments. An evaluation of the δ³⁴S data and sulfate concentrations indicated that sulfate reduction was also occurring at the site. The δ¹³C ranged from -15 to +11.7 ‰ and supported the occurrence of sulfate reduction in the down gradient wells. The δ¹⁸O and δD plotted near the global meteoric water line and did not indicate any significant mixing of regional geothermal waters associated with geysers with the local groundwater at the site. The ¹⁴C activity and ³H concentration were significantly larger in the down gradient wells compared to the up gradient well and suggested that landfill leachate had impacted some of the down gradient wells. Thus, the suite of isotopic data indicated that landfill leachate had impacted some down gradient wells but the large sulfate concentrations detected were from natural sources and did not originate from the landfill refuse.