At an urban landfill facility located in central Italy, we investigate the processes leading to moderate amounts of VOC in groundwater (benzene), at one downgradient monitoring well. Increased alkalinity and sulfates, lower pH and high levels of redox-sensitive elements (Mn, Fe, As), might support both the hypothesis of leachate loss to groundwater or landfill gas-to-water transfer.
In this study, we analyze the available information concerning geochemistry, organic contaminants, environmental isotopes and landfill gas in groundwater to shed light on the possible mass transfer processes. We test the hypothesis that landfill gas migrating from the landfill at the downgradient end of the plant be responsible for high dissolved organic carbon (from methane and other organic compounds), lower pH (due to CO2), and the presence of limited amounts of VOCs transferred from the landfill gas to the groundwater.
Periodical groundwater monitoring campaigns have been conducted since 2016 at the site. Field parameters (T, EC, pH, DO, ORP) are measured with probes in a flow-through cell. Groundwater sampling is performed with an in-line device in order to minimize sample disturbance. Lab analysis are performed for major and trace elements. Organic compounds include DOC, PAH, PCB, VOC. Ammonia and sulphide are measured in the field with a portable UV-VIS. Environmental isotopes (18O, 2H, Tritium, 13C) have been assessed twice. The pressure of CH4, CO2, O2 in the piezometer free gas phase are measured in situ with Draeger X-am 7000 (CO2, O2) and laser INSPECTRA® LASER (GAZOMATTM) (CH4); dissolved gas in groundwater is analyzed by gas cromatography on the headspace.
Groundwaters are mostly anoxic (DO < 1 mg/L), with neutral to slightly basic pH and calcium-bicarbonate facies. Iron, manganese and arsenic are above national standards for groundwater. PAH, PCB and VOC were always below the standards except benzene at one downgradient well. The downgradient well stands out also for high EC, high alkalinity, low pH, very high As, Fe and Mn, above average DOC. Preliminary results on the landfill gas measurements indicate that methane is present both in the free phase in the wells’ headspace and as dissolved phase in groundwater.
The high values of methane support the hypothesis of a gas-phase transport of VOC localized at the downgradient end of the facility. Landfill gas CO2 might enhance the dissolution of carbonate minerals and increase alkalinity. The reducing capacity of CH4 favours the high levels of redox sensitive elements in groundwater. While the plant manager has already improved the gas extraction system, a sampling campaign is planned in spring 2019 to further investigate the impact of landfill gas on groundwater and verify the efficacy of the gas recovery measures.