Effects of carbon dioxide variations in the unsaturated zone on water chemistry in a glacial-outwash aquifer

The research site at Otis Air Base, Cape Cod, Massachusetts, has been developed for hydrogeological and geochemical studies of sewage-effluent contaminated ground water since 1982. Research of hydrologic properties, transport, and chemical and biological processes is ongoing, but the origin of background water chemistry has not been determined.The principal geochemical process giving rise to the observed background water chemistry is CO₂-controlled hydrolysis of Na feldspar. Geochemical modeling demonstrated that CO₂ sources could vary over the project area. Analyses of unsaturated zone gases showed variations in CO₂ which were dependent on land use and vegetative cover in the area of groundwater recharge. Measurements of CO₂ in unsaturated-zone gases showed that concentrations of total inorganic C in recharge water should range from about 0.035 to 1.0 mmoles/L in the vicinity of Otis Air Base. Flux of CO₂ from the unsaturated zone varied for 4 principal land uses, ranging from 86 gC/m²/yr for low vegetated areas to 1630 gC/m²/yr for a golf course. Carbon dioxide flux from woodlands was 220 gC/m²/yr, lower than reported fluxes of 500 to 600 gC/m²/yr for woodlands in a similar climate. Carbon dioxide flux from grassy areas was 540 gC/m²/yr, higher than reported fluxes of 230 to 490 gC/m²/yr for grasslands in a similar climate.