GEOCHEMICAL EVOLUTION OF A SEWAGE-CONTAMINATED, SAND AND GRAVEL AQUIFER, CAPE COD, MASSACHUSETTS

Land disposal of treated sewage effluent created a plume of reduced and contaminated ground water that makes part of the sole-source aquifer of western Cape Cod, Massachusetts, unavailable for water supply. Mineralogy of the sand and gravel aquifer is dominated by quartz; chemical reactivity is dominated by poorly crystalline iron and aluminum oxyhydroxides that coat mineral grains. Chemical contrasts between the sewage effluent and the acidic (pH 5.0-5.8), dilute, native ground water, along with microbial metabolic processes within the aquifer stimulated by the oxygen, nitrate, and organic carbon in the effluent, resulted in vertical and horizontal gradients in ground-water chemistry. The sewage plume had a core characterized by near-neutral pH values, dissolved iron(II) concentrations of >500mM, and moderate concentrations of ammonium. The core was surrounded by ground water with pH values of 5.8-6.2, very low concentrations of dissolved oxygen, low concentrations of dissolved manganese, and high concentrations of nitrate. The aquifer within 600 m of the source was contaminated with phosphate and a more limited region was contaminated with sewage-derived zinc and copper.

In December 1995, sewage disposal ceased and the portion of the plume near the disposal beds has been allowed to evolve naturally. Some sewage-plume constituents, such as boron, have flushed out of the region immediately downgradient of the disposal beds. Nitrate concentrations have attenuated by flushing away from the disposal beds and by slow denitrification, but have persisted in a limited region near the beds as a result of nitrification of material stored in the sediments. Redox conditions and the pH gradients within the plume have changed little since disposal ceased owing to consumption of dissolved oxygen in the inflowing ground water by high levels of biological activity fueled by organic carbon sorbed to the aquifer sediments near the disposal beds. Iron concentrations directly under the disposal beds have actually increased. Phosphorus and dissolved metal concentrations have changed little. Reactive transport modeling indicates that the large mass of constituents sorbed to sewage-contaminated sediments will prevent significant decreases in their concentrations and that the iron-reducing zone will persist for many decades.