NATURAL RESTORATION OF A TREATED-WASTEWATER PLUME IN A SAND AND GRAVEL AQUIFER, CAPE COD, MASSACHUSETTS

Land disposal by rapid infiltration of treated wastewater to a sand and gravel aquifer on Cape Cod, Massachusetts, for 60 years formed a groundwater contaminant plume that is about 23 meters thick, 1 kilometer (km) wide, and more than 10 km long. The plume is characterized by steep geochemical gradients, with a suboxic transition zone that contains less than 0.06 µM (micromolar) dissolved oxygen and elevated nitrate, and an anoxic core that contains ammonium and ferrous iron (Fe(II)). Since disposal ended in December 1995, the natural restoration of the plume has been monitored by intensive spatial and temporal sampling of wells and multilevel samplers along a 600-m-long transect that is aligned in the general direction of groundwater flow. Conservative, non-reactive chemical species, such as boron and chloride, were flushed from the aquifer by the inflow of uncontaminated groundwater within 5 years after disposal ended. Strongly sorbing chemical species, such as phosphate and zinc, have persisted at substantially elevated levels despite the high groundwater-flow velocity (about 0.4 meters per day). Dissolved oxygen levels in the core of the plume remain near zero more than 17 years after disposal ended owing to the large residual oxygen demand caused by sorbed organic matter associated with the sediments. Natural aquifer sediments are very low in organic carbon, but desorption of wastewater-derived organic matter continues to supply dissolved organic carbon (DOC) to the downgradient portions of the plume, while degradation of the organic matter produces long-term sources of Fe(II), ammonium, nitrate, and total carbon dioxide. Changes in the geochemical composition of the plume along the slowly advancing oxygen front are being closely monitored to document changes in the mobility of redox-active and pH-sensitive species and to predict when, if ever, distant portions of the contaminant plume will return to pre-contaminated oxic conditions. The results of this investigation indicate that the geochemical effects of the wastewater disposal will persist for decades, even near the disposal site, and have important implications nationally for the attenuation of millions of septic and wastewater plumes present in unconsolidated formations.