RAPID INFILTRATION WASTEWATER DISPOSAL OR AQUIFER RECHARGE BY RECLAIMED WATER THE CONTAMINANTS DON'T CARE

Increasing demands on water supplies resulting from shifting climate patterns and population growth will increase the need for reuse of wastewater treatment plant (WWTP) effluents. One form of direct WWTP reuse is rapid infiltration recharge of shallow aquifers to replenish depleted groundwater, a common practice in the arid southwestern United States. The same practice is used in many other areas of the country for WWTP effluent disposal, resulting in de facto reuse. A distinguishing characteristic of reclaimed WWTP effluent is the trace organic chemical composition resulting from use of consumer products. For 30 years, studies have been conducted at the U.S. Geological Survey’s Cape Cod Toxics Substances Hydrology Research Site on the subsurface fate and transport of a complex mixture of trace organic compounds in an extensive contamination plume that developed over 60 years of WWTP effluent disposal. Among the major groundwater contaminants detected is the endocrine disrupting compound 4-nonylphenol, a degradation product of nonionic surfactants. Although many of the more water soluble contaminants such as boron, 1,4-dichlorobenzene, and sulfamethoxazole are being flushed from the aquifer as a consequence of natural-attenuation processes following removal of the WWTP discharge source, 4-nonylphenol retains the same general footprint that was mapped out in the mid-1980s, showing long-term persistence and lack of mobility in the aquifer. Results of this long-term research, conducted under well-defined hydrological and geochemical conditions, suggest that similar groundwater contaminant plumes will develop over time in other aquifers receiving reclaimed water via rapid infiltration. The extent of such plumes will be governed by groundwater transport velocities, relative sorption of the diverse chemicals present, groundwater oxygenation conditions, and subsurface microbial populations. This research is relevant to disposal of both municipal WWTP effluent and domestic wastewater, because similar complex contamination plumes can develop in shallow groundwater impacted by onsite disposal through septic tanks. Over long periods, wastewater recharged into aquifers will introduce a complex chemistry that has the potential to result in large-scale impacts on groundwater quality.