As urbanization continues so to does the generation of wastewater, much of which is discharged into the nearest stream channel. These effluent dependent waterways fundamentally differ from non-effluent rivers because the wastewater is nutrient rich and steadily released, often remaining disconnected from the floodplain aquifer. The outcomes of the introduced wastewater for riparian ecosystems vary depending on hydrogeomorphic traits of the receiving stream. Uncertainty remains over the temporal persistence, spatial extent, and biological diversity of these riparian communities. Two central questions drive our research: 1) Does development of riparian ecosystems along effluent-dependent waterways correlate with degree of connectivity between the surface flow and groundwater? 2) Are riparian plant communities of effluent-dependent waterways different from non-effluent rivers in community structure, composition, or richness? Using aerial photographs and field sampling, we examine changing spatial patterns of effluent discharge and effluent dependent waterways over the past forty years in Arizona. We also present a framework for categorizing the fate of municipal effluent with respect to the hydrogeomorphology of the receiving waterway, recognizing that each type has varying capacity to sustain riparian vegetation.
Results/Conclusions
Data compiled from state agencies show that of all major (>1MGD) effluent discharge points in Arizona, over 85 percent are municipal. Before 1970, fewer than twenty wastewater treatment plants were in operation; today, there are more than 160. We developed a scheme to classify the fate of municipal wastewater, quantifying the degree to which effluent is reused directly, recharged to aquifers, or used to create wetland/riparian habitat. We also categorize based on hydrogeomorphic setting: 1) ephemeral channels within a) an endogenic dryland river and b) an allogenic dryland river; 2) channels of perennial or spatially intermittent rivers; 3) stream aquifers; 4) regional aquifers; 5) urban lakes; and 6) reuse. Using a case-study approach, we quantified vegetation change through time for the effluent-dependent Santa Cruz River. Despite increases in effluent discharge, riparian forest area has remained fairly constant on the Upper Santa Cruz River. There have, however, been recent forest die-offs near the effluent release point for reasons that may relate to altered surface-groundwater connectivity. On the effluent-dependent Lower Santa Cruz River, riparian forests are younger, narrower, and willow-dominated. The goal is to establish policy recommendations for wastewater, with emphasis on assessing its role in restoring or sustaining riparian ecosystems.
