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Abstract Miami‐Dade County (MDC) has over 112,000 septic systems, some of which are at risk of compromise due to water table rise associated with sea level rise. MDC is surrounded by protected water bodies, including Biscayne Bay, with environmentally sensitive ecosystems and is underlain by highly transmissive karstic limestone. The main objective of the study is to provide first estimates of the locations and magnitudes of septic return flows to discharge endpoints. This is accomplished by leveraging MDC's county‐scale surface‐groundwater model using pathline analysis to estimate the transport and discharge fate of septic system flows under the complex time history of groundwater flow response to pumping, canal management, storms, and other environmental factors. The model covers an area of 4772 km²in Southeast Florida. Outputs from the model were used to create a 30‐year (2010 to 2040) simulation of the spatial–temporal pathlines from septic input locations to their termination points, allowing us to map flow paths and the spatial distribution of the septic flow discharge endpoints under the simulated conditions. Most septic return flows were discharged to surface water, primarily canals 52,830 m³/d and Biscayne Bay (5696 m³/d), and well fields (14,066 m³/d). Results allow us to identify “hotspots” to guide water quality sampling efforts and to provide recommendations for septic‐to‐sewer conversion areas that should provide most benefit by reducing nutrient loading to water bodies. 

Abstract High nighttime urban air temperatures increase health risks and economic vulnerability of people globally. While recent studies have highlighted nighttime heat mitigation effects of urban vegetation, the magnitude and variability of vegetation-derived urban nighttime cooling differs greatly among cities. We hypothesize that urban vegetation-derived nighttime air cooling is driven by vegetation density whose effect is regulated by aridity through increasing transpiration. We test this hypothesis by deploying microclimate sensors across eight United States cities and investigating relationships of nighttime air temperature and urban vegetation throughout a summer season. Urban vegetation decreased nighttime air temperature in all cities. Vegetation cooling magnitudes increased as a function of aridity, resulting in the lowest cooling magnitude of 1.4 °C in the most humid city, Miami, FL, and 5.6 °C in the most arid city, Las Vegas, NV. Consistent with the differences among cities, the cooling effect increased during heat waves in all cities. For cities that experience a summer monsoon, Phoenix and Tucson, AZ, the cooling magnitude was larger during the more arid pre-monsoon season than during the more humid monsoon period. Our results place the large differences among previous measurements of vegetation nighttime urban cooling into a coherent physiological framework dependent on plant transpiration. This work informs urban heat risk planning by providing a framework for using urban vegetation as an environmental justice tool and can help identify where and when urban vegetation has the largest effect on mitigating nighttime temperatures.