Abstract
The aim of this paper is to explore patterns of wastewater infrastructures (sewers vs. septic tanks) in urbanizing watersheds across a coastal metropolitan region. This research combines an urban-rural gradient with spatial metrics at the patch and watershed scale (proportion of parcels on a treatment system, septic density, lot size and percent imperviousness) to analyze wastewater patterns in the Puget Sound, WA, USA. Results show that most urban residential parcels are hooked up to a sewer, although there remain urban residences on a septic tank with small lots. I find a complex arrangement of wastewater treatment in suburban watersheds representing a patchwork of parcels on sewers and septic tanks. Sewers dominate in total numbers, while the density of septic tanks is highest in this portion of the urban gradient. Lot size decreases from rural to urban; however, it varies depending on the type of wastewater treatment system. In urban watersheds, lots on septic tanks are significantly smaller than lots in suburban and rural watersheds and of a similar size compared to lots on sewers. I also find a significant difference in the amount of impervious surfaces in watersheds dominated by sewers vs. septic tanks. In the urban portion of the gradient, the amount of paved surfaces in parcels with septic tanks is also similar in level as parcels with sewers. I discuss how these patterns emerge from the interplay of biophysical, socio-economic and technological factors and how different regulatory regimes for septic tanks and sewers may further induce these patterns.
Highlights
Coastal areas in the United States and in the world are experiencing some of the highest rates of population growth and land use change [1,2]
The Puget Sound metropolitan region is located in Western Washington and comprises 12 counties in total, of which this study focuses on seven with varying land uses and population density (Figure 2)
The formulation of septic regulations is generally confined to the narrow task of site-scale controls based on bio-physical characteristics and concerns for human health and water quality
Summary
Coastal areas in the United States and in the world are experiencing some of the highest rates of population growth and land use change [1,2]. Centralized wastewater infrastructures collect sewage from multiple sources and transport it through a network of sewers and pumps to be treated by a central plant before discharging effluent to surface and/or groundwater. Since the passage of the Clean Water Act (1972) and subsequent environmental policies, sewers and wastewater treatment plants have improved coastal water conditions [13]. Despite this improvement, sewage and non-point sources of pollution continue to be leading causes of coastal contamination [9,14,15]. There is evidence that septic density in watersheds with low amounts of urban development lead to contamination in coastal waters [19,20]. Urban land cover, impervious surfaces and septic density are found to positively correlate with fecal pollution [21]
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