Examining the Effects of Wastewater Infrastructure on Puget Sound Near-shore Water Quality Across a Gradient of Urbanization
Spirandelli, Daniele J.
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As shorelines develop, wastewater infrastructure systems are designed to simultaneously support urban development and protect human and ecosystem health from sewage contamination. The ability to effectively manage coastlines in an urban setting depends on understanding the interactions between human and biophysical systems, and to incorporate that information into policy and planning decisions that support regional water quality goals. Past research has linked the contamination of coastal and estuarine ecosystems to upland land use, as well as centralized and decentralized wastewater systems. Missing from this research is an evaluation of how urban development relates to alternative wastewater infrastructures and the links between coastal water quality, wastewater infrastructure and patterns of urbanization. My research sought to systematically assess alternative wastewater infrastructures across an urban gradient and to assess the relationships between wastewater disposal type, land use-cover pattern, and near-shore water quality. I evaluate the spatial characteristics of parcels with sewers vs. onsite septics in sub-watersheds and link wastewater type to urban patterns to answer two questions: (1) How are alternative wastewater infrastructures spatially distributed across an urban gradient? (2) How do different wastewater infrastructures relate to land-use and land-cover patterns? At a larger landscape-scale, I relate indicators of near-shore water quality to wastewater infrastructure and land use-land cover patterns to address a third question: (3) How does near-shore water quality relate to wastewater infrastructure and urban development patterns? This research relies on a three-part approach that uses quantitative measures of wastewater patterns, landscape characteristics, and microbial conditions for shellfish growing areas. To answer the first question, I quantify and compare spatial measures of parcels on two types of wastewater systems. I address the second question by relating land-use and land-cover pattern (composition and configuration) to wastewater type (septic vs. sewer). I answer the third question by combining spatial information about land-use, land-cover and wastewater in near-shore basins to assess how these factors influence water quality in shellfish growing areas. The results from the study in the Puget Sound support previous findings that suburban sub-basins with moderate amounts of development contain significantly higher counts and densities of residential parcels with onsite-septics than urban or rural counterparts. However, I did not find septic density to be a significant predictor of fecal coliform (FC) in shellfish growing areas; the number of septics was only moderately related to FC. The total amount of high intensity urban land cover was the single best predictor of near-shore water quality. I found significant associations between wastewater treatment type and the composition and configuration of land cover. On average, Puget Sound basins with 20% of their area or more covered by high urban land cover are more likely to be dominated by parcels on a sewer system.
- Urban planning