Landscape patterns and wastewater infrastructure: Are there ecological trade-offs to alternative wastewater infrastructures?
Wastewater is a leading cause of water contamination in urbanizing coastal regions and a key contributor to the loss of nearshore ecosystem functions. Previous studies examining the impacts of urbanization on coastal ecosystems have identified both sewers and on-site septics as leading stressors. Little empirical research has been done to study the land use and land cover patterns associated with different wastewater disposal types and the implications for coastal ecosystems. As urbanization increases in coastal regions worldwide, we must assess the trade-offs of our wastewater infrastructure decisions, including ecological trade-offs. We conducted this study in seven counties across a gradient of urbanization in the Puget Sound, WA., one of the most rapidly developing shorelines in the U.S. We addressed three 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? (3) How do indicators of near-shore water quality relate to wastewater infrastructure and urban development patterns? This research relies on a landscape approach that uses spatial measures of wastewater patterns, landscape characteristics, and relates them to microbial conditions for shellfish growing areas.
This study finds that there are distinct patterns of wastewater infrastructures across a gradient of urbanization. Parcels on sewers are found in small lots across urban, suburban and rural coastal basins. Sub-watersheds dominated by sewers also contain the highest amounts of impervious surfaces regardless of where they are on the urban gradient. The results also support previous findings that suburban sub-basins with moderate amounts of development contain significantly higher counts and densities of onsite-septics than urban or rural counterparts. However, we did not find septic density to be a significant predictor of contamination in shellfish growing areas. The total amount of high intensity urban land cover was the single best predictor of near-shore water quality. We also found significant associations between wastewater treatment type and the composition and configuration of land cover. On average, Puget Sound basins with as little as 20% of their area covered by intense urban land cover are more likely to be dominated by parcels on a sewer system than a septic system.