Sewer construction, decay, and stream burial: Implications for urban ecosystems

Background/Question/Methods

Aquatic ecosystems provide vital habit, regulate nutrient and hydrologic regimes and provide drinking water sources essential to human society. Urbanization of the landscape has disrupted natural stream function and form, leading to a consistent degradation of stream ecosystems, often referred to as the “urban stream syndrome.” With an estimated 87% of the U.S. population expected to reside in urban centers by 2030, the impacts of sprawling develop on stream ecosystems will likely increase. An understanding of how existing infrastructure systems evolved is vital to successful water resources management and stream rehabilitation. This study examines the spatiotemporal dynamics of sewer infrastructure construction and decay and the implications of these processes to urban ecosystems. Existing sewer networks were compared with synthetic renderings of historic drainage networks to identify sewers in close proximity to natural drainage systems. Flow through each sewer reach was estimated using flow data gathered at specific points and extrapolated across the network. Hot spots for sewer leakage were identified based on the deterioration rates of various pipe materials and sewer age.  

Results/Conclusions

Mapping identified over 123 kilometers of brick sewer mains that were constructed during the late 19^th and early 20^th centuries, making these sewers over 100 years old. The majority of these brick sewers were placed in topographically advantageous positions, often coinciding with historic stream channels. Average deterioration rates indicate these brick combined sewers are leakage hot spots. These hot spots facilitate the transfer of bacteria, metals, organic compounds including pharmaceutical and personal care products (PPCPs), and nutrients to subsurface and surface flowpaths. These hot spots result from systematic human decision making on infrastructure maintenance over the last 100 years. If deferred sewer maintenance continues, urban ecosystems face an increasingly important contaminant source. To quantify the implications of this sewer leakage on the urban system, scenarios of sewer replacement and repair were explored to quantify total reductions in flux from varied replacement strategies. While it’s clear that urban systems can benefit from rehabilitation of degraded surface waterways, these surface oriented projects can be used to treat leaking sewer infrastructure, a dubious practice. Eliminating contaminants sourced from leaking sewers hot spots may yield more reductions in priority pollutants and nutrients than costly and potentially unstable urban stream restoration.