Captured streams and springs in combined sewers: A review of the evidence, consequences and opportunities

Abstract

Captured streams and springs may be flowing in combined sewers, increasing clean baseflow in pipes and wastewater treatment works (WwTWs), reducing pipe capacity and increasing treatment costs. The UK water industry is aware of this in principle, but there has been no explicit discussion of this in the published literature, nor have there been any known attempts to manage it. Instead, the current focus is on the similar intrusion of groundwater infiltration through pipe cracks and joints. We have conducted a thorough review of literature and international case studies to investigate stream and spring capture, finding several examples with convincing evidence that this occurs. We identify three modes of entry: capture by conversion, capture by interception, and direct spring capture. Methods to identify and quantify capture are limited, but the experience in Zurich suggests that it contributed 7-16% of the baseflow reaching WwTWs. There are negative impacts for the water industry in capital and operational expenditure, as well as environmental and social impacts of loss of urban streams. For a typical WwTW (Esholt, Bradford) with 16% of baseflow from captured streams and springs, we conservatively estimate annual costs of £2 million to £7 million. A detailed case study from Zurich is considered that has successfully separated captured baseflow into daylighted streams through the urban area, with multiple economic, environmental and social benefits. We conclude that there is a strong case for the UK water industry to consider captured streams and springs, quantify them, and assess the merits of managing them.

Urban Evolution: The Role of Water

“…For example, there were questions regarding the biogeochemical impacts of engineered waterways at a watershed scale [41,42]. As urban watershed concepts further developed, it was suggested that engineered waterways should explicitly be considered as integral components of the urban ecosystem due to their immense spatial distribution and importance in watersheds and drainage networks [43][44][45]. In many cities, engineered waterways have actually surpassed "natural streams" in spatial extent and watershed drainage area [41,44,46].…”

“…As urban watershed concepts further developed, it was suggested that engineered waterways should explicitly be considered as integral components of the urban ecosystem due to their immense spatial distribution and importance in watersheds and drainage networks [43][44][45]. In many cities, engineered waterways have actually surpassed "natural streams" in spatial extent and watershed drainage area [41,44,46]. Thus, there is a need to better detect changes in ecosystem structure, function, and services along both natural and engineered hydrologic flowpaths, from roof tops to ground water, as an entire urban ecosystem evolving across space and time [4,[43][44][45]47].…”

“…In many cities, engineered waterways have actually surpassed "natural streams" in spatial extent and watershed drainage area [41,44,46]. Thus, there is a need to better detect changes in ecosystem structure, function, and services along both natural and engineered hydrologic flowpaths, from roof tops to ground water, as an entire urban ecosystem evolving across space and time [4,[43][44][45]47].…”

“…Drinking water and stormwater networks expanded during urban growth/expansion transitions (transition periods are indicated in Figure 4) and have aged over time following decay/repair transitions [4,44,90]. This expanding vascular infrastructure across all stages of urban evolution includes ditches, storm drains, leaking water and sewer pipes, roofs, and gutters.…”

“…For example, the amplified hydrologic pulse of urban ecosystems is expected to change due to the interactive effects of land use and climate change across different areas of the U.S. and elsewhere [79,[93][94][95][96], and this may trigger urban adaptation and evolution of ecosystem structure and function of urban drainage [44,90]. As another example, the number and concentrations of pharmaceutical and personal care products (PPCPs) in the chemical diet of urban watersheds can be expected to increase in the future, and there are questions regarding urban adaptation from individual organisms and ecosystem responses to environmental regulations globally [128][129][130][131].…”