Captured streams and springs may flow in combined sewers, increasing clean baseflow in pipes and wastewater treatment works, but reducing pipe capacity and increasing treatment costs. This review investigates stream and spring capture, finding several examples with convincing evidence that this occurs.
Captured streams and springs may be flowing in combined sewers, increasing clean baseflow in pipes and wastewater treatment works (WWTWs), but 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 it 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.
Researchers at the University of Sheffield have now conducted a thorough review of the literature and international case studies, to investigate stream and spring capture, finding several examples with convincing evidence that this occurs.
Three modes of entry are identified: capture by conversion, capture by interception, and direct spring capture.
Methods to identify and quantify capture are limited, but experience in Zürich, Switzerland suggests that it contributes 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 in Bradford, UK) with 16% of baseflow from captured streams and springs, annual costs are conservatively estimated at £2–7 million (US$3–11 million, €2.3–8 million).
A detailed case study from Zürich is considered that has successfully separated captured baseflow into daylighted streams through the urban area, with multiple economic, environmental, and social benefits.
The researchers 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.
