HYDROLOGICAL PROCESSES Hydrol. Process. 23, 1701–1713 (2009) Published online 1 April 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.7305 Thermal influence of urban groundwater recharge from stormwater infiltration basins Arnaud Foulquier, 1 * Florian Malard, 1 Sylvie Barraud, 2 and Janine Gibert 1 1 Universit´ e de Lyon, Lyon, F-69622, France; universit´ e Lyon 1, Lyon, F-69622, France; CNRS, UMR 5023, Ecologie des Hydrosyst` emes Fluviaux, Villeurbanne, F-69622, France 2 Universit´ e de Lyon, universit´ e Lyon 1, Lyon, F-69622, France; INSA de Lyon, LGCIE, Villeurbanne, F-69621, France Abstract: Groundwater warming below cities has become a major environmental issue; but the effect of distinct local anthropogenic sources of heat on urban groundwater temperature distributions is still poorly documented. Our study addressed the local effect of stormwater infiltration on the thermal regime of urban groundwater by examining differences in water temperature beneath stormwater infiltration basins (SIB) and reference sites fed exclusively by direct infiltration of rainwater at the land surface. Stormwater infiltration dramatically increased the thermal amplitude of groundwater at event and season scales. Temperature variation at the scale of rainfall events reached 3 ° C and was controlled by the interaction between runoff amount and difference in temperature between stormwater and groundwater. The annual amplitude of groundwater temperature was on average nine times higher below SIB (range: 0Ð9–8Ð6 ° C) than at reference sites (range: 0–1Ð2 ° C) and increased with catchment area of SIB. Elevated summer temperature of infiltrating stormwater (up to 21 ° C) decreased oxygen solubility and stimulated microbial respiration in the soil and vadose zone, thereby lowering dissolved oxygen (DO) concentration in groundwater. The net effect of infiltration on average groundwater temperature depended upon the seasonal distribution of rainfall: groundwater below large SIB warmed up (C0Ð4 ° C) when rainfall occurred predominantly during warm seasons. The thermal effect of stormwater infiltration strongly attenuated with increasing depth below the groundwater table indicating advective heat transport was restricted to the uppermost layers of groundwater. Moreover, excessive groundwater temperature variation at event and season scales can be attenuated by reducing the size of catchment areas drained by SIB and by promoting source control drainage systems. Copyright  2009 John Wiley & Sons, Ltd. KEY WORDS urban groundwater; stormwater infiltration; groundwater temperature; dissolved oxygen Received 2 May 2008; Accepted 9 February 2009 INTRODUCTION Half of the world’s population lives in cities and this number is predicted to reach 60% in 2030 (United Nations, Department of Economic and Social Affairs, Population Division, 2007). This proportion is even higher in European countries where 80% of the popula- tion lives in the cities (European Environmental Agency, 2006). Many cities are groundwater dependent and it is estimated that 2 billion people in the world rely on groundwater for drinking water (Morris et al., 2003). Any increase in urban groundwater temperature is a crit- ical issue because temperature is a key variable affecting most physicochemical and biological processes including groundwater flow, gas solubility, and microbial activity (Chapelle, 1993; Jones and Mulholland, 2000; Ander- son, 2005). Groundwater temperature below major cities was found to be 2 to 3 ° C higher and typically more heterogeneous over space and time than in surrounding areas (Allen et al., 2003; Ferguson and Woodbury, 2004; SDUD, 2005; Taniguchi, 2006). Groundwater warming in * Correspondence to: Arnaud Foulquier, UMR CNRS 5023, Ecologie des Hydrosyst` emes Fluviaux, Universit´ e Claude Bernard Lyon 1, Bˆ atiment Forel (403), 43 Bd 11 Novembre 1918, F-69622 Villeurbanne Cedex, France. E-mail: foulquier@univ-lyon1.fr urban environments was attributed to the increase in air temperature within cities (i.e. the urban heat island effect, Bornstein, 1968; Karl et al., 1988) but also to multiple local sources of heat including heat loss from buildings (Ferguson and Woodbury, 2004), increase in soil tem- perature as a result of pavement (Dorava et al., 2003; Thompson et al., 2007), and heat transfer due to ground- water heat pumps used for air conditioning (SDUD, 2005). Stormwater infiltration practices, which consist of infil- trating rapidly large quantities of runoff water into local- ized areas, potentially contribute to the warming of urban groundwater by increasing advective heat transfer in the subsurface. Urban stormwater infiltration into the ground is increasingly used as an alternative to traditional stormwater network for many reasons. It is recognized to limit peak flows and volumes, reducing hydraulic and flooding impact. It is supposed to improve the quality of watercourses by limiting pipe discharge (Pitt et al., 1999). Moreover, it can compensate for reduced groundwater recharge caused by the sealing of urban surfaces, replen- ish urban groundwater with dissolved oxygen (DO), and promote the retention and degradation of contam- inants in the soil and vadose zone (Ferguson, 1994; Pitt et al., 1999; Fischer et al., 2003). Although much Copyright  2009 John Wiley & Sons, Ltd.