Estimating natural recharge by means of chloride mass balance in a volcanic aquifer: northeastern Gran Canaria (Canary Islands, Spain)

The chloride mass balance method was used to estimate the average diffuse groundwater recharge on northeastern Gran Canaria (Canary Islands), where the largest recharge to the volcanic island aquifer occurs. Rainwater was sampled monthly in ten rainwater collectors to determine the bulk deposition rate of chloride for the 2008-2014 period. Average chloride deposition decreases inwardly from more than 10 g center dot m(-2)center dot year(-1) to about 4 g center dot m(-2)center dot year(-1). The application of the chloride mass balance method resulted in an estimated average recharge of about 28 hm(3)/year or 92 mm/year (24% of precipitation) in the study area after subtracting chloride loss with surface runoff. The average storm runoff was estimated to be 12 hm(3)/year (9% of precipitation) for the 1980-2014 period. Runoff was sampled during scarce rainy periods, which produce surface water flow. Average recharge varies from less than a few mm/year near the coast up to 270 mm/year in the highlands (about 33% of average rainfall), with a close-to-linear increase inwardly of about 18 mm center dot year(-1)center dot km(-1). Recharge rate uncertainty corresponds to an estimated CV of 0.3-0.4 because of the short data series available.Peer Reviewe