The measurement of the atmospheric chloride deposition on land allows the estimation of long-term average aquifer recharge when the average chloride content in the recharge water is known. This is a well-established, simple method, although not exempt from theoretical and practical difficulties. In spite of its potential, it is seldom applied. Under favorable circumstances, the results are reliable and the associated uncertainty can be determined. The chloride content of recharge water can be obtained from sampling the top of the water table. The method and its improvements have been the subject of extensive research and it has been applied to several aquifers throughout Spain, in the context of applied research projects and in doctoral thesis studies, by the Groundwater Hydrology Group of the Geoengineering Department of the Technical University of Catalonia (UPC). For aquifers discharging into springs or developed by means of deep wells, the vertical mixing of groundwater recharged at different altitudes is important. Thus, the chloride content may overestimate aquifer recharge and corrections are needed. In the present study the method has been developed and improved. It compares favorably to other methods, although it yields diverse recharge estimations that have to be referred to the appropriated site and time.

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