Comparison of Selected Methods for Estimating Groundwater Recharge In Humid Regions

Method:	Category:	Spatial Scale:	Temporal Scale:	Quantity Estimated:	Ease of Use:	Data Needs:	Cost:
Chlorofluorocarbons	Tracer	Local	Month to Years	Recharge	Difficult	Moderate	High
Chlorofluorocarbons (CFCs) are man-made compounds introduced into the atmosphere that can be used to estimate the date at which groundwater was recharged during about the past 50 years. Near groundwater divides, the recharge date can be estimated by sampling CFCs in groundwater and relating the concentration to the known relation of atmospheric content versus time. Local estimates of groundwater recharge are determined as the volume of water above the depth of peak concentration (requiring estimate of effective porosity) divided by the recharge date. The method is difficult because of special procedures for sampling CFCs to obtain samples without contact with the atmosphere and need for complex interpretation of results. Laboratory analysis is expensive and analysis of results requires knowledge of the properties of the different CFCs.