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:
Tritium	Tracer	Point	Month to Years	Recharge	Moderate	Moderate	High
Tritium is a naturally occurring radioactive isotope of hydrogen with a half-life of 12.3 years introduced to the atmosphere in large quantities from atmospheric nuclear testing, with peak concentrations occurring about 1962. The tritium method is conducted by measuring tritium concentrations in groundwater at various depths and locating the depth of peak tritium concentration. Local estimates of groundwater recharge are determined as the volume of water above the depth of peak tritium concentration (requiring estimate of effective porosity) divided by the time between the peak atmospheric concentration and the sampling. Because tritium is part of the water molecule, it is an ideal tracer. The usefullness of envirionmental tritium has lessened greatly in recent years because the concentration of the bomb-pulse signature has decayed to near background levels. The method is not conceptually difficult, but costs are moderate because of the need for wells and lab analysis.