The Reston Groundwater Dating Laboratory

Why collect samples for N2, Ar, CH4, O2, CO2

To determine the recharge temperature the solubility of gases varies as a function of temperature. The concentration, as well as the ratios of concentrations, can be used to calculate the recharge temperature of a ground water sample. This may give clues as to where the recharge occurred.

To determine the amount of excess air - air trapped in pores dissolves in the ground water after a rapid rise of the water table causing the ratio of N2 to Ar in air to be different from the equilibrium solubility-ratio of N2 to Ar in water. High concentrations of excess air are common in fractured rock aquifers and in aquifers in semi-arid areas.

To determine the amount of excess N2 - Excess N2 indicates de-nitrification has taken place in the ground water. The total initial nitrate concentration will equal the nitrate present in the water plus calculated nitrate from excess N2 (from denitrification, not from excess air).

To determine if gas loss from the water has occurred- high apparent recharge temperatures with "negative excess air concentrations"; indicate degassing has taken place in the aquifer or during sampling.

To determine methane concentrations- the presence of methane indicates very reducing conditions somewhere along the flow path.

To determine oxygen concentrations- oxygen is determined by difference in our method from Ar+O2 minus Ar concentrations. Oxygen should be measured in the field since oxygen concentrations can decrease (and CO2 concentrations increase) during sample storage.