Response to Concept 4, One of Many Possible Responses

There is both a direct and indirect approach to measurement of solute loss as a gas phase. In the direct method a flux box is placed over a representative portion of the system and concentration of the gas of interest is measured at regular intervals over a 24-hour period at several locations during different seasons. Those data are used along with the area of the system to calculate the annual flux of the gas out of the system. A less direct, but perhaps better-integrated value might be obtained by using the geochemistry or isotopes to identify changes that reflect gas loss. For example, of the major ions in groundwater carbon, nitrogen, and sulfur have gases phases and their phase change would generate an isotope change in the remaining solute. We know that most groundwater has a CO₂ concentration that is tens of times greater than the atmospheric concentration of CO₂ and CO₂ degases when the groundwater reaches the atmosphere. Thus, changes in pH, calcium, and bicarbonate concentration relative to the recharge values would suggest the loss of both CO₂ to gas and carbon loss to mineral precipitation. This would be reflected in the isotopes as well as in the solute balance. The relative amount of carbon in each phase would need to be determined by a thermodynamic speciation model. The loss of N₂ gas by reduction of nitrate (NO₃^-1) might be estimated by the change in nitrogen isotopes relative to the recharge concentration to the system. Similarly, the loss of sulfate to H₂S gas might be evaluated by sulfur isotopes.

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