The effects of surface area and volume on the rate of isotopic exchange between water and water vapor were quantified by allowing beakers containing waters with differing D, 18O, and tritium contents to equilibrate under controlled conditions in a closed system. Over a 2-month period the waters shifted by as much as 80%0 in ΔD, 13%0 in Δ18O, and 57,000 tritium unit (TU), sometimes in opposite directions (either toward being more enriched or more depleted) depending upon initial compositions of the juxtaposed waters. Over time the concentrations of the stable and radioactive isotopes approach a common value; which in an ideal closed system represents the mean composition on a volumetric basis. The exchange rate is represented by dRX/dt=-(RX-RM)ksx/vx where RX represents the actual isotopic ratio of the water at time t, RM is equal to αeq*Rv and represents the isotopic ratio of the water that would at any instant be in equilibrium with ambient vapor with an isotopic ratio of Rv, and sX and vX refer to the surface area and volume of the water, respectively.

In our 20¿C experiments the constant k ranged from 0.12 to 0.18 cm/d and was identical for D/H, T/H, and 18O/16O exchange for a given set of conditions. In cases where RM is constant, as for example, occurs if the atmospheric reservoir is infinite or if water samples having identical surface areas and volumes undergo exchange, the isotopic ratio of the vapor is constant and the exchange equation is (Δx-Δ∝)/Δi-Δ∝)= e-ksxt/vx where Δi, Δ∝, and Δx refer to the Δ value of the water at the initiation of the experiment, at equilibrium, and any time. In the case of exchange between samples having different surface areas and/or volumes, the isotopic ratio of the vapor is variable and the value for RM is RM=(sX(RX)+sY(RY) +(sZ(RZ)+...)/(sX+sY+sZ...). In the case of identical surface areas and volumes the composition of the waters may change, but the vapor to which the waters respond remains the same during the exchange process. In the case of differing surface areas, or differing volumes, the composition of the vapor and RM will both change. In any case, after complete exchange, all waters will attain the same equilibrium isotopic ratio. ¿ American Geophysical Union 1993