USING STABLE ISOTOPES OF WATER AND STRONTIUM TO INVESTIGATE THE HYDROLOGY OF A NATURAL AND A CONSTRUCTED WETLAND

Wetlands cannot exist without water, but wetland hydrology is difficul t to characterize. As a result, compensatory wetland mitigation often only assumes the proper hydrology has been created. In this study, wat er sources and mass transfer processes in a natural and constructed we tland complex were investigated using isotopes of water and strontium, Water isotope profiles in the saturated zone revealed that the natura l wetland and one site in the constructed wetland were primarily fed b y ground water; profiles in another constructed wetland site showed re cent rain was the predominant source of water in the root zone. Water isotopes in the capillary fringe indicated that the residence time for rain is less in the natural wetland than in the constructed wetland, thus transpiration (an important water sink) was greater in the natura l wetland, Strontium isotopes showed a systematic difference between t he natural and constructed wetlands that we attribute to the presence or absence of peat, In the peat-rich natural wetland, delta(87)Sr in t he pore water increased along the flowline due to preferential weather ing of minerals containing radiogenic Sr in response to elevated Fe co ncentrations in the water, In the constructed wetland, where peat thic kness was thin and Fe concentrations in water were negligible, delta(8 7)Sr did not increase along the flowline, The source of the peat (on-s ite or off-site derived) applied in the constructed wetland controlled the delta(87)Sr, but the top of the profile, but the effects were res tricted by strong cation exchange in the underlying fluvial sediments. Based on the results of this study, neither constructed wetland site duplicated the water source and weathering environment of the adjoinin g natural wetland. Moreover, stable isotopes were shown to be effectiv e tools for investigating wetlands and gaining insight not easily obta ined using non-isotopic techniques. These tools have potential widespr ead application to wetlands that have distinct isotopic endmember sour ces.