Solute isotopes as hydrologic tracers: Examples from the Glacial Lake Agassiz peatlands, Minnesota, Hubbard Brook, New Hampshire and the Fresh Kills landfill, New York

Natural variation in the isotopic composition of solutes contains information about the path that water takes as it moves through the environment, a parameter that is often difficult to observe or measure directly. Three separate studies, in three distinct hydrogeologic environments, were conducted employing solute isotopes as hydrologic tracers.; At the Glacial Lake Agassiz peatlands of northern Minnesota, a contrast in the 87Sr/86Sr ratio of atmospherically derived Sr (∼0.710) and Sr from regional bedrock (>0.730) allows quantification of the proportions of precipitation recharge and groundwater discharge into the peat columns of bog and fen wetlands. Bog pore waters consist of 90–100% precipitation in the upper 2 meters, below which the pore waters mix with groundwater. Adjacent fens consist mostly of groundwater, ranging from ∼30% near the surface to ∼100% with depth. Groundwater discharging into bog and fen sites has distinct 87Sr/86Sr and [Ca]/[Sr] ratios, and is likely indicative of flowpaths at two different length scales.; Sr isotopes were used to trace changing hydrologic flowpaths during storm events within an 11.8 ha first order catchment (Watershed 1) of the Hubbard Brook Experimental Forest, New Hampshire. Shallow flowpaths have 87 Sr/86Sr ratios closer to the atmospheric deposition end member (∼0.710), whereas deeper flowpaths have more of a mineral weathering component (∼0.725). Distinct changes in 87Sr/86 Sr ratios and δ18O values for stream water during storm events indicate that the two isotopic systems record complementary information about watershed hydrology. Hydrograph separation using 87Sr/ 86Sr ratios indicates that shallow subsurface flow (soil water) can contribute significantly to storm flow (up to 40%), however the percentage appears to be dependant on soil moisture conditions.; Boron and lithium are two isotopic systems that display large natural variation but have not been widely utilized in environmental studies. A pilot study was conducted at the Fresh Kills landfill, Staten Island New York to assess the ability of these isotopes to act as hydrologic tracer. Both isotopic systems exhibit non-conservative behavior. In particular, dilution of the sea water end member resulted in heavier boron and lithium isotopic compositions. Cation exchange reactions with the aquifer material are the most likely cause of this fractionation.