Boron and lithium isotopes as groundwater tracers: a study at the Fresh Kills Landfill, Staten Island, New York, USA

A study was conducted at the Fresh Kills landfill, Staten Island, New York to investigate the use of B and Li isotopes as tracers of mixing and flow in the groundwater environment. Four end-member waters are present at the Fresh Kills: freshwater, seawater, a geochemically distinct transitional groundwater (that occurs in the zone of mixing between seawater and freshwater) and landfill leachate. The δ11B and δ6Li values of end-member waters are distinct and have isotopic compositions that reflect the solute sources: freshwater δ11Bnot, vert, similar+30‰, δ6Linot, vert, similar−22‰; transition zone groundwaters δ11Bnot, vert, similar+20‰, δ6Linot, vert, similar−27‰; seawater δ11B+40 to +75‰, δ6Li−37 to−44‰; leachate δ11Bnot, vert, similar+10‰ (δ6Li not determined). Those wells influenced by seawater exhibited a clear chemical mixing trend, with seawater contributions ranging from 3 to 85%. Well waters with a high percentage of seawater (>30%) had δ11B values that were within 1‰ of the seawater value (+40‰), whereas a trend of increasing δ11B values (+55 to +75‰) was observed for wells with a lower percentage of seawater (<30%). δ6Li values for well waters impacted by mixing with seawater ranged from−37 to−44‰, significantly more negative than pure seawater (−31‰). This deviation from the isotopic composition of seawater, for both δ11B and δ6Li values, represents non-conservative behavior and is likely the result of isotopic fractionation during ion exchange reactions. The wide range of δ11B and δ6Li values and the distinct isotopic compositions of end-member waters makes B and Li isotopes useful for recognizing solute sources, however isotopic fractionation may limit their use as simple tracers of groundwater flow and mixing.