Monitoring reverse osmosis treated wastewater recharge into a coastal aquifer by environmental isotopes (B, Li, O, H)

Artificial recharge (AR) is gaining importance as a management tool in water stressed regions. The need to prove recovery performance requires new monitoring tools for AR systems. A novel combination of environmental isotope tracers (B, Li, O, H stable isotopes) was tested for the monitoring of AR of tertiary treated, desalinated domestic wastewater into a coastal dune aquifer in Flanders, Belgium. No significant isotope fractionation was observed for the treatment process, which includes low pH RO desalination. The wastewater, after infiltration through ponds and before recovery through pumping wells is characterized by low molar Cl/B ratios (3.3 to 5.2), compared to 130 to 1020 in the wider study area, delta(11)B values close to 0% per hundred, rather homogeneous delta(7)Li values (10.3 +/- 1.7% per hundred), and a 18O and 2H enrichment with respect to ambient groundwater due to evaporation in the infiltration ponds. This confers to the AR component a unique isotopic and geochemical fingerprint. Immediately downstream of the pumping wells and in the deeper part of the aquifer no evidence of AR wastewater could be found, indicating a high recovery efficiency. In the wider area and in the deeper part of the aquifer, isotopes evidence mixing of coastal rain and a fresh paleo-groundwater component with residual seawater as well as interaction with the aquifer material. Combining several isotope tracers provides independent constraints on groundwater flow and mixing proportions as a complement to hydrodynamic modeling and geochemical studies.