Stable and other isotope techniques for perchlorate source identification

Stable isotopes of chlorine, oxygen, nitrogen, strontium, and hydrogen can be used as forensic indicators to fingerprint and possibly differentiate perchlorate plumes. Direct analytical methods involve isotope ratio mass spectrometry (IRMS) of delta Cl-37 and delta O-18, which comprise the perchlorate anion (ClO4-). Plots of delta Cl-37(perchlorate) versus delta O-18(perchlorate) have been used to differentiate between anthropogenic and geogenic (natural) perchlorate sources. Indirect analytical methods involve the stable isotopic analysis of substances associated with perchlorate. These can be used as surrogates and include delta N-15 and delta O-18 in dissolved nitrate, and Sr-87/Sr-86 in solid and/or dissolved strontium nitrate associated with potassium perchlorate in safety flares. Strontium compounds may also exist as impurities in ammonium and other perchlorate compounds. Stable isotopes of delta O-18 and delta H-2 in water can be used to differentiate different groundwater sources such as those from river recharge or precipitation. Other indirect methods include analysis of radiogenic isotopes such as tritium (T) and helium-3 (He-3), which can be used to approximate the mean residence time ("age") of relatively young groundwater.