Application of Boron Stable Isotope and Strontium Isotopic Ratios to Evaluate Coal-Combustion Product Leachate Effects on Groundwater Quality

Boron (B) and strontium (Sr) concentrations are potential indicators of the influence of leachate from coal combustion products (CCP) on groundwater quality.  Ratios of B stable isotopes (¹¹B/¹⁰B, expressed as d¹¹B) and Sr isotopes (⁸⁷Sr/⁸⁶Sr) in groundwater in combination with B and Sr concentrations and concentrations of additional major ions can provide further detail to understand the relative contribution of B and Sr from CCP leachate and other sources.  Background information relevant to successful interpretation of the isotope data includes (1) locations of sampled wells and groundwater in the groundwater flow system relative to potential contaminant sources in space and time, (2) relevant geochemical characteristics such as pH and major element concentrations from potential contaminant sources and ambient groundwater, and (3) the precision of the d¹¹B and ⁸⁷Sr/⁸⁶Sr isotopic ratio analyses and the B and Sr concentration analyses. 

B and Sr isotope ratios and B and Sr concentrations that are used to represent CCP-affected groundwater should represent a relatively stable, equilibrium condition, such as leachate from a collection system or CCP-affected groundwater on the downgradient perimeter of a landfill.  Temporal changes in d¹¹B and ⁸⁷Sr/⁸⁶Sr  during leaching of small CCP samples gradually approach a steady condition and indicate the need to collect several samples to represent isotope ratios in leachate.   Binary plots of d¹¹B and B concentrations and ⁸⁷Sr/⁸⁶Sr  and Sr concentrations are demonstrated to illustrate use of isotopes as source fingerprints of potential mixtures among CCP impacted and ambient waters.  Additionally, use of groundwater age dates to rule out implausible mixtures is demonstrated using data from a USGS study in northwestern Indiana.  The precision of isotope ratio analyses is demonstrated as a critical factor in study designs that use d¹¹B and ⁸⁷Sr/⁸⁶Sr  ratios to evaluate CCP leachate contributions to water quality.