New indexes for compound prioritization and complexity quantification on environmental monitoring inventories

Lists of compounds resulting from environmental monitoring may be conveniently represented in a very general way using Pareto distributions, after ranking them on descending order according to their concentration or hazard quotient expressed as percentages, depending on whether the objective of the monitoring is focussed on mass load occurrence or risk assessment respectively. Ranked distributions are characterized using appropriate indexes, such as h (Hirsch), well known in other disciplines like bibliometry. Furthermore, to such ordered distributions, simple numerical power type equations relating rank order and occurrence probability can be fitted, following the so-called power or Zipf law. Both h indices and the characteristic power law exponents are interpreted as measures of complexity of the overall mixture. On the other hand, compounds included within the h index may be seen as the most relevant in the mixture, thus providing a reasonable indication of what is worth analyzing. These concepts have been applied, as case study, to the characterization of the pharmaceutical compounds found in the input and output streams of wastewater treatment plants. Whereas both the concentration load and ecotoxicity of pharmaceuticals in WWTPs obviously decrease in the output of the treatment (influent > effluent, sludge), complexity quantified using the proposed indexes does not follow the same trend, being this behaviour common to the three plants examined. The joint combination of h compounds of the three plants studied allowed optimizing the list of compounds to be analyzed, which must be considered the key ones for the scenario under study.