Background Concentration of Pollutants

Introduction. A pollutant is a substance introduced in a water system that has undesirable effects, especially to living species and ecosystems, and that ultimately may turn water resources useless. Pollutants may be divided into two main groups: inorganic and organic. Several physical and chemical parameters determine the concentration of a pollutant substance in aqueous environments. The primary controlling factor is the aqueous solubility of the substance. Other parameters may control solubility, such as the redox potential, pH, organic and inorganic particulate matter, and kinetic factors. In terms of source, pollutant substances may originate from natural processes or from anthropogenic activities.

The case of arsenic contamination in Bangladesh groundwater is an example of a natural process causing population poisoning because of the high levels of contaminated wells for drinking water. Therefore, determination of background concentration of pollutants in waters is fundamental to establish water quality standards and goals to meet those standards. For that purpose, current studies apply the definition of baseline concentration: “the range of concentrations of a given element, isotope or chemical compound in solution, derived entirely from natural, geological, biological or atmospheric sources, under conditions not perturbed by anthropogenic activity”.

This definition is stricter than the definition for background because the latter should be understood irrespective of the source of pollutant substances. Many places with abandoned industries, mines, and reclaimed land, for example, have background concentration of several substances for which the anthropogenic source cannot be put aside. The overwhelming majority of organic pollutants is also out of the notion of baseline concentration by obvious reasons.

Determining the background concentration of a pollutant substance in an aquatic environment seems no less than an extraordinary task. Pollutants occur in surface and ground waters in a range of concentrations, spanning several orders of magnitude, exclusively related to natural phenomena. On the other hand, anthropogenic activities induce large perturbations in most water systems, which complicates the determination of their background concentration in several substances.

An additional difficulty in the chemical characterization of surface and groundwater systems is their dynamic nature, since concentration patterns change with time, requiring at least seasonal sampling and long time-series for statistical significance. Once introduced in the water system, the fate and spatial distribution of a pollutant are determined by factors such as advection, diffusion, chemical reactivity, and biodegradation.