Background Concentration of Inorganic Pollutants

Inorganic pollutants are composed of elements that cannot be broken-down as organic pollutants in general are. These are elements that occur naturally on Earth, that become “pollutants" or dangerous when they are concentrated or combined with other elements or compounds. The only effective removal mechanism from aqueous solutions is by precipitation and/or incorporation into mineral solid phases. Many trace metals are nutrients and essential to living organisms at trace concentrations, but become highly toxic if excessively abundant.

Therefore, these situations reveal an ambiguous character of the term “pollutant" if exclusively associated to the identity of an element or compound. Therefore, the same element might have a high background concentration in an aqueous solution and be considered a pollutant substance, while it may have a lower background concentration in another aqueous solution and not qualifying as a pollutant substance.

Also the redox potential may determine an element’s toxicity, such as the case of Cr⁶⁺ which is very toxic, whereas in its less oxidized state, Cr³⁺ is much less toxic and is essential for human health because of its role in glucose metabolism [16] and incorporation in vitamins. Phosphorous (P) is another example of a beneficial element actively present in all living matter (in marine phytoplankton occurs in the Redfield molar ratio of 117:16:1 - C:N:P), but in simple terms if phosphorous compounds dissolve in water they can be deadly.

Nitrate, ammonia, and trace metals are the most important inorganic pollutants present in waters. Several surface and groundwaters contain natural background (baseline) concentrations of one or more of these and other chemical elements and species exceeding, for example, the U.S. EPA drinking water standards for reasons totally unrelated to human activities. Identifying such natural sources is fundamental for regulatory decisions to avoid the assignment of unrealistic cleanup goals below such natural background concentrations.

One of the most striking examples comes from groundwater arsenic contamination in Bangladesh, whose mechanism, most probably attributable to iron oxide reductive dissolution, is still a matter of debate. Of the millions of tube wells, about one-third have arsenic concentrations above the local drinking water standard (50 µg L^-1), and half of them do not meet the 10 µg L^-1 guideline value of the World Health Organization (WHO).

Whatever the approach to determine background concentrations of inorganic pollutants in water systems requires the sampling of stream and surface waters and/or ground waters from boreholes in areas with similar geological settings, and where water chemistry has presumably not been affected by human activity. This may be difficult to achieve, and thus it is important to know the extent of chemically evolved waters and differentiate and recognize the degree of human impacts on the baseline concentration.

Therefore, to extract these baseline characteristics researchers use tracers, reactive and inert. This approach has been mostly applied to groundwater systems. Inert tracers include halogens, especially chlorine (Cl^-), and nitrate (NO₃^-), which is highly mobile because it is not limited by solubility constraints. Reactive tracers are usually major ions, especially derived from water-rock interaction. Stable isotopes are also among both groups of inert and reactive tracers. This allows the use of geochemical modelling of processes that control the baseline water chemistry, and thus help establish the range of background concentrations of several groundwater systems (e.g., 18), and identify distinct natural and anthropogenic sources.

Whenever this approach is not possible it might be reasonable assuming natural background concentrations equal to the measured concentrations in streams and groundwaters in adjacent or nearby areas sharing similar geologic environments. One should not forget that factors such as climate, relief, aquifer recharge rate, and hydraulic properties must be evaluated. Data sources on the “typical" chemical composition of ground waters from different rock types also exist.