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Polycyclic aromatic hydrocarbons (PAHs) are a large group of chemical substances with a similar structure comprising two or more joined aromatic carbon rings. PAHs vary both in their chemical characteristics and in their environmental sources, and they are found in the environment both as gases and associated with particulate material.

The antimicrobial effects of silver (Ag) have been recognized for thousands of years. In ancient times, it was used in water containers () and to prevent putrefaction of liquids and foods. In ancient times in Mexico, water and milk were kept in silver containers (). Silver was also mentioned in the Roman pharmacopoeia of 69 b.c. ().

Concern about the input of metals to terrestrial ecosystems is related to (i) the ecotoxicological impact on soil organisms and plants (; ) and also on aquatic organisms resulting from runoff to surface water and (ii) the uptake via food chains into animal tissues and products, which may result in health effects on animals and humans (). Effects on soil organisms, including microorganisms/macrofungi and soil fauna, such as nematodes and earthworms, are reduced species diversity, abundance, and biomass and changes in microbe-mediated processes (; ; ). Effects on vascular plants include reduced development and growth of roots and shoots, elevated concentrations of starch and total sugar, decreased nutrient contents in foliar tissues, and decreased enzymatic activity (; ). A review of these phytotoxic effects is given by Balsberg-Påhlsson (1989). Effects on aquatic organisms, including algae, Crustacea, and fish, include effects on gill function (), nervous systems (), and growth and reproduction rates (). Environmental quality standards or critical limits, often also denoted as Predicted No Effect Concentrations, or PNECs, for metals in soils and surface waters related to those effects serve as a guide in the environmental risk assessment process for those substances.

To assess the impact of elevated concentrations of metals in terrestrial ecosystems, a major distinction should be made in risks/effects of heavy metals related to (i) the soil ecosystem (soil organisms/processes and plants) and (ii) human health or animal health resulting from bioaccumulation. The latter effect is related to the phenomenon that a chemical accumulates in species through different trophic levels in a food chain, or secondary poisoning. Heavy metal accumulation in the food chain is specifically considered important with respect to cadmium (Cd), mercury (Hg), and, to a lesser extent, lead (Pb). Accumulation ultimately causes toxic effects on (i) humans by affecting food quality of crops (Kawada and Suzuki 1998) and animal products, as well as drinking water quality, and (ii) animal health by affecting fodder quality and by direct intake of contaminated soil (). For both humans and animals, health effects arise mainly through accumulation in target organs such as kidney and liver (). Apart from direct health effects related to intake of food and soil, elevated metal levels in soil also lead to an increase in leaching losses of metals to groundwater and surface water, which will, after a considerable delay time, affect both drinking water quality and aquatic organisms ().

The occurrence of human and veterinary pharmaceuticals in soil and water (; ; ; ) has led to increased research activities among environmental scientists to find out their possible environmental threats. As antibiotics are used for human and animal medical care, there is a possibility for these drugs to reach the environment via direct or indirect contamination (; ). As they are produced and applied with the aim of being biologically highly effective, their occurrence is of ecotoxicological interest. In Berlin, Germany, the groundwater wells located near contaminated surface waters showed a concentration of pharmaceuticals at the µg/L level (). Thus, at such a level of contamination, the use of groundwater for drinking purposes may pose a potential risk. Studies on the behavior of animal drugs and their metabolites after excretion, along with their transport from agricultural sources into surface water and groundwater by overland-flow runoff and leaching, are of utmost importance at present.

Explosive, or energetic compounds, may be defined as chemicals that, under the influence of thermal or chemical shock, decompose rapidly with the evolution of large amounts of heat and gas (). Numerous energetic compounds have been produced for varying industrial uses; however, secondary explosives pose the largest potential environmental concern because they are produced and used in defense activities in the greatest quantities. Secondary explosives may enter the environment following explosives manufacture, assembly, and packing, and explosives detonation. During these activities, soil, sediment, and water may become contaminated with energetic and related compounds with potential impacts on environmental and human health. Of the secondary explosives, trinitrotoluene (TNT) and Royal Demolition Explosive (hexahydro-1,3,5-trinitro-1,3,5-triazine) (RDX) production outweigh other secondary explosives as they are the major ingredients in nearly every munition formulation (). In addition to chemicals added to explosive formulations, residues may contain compounds such as production impurities or decomposition by-products. For example, High Melting Explosive (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) (HMX) may be found as an impurity in RDX (), and TNT may contain dinitrotoluene and trinitrotoluene isomers (Legett et al. 1977).