The increasing worldwide contamination of freshwater systems with thousands of industrial and natural chemical compounds is one of the key environmental problems facing humanity. Although most of these compounds are present at low concentrations, many of them raise considerable toxicological concerns, particularly when present as components of complex mixtures. Here we review three scientific challenges in addressing water-quality problems caused by such micropollutants. First, tools to assess the impact of these pollutants on aquatic life and human health must be further developed and refined. Second, cost-effective and appropriate remediation and water-treatment technologies must be explored and implemented. Third, usage and disposal strategies, coupled with the search for environmentally more benign products and processes, should aim to minimize introduction of critical pollutants into the aquatic environment.

Measuring stable isotope fractionation of carbon, hydrogen, and other elements by Compound Specific Isotope Analysis (CSIA) is a new, innovative approach to assess organic pollutant degradation in the environment. Central to this concept is the Rayleigh equation which relates degradation-induced decreases in concentrations directly to concomitant changes in bulk () average over the whole compound) isotope ratios. The extent of in situ transformation may therefore be inferred from measured isotope ratios in field samples, provided that an appropriate enrichment factor ( bulk ) is known. This bulk value, however, is usually only valid for a specific compound and for specific degradation conditions. Therefore, a direct comparison of bulk values for different compounds and for different types of reactions has in general not been feasible. In addition, it is often uncertain how robust and reproducible bulk values are and how confidently they can be used to quantify contaminant degradation in the field. To improve this situation and to achieve a more in-depth understanding, this critical review aims to relate fundamental insight about kinetic isotope effects (KIE) found in the physico(bio)chemical literature to apparent kinetic isotope effects (AKIE) derived from bulk values reported in environmentally oriented studies. Starting from basic rate laws, a quite general derivation of the Rayleigh equation is given, resulting in a novel set of simple equations that take into account the effects of (1) nonreacting positions and (2) intramolecular competition and that lead to position-specific AKIE values rather than bulk enrichment factors. Reevaluation of existing bulk literature values result in consistent ranges of AKIE values that generally are in good agreement with previously published data in the (bio)-chemical literature and are typical of certain degradation reactions (subscripts C and H indicate values for carbon and hydrogen): AKIE C ) 1.01-1.03 and AKIE H ) 2-23 for oxidation of C-H bonds; AKIE C ) 1.03-1.07 for S N 2-

Water quality issues are a major challenge that humanity is facing in the twenty-first century. Here, we review the main groups of aquatic contaminants, their effects on human health, and approaches to mitigate pollution of freshwater resources. Emphasis is placed on chemical pollution, particularly on inorganic and organic micropollutants including toxic metals and metalloids as well as a large variety of synthetic organic chemicals. Some aspects of waterborne diseases and the urgent need for improved sanitation in developing countries are also discussed. The review addresses current scientific advances to cope with the great diversity of pollutants. It is organized along the different temporal and spatial scales of global water pollution. Persistent organic pollutants (POPs) have affected water systems on a global scale for more than five decades; during that time geogenic pollutants, mining operations, and hazardous waste sites have been the most relevant sources of long-term regional and local water pollution. Agricultural chemicals and wastewater sources exert shorter-term effects on regional to local scales. 109 Annu. Rev. Environ. Resourc. 2010.35:109-136. Downloaded from www.annualreviews.org by University of Wisconsin -Madison on 10/07/12. For personal use only.