Introduction to bioanalytical tools in water quality assessment

BACKGROUND

Chemical monitoring provides a quantitative assessment of single organic contaminant concentrations in a water sample but does not account for the presence of unknown compounds such as transformation products, untargeted chemicals (i.e., not previously known to be present) or for interactions among chemicals. Biological-based monitoring is complementary to chemical analysis and provides information on all bioactive micropollutants in a sample ranked according to potency, i.e., more toxic chemicals are weighted higher than less toxic chemicals.

Classical aquatic toxicity tests used in water quality assessment include in vivo assays with fish and aquatic invertebrates that measure endpoints such as mortality, growth, reproduction and feeding responses. Ecosystem level assessment is based on functional or structural measures of whole ecosystems such as species diversity or genetic structure. The sensitivity of in vivo tests has been greatly improved with the development of early life-stage tests (e.g., the zebrafish embryo toxicity test), which is considered to be ethically more acceptable than adult fish tests. In vitro molecular and cell-based assays are sensitive, cost- and time-efficient alternatives to whole animal testing. Implementation of human and other mammalian cell lines in water testing has facilitated evaluation of toxicological endpoints relevant for assessing the potential for deleterious effects on human health.

For the purpose of this book, we define 'bioanalytical tools' as cell-based and low-complexity in vitro bioassays indicative of specified endpoints relevant for human and/or environmental health. These tools include whole cell assays and assays with genetically modified cells, where natural features have been over-expressed to enable more sensitive detection and/or where foreign features have been added for visualisation of effects. Further, we include assays with unicellular organisms, such as algae, yeast or bacteria, and some enzymes but exclude other cell-free assays (immunoassays and direct receptor binding assays).

A major advantage of bioanalytical tools is the ability to detect the toxicity of mixtures of known and unknown compounds, while chemical analysis can only quantify the concentration of known, targeted chemicals irrespective of toxicity. By measuring the mixture toxicity of a water sample, the bioassay approach is weighted by risk as it explicitly accounts for the differential in toxicity across different chemicals and for interactions among chemicals in a mixture. Many bioassays yield specific information on a given mode of toxic action rather than merely answering whether or not the cells are dead or alive after exposure to the sample. This mechanistic information can be exploited by running a series of bioassays indicative of a range of different modes of toxic action in parallel. In this way, a comprehensive bioanalytical test battery provides an integrated measure of the toxicity of the biologically active substances in a water sample. A bioassay can also be selected to target a specific protection goal such as maintenance of hormonal balance.

This book focuses exclusively on organic chemicals. Although many of the bioassays discussed here are also applicable for metals and inorganic pollutants, sample treatment and data interpretation differs between organics and inorganics. In addition, while there are millions of organic chemicals, many of which may never be identified by chemical analysis, the limited number of elements allows comprehensive chemical analysis of metals and inorganics, reducing the need for effect-based analysis.

1.2 ORGANIC MICROPOLLUTANTS

1.2.1 Defining the issue

Organic micropollutants are a group of man-made chemicals such as pesticides, industrial chemicals, consumer products and pharmaceuticals but also include natural compounds such as hormones (Schwarzenbach et al., 2006) (Table 1.1). As the name implies, micropollutants occur in water and the environment in the microgram per litre concentration range (1 µg/L = 10-6 g/L = 0.000001 g/L) or even lower, in the nanogram to picogram range (1 ng/L = 0.000000001 g/L; 1 pg/L = 0.000000000001 g/L).

In contrast to micropollutants, macropollutants are naturally occurring compounds that exist locally in excess concentration, e.g., phosphate and nitrogen compounds, which can lead to eutrophication of surface waters (Schwarzenbach et al., 2006). Macropollutants were the big environmental problem of the 1960s and 1970s in the developed world. Nowadays macropollutants are managed quite well with the introduction of source controls and additional wastewater treatments. As a result, attention has shifted to micropollutants, including inorganic and organic chemicals.

The widespread distribution of organic micropollutants in our waterways presents a hazard to aquatic life. Risks to humans can occur through the consumption of food and drinking water and through other exposure routes such as inhalation and dermal contact. Micropollutants enter the aquatic environment via direct sources, such as industry and sewage effluent discharge, and via non-point sources such as urban runoff and agricultural applications. Due to the complex nature of the chemical mixtures occurring in domestic wastewater used for water recycling schemes, conventional treatment is not always sufficient to remove the entire contaminant load. Disinfection steps such as chlorination have been introduced to wastewater and recycled water treatment to control human pathogens (microorganisms that cause diseases). While conventional biological treatment and advanced treatment processes are very effective in eliminating most unwanted pathogens and many micropollutants, they may also introduce other potentially harmful substances such as disinfection by-products and transformation products. Membrane processes such as reverse osmosis and adsorptive processes such as activated carbon filtration reduce a wide range of micropollutants to safe levels. Removal efficiency is, however, related to the chemical structure and properties, and some compounds may not be fully removed by these processes alone. Further remains the problem of contaminated waste generated during filtration and adsorption processes.

1.2.2 Transformation...