Ambient Water Quality Criteria for Colour

1. Definitions, Concepts and Analytical Measurements

1.1 Key Concepts

The observed colour of water is the result of light back scattered upward from the water after it has passed through to various depths and undergone selective absorption. Colour and turbidity determine the depth to which light penetrates in water systems. In water, the light intensity or irradiance at a particular depth (I_z) is a function of the intensity at the surface (I_o) to the exponent of the negative extinction coefficient at the depth distance z which is called the Beer-Lambert's Law (Freifelder 1985).

The extinction coefficient is a constant that predicts the attenuation or dissipation of light at a certain wavelength. In pure water, light is highly absorbed in the infrared region of the light spectrum and poorly absorbed in the blue region. Extinction coefficients are influenced by water absorption, suspended organic and inorganic particles, and dissolved compounds (Jerome et al. 1994a; Jerome et al. 1994b). Thus, the visible colour in a water sample is the light that is refracted, reflected or re-emitted by substances in water because it has not been absorbed to produce heat or chemical reactions.

The colour of water and other materials has three main attributes: hue, brightness and saturation (Davies-Colley et al. 1988). Hue refers to whether the water colour is described as blue, green or yellow, for example, and is determined by the dominant wavelength in the visible spectrum. Brightness depends on the amount of energy detected by the human eye, which is most sensitive to green light of wavelength 555 nm. The saturation depends on the spread of energy around the dominant wavelength. Saturation is also referred to as spectral purity (Jerome et al. 1994b).

True colour is due to natural minerals such as ferric hydroxide and dissolved organic substances such as humic or fulvic acids (Hongve and Akesson1996). A great variety of dissolved organic substances originating from anthropogenic sources such as dyes can also contribute to water colouration (McCrum 1984; Brown 1987; Borgerding and Hites 1994). True colour can only be measured once a sample has been centrifuged or filtered (APHA 1992; Environment Canada 1989; Bennett and Drikas 1993).

Colour measured in water containing suspended matter is defined as apparent colour (APHA 1992; Bennett and Drikas 1993). Suspended matter can be in the form of large organic particulates such as plant debris, phyto- and zooplankton (Effler and Auer 1987). For example, a blue-green colour can be due to blue-green algal blooms, a yellow-brown colour to diatoms or dinoflagellates, and reds and purples to Daphnia or copepods (Chapman 1992). Because some of these organisms thrive on anthropogenic releases or disturbances (e.g., fertilizers and forest activities), polluted waters may have a strong apparent colour.

1.2 True Colour

The amount of dissolved organic carbon in streams and lakes is typically about ten times the amount of particulate organic carbon. Dissolved organic compounds or "dissolved colour" greatly affects the absorption of light compared to suspended particles. Organic compounds such as humic acids absorb light and reduce its transmission relative to distilled water (James and Birge 1938). As well, the adsorption will be shifted selectively (Figure 1). Organic compounds contributing to the "dissolved colour" are highly selective and marked by UV, blue and green wavelengths and less so in the red and infrared regions of the light spectrum (see curve C, Figure 1). Light absorption by humic acids has been used extensively in the determination of their concentration in water systems (Reckhow and Singer 1990; Mierle and Ingram