Authors: PSJ Coleman,
The color of natural waters varies considerably, and this can indicate the chemicals dissolved in it, organic materials such as peat, blooms of phytoplankton, and soil erosion. Field measurements of colour tend to be subjective, and a wide range of colour scales have been developed for measuring colour in water, ranging from the platinum scale, to the Forel-Ule scale and the Borger and Munsell colour systems. The following methods may be used for measuring colour in salty waters.
"Apparent" color/turbidity measurement using a spectophotometer:Edit
Color as APHA 'color units' or 'hazen' (see Eaton et al, 1995) may be measured using a spectrophotometer. A sample is placed in a cell and a beam shone through it. The machine may need calibration by zeroing against a blank.
Record the absorbance of the sample at 425nm and 580nm. As color and turbidity affect each other, correct by plotting the readings on the Nomogram Chart below. The chart is derived from AT Palin (1955) "Photometric determination of the colour and turbidity of water." Water & Water Engineering, 59:341-345.
To plot the turbidity and color onto the nomogram, mark the long scale on the left side of the nomogram with the reading you obtained when you used a wavelength of 425nm. Mark the long scale on the right hand side with the reading you obtained when you used a wavelength of 580nm. Take a ruler and carefully rule a straight line that connects the two readings. Where this line intersects the color scale and the turbidity scale in the centre of the nomogram, the intersections mark the values to record for each parameter.
Record colour as Hazen's or Color Units.
Note: The nomogram is dependent on the path length of the spectrophotometer cell. This chart was developed for a 10mm cell path length. Should you have a different cell path length the chart will need adjusting.
"Apparent" color reflected from a Secchi disk:Edit
A rapid method for use in the field, is to observe the apparent color of water reflected from the white quadrants of a secchi disk. The color may be compared to a set of color standards.
Gently lower the secchi disk into the water until the disk is one meter below the surface. Note whether you are looking at a surface scum (opaqhe, deep colored material that floats on the water surface) or whether you are looking at the color of the main water column. Look at the color reflecting back from the white quadrant of the disk and visually compare this colour with your color standard. Record the color, and the name of the standard you were using.
Useful color standards for reflected apparent color measurementsEdit
- Forel-Ule scale - a set of glass vials filled with colored water, developed for estuarine studies
- Permanent glass standards such as those made by Lovibond, in either the Pt-Co scale, Hazens or Color Units
- Munsell's color books - there are some choices - the full 2-volume Book of Color, the single volume 'Soil Color Charts' (useful for muddy water, and the single volume 'Plant Tissue Color Charts' (useful for yellow-green water or algal blooms)
- Globe put out color books for soil and plants too, that are less expensive
- A chart such as the one shown here that uses HTML-RGB codes to define its colors (selected from the 216 non-dithering color palette), and which includes a similar range of colours to the Forel-Ule scale used in estuarine studies. This chart was initially developed for use by community Waterwatch groups in South Australia, by Coleman and Cook (2007) and has been extended to display colours found in hypersaline conditions.
On the HTML-RGB chart shown here, the yellowy colors across the top row may represent dissolved organic (humic) substances. The unusual blues in the left-most column are likely to reflect dissolved minerals like calcite. Water colors in the 2c-3e area may represent cyanobacterial blooms or blooms of other extremely small chlorophyllous bacteria in the water column. Green flagellated algae such as Chlamydomonas may cause colors such as those in the 3b-5c region, while dinoflagellate and euglenoid blooms may develop colors in the 4c-4e and 5c areas. Muddy waters may have colors similar to 5d and column 6, while grey colors such as 5e suggest anaerobia has recently occurred in the waterbody. Column 7 shows colors frequently found in very hypersaline situations. The lighter, pinker colors may represent bacterial blooms, while the more orange, intense colors suggest blooms of the unicelluar algae Dunaliella salina.
"True" color using a spectrophotometer:Edit
Should a sample need testing for hue, saturation and luminance, the method as specified in Standard Methods for the Examination of Water and Wastewater (Eaton et al, 1995) should be followed. The method measures absorbance at a minimum of thirty different wavelengths, and then uses calculations to determine hue, saturation and luminance. This method provides a quantitative, rather than a qualitative measure of color.