Citizen Science - Turbidity (Part 2)

 In an earlier post (Citizen Science - Turbidity (Part 1)), I gave a brief introduction on why we choose to measure turbidity as part of the citizen science project monitoring pollution in the River Wye. 

Photo 1: Secchi Tube (use in upright position when filled with water!)

The majority of CSs use a Secchi Tube with a turbidity range of 12 - 240 NTU. Secchi tubes (Photo 1) are inexpensive and easy to use. Simply fill with river water until the black & white quadrants on the Secchi disc at the bottom of the tube are just no longer visible. Read off the turbidity value (NTU) printed on the side of the tube at the meniscus level. This simple procedure has low precision and accuracy because of the non-linear turbidity scale and the subjective nature of the test.

Note 1: Nephelometric Turbidity Units (NTU) are the most widely used units for quantifying turbidity and are based on 90⁰ scattering of white light (EPA Method 180.1). Formazin Nephelometric Unts (FNU) are similar (and often used interchangeably with NTU) and are based 90⁰ scattering of near infrared light (860 nm) according to the ISO 7027 method.

The only practical alternative is an optical instrument: either a turbidimeter or a nephelometer. The turbidimeter measures the amount of light passing through (transmitted by) a sample while the nephelometer measures the amount of light (usually at 90⁰ to the incident light) scattered by the particles in the liquid (Scheme 1). Turbidimeters are best suited to highly turbid fluids (> 100 NTU) whereas nephelometers work better with low turbidity fluids (< 50 NTU).

Scheme 1: Difference between Turbidimeter and Nephelometer

Professional-quality turbidimeters/nephelometers do not come cheap, typically >£1000. Fortunately, there are a range of Chinese-made instruments that are more reasonably priced for citizen science projects. The ZD2A nephelometer that I purchased is currently available for the relatively low price of £70-75 including shipping.

Photo 2: Generic Chinese Nephelometer (Model ZD2A)

My ZD2A nephelometer requires a 0.0 NTU turbidity standard and a 100 NTU turbidity standard to calibrate the nephelometer. I prepared my own nominal 0.0 NTU standard by filtering de-ionised water through a 0.22 μm membrane. The instrument came with a small bottle of 400 NTU formazin standard along with instructions for diluting to a 100 NTU standard. This is a simple dilution procedure provided you have the necessary volumetric glassware (or an accurate balance) to, for example, take a 2.5 ml aliquot and make up to 10 ml with distilled or de-ionised water. I chose to purchase a Stablcal 100 NTU standard directly to simplify the calibration procedure.

Note 2: Turbidity is calibrated using Formazin standards that have short shelf lives (from days to a few months). Stabilised Formazin Standards (e.g. Stablcal and T-CAL) have longer shelf lives (typically 1 - 2 years).

Since we have several years of turbidity data measured using Secchi tubes, one of the first tasks was to compare turbidity values measured using both techniques (Secchi vs Nephelometry). Along with a couple of fellow citizen scientists (Alan & Maggie), I collected river samples in clean 100 ml polyethylene bottles (the retention sample). To make the comparison as valid as possible, the retention samples were taken directly from the Secchi tube used to measure and record the turbidity of the river sample.

On returning home, the retention samples were analyzed using the ZD2A nephelometer. First the nephelometer was calibrated with the 0.0 and 100 NTU standards, followed by analysis of the retention samples. Retention samples were vigorously shaken for 10 seconds (to resuspend any settled particles), the sample cell was rinsed three times with sample, filled, capped, inverted 5 times and placed in the sample compartment. Turbidity readings were taken after 30 seconds, once the instrument had stabilised, and are collated in Table 1.

TABLE 1: Turbidity Values (NTU) Measured By Secchi Tube and Nephelometry
Secchi Tube (NTU)	Nephelometry (NTU)
<12	7
<12	11
15	14
21	19
25	26
25	41
40	69
40	75
50	56
75	122
100	134
190	221*
200	228*
>240	325*

Note 3: Samples with Secchi Tube values were excluded from the linear regression analysis (Figure 1)

Note 4: Samples marked with an asterisk (*) were accurately diluted by weight with de-ionised water before applying a dilution factor to the nephelometry result.

Figure 1 is the resultant linear regression analysis of the data from Table 1 (n= 11). The correlation (R² = 0.964) is better than I expected considering the relatively poor precision of the Secchi Tube method.

Despite some valid concerns regarding the accuracy and precision of the Secchi tube method, it does a remarkably good job of quantifying turbidity for this suite river samples. Further work is planned.