3.2.9.4. Determination of Suspended Matter in Sea and Drinking Water

We shall compare the performance of flow nephelometry with stop flow turbidimetry by analyzing water samples collected from different sources.  This is a critical test because the instrument is calibrated by a homogenous stabilized suspension of standard material, while sea water contains particles of sand, clay, and organic matter of different sizes and optical properties.

The turbidity of sea water changes considerably between the open ocean where it can be as low as 0.05 NTU and as high as 60 NTU in an estuary if sediment is stirred by tidal activity. In coastal waters, the turbidity increases during the rainy season: in Hawaii, it may be as high as 5 NTU due to erosion of soil from the mountainside. Freshwater in clear rivers and lakes is about 10 NTU, and groundwater is as low as 2 NTU. Drinking water should be below 1 NTU (EPA) and never more than 4 NTU (WHO). To accommodate such a wide range of concentrations, we prepared calibrations in 0 to 20 NTU (P) and 0 to 2 NTU ranges and analyzed samples collected in Hawaii Kai Marina (U) and water supply in Honolulu while using ‘Analysis’ software (Appendix 1).

Suspended Matter concentrations determined by nephelometry (V left) and turbidimetry (right) follow a similar pattern: higher turbidity in Marina, which is heavily silted, and lower turbidity outside Marina by the bridge, where sea water from the bay dilutes water from the Marina at flood. The turbidity at Dock is high because a culvert in its vicinity brings in muddy water from the mountainside.  (Response to drinking water sample (TAP) is included to show baseline – note absence of schlieren effect).

The unexpected, striking difference between large fluctuations of nephelometric and well stabilized turbidimetric data is due to a large difference in volumes of interrogated suspensions. In turbidimetry, the illuminated and monitored volume is 50× larger than that for nephelometry, while the light path of 10 cm is 100× longer (B right). Therefore, in turbidimetry, the beam, passing through the vertical column, interrogates a larger volume of particles and integrates their individual responses, which vary due to their size, shape, and optical density, while the sample is arrested in the flow cell. In contrast, in nephelometry mode, a stream of heterogenous particles reflects light, which is monitored as large fluctuations in signal amplitude are scanned by a 1 μL flow cell. The inevitable conclusion is that particles of sediment passing through the flow cell reflect light in flow nephelometry differently than particles of Formazin (compare V left with I) while in flow turbidimetry, responses to Formazin and sediment are similar (compare V right with S). This observation argues against the use of flow nephelometry for monitoring the turbidity of sediments because there's no way how to increase the illuminated volume of the flow cell constructed by an orthogonal configuration of optical fibers.

To verify that turbidimetry is suitable for determination of Suspended Matter by using  calibration with Formazin (P), it must be shown that suspended particles of SM scatter light in the same way as particles of Formazin. This is confirmed in two ways: 1) by comparing spectra of Formazin with spectra of Suspended Matter recorded at a wide range of concentrations, and 2) by correlation of slopes of calibration lines of Formazin with calibration line obtained by dilution of particles of Suspended Mater.

Spectra of Formazin in DI (N) and in SSW (R) are the same as of unfiltered SM spectra in SSW (X and Y right). Also Formazin in DI and unfiltered  SM yield identical calibration lines (AA) and spectra (Z). This confirms that Formazin simulates optical properties of HK Suspended Matter and  that both suspensions scatter light in a way defined by Beer’s Law.

Filtration and particle size

Prior to chemical analysis sea water is routinely filtered through 0.5 μm filter that allows soluble species and colloids to pass into a filtrate. To gain insight into effect of this sample treatment, we collected three SW samples (not the same as the samples analyzed above) selected to encompass a wide NTU range and analyzed them unfiltered and filtered (Y).  Surprisingly while the unfiltered Dock samples contained 23.9, 9.8 and 4.9 NTU, the filtered Dock samples contained the same concentration of (colloidal ?) particles (1.6 NTU blue lines). In comparison, a 0.5 μL filtrate from 2 the 20 NTU standard contains 0.25 NTU. The spectra of Suspended Matter (Y right) support the calibration data and are very similar to and SM data while filtered SW shows only the 245 nm arbitrary peak on all blue lines.

These findings lead to the conclusion discussed in Section 3.2.9.5., but also open Pandora’s Box of uncomfortable questions: what are the effects of sample filtering? Is it necessary, and if so, is a 0.5 μm filtration sufficient cutoff when leaves as much as 1.6 NTU (or more) solids in filtrate? Considering a vast number of samples filtered manually each day on a cruise or in the laboratory, screening of NTU level should be considered because it will identify large blank due to high concentration of Suspended Matter or eliminate the need to filter samples with NTU below level of high blank (Appendix 3).

Analysis of drinking water

Since the quality of potable water is evaluated by measuring suspended matter in the 0 to 2 NTU range, we ran a calibration protocol (L) by using a 2.0 standard prepared in DI. The resulting outputs: 1/ absorbance vs. time (AB left) X and 2/ calibration graph (AB right) are similar to data in the range 0 to 20 NTU, with strictly linear calibration response. However, the higher values of LOD and BLANK (Q) compared to those obtained in 0 to 20 NTU (P) are likely due to the varying quality of the daily output of our DI source (avoidable by additional filtration). The calibration (AB right) was used with the software protocol in the (Appendix 1) to analyze the following samples of drinking water: 1) Tap water UH, 2) Tap water from home, 3) PELLEGRINO^® bottled water, 4) Laboratory DI water, 5) Drinking water fountain UH, and 6) SMART^® bottled water. Results (AD) show that unfiltered tap water did not meet the strict EPA standard, but the filtered tap (UH Font.) and all bottled waters did. Interestingly, PELLEGRINO® is carbonated mineral water, while SMART^® is tap water, distilled with taste adjusted by the addition of inorganic salts and by carbonation.

As noted above, our DI water would benefit from additional filtration. Spectra of suspended matter in tap waters (yy) are similar to spectra, while spectra below 1 NTU lack resolution.