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possible as the differentiation is defined un-changeable by the resistor and the capacitor of thedifferentiating circuit [7].

Analogous to the adjustment of the window width,different modulation amplitudes a [8] can be chosenin optical derivative spectroscopy. According to [12],larger derivative signals and hence better signal-

to-noise ratio can be gained by increasing a. This wasconfirmed experimentally; however, a signal increasecomes along with a washing out of derivative shapesand minute derivative features are lost (Figure 7).The values for the modulation amplitude given inFigure 7 are typical examples. Thus, the modulationamplitude can be optimized for different applica-tions: if several analytes in mixtures have very similarand overlapping absorption spectra, a small modu-lation amplitude helps to pronounce minor differ-ences for discrimination. If, however, clearly differentabsorption spectra are present, a large modulation

amplitude is selected for improved signal-to-noiseratio.

See also : Chemometrics and Statistics: Multivariate

Calibration Techniques. Optical Spectroscopy: Radia-

tion Sources; Wavelength Selection Devices; Detection

Devices. Spectrophotometry: Overview.

Further Reading

Bosch OC, Sanchez RF, and Cano PJ (1995) Recentdevelopments in derivative ultraviolet/visible absorption

spectrophotometry. Talanta 42: 1195–1214.Brown C, Vega-Montoto L, and Wentzell P (2000)Derivative preprocessing and optimal correction for

baseline drift in multivariate calibration. Applied Spectro-scopy 54: 1055–1068.

Dixit L and Ram S (1985) Quantitative analysis byderivative electronic spectroscopy. Applied SpectroscopyReview 21: 311–418.

Fell A (1983) Biomedical applications of derivative spectro-scopy. Trends in Analytical Chemistry 2(3): 63–66.

Hager R and Anderson R (1970) Theory of the derivativespectrometer. Journal of the Optical Society of America60: 1444–1449.

Hawthorne A and Thorngate J (1978) Improving analysisfrom second-derivative UV-absorption spectrometry. Ap- plied Optics 17: 724–729.

Knowles A and Burgess C (1984) Practical AbsorptionSpectrometry/Ultraviolet Spectrometry. New York: Chap-man & Hall.

O’Haver TC and Begley T (1981) Signal-to-noise in higherorder derivative spectrometry. Analytical Chemistry 53:1876–1878.

Sassenscheid K, Klocke U, Marb C, et al. (1998) Dynamic

derivative UV-spectroscopy for combustion monitoring.Proceedings of SPIE 3535: 204–214.Sassenscheid K, Klocke U, Marb C, et al. (1998) Enhanced

selectivity and sensitivity in UV-analysis of volatile org-anic compounds. Proceedings of SPIE 3533: 222–233.

Savitzky A and Golay M (1964) Smoothing and differen-tiation of data by simplified least squares procedures.Analytical Chemistry 36: 1627–1639.

Steiner J, Termonia Y, and Deltour J (1972) Smoothing anddifferentiation of data by simplified least square proce-dure. Analytical Chemistry 44: 1906–1909.

Vogt F, Klocke U, Rebstock K, et al. (1999) Optical UVderivative-spectroscopy for monitoring gaseous emis-

sions. Applied Spectroscopy 53: 1352–1360.Williams D and Hager R (1970) The derivative spectro-meter. Applied Optics 9: 1597–1605.

Turbidimetry and Nephelometry

D M Lawler, University of Birmingham, Birmingham, UK

& 2005, Elsevier Ltd. All Rights Reserved.

Introduction

Turbidity is an expression of the optical property of amedium, which causes light to be scattered and ab-sorbed rather than transmitted in straight linesthrough the sample. The International Organizationfor Standardization (ISO) defines turbidity as the‘reduction of transparency of a liquid caused bythe presence of undissolved matter’. It is, therefore,the opposite of clarity. The medium concerned is

normally a fluid (but may be solid) in which light is

scattered by matter – usually small particles – sus-pended in the light path. Measurements of turbiditycan be used in many analytical fields to determine the

mass concentration of suspended particles in a sam-ple and, for some simple contexts, particle size dis-tributions. The field is hampered, however, by a lackof standardization in units, measurement devices andcalibration techniques. Analytical determinations of concentrations tend to be empirical. Such method-ological problems have recently driven a profusion of technical papers.

This article reviews turbidity theory, measurementprinciples, instrumentation systems, and applica-tions, with particular reference to suspended sedi-ment concentrations (SSCs) in natural waters (e.g.,

rivers, estuaries, and nearshore zones).

SPECTROPHOTOMETRY / Turbidimetry and Nephelometry 343



Definitions and Applications

Measurement Definitions

Turbidity can be measured using the techniques of turbidimetry or nephelometry (from nephelo ¼ cloud(Greek)). Turbidimetry is the measurement of tur-bidity by quantifying the degree of ‘attenuation’ of a

beam of light of known initial intensity. It is usuallyapplied to media of fairly high turbidity in which thescattering particles are relatively large (e.g., naturalwaters), for reasons, which will be addressed below.

Nephelometry is the measurement of turbidity bythe direct evaluation of the degree of light ‘scattering’taking place in the medium. It is much more appro-priate to media of lower turbidity in which thesuspended particles are small. Turbidimetry andnephelometry can offer considerable time-savingadvantages over gravimetric methods for the deter-mination of particle concentrations, and are non-destructive techniques.

Typical Applications

Turbidimetry and nephelometry have found manyapplications in scientific laboratories and in thechemical, pharmaceutical, foodstuffs, and beverageindustries. In addition, turbidimetry and nephelome-try are well-established procedures wherever filtra-tion processes have to be effected, monitored, andcontrolled. Within the hydrological sciences, andwater supply and wastewater management indus-tries, turbidity values can act as simple and conveni-ent surrogate measures of the concentration of suspended solids, sulfate ions (which are precipitat-ed as BaSO4 in acidic media (HCl) with bariumchloride), and other particulate material, and remainone of the most common applications of turbidime-try. Also, atmospheric and space physicists effect ne-phelometric analyses because of the importance of dust particles to radiation and other processes. Inquantitative chemical and biological analysis, appli-cations are common, especially the calculation of

absolute molecular weights and dimensions of pol-ymers in solution, as well as particle size determina-tions of suspended matter. Chemical profiles can alsobe obtained by observing turbidity changes deliber-ately induced by the addition of specific substancesto the solution. Within microbiology, cell and bac-teria growth can be monitored through the mediaturbidity changes such activity causes. In foodstuff manufacturing, turbidimetry is often used to monitorproduct quality and treatment process efficiency, es-pecially in the dairy and brewing industries. Clarity(and hence turbidity) is also a key concern in the

petrochemical industries. Determining turbidity

components – turbidity apportionment – has advan-ced with recent technological developments.

Principles and Theory

Light passing through a liquid medium may be scat-tered and absorbed by inhomogeneities in the light

path, especially suspended particles of silt, clay, algaeand other plankton, microbes, organic matter, andother fine insoluble particulate substances. Bubblesand density discontinuities can also scatter light.Scattering occurs when ‘a minute particle interactswith incident light by absorbing the light energy andthen, as if a point light source itself, reradiating thelight energy in all directions’. Absorption takes placewhen light is converted to other energy forms (e.g.,heat) within the particle. Scattered light includes thatreflected from the surface of the particle and thatrefracted within the particle, possibly after many in-

ternal reflections. Scattering is often accompanied byabsorption.

The direct relationship, however, between turbid-ity data and suspended solids concentrations isweakened by the complex interactions of light energywith suspended particles. This interplay is heavilydependent on many factors, including:

* concentration of scattering particles suspended inthe medium;

* size distribution of the scattering particles;* shape, orientation, and surface condition of the

scattering particles;* refractive index of the scattering particles;* refractive index of the suspension medium;* wavelength of the light source employed.

Consequently, separate bodies of theory have beendeveloped to describe the many different processesthat result. At its simplest level, light intensity is re-duced during transmission through a collection of scattering particles in a sample according to an at-tenuation function of the form:

I ¼ I 0eÀtl ½1

where I 0 is the initial beam intensity, I the beam in-tensity after passing through a medium of length l ,and t the turbidity coefficient of the medium. Equa-tion [1] ignores losses of light through true absorp-tion by suspended particles or reflection from thesides of the sample container.

Light-Scattering Theory and the Influence

of Particle Size

Appropriate light-scattering theory is governed by

the diameter, D, of the scattering elements in relation

344 SPECTROPHOTOMETRY / Turbidimetry and Nephelometry



to the wavelength, l, of the light emitted by the me-asuring instrument. Indeed, theory is often specifiedin terms of the Mie size parameter ða ¼ 2pR=lÞ,where R is particle radius. Particle size thus forms anappropriate basis for the subdivision of the theoret-ical discussion that follows.

Small Particles

For particles where Do0.05l, Rayleigh scatteringtheory of 1871, originally developed for gases, isapplicable to liquids with low concentrations of sus-pended particles which do not interact with eachother. For such small particles, relatively symmetricallight-scattering distributions are obtained (Figure

1A). If a visible light source (i.e., l¼ 0.4–0.7mm) isemployed, then it follows that this theory is appli-cable for particles where DoB0.03 mm.

The Rayleigh equation describing the angular dis-tribution of resultant scattering is:

iy=I 0 ¼ fðn0=nÞ À 1g2ðNV 2=l4r2Þð1 þ cos2 yÞ ½2

where iy is the intensity of light scattered at angle y,I 0 is the initial light source intensity, n0 is the refr-active index of the particles, n is the refractive indexof the suspension medium, r is the distance from theparticles to the point of measurement, in terms of thenumber, N , of particles, each of volume V . Rayleigh’swork thus shows that the intensity of scattered lightvaries: (1) with the square of the particle volume –and thus with the sixth power of the particle radius,assuming spherical shapes; and (2) inversely with thefourth power of the light wavelength used. Rayleightheory has since been developed to allow relativemolecular masses and sizes to be determined.

Large Particles

For larger particles, however, where 0.1loDo0.8l,the angular distribution of scattered light becomesasymmetrical. Destructive interference of light scat-tered in the backward direction leads to a bias inforward-scattered light (Figure 1B). In these con-texts, Mie scattering theory of 1908 for larger

spheres becomes more appropriate. For such larger

particles, scattering intensity is less dependent onwavelength.

Very Large Particles

For larger particles still, where D40.8l, the Mieequations are still workable, although for particleslarger thanB0.4mm in diameter wide oscillations inscattering patterns emerge. For particles of D41mm,extreme concentration of scattering in the forward

direction emerges (because of mutually destructivebackscattering effects), along with secondary peaksin the angular distribution of scattered light (Figure 1C).

Theory and practice also demonstrate that themost efficient scattering elements are those of a dia-meter similar to the light wavelength used. Also, agiven mass of small particles causes much greaterlight attenuation than the same mass of large parti-cles (Figure 2).

One complexity is that much classical theory hasbeen developed for identically sized spherical parti-cles – conditions that may not be obtained in all

laboratory or field situations. Indeed, many natural

Incident beamIncident beam Incident beam

(A) Small particles (B) Large particles (C) Larger particles

Figure 1 Influence of particle size on the angular distribution of scattered light: (A) small particles ( D o 0.1l); (B) large particles

(D B

0.25l); and (C) larger particles (D 4 1l). (From Vanous RD, Larson PE, and Hach CC (1982) The theory and measurementof turbidity and residue. In: Minear RA and Keith LH (eds.) Water Analysis , vol. 1, pp. 163–234. New York: Academic Press.)

1.0

0.3

0.1

0.0330 100 300 1000 3000 10 000

Suspended sediment concentration (mg l−1

)

12−18µm30−50µm

6−8µm

<5µm

L

i g h t a

b s o r b a n c e

( i c )

Figure 2 Influence of sediment particle diameter on light ab-

sorbance by samples of different concentrations. Note how a

given concentration effects much greater absorbance at the

smaller particle diameters. (Reproduced with permission from

Ward PRB and Chikwanha R (1980) Laboratory measurement of

sediment turbidity. Proceedings of the American Society of Civil

Engineers, Journal of the Hydraulics Division 106: 1041–1053.)




waters, like the atmosphere, contain an ensemble of variably sized, irregularly-shaped, and randomlyoriented particles, for which theory is still beingdeveloped. Furthermore, processes become highlycomplex when concentrations are so great that mul-tiple scattering occurs (i.e., particles receive lightpreviously scattered from other particles: this nor-

mally increases opportunities for light absorption).

Instrumentation

Range of Turbidimetric and Nephelometric

Systems

Early procedures were based on manual operation of analytical systems and visual turbidity assessment(e.g., the Secchi disk). Several instruments are nowavailable, however, for quantitative turbidity deter-mination in a variety of scientific, industrial, and

process management applications. Choice will de-pend largely on the analytical aims (e.g., mass con-centration, particle size distribution, moleculardimensions, or crystal/cell growth), the nature of the scattering elements and suspension medium, andwhether field or laboratory measurement is needed.Turbidimeters also vary in optical geometry, mode of operation, sample handling capabilities, data record-ing options (e.g., automatic/manual or analogue/ digital) and portability. Ultraviolet-visible spectro-photometry can also be used for turbidimetricmeasurements by measuring the absorption of light

by particles at a fixed wavelength or full spectrum(e.g., for kinetic studies of the time decay of species).The discussion that follows focuses on the use of turbidimetric instrumentation to estimate the massconcentration of suspended matter in liquid samples,with particular reference to sediment in naturalwaters.

Basic Elements of Measurement Systems

Modern measurement devices use photosensitivecells to quantify scattered and/or transmitted light.

Figure 3 illustrates that most laboratory bench in-struments usually have five basic components: a lightsource of known, constant intensity, and givenwavelength characteristics; a lens to collimate thelight beam; a sample cell; photosensor(s); and a me-ter or logger to record the output signals from thephotosensor(s). Versions for continuous monitoringof turbidity values (e.g., for online industrial systemsor process measurement in environmental sciences)include some kind of flow-through measurementchamber (instead of a ‘static’ sample cell) and out-puts for a datalogger. The two basic measuring in-

struments are the nephelometer and the turbidimeter.

A ‘nephelometer’ measures directly the intensity of light scattered by the sample, which is proportionalto the amount of matter suspended in the light path,though the influence of size, shape, and refractiveindex of the scattering particles is also important.With nephelometers, the sensor is mounted at anangle to the traversing beam (often 901) to recordscattered light in one part of the angular distribution(Figure 3A). Some more sophisticated versions canmonitor scattering intensity at many different angles:this allows angular summation values to be checkedagainst initial and attenuated signals. Nephelometersusually provide better precision and sensitivity thanturbidimeters and are normally used for samples of low turbidity containing small particles.

A ‘turbidimeter’, sometimes called a transmisso-meter, absorptiometer, or turbidity meter (the latterterm is commonly used for field instruments in the

earth and environmental sciences), measures the

5.9

(A)

(B)

(C)Filter

90° Detector

Lamp

LensSample

cell

Transmitteddetector

Forwardscatter

LampLens

Photocell

Sample cell(vertical view)

Filter

Lamp

LensSample

cell

Transmitteddetector

Figure 3 Three basic designs of turbidity meter: (A) the ne-

phelometer, which directly measures light scattered (usually at

901 to the beam direction) by suspended particles; (B) the tur-

bidimeter, where the transmitted light is detected, in relation to

initial beam intensity; (C) the ratio turbidimeter in which both

transmitted and scattered light is detected. (Reproduced withpermission from Hach CC, Vanous RD, and Heer JM (1982)

Understanding turbidity measurement. Technical Information

Series , Booklet No. 11, 1st edn., 11pp. Hach Chemical Co.)




intensity of the beam after it has passed through thesample, i.e., it quantifies the amount of transmittedlight remaining (Figure 3B). Suspended matter in thelight path causes scattering and absorption of somelight energy, which reduces the incident illuminationfalling on the photocell. These instruments are moreappropriate for relatively turbid samples in which the

scattering particles are large in relation to the lightwavelength used. This is because a significant reduc-tion in the intensity of incident light is needed toyield precise results.

Some newer instruments, called ratio turbidi-meters, incorporate measurement systems for lightwhich is side-scattered (usually at 901), forward-scattered, and transmitted (Figure 3C). The turbidityvalue is obtained as the ratio of the 901 signal tothe sum of forward-scattered and transmittedvalues. The ratio feature has a number of advantages:it increases the long-term stability of the sensor

(by reducing effects of instrumental drift); it com-pensates for ageing of, and deposits on, the optics;it reduces the influence of temperature changes inthe electronics; it minimizes the need for repeatedrecalibration; and it limits the effect of sample coloron readings. This can be more appropriate forstrongly and/or variably colored liquids, or for sam-ples of high turbidity. A four-beam instrument vers-ion has emerged recently, which reduces error stillfurther.

Recent developments include laser-based turbidi-meters, reflectometers, or fiber-optic systems. Thedevelopment of the optical backscatter sensor (OBS)has become popular for field deployment in the hy-drological and oceanographic sciences: this instru-ment monitors water turbidity through thebackscattering of pulsed infrared light emitted fromthe OBS instrument head. Also, remotely sensed tur-bidity measurement, using satellite or air borneinstruments (e.g., the CASI (Compact Airborne Spec-trographic Imager) system deployed by the UKNatural Environment Research Council), has recent-ly eased the mapping of turbidity patterns over largespatial scales.

There is a strong dependency of scattering effi-ciency on light wavelength (see above). Consequent-ly, for a given detector, light sources of shortwavelength are more sensitive to, and therefore moreuseful for, the detection of small particles. Convers-ely, longer wavelengths are more appropriate forsamples containing large particles (e.g., sediment inmany earth or environmental science systems). Thesource–detector relationship can vary widely be-tween instruments, and is cited as the key reasonexplaining the different readings obtained on the

same sample by different devices.

Units of Measurement and Instrument Calibration

The field is hampered by a nonstandard, ill-defined,and historically changing unit of measurement. TheNephelometric Turbidity Unit (NTU) is the mostcommon unit employed. The precision with whichturbidity data should be reported depends on howturbid the sample is, but should be to the nearest1–10%, approximately, of the NTU value deter-mined. For example, NTU values for distilled water,tap water, and raw water are 0.08, 0.54, and3.52, respectively, but much higher values, wellabove 150 NTU, are common in many hydrologicalsystems.

Formazin polymer, developed in 1926, can be usedfor turbidimeter calibration, and is straightforwardto prepare, control, and reproduce. Standard proce-dures for the production of a stock formazin turbid-ity suspension of 400 NTU are given in American

Public Health Association. Other calibration materi-als can be used (e.g., Fullers Earth or Hach Gelex‘fixed’ standards – metal oxide particles permanentlyand statically suspended in silica gel) and mayprovide suitable alternatives, especially given thehealth concerns voiced in some quarters over form-azin use.

Field Calibration

In natural waters, suspended material may largelyconsist of particles in the size range of clay(Do2 mm), silt (2oDo63mm), or even sand(63oDo2000mm). It may also include organic mat-ter and compounds and microscopic organisms. Forfield applications in hydrology or oceanography, an-alysts should preferably calibrate turbidity readingsagainst known mass concentrations of the suspendedsediment typical of that context, and declarethe strength of the diagnostic statistics for derivedrelationships. Such correlations can be weak,reflecting temporal changes in suspended loadcomposition (and hence its light-scattering efficien-

cy), water color, or bubble presence. Predictiverelationships can be strengthened by accountingfor such changes (especially in sediment load parti-cle size distribution), which can occur over varioustimescales (e.g., interannual, seasonal, subseasonal,flood event). It may even be necessary to producemultivariate or separate calibration equations to in-corporate the effects of, for example, changing flowlevels, sediment source areas, and season on sedimentload constitution. For such field applications, theturbidimeter reading (often in arbitrary units) isconverted to estimated SSC using a site-specific

calibration curve.




Turbidity Meters for Continuous Field Operation

For unattended automated field use a range of suit-able turbidity meters with additional instrumentalfeatures are available. The measurement cell is re-placed by a flow-through chamber, which mustprevent stray light from reaching the photosensor.

Instruments can be boom-mounted directly in theflow, or at the end of pump-lines connecting sa-mpling point to measurement system.

Turbidity meters with narrow-band near infrared(NIR) light sources (peak output at 0.86 mm; spectralbandwidth o0.06mm) are recommended by ISO.Such instruments reduce problems of algal build-upon the optical surfaces, are less affected by color, andare more sensitive to the slightly larger particles typ-ical of sediment transport systems. However, somerelaxation of the infrared protocol is tolerable forfield instruments operating in continuous monitoring

mode.To limit further the impact of problematic algal

growth on the optics, field turbidimetric systems canbe equipped with a pulsed light source, antifoulingchemicals or films, and/or ‘wiper’ blades for conveni-ent (sometimes automated) cleaning of opticalsurfaces. Alternatively, a dual-beam (twin-gap)instrument to compensate for these effects can bedeployed.

Power requirements for field instruments are im-portant considerations. The use of low-consumptionlight-emitting diodes (LEDs) in sensors, and pho-tovoltaic sensors which convert incident light directlyinto electrical energy, reduces power needs to a min-imum. Solar panels are useful to trickle-charge in-struments and dataloggers.

Some instruments are temperature sensitive, lar-gely because LEDs can emit more strongly whenwarm, and photovoltaic detectors convert photonsinto electrons more efficiently at low temperatures.Given, for example, the annual range of rivertemperature in the UK is typically B20–25 K, somecorrection procedures may be necessary.

Figure 4 also demonstrates that many turbidimeters are relatively insensitive to very fine or verycoarse particulate matter. Because most instrumentswork in the visible or NIR spectrum this means thatthe most readily detectable particles are those where,B0.2oDo1.8 mm. In standard sedimentologicaland engineering classifications, these are clay-sizedparticles, and hence the occasionally used term ‘silt-meter’ for turbidimetric instruments is not entirelyappropriate.

Turbidity meters need to be interfaced with port-able multimeters, dataloggers, or computers for

recording and storage of turbidity data. Dataloggers

equipped with an appropriate input channel andneeding minimal power for operation are heavilyused today. They provide quasi-continuous digitaldata on temporal variations in turbidity, which areideal for computational analysis and the study of turbidity dynamics. Telemetry systems for real-timedata acquisition, alarm facilities, and remote down-loading capabilities are becoming increasinglycommon.

Many field scientists, however, despite the range of commercially available turbidity instrumentation,still recognize a need for low cost, rugged, and re-liable systems. This is especially so when a networkof instruments is required for permanent installationto define a turbidity field, for simultaneous manualturbidity measurements by a research team, or forspecialist applications such as in subglacial environ-ments. This has led many researchers to custom-build their own instruments.

Hydrological Applications

of Turbidimetry

The monitoring of turbidity and SSCs in rivers, es-tuaries, lakes, reservoirs, nearshore zones, etc. is att-racting increasing attention from hydrologists,limnologists, geomorphologists, freshwater ecolo-gists, engineers, oceanographers, glaciologists, waterresource managers, and policy makers. Such meas-urement programs can allow inferences to be made

about upstream hydrogeomorphological processes,

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

200

100

0

300

400

I n s

t r u m e n t r

e s p o n s e

( N T U )

Hach 2100A

5×103 wt. % conc.

Particle diameter (µm)

HF DRT-1000

Figure 4 Sensitivity of two turbidity meters to particle diameter

(HF ¼ H.F. instruments). Test material here is spherical latex

particles of very narrow size distribution. (From Vanous RD,

Larson PE, and Hash CC (1982) The theory and measurement of

turbidity and residue. In: Minear RA and Keith LH (eds.) Water Analysis , vol. 1, pp. 163–234. New York: Academic Press.)




catchment erosion rates, downstream fluvial proc-esses and sedimentation impacts, pollutant and con-taminant transfer, and aquatic habitat quality. Therecreational value of water bodies can be partlylinked to their clarity, as demonstrated in the LakeTahoe turbidity reports of 2001–02. Increasingly,there is a legal requirement for environmental impact

assessments and water supply managers to considerthe possibility of short- or long-term turbidityincreases resulting from proposed developmentschemes.

Automated, in-stream, high-frequency turbiditymonitoring has become increasingly popular, main-ly because the alternative practice of sampling andsubsequent laboratory processing of samples is labo-rious and resource-intensive. Sampling approachesthus inevitably constrain the level of temporal andspatial detail possible, making it difficult to revealthe patterns, dynamics, and processes present. Re-

cent advances in the understanding of the hydrody-namics of fine-sediment transport in river, tidal, andnearshore environments, for example, would havebeen impossible without very high frequency (e.g.,5 Hz) monitoring of transient turbidity changes. Inremote environments, and in the developing world, alikely paucity of suitable sample analysis facilitiesunderlines the need for an automated and directfield-based method.

Three example applications of continuous moni-toring are outlined below: river turbidity varia-tion during rainstorms; very short-lived turbiditypulsing in glacial and coastal waters; and thedefinition of the estuarine turbidity maximum. Theseillustrate the many advantages of turbidity instru-mentation over sampling-based approaches inquantifying and understanding complex temporaland spatial patterns of suspended sediment fluxes.They also demonstrate the substantial variations of turbidity and SSCs in natural systems in space andtime.

River Turbidity Variation through Individual

Storm Events

SSC in streams can change appreciably over seasonaltimescales and during high-flow events. Automatedriver turbidity monitoring is very useful in refiningcalculations of suspended sediment loads, because itdetects the short-lived, but very important order-of-magnitude changes in SSC that occur in many riversduring complex storm events (e.g., Figure 5). The1-min datalogging scan interval used in Figure 5

defined all peaks and troughs in turbidity for mostevents. Following calibration, this permitted the full

definition of exhaustion phenomena in SSCs, and

hysteresis effects in relation to the discharge series(hitherto undetected for the system by earlier work-ers using a conventional sampling program). Thisled, in turn, to more securely based explanations of fine-sediment delivery processes.

Turbidity Pulsing in Rivers and Nearshore Zones

Turbidimetric instrumentation also facilitates the de-tection of very short-lived pulsing of suspended sed-iment, which characterizes many systems, especiallyproglacial meltwater environments. The example inFigure 6, for the Jo ¨ kulsa ´ a ´ So ´ lheimasandi glacial riverin southern Iceland, shows two substantial sediment-pulsing events detected by a 2-min turbidity scanningprogram. These were unrelated to river flow variat-ions, and showed that other significant sedimentmobilization processes were present in the system.The alternative approach of flow-triggered automat-

ed sampling is unsatisfactory in these situationswhere many sediment–flux perturbations are un-related to water discharge.

One strength of automated turbidity monitoring isthat the logging system can also be used to record, onthe same time base, data on associated or explana-tory variables. For glacial meltwater studies (Figure

6), these typically include energy budget componentsrelevant to glacial ablation, rainfall intensity, riverdischarge, temperature, and electrical conductivity,and, with Photo-Electronic Erosion Pin (PEEP) sen-sors, even the erosion events themselves which gene-

rate sediment plumes. Such data can strengthen the

12 12 12

Sediment concentration

C o n c e n

t r a t i o n ( m g

I − 1 ) D i s c

h a r g e

( I s −

1 )

Discharge

18.11.1986 19.11.1986 20.11.1986

0

25

50

75

100

Figure 5 Continuous record of stream suspended sediment

concentration, in relation to river discharge changes during storm

events, derived from calibration of a Partech turbidity record.

(From Gippel CJ (1989) The use of turbidity instruments to

measure stream water suspended sediment concentration,

Monograph Series No. 4, Department of Geography and

Oceanography, University College, University of New SouthWales, Australian Defence Force Academy, Canberra, 204pp.)




process-inference capabilities of the whole exercise.Thus, the correlation of high-frequency velocity andOBS turbidity series (Figure 7) allowed a clearer un-derstanding of river sediment transport events to begained. In coastal zones, very high frequency (5 Hz)monitoring of OBS turbidity, wave height, andcurrents (Figure 8), facilitated the definition of thecritical flow velocities required to mobilize bed sed-iment. Knowledge of these threshold conditions isimportant for the stability, engineering, and protec-tion of coastlines and their ecosystems.

Estuarine Turbidity Maxima

The ‘estuarine turbidity maximum’ is the term givento the clear peak in mean SSC observable in manyestuaries around the limit of saline intrusion. Im-proved explanations of turbidity fields in estuarinesystems, including the nature, location, and migrationof the turbidity maximum, and the tidal pumpingprocesses responsible, have recently been obtained bysupplementing water sampling approaches with de-tailed automatic turbidity monitoring. Furthermore,

18:00 20:00 22:00 00:00 02:00

100020

0 0

40

60

80

100

S u s p e n

d e

d s e

d i m e n

t c o n c e n

t r a t i o n

( m g

I − 1 )

2000

3000

4000

5000

D i s c

h a r g e

( m 3

s −

1 )

GMT (also Icelandic time)

Concentration

Discharge

Figure 6 Two-minute scanning of turbidity and river discharge (8–9 Aug 1988) showing a compound pulsing of suspended sediment

concentration, unrelated to flow variations, in the Jokulsa a Solheimasandi glacial river in southern Iceland. (Reproduced with

permission from Lawler DM and Brown RM (1992) A simple and inexpensive turbidity meter for the estimation of suspended sedimentconcentrations. Hydrological Processes 6: 159–168; & John Wiley and Sons Ltd.)

2 0

1 0

6 0

8 0

1 0 0 1 2 0

1 7 0 0

1 9 5 0

2 2 0 0

0

− 1 0

0 1 2 3 4 5 6 7 8 9 10

− 2 0

v ( c m

s −

1 )

Time (min, from 16:57 h PST)

OBS output

V

U

U

( c m s −

1 )

O B S o u

t p u

t ( m V )

Figure 7 A high-frequency, 5 Hz, 10-min time series of OBS turbidity alongside river flow components (u, streamwise; v , normal to

the bed) for the Fraser River, near Mission, BC, Canada. (Reproduced with permission from Lapointe M (1992) Burst-like sediments

suspension events in the sand bed river. Earth Surface Processes and Landforms 17: 253–270; & John Wiley and Sons Ltd.)




for any spatial survey, the fact that turbidity valuesare obtained at the field sites themselves, ratherthan in the laboratory subsequently, can allow in-stant decisions to be made regarding any furtherenvironmental sampling (including turbidity) thatmay be desirable. Figure 9 illustrates the value of repeated estuarine turbidity measurement in revea-

ling the spatial and temporal structure of the turbi-dity maximum.

See also : Color Measurement. Environmental Analy-

sis. Geochemistry: Sediment. Particle Size Analysis.

Sensors: Photometric. Water Analysis: Particle Charac-

terization.

Further Reading

American Public Health Association (1998) Turbidity.Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Amer-ican Water Works Association, and Water PollutionControl Federation.

Davies-Colley RJ and Smith DG (2001) Turbidity, suspe-nded sediment, and water clarity: a review. Journal of American Water Resources Association 37(5):1085–1101.

ISO (International Organisation for Standardization)(1984). Water Quality – Determination of Turbidity,1st edn., 5pp. ISO 7027 – 1984(E).

Mitchell SB, Lawler DM, West JR, and Couperthwaite JS(2003) Use of continuous turbidity sensor in the predic-tion of fine sediment transport in the turbidity maximumof the Trent Estuary, UK. Estuarine Coastal and Shelf Science 58: 643–650.

Schuerman DW (ed.) (1980) Light Scattering by Irre- gularly Shaped Particles, 334pp. New York: PlenumPress.

Van de Hulst HC (1981) Light Scattering by Small Particles, 2nd edn., 470pp. New York: Dover Publica-tions.

Walling DE (1977) Limitations of the rating curve tech-nique for estimating suspended sediment loads, withparticular reference to British rivers. Erosion and Solid Matter Transport in Inland Waters. InternationalAssociation of Hydrological Sciences Publ. No. 122,pp. 34–48.

Inorganic Compounds

M A Zezzi-Arruda and R J Poppi, University of

Campinas, Campinas, Brazil

& 2005, Elsevier Ltd. All Rights Reserved.

Introduction

Spectrophotometry is an excellent alternative forthe determination of inorganic compounds. It ischaracterized by a wide analytical working range,

Distance from mouth (km)

T u r b

i d i t y ( N T U )

Bottom

Top

0 2 4 6 8 10 12 14 16 18

120

140

160

180

100

80

60

40

20

0

Figure 9 Relationship between estuarine turbidity in NTU and

distance from the mouth of St. Lucia Estuary, Natal, on a rising

tide on 20 March 1981, showing a clear turbidity maximum

B8–12 km from the estuary mouth for both near-bed (bottom)

and surface (top) waters. (From Cyrus DP (1988) Turbidity and

other physical factors in Natal estuarine systems. Part 1: selected

estuaries. Journal of the Limnological Society of southern Africa

14(2): 60–71.)

0 50 100 150 200 250 300 350 400

Time (h)Start04.02.9612:00

H S

( m )

Finish19.02.9608:00

S

S C

( m g

l − 1 )

U ( m s −

1 )

0

100

200

−0.2

00.20.5

0

−0.5

2

3

1

0

V ( m s −

1 )

H s

SSC

V

U

Figure 8 A high-frequency, 5 Hz record averaged to hourly time

series of OBS turbidity (SSC), in relation to significant wave

height, H s; cross-shore current, U ; and longshore current, V , for

the North Sea nearshore zone at Holderness, UK. Data from Feb.

1996; water depth 16.8 m. The OBS here has been deployed

within BLISS (Boundary Layer Intelligent Sensor System). (From

Blewett J and Huntley D (1999) Measurement of suspended

sediment transport processes in shallow water off the Holderness

coast, UK. Marine Pollution Bulletin 37(3–7): 134–143.)

SPECTROPHOTOMETRY / Inorganic Compounds 351

turbidimetry and nephelometry sd

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