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Light transmissivity, suspended sediments and the legal definition of turbidity
Estuarine and Coastal Marine
Science (1974) 2, 291~299
Light Transmissivity, Suspended Sediments and the Legal Definition of Turbidity John C. McCarthy”, George M. GriiTS
Thomas
E. Pyle” and
‘Department of Marine Science, University of South Florida, St. Petersburg, Flu 33701, U.S.A. bHarbor Branch Foundation Laboratory, Box 196, Ft. Pierce, Fla 33450, and Geology Department, University of Florida, Gaine.wille, Flu 32601, U.S.A. Received 6 November 1973 and in revised form 8 February 1974
Evaluation of current methods of measuring turbidity in estuaries indicates a need to calibrate rapid, in situ optical measurements of beam transmittance with the units specified in water quality standards and with quantitative and qualitative data on the composition of suspended matter. Our results include curves relating per cent light transmission to Formazin Turbidity Units and the concentrations of monomineralic suspensions of Ca-montmorillonite and kaolinite. Beam attenuation coefficients calculated from transmissivity measurements vary linearly with concentrations (mg/l) of suspended clay particles measured in the laboratory. Field measurements of total suspended load (C in mg/l) and attenuation coefficients (a) in a west Florida estuary plot between the extremes defined by these laboratory data on two clay minerals. The relationship CI! 3.3~ determined for the Anclote estuary in August r97r, is compared with the results of previous studies. Such comparisons may be appropriate in restricted geographic areas or during certain seasons but the multitude of factors influencing natural turbidity measurements indicates the futility of searching for a universal equation applicable outside the laboratory. Instead of trying to apply instruments developed for optical oceanographers and geologists to the basically biological problems of estuarine turbidity variations, we recommend a long-term effort to determine the light and siltation tolerances of the organisms we are trying to protect. Field and laboratory measurements of light received or silt deposited, rather than tenuously related parameters, should determine the appropriateness of existing arbitrary water quality standards. Introduction Turbidity is a water quality parameter that is routinely measured as part of innumerable estuarine and coastal research programs and monitoring surveys. The ecological aspects of excess turbidity have been discussed by Biggs (1968, 1970), Ingle (1952), Johannes (1972), Robinson (1957), Saila et al. (1972) and Windom (1972) among others. Turbidity levels in discharges and effluents are regulated by a number of federal and state laws in the United States (e.g. F. W. P. C. A., 1968). As an optical property of water masses, turbidity expresses the attenuation of light due to scattering and absorption by the water itself, by dissolved substances and by organic and inorganic suspended matter. Despite the number of factors affecting its measurement,
292
J. C. McCarthy,
T. E. Pyle & G. M. Grjjin
turbidity has historically been determined by such apparently simple means as the Secchi disc (Tyler, 1968). The lack of specifications for this early device and the long period of time since its development in the mid-r8oos are at least partially responsible for a proliferation of turbidity measurement techniques. The situation is further complicated by intentionally different or confused definitions of turbidity employed in different fields of science and engineering (Austin, 1973a). As a consequence, many of today’s nominally equivalent measurements express different properties of natural waters. Recently we were confronted with the need for some form of turbidity measurement as part of an effort to assess the environmental impact of design, construction and operation of a coastal power-generating facility (Humm et al., 1971; Baird et al., 1972, 1973 ; Pyle et al., 1972; Pyle & McCarthy, 1973). The power plant is located at the mouth of the Anclote River, near Tarpon Springs in west-central Florida, U.S.A. Measurements of the natural extremes of turbidity distributions over an area of 50 km2 for three to four years were necessary to evaluate properly possible man-made perturbations. Inspection of construction plans suggested that these would include the dredging of water intake and discharge canals, run-off from settling ponds and construction activities and the likely erosion of canal walls by flow of cooling water which would be discharged in an area of seagrass beds. In this environmentally and publicly sensitive situation, where anticipated spatial variations were estimated to range from a few meters to 2 or 3 km, the desire for accuracy and precision in scientific measurement tended to conflict with the need for extensive data reflecting tidal, daily, seasonal and other variations in natural turbidity. The thoroughness of measurements required by optical oceanographers seemed incompatible with the requirement to determine ‘any and all’ ecological effects in an area whose center was known but whose radius could not be specified. We discuss one approach toward resolution of this problem and point out the shortcomings in current environmental protection standards which this approach has made apparent. Evaluation of techniques and instrumentation The following requirements dictated our choice of instrumentation : (I) the sensor should be able to make continuous, in situ measurements over a large area in minimum time with operational ease under a variety of sea and weather conditions; (2) it should have reasonable cost and maintenance requirements; (3) the parameter measured should be environmentally relevant, reproducible and consistent over the range and types of turbidity encountered in the study area. The Secchi disc is a simple kind of irradiance meter whose readings have been correlated with turbidity in the form of total suspended matter (Jones & Wills, 1956; Postma, 1961; Manheim et al., 1970, 1972) and attenuation coefficients (Jones & Wills, 1956; Tyler, 1968; Holmes, 1970). Despite such evidence that this time-honored technique should not be abandoned, we chose not to use it because of our need for continuous measurements and because of the increased subjectivity of Secchi depth (OS) versus suspended matter correlations in very turbid water (OS< I m; Manheim et al., 1972). Other instruments for turbidity measurement include absorptometers, nephelometers, light-scattering photometers and irradiance meters (Jerlov, 1968). Although each class has certain advantages, the latter three groups, as well as gravimetric techniques, require separate field stations or discrete water samples which must be transported to a laboratory. In addition to sample storage problems, our experience indicates that such sampling in estuaries does not adequately reflect abrupt variations in turbidity associated with foam lines,
Light transmissivity,
suspended sediments and turbidity
293
boundariesof small water masses,and the edgesof shoals,spoil islands, etc. Some instruments in the first class,commonly called transmissometers,can provide continuous in situ measurementsand generally meet the three criteria but they do not provide a read-out in legally acceptableunits such as the arbitrary JacksonTurbidity Unit (JTU) or Formazin Turbidity Unit (FTU) used in the United States. Our ultimate, though not ideal, choice was a commercial beam transmittance meter (Hydro Products Model 612 transmissometer).The per cent transmissionof an air-calibrated tungsten light source is measuredin the visible range with a peak photocell responseat 550 nm. These data can be converted to beam attenuation coefficients by means of the relationship T = Iooe-aL, where T = o/otransmissivity, a = beam attenuation coefficient, L = optical path length (m). Continuous measurementof y. light transmittance can be made while towing the instrument from a small boat at speedsup to 4 or 5 knots. The light source and detector are mounted in .an internally baffled tube to minimize ambient light effects and to facilitate towing. A removable center section allows optional use of either a r-m or ro-cm light path. Flushing time is lessthan I s at normal towing speeds.To alleviate problemsassociatedwith needle movement during rougher weather, stripchart recording of the data should be used insteadof dial readout. With a r-m optical path, the attenuation coefficient can be measuredfrom 0-a to 2.5 m-l and with a ro-cm path from 2.0 to 25 m-l. The ro-cm version is normally usedin our study of the Anclote estuary where the attenuation coefficient hasbeen observedto vary between 2 and 12 m-l (about 80 to 30% T/IO cm). It should be noted that studiesin other coastal locations have required useof a r-m optical path (e.g. at Crystal River, Florida, we observed a range in a from 0.5 to 0.9 m-l). In terms of those studying optical properties of water, this instrument, in effect, measures the sum of absorption and scattering coefficients.From a biological viewpoint, the measurements are affected by water color and suspendedmineralsas well as plankton. Data reduction There are a number of ways to handle data of the type generatedby this system.By virtue of simplicity and direct objective utilization of instrument readout, we have preferred to simply map and contour transmissivity values for each survey. This type of presentation allows a number of generalized qualitative assessments of turbidity sourcesand background levels, but one should have complementary hydrographic, meteorologic, biologic and geologicdata for complete interpretations. In someapplications it may be desirableto convert transmissivity data and then plot and contour the equivalent attenuation coefficients. These data could be further interpreted in terms of underwater visibility (Duntley, 1963 ; Briggs & Morris, 1966). For geological studies,it would be valuable to be ableto interpret the data in terms of the approximate mass concentration of suspendedmaterials. If compliancewith water-quality standardsmust be checked, it is necessaryto relate the data to the turbidity units of current legaland engineering usage.Certain aspectsof the latter two problemsare consideredin the following paragraphs. Calibration Water quality standards Turbidity haslong beenlegally characterized in the U.S. by JacksonTurbidity Units (JTU) and more recently by Formazin Turbidity Units (FTU). A brief outline of the development
J. C. McCarthy, T. E. Pyle & G. M. Grj&
294
and significanceof theseunits is given by Hach (1972) and in the referenceStandard Methods (1971, pp. 34.+-356), jointly published by the American Public Health Association, the American Water Works Association and the Water Pollution Control Federation. The prescribedmethods for turbidity determination are intended for the ‘examination of natural and treated waters in the absenceof gross pollution’. We are not aware of any further specification of the environments these arbitrary units were intended to cover or of the scientific basisfor allowable turbidity increments. In an attempt to reconcile our needfor rapid area1coverageof a quasi-synoptic nature with the legal definition of turbidity, we have accomplishedan approximate calibration of our transmissometeragainst Formazin Turbidity Units (FTU). This was conducted in laboratory tanks using Formazin standardsprepared according to Standard Methods (1971, pp. 351-352) and checked by a Hach Model 21ooA turbidimeter. A plot of yO transmissivity (%T) against FTU [Figure I(a)] shows that the useful responseof our transmissometer with a IO-cm path is from approximately IO to 80% T/IO cm or about 50 to 5 FTU.
80
0.1
I .o
FTU Figure I. Laboratory calibrations of transmissometer against (a) Formazin Turbidity Units (FTU) determined meter; (b) concentrations of standard clay minerals Ca-montmorillonite).
IO
100
1000
C (mg/l) (with ro-cm path length) by Hach zrooA turbidi( . . . ., kaolinite; - -,
Suspended minerals Suspensionsof standard clay minerals (Table I) in tap water were prepared for comparison with the Formazin calibration. Figure 1(b) relates %T/Io cm to the massconcentration (C in mg/l) of two common clay species.The dotted curve representsthe function for kaolinite (kandite) and the dashed curve for Ca-montmorillonite (smectite). Note that for a given concentration, Ca-montmorillonite allowsgreater light transmissionthan doeskaolinite and that the curve for kaolinite approximates the %T versusFormazin function in Figure r(a). Comparison of Figures r(a) and (b) suggeststhat a mono-minerallic suspensionof kaolinite would be in a I : I ratio of massconcentration (mg/l) to FTU while Ca-montmorillonite approximatesa 2 : I ratio. Similar compositionalor grain size variations, but not identical relationships, are shown by Postma (1961), Duchrow & Everhart (1971) and are probably present in the data of Jones& Wills (1956). The magnitude of variability of optical
Light transmissivity,
suspended sediments and turbidity
295
attenuation due to differences in size and refractive index of suspendedmaterials becomes apparent from these independent studies. Beam attenuation
coeficient
(a)
Alpha asa function of massconcentration for a specificsuspendedmaterial under controlled conditions is observed to be essentially linear. This relationship is shown in Figure z for the sametwo clays and Formazin usedto generatethe curves in Figures r(a) and (b). Firstorder regressionanalysisof thesedata suggeststhat FTU and kaolinite (mg/l) approximate z a and montmorillonite 4 a (correlation coefficientsr>o.99 for all three curves).
TABLE
I. Data
for clay mineral
standards
Ca-montmorillonite I.
Origin
2.
Chemical SiOz Al@, FedA Fe0 FeS MgO CaO Na,O K,O so,
3.
White Springs, ‘Panther Creek Bentonite’ analyses :
(smectite) Miss. Sold as Southern
56.77 20.33 8.42 0.37 0.45 3’10 “35 0.34 0.73 0.16 n.r.
2iFF2
0.85
co,
0.45
Ignition loss H,O Minor Total Particle size characteristics A. Manufacturers’ data >SW 0.5-s Pm cc.5 w B. Pipette analysis >62 31-62 16-31 8-16 4- 8 2- 4 I-
Kaolinite
(kandite)
Putnam Co., Fla Sold as ‘Edgar Plastic Kaolin’ 46.0 37.8 0.60 n.r. n.r. 0’15
0’10 0’20 0’25
0.035 <0.05 n.r. n.r. 14.0
1:;;; 0’03 100’00
(wt
2
“;,) 30% 35% 35%
37% 45 %
0’0 7’1 7” 3.6 7’1 7’1 42’9
0’0
25’0
diameter
Total suspended load in natural
n.r. n.r. 99.18
9.6~
18%
7’0 4.6 7’0 7’0 11.6 9’3 53’4 rc’3cP
waters
We have measuredtotal suspendedload (C in mg/l) by membranefiltration (0.45 p) of water samplestaken simultaneouslywith transmissometersurveys at the Anclote site. Scatter plots of C versr~~attenuation coefficient (from o/oT) have generally fallen betweenthe kaolinite and
296
J. C. McCarthy,
T. E. Pyle & G. M. Grjjin
montmorillonite regressionlines in Figure 2. The relationship CE 3.3~ wasfound to hold for a survey in August 1971 (rangeof C, 22.1-47.2 mg/l). A comparisonof theseresultswith those of previous workers is given in Table 2. %T/lOcm
Figure 2. First-order regression of concentration (mg/l) minerals ( . . . ., kaolinite; - -, Ca-montmorillonite) attenuation coefficient (a) computed from transmissivity
of standard and data.
clay minerals FTU against
Although the transmissometeris useful in making approximations of particle concentration and distribution while providing rapid area1coverageand high density of data, it should be emphasizedthat the concentration of suspendedmaterial inferred by this method is only semi-quantitative. When parameterssuch as water color, particle size, composition and morphology are at minimum variance, the estimates may approach a higher degree of absoluteaccuracy.
Discussion
and recommendations
Turbidity as currently defined by regulatory agenciesin the United States is not a very effective measurementof water quality in many cases.Probably the Formazin or Jackson Turbidity Unit can consistently and reliably be related to water and sediment properties only in the laboratory. To relate field measurementsof water quality to light-dependent phenomenasuch as primary productivity, the establishmentof light attenuation standards for environmental protection would seemmore appropriate. To regulate the addition of suspendedmaterials(asin dredging operations)to the environment, the standard parameter should relate better to actual mass concentration to the exclusion of material types and water color. As described above, the same FTU reading could be obtained from a given concentration of kaolinite and twice that concentration of Ca-montmorillonite. Becausetwice asmuch material is in suspension,the resulting siltation problem would be significantly greater for a Ca-montmorillonite discharge than for a kaolinite discharge. We also believe the current turbidity standards are practically and environmentally unrealistic in many situations. As an example, dredge operators in the U.S. are allowed by
Light transmissivity,
suspended sediments and turbidity
297
law to makeadditions of up to 50 FTU above background turbidity. For a background level of IO to 15 FTU in the Anclote study area,this arbitrary numerical standard would allow a three- to five-fold increaseover natural levels (increasein a from 5 to 30). In an areasuch as Crystal River, Florida, the allowable increasesin alpha could be two orders of magnitude over background. In this latter case,the law seemsto imposeno restrictions at all because such levels might be impossibleto attain by even the most carelessand uncontrolled operations. At the other extreme, it may not be realistic to limit turbidity additions to an arbitrary number such as 50 or IOO FTU in areasof very high natural turbidity. TABLE 2. Relations between suspended sediment attenuation coefficient, a, in marine waters Reference
Area
This paper Gordon & Smith (1972) Drake et al. (1972) Jones & Wills
Anclote estuary, Fla Biscayne Bay, Fla Santa Barbara-Oxnard Shelf, Calif. Thames estuary Plymouth Sound Hamoaze
(1956)
11Not
Time
given
by authors,
calculated
Function
August 1971 May-June 1970 February I 970 July May May
1953 1953 1953
from
plotted
concentration,
Cz3.3a C=2,6a
C, (mg/l) Range
and (mg/l)
22'1-47'2
I -15
CE2.7a"
1’5-
Cz7’7a”
7'0-27'3
C2:2.2a' Cz5.8a"
0.6-
5’7 3.5
2.2-24'9
data or tables.
Several approachesshouldbe followed in the future-some simultaneously.We agreewith Austin (1973a) that ‘To perpetuate the useof the JTU or FTU in measurementproblemsof coastal and offshore monitoring would seem . . . to be a path of dubious wisdom’. His suggestion(Austin, npja, 6) that the attenuation coefficient, a well-defined, widely-accepted physical unit, be consideredas a more logical substitute has great merit. However, we feel that turbidity standardsultimately should be defined in terms of the light requirements or silt tolerancesof the organismswe are attempting to protect (e.g. Davis & Nudi, 1971). Field and laboratory measurementsof light received or silt deposited versusplant and animal condition, rather than measurementof tenuously related parameters,should be conducted to determine the appropriatenessof existing water quality standards.At present, it may be more realistic to redefine allowable turbidity increasesin terms of percentage above background rather than an arbitrary numerical increment. Such a standard should, of course, consider the level and duration of the maximum turbidity to which an environment is naturally subjected aswell asthe background or ‘average’ level. However, as geologistswe are not prepared to suggestwhat that percentage increase should be. Until the studies outlined above are conducted, establishmentof such a percentage,though seemingly more logical, would be as arbitrary as the current absolute numerical standards. Obviously, no simple solutions exist, but it is just as obvious that further refinement and standardization of turbidity measurementtechniques is required for environmental impact studiesin estuariesand coastalregions. In addition to agreeingon a replacementfor units of vague origins and dubious merit (JTU, FTU), we must determine more realistic standards for turbidity increases,either in terms of percentage increase or effect upon the biota. Modifications may have to be made to present standards which enjoy political and legal sanctity to the possibledetriment of the environment. Conclusion We have describedan expedient method for satisfying someof the demandsof estuarineand coastalturbidity studieswhile estimating turbidity in legally acceptableunits. We emphasize
J. C. McCarthy,
T. E. Pyle
@ G. M.
Gr~%n
that the specific correlations given here should not be used directly in other areas or with other instruments, but that the strategy outlined should be valuable if individual calibrations are carried out. Such calibrations also serve to emphasize the number of factors affecting turbidity at any one site and the need for additional study of turbidity and siltation effects upon organisms. Acknowledgements This study was supported by the Florida Power Corporation. Assistance in the field and laboratory was provided by Don Eggimann, Phyllis Jones, Jack Dyer and Richard Clingan. David Wallace provided valuable assistance and discussion in all phases of our study. Helpful criticisms and suggestions were given by Larry Doyle, Frank Manheim and Ronald Baird. We are grateful to R. W. Austin for providing very helpful comments as well as copies of unpublished manuscripts which clarified our understanding of the physical aspects of turbidity. References Austin, R. W. r973a Turbidity as a water quality parameter. Proceedings of the Seminar on Methodology for Monitoring the Marine Environment, Seattle, Washington, Oct. 16-18, Environmental Protection Agency (In press). Austin, R. W. r9736. Visibility Laboratory, Scripps Institution of Oceanography, personal communication. Baird, R. C., Carder, K. L., Hopkins, T. L., Pyle, T. E. & Humm, H. J. 1972 Anclote Environmental Project, Annual Report 1971. University of South Florida, Marine Science Institute, Technical Report 12, 251 pp. Baird, R. C., Carder, K. L., Hopkins, T. L., Pyle, T. E. & Humm, H. J. 1973 Anclote Environmental Project, Annual Report 1972. University of South Florida, Department of Marine Science, Contribution 41, 220 pp. Biggs, R. B. 1968 Environmental aspects of overboard spoil disposal. Proceedings of American Society of Civil Engineers, Journal of Sanitary Engineering Division 94 (SA3), 477-487. Biggs, R. B. 1970 Geology and hydrography. In Gross Physical and Biological Eflects of Overboard Spoil Disposal in Upper Chesapeake Bay University of Maryland, Natural Resources Institute Special Report No. 3, 7-15. Briggs, R. 0. & Morris, G. 1966 Instrumentation for the prediction of underwater visibility range as a function of water condition. Proc. Underwater Photo-Optics Seminar, Society of Photographic Instrument Engineers C-XVII-r to C-XVII-7. Davis, C. R. & Nudi, F. A. Jr 1971 Turbidity bioassay method for the development of prediction techniques to assess the possible environmental effects of marine mining. Preprints Oflshore Technology Conference Houston, Texas I, 881-888. Drake. D. E.. Kolaack. R. L. & Fischer. P. 1. 1q72 Sediment transport on the Santa Barbara-Oxnard shelf, Santa Barbara Channel, California. In Shelf Sediment Transport, Process and Pattern. 656 pp. (Swift, D. J. P., Duane, D. B. & Pilkey, 0. H., eds) Dowden, Hutchinson & Ross, Inc., Stroudsberg, Pa. Duchrow, R. M. & Everhart, W. H. 1971 Turbidity measurement. Transactions of American Fisheries Society IO0 (4), 682-690. Duntley, s. Q. 1963 Light in the sea. ~ournol Optical Society of America 53, 2X4-233. F.W.P.C.A. [Federal Water Pollution Control Administration) 1968 Water Oualitv U. S. - Criteria Government Printing Office, Washington, D.C., 234 pp. ’ . Gordon, H. R. & Smith, J, M. 1972 A time series study of beam transmittance in Biscayne Bay. Abstract, EOS 539 4, 400. Hach, C. C. 1972 Understanding turbidity measurement. Industrial Water Engineering February/ March, 18-22. Holmes, R. W. 1970 The Secchi disc in turbid coastal waters. Limnology and Oceanography 15~688-694. Humm, H. J., Baird, R. C., Carder, K. L., Hopkins, T. L. & Pyle, T. E. 1971 Anclote Environmental Project, Annual Report 1970. University of South Florida, Marine Science Institute, Technical Report I, 171 pp. Ingle, R. M. 1952 Studies on the effect of dredging operations upon fish and shellfish. Florida Board of Conservation, Tallahassee, Technical Series No. 5, 26 pp. ,
~I
.
I
-
,,
Ligkt
transmissivity,
suspended
sediments
and turbidity
299
Jerlov, N. G. 1968 Optical Oceanography. 194 pp. Elsevier, London. Johannes, R. E. 1972 Coral reefs and pollution. In Marine Pollution and Sea Life (Ruivo, M., ed.) Fishing New (Books), Ltd., London. Jones, D. & Wills, M. S. 1956 The attenuation of light in sea and estuarine waters in relation to the concentration of suspended solid matter. Journal of Marine Biological Association of the United Kingdom 35,43 I-+++. Manheim, F. T., Meade, R. H. & Bond, G. C. 1970 Suspended matter in surface waters of the Atlantic Continental margin from Cape Cod to the Florida Keys. Science 167, 371-376. Manheim, F. T., Hathaway, J. C. & Uchupi, E. 1972 Suspended matter in surface waters of the northern Gulf of Mexico. Limnology and Oceanography 17 (I), 17-27. Postma, H. 1961 Suspended matter and Secchi disc visibility in coastal waters. NetherlandsJournal Sea Research I (3), 359-390. Pyle, T. E., Baird, R. C., Carder, Ii. L., Hopkins, T. L. & Wallace, D. W. 1972 The making of a power plant I: environment and design. Abstract, QuarterlyJournal of Florida Academy of Sciences 35 (I), 30. Pyle, T. E. & McCarthy, J. C. 1973 The use of historical bathymetric data in assessing environmental impact. Enoironmental Letters 5 (I), 63-69. Robinson, M. 1957 The effects of suspended materials on the reproductive rate of Dapknia magna. University of Texas, Publications of Institute of Marine Science 4 (z), 265-277. Saila, S. B., Pratt, S. D. & Polgar, T. T. 1972 Dredge spoil disposal in Rhode Island Sound. Uniwersity of Rhode Island Marine Technical Report No. 2, 48 pp. Standard Methods 1971 13th edition, 874 pp. American Public Health Association, New York, New York. Tyler, J. 1968 The Secchi disc. Limnology and Oceanography 13, 1-6. Windom, H. L. 1972 Environmental aspects of dredging in estuaries. Proceedings of American Society of Civil Engineers, rournal of Waterways, Harbors and Coastal Engineering Division 98 (WWq), 475-487.
Science (1974) 2, 291~299
Light Transmissivity, Suspended Sediments and the Legal Definition of Turbidity John C. McCarthy”, George M. GriiTS
Thomas
E. Pyle” and
‘Department of Marine Science, University of South Florida, St. Petersburg, Flu 33701, U.S.A. bHarbor Branch Foundation Laboratory, Box 196, Ft. Pierce, Fla 33450, and Geology Department, University of Florida, Gaine.wille, Flu 32601, U.S.A. Received 6 November 1973 and in revised form 8 February 1974
Evaluation of current methods of measuring turbidity in estuaries indicates a need to calibrate rapid, in situ optical measurements of beam transmittance with the units specified in water quality standards and with quantitative and qualitative data on the composition of suspended matter. Our results include curves relating per cent light transmission to Formazin Turbidity Units and the concentrations of monomineralic suspensions of Ca-montmorillonite and kaolinite. Beam attenuation coefficients calculated from transmissivity measurements vary linearly with concentrations (mg/l) of suspended clay particles measured in the laboratory. Field measurements of total suspended load (C in mg/l) and attenuation coefficients (a) in a west Florida estuary plot between the extremes defined by these laboratory data on two clay minerals. The relationship CI! 3.3~ determined for the Anclote estuary in August r97r, is compared with the results of previous studies. Such comparisons may be appropriate in restricted geographic areas or during certain seasons but the multitude of factors influencing natural turbidity measurements indicates the futility of searching for a universal equation applicable outside the laboratory. Instead of trying to apply instruments developed for optical oceanographers and geologists to the basically biological problems of estuarine turbidity variations, we recommend a long-term effort to determine the light and siltation tolerances of the organisms we are trying to protect. Field and laboratory measurements of light received or silt deposited, rather than tenuously related parameters, should determine the appropriateness of existing arbitrary water quality standards. Introduction Turbidity is a water quality parameter that is routinely measured as part of innumerable estuarine and coastal research programs and monitoring surveys. The ecological aspects of excess turbidity have been discussed by Biggs (1968, 1970), Ingle (1952), Johannes (1972), Robinson (1957), Saila et al. (1972) and Windom (1972) among others. Turbidity levels in discharges and effluents are regulated by a number of federal and state laws in the United States (e.g. F. W. P. C. A., 1968). As an optical property of water masses, turbidity expresses the attenuation of light due to scattering and absorption by the water itself, by dissolved substances and by organic and inorganic suspended matter. Despite the number of factors affecting its measurement,
292
J. C. McCarthy,
T. E. Pyle & G. M. Grjjin
turbidity has historically been determined by such apparently simple means as the Secchi disc (Tyler, 1968). The lack of specifications for this early device and the long period of time since its development in the mid-r8oos are at least partially responsible for a proliferation of turbidity measurement techniques. The situation is further complicated by intentionally different or confused definitions of turbidity employed in different fields of science and engineering (Austin, 1973a). As a consequence, many of today’s nominally equivalent measurements express different properties of natural waters. Recently we were confronted with the need for some form of turbidity measurement as part of an effort to assess the environmental impact of design, construction and operation of a coastal power-generating facility (Humm et al., 1971; Baird et al., 1972, 1973 ; Pyle et al., 1972; Pyle & McCarthy, 1973). The power plant is located at the mouth of the Anclote River, near Tarpon Springs in west-central Florida, U.S.A. Measurements of the natural extremes of turbidity distributions over an area of 50 km2 for three to four years were necessary to evaluate properly possible man-made perturbations. Inspection of construction plans suggested that these would include the dredging of water intake and discharge canals, run-off from settling ponds and construction activities and the likely erosion of canal walls by flow of cooling water which would be discharged in an area of seagrass beds. In this environmentally and publicly sensitive situation, where anticipated spatial variations were estimated to range from a few meters to 2 or 3 km, the desire for accuracy and precision in scientific measurement tended to conflict with the need for extensive data reflecting tidal, daily, seasonal and other variations in natural turbidity. The thoroughness of measurements required by optical oceanographers seemed incompatible with the requirement to determine ‘any and all’ ecological effects in an area whose center was known but whose radius could not be specified. We discuss one approach toward resolution of this problem and point out the shortcomings in current environmental protection standards which this approach has made apparent. Evaluation of techniques and instrumentation The following requirements dictated our choice of instrumentation : (I) the sensor should be able to make continuous, in situ measurements over a large area in minimum time with operational ease under a variety of sea and weather conditions; (2) it should have reasonable cost and maintenance requirements; (3) the parameter measured should be environmentally relevant, reproducible and consistent over the range and types of turbidity encountered in the study area. The Secchi disc is a simple kind of irradiance meter whose readings have been correlated with turbidity in the form of total suspended matter (Jones & Wills, 1956; Postma, 1961; Manheim et al., 1970, 1972) and attenuation coefficients (Jones & Wills, 1956; Tyler, 1968; Holmes, 1970). Despite such evidence that this time-honored technique should not be abandoned, we chose not to use it because of our need for continuous measurements and because of the increased subjectivity of Secchi depth (OS) versus suspended matter correlations in very turbid water (OS< I m; Manheim et al., 1972). Other instruments for turbidity measurement include absorptometers, nephelometers, light-scattering photometers and irradiance meters (Jerlov, 1968). Although each class has certain advantages, the latter three groups, as well as gravimetric techniques, require separate field stations or discrete water samples which must be transported to a laboratory. In addition to sample storage problems, our experience indicates that such sampling in estuaries does not adequately reflect abrupt variations in turbidity associated with foam lines,
Light transmissivity,
suspended sediments and turbidity
293
boundariesof small water masses,and the edgesof shoals,spoil islands, etc. Some instruments in the first class,commonly called transmissometers,can provide continuous in situ measurementsand generally meet the three criteria but they do not provide a read-out in legally acceptableunits such as the arbitrary JacksonTurbidity Unit (JTU) or Formazin Turbidity Unit (FTU) used in the United States. Our ultimate, though not ideal, choice was a commercial beam transmittance meter (Hydro Products Model 612 transmissometer).The per cent transmissionof an air-calibrated tungsten light source is measuredin the visible range with a peak photocell responseat 550 nm. These data can be converted to beam attenuation coefficients by means of the relationship T = Iooe-aL, where T = o/otransmissivity, a = beam attenuation coefficient, L = optical path length (m). Continuous measurementof y. light transmittance can be made while towing the instrument from a small boat at speedsup to 4 or 5 knots. The light source and detector are mounted in .an internally baffled tube to minimize ambient light effects and to facilitate towing. A removable center section allows optional use of either a r-m or ro-cm light path. Flushing time is lessthan I s at normal towing speeds.To alleviate problemsassociatedwith needle movement during rougher weather, stripchart recording of the data should be used insteadof dial readout. With a r-m optical path, the attenuation coefficient can be measuredfrom 0-a to 2.5 m-l and with a ro-cm path from 2.0 to 25 m-l. The ro-cm version is normally usedin our study of the Anclote estuary where the attenuation coefficient hasbeen observedto vary between 2 and 12 m-l (about 80 to 30% T/IO cm). It should be noted that studiesin other coastal locations have required useof a r-m optical path (e.g. at Crystal River, Florida, we observed a range in a from 0.5 to 0.9 m-l). In terms of those studying optical properties of water, this instrument, in effect, measures the sum of absorption and scattering coefficients.From a biological viewpoint, the measurements are affected by water color and suspendedmineralsas well as plankton. Data reduction There are a number of ways to handle data of the type generatedby this system.By virtue of simplicity and direct objective utilization of instrument readout, we have preferred to simply map and contour transmissivity values for each survey. This type of presentation allows a number of generalized qualitative assessments of turbidity sourcesand background levels, but one should have complementary hydrographic, meteorologic, biologic and geologicdata for complete interpretations. In someapplications it may be desirableto convert transmissivity data and then plot and contour the equivalent attenuation coefficients. These data could be further interpreted in terms of underwater visibility (Duntley, 1963 ; Briggs & Morris, 1966). For geological studies,it would be valuable to be ableto interpret the data in terms of the approximate mass concentration of suspendedmaterials. If compliancewith water-quality standardsmust be checked, it is necessaryto relate the data to the turbidity units of current legaland engineering usage.Certain aspectsof the latter two problemsare consideredin the following paragraphs. Calibration Water quality standards Turbidity haslong beenlegally characterized in the U.S. by JacksonTurbidity Units (JTU) and more recently by Formazin Turbidity Units (FTU). A brief outline of the development
J. C. McCarthy, T. E. Pyle & G. M. Grj&
294
and significanceof theseunits is given by Hach (1972) and in the referenceStandard Methods (1971, pp. 34.+-356), jointly published by the American Public Health Association, the American Water Works Association and the Water Pollution Control Federation. The prescribedmethods for turbidity determination are intended for the ‘examination of natural and treated waters in the absenceof gross pollution’. We are not aware of any further specification of the environments these arbitrary units were intended to cover or of the scientific basisfor allowable turbidity increments. In an attempt to reconcile our needfor rapid area1coverageof a quasi-synoptic nature with the legal definition of turbidity, we have accomplishedan approximate calibration of our transmissometeragainst Formazin Turbidity Units (FTU). This was conducted in laboratory tanks using Formazin standardsprepared according to Standard Methods (1971, pp. 351-352) and checked by a Hach Model 21ooA turbidimeter. A plot of yO transmissivity (%T) against FTU [Figure I(a)] shows that the useful responseof our transmissometer with a IO-cm path is from approximately IO to 80% T/IO cm or about 50 to 5 FTU.
80
0.1
I .o
FTU Figure I. Laboratory calibrations of transmissometer against (a) Formazin Turbidity Units (FTU) determined meter; (b) concentrations of standard clay minerals Ca-montmorillonite).
IO
100
1000
C (mg/l) (with ro-cm path length) by Hach zrooA turbidi( . . . ., kaolinite; - -,
Suspended minerals Suspensionsof standard clay minerals (Table I) in tap water were prepared for comparison with the Formazin calibration. Figure 1(b) relates %T/Io cm to the massconcentration (C in mg/l) of two common clay species.The dotted curve representsthe function for kaolinite (kandite) and the dashed curve for Ca-montmorillonite (smectite). Note that for a given concentration, Ca-montmorillonite allowsgreater light transmissionthan doeskaolinite and that the curve for kaolinite approximates the %T versusFormazin function in Figure r(a). Comparison of Figures r(a) and (b) suggeststhat a mono-minerallic suspensionof kaolinite would be in a I : I ratio of massconcentration (mg/l) to FTU while Ca-montmorillonite approximatesa 2 : I ratio. Similar compositionalor grain size variations, but not identical relationships, are shown by Postma (1961), Duchrow & Everhart (1971) and are probably present in the data of Jones& Wills (1956). The magnitude of variability of optical
Light transmissivity,
suspended sediments and turbidity
295
attenuation due to differences in size and refractive index of suspendedmaterials becomes apparent from these independent studies. Beam attenuation
coeficient
(a)
Alpha asa function of massconcentration for a specificsuspendedmaterial under controlled conditions is observed to be essentially linear. This relationship is shown in Figure z for the sametwo clays and Formazin usedto generatethe curves in Figures r(a) and (b). Firstorder regressionanalysisof thesedata suggeststhat FTU and kaolinite (mg/l) approximate z a and montmorillonite 4 a (correlation coefficientsr>o.99 for all three curves).
TABLE
I. Data
for clay mineral
standards
Ca-montmorillonite I.
Origin
2.
Chemical SiOz Al@, FedA Fe0 FeS MgO CaO Na,O K,O so,
3.
White Springs, ‘Panther Creek Bentonite’ analyses :
(smectite) Miss. Sold as Southern
56.77 20.33 8.42 0.37 0.45 3’10 “35 0.34 0.73 0.16 n.r.
2iFF2
0.85
co,
0.45
Ignition loss H,O Minor Total Particle size characteristics A. Manufacturers’ data >SW 0.5-s Pm cc.5 w B. Pipette analysis >62 31-62 16-31 8-16 4- 8 2- 4 I-
Kaolinite
(kandite)
Putnam Co., Fla Sold as ‘Edgar Plastic Kaolin’ 46.0 37.8 0.60 n.r. n.r. 0’15
0’10 0’20 0’25
0.035 <0.05 n.r. n.r. 14.0
1:;;; 0’03 100’00
(wt
2
“;,) 30% 35% 35%
37% 45 %
0’0 7’1 7” 3.6 7’1 7’1 42’9
0’0
25’0
diameter
Total suspended load in natural
n.r. n.r. 99.18
9.6~
18%
7’0 4.6 7’0 7’0 11.6 9’3 53’4 rc’3cP
waters
We have measuredtotal suspendedload (C in mg/l) by membranefiltration (0.45 p) of water samplestaken simultaneouslywith transmissometersurveys at the Anclote site. Scatter plots of C versr~~attenuation coefficient (from o/oT) have generally fallen betweenthe kaolinite and
296
J. C. McCarthy,
T. E. Pyle & G. M. Grjjin
montmorillonite regressionlines in Figure 2. The relationship CE 3.3~ wasfound to hold for a survey in August 1971 (rangeof C, 22.1-47.2 mg/l). A comparisonof theseresultswith those of previous workers is given in Table 2. %T/lOcm
Figure 2. First-order regression of concentration (mg/l) minerals ( . . . ., kaolinite; - -, Ca-montmorillonite) attenuation coefficient (a) computed from transmissivity
of standard and data.
clay minerals FTU against
Although the transmissometeris useful in making approximations of particle concentration and distribution while providing rapid area1coverageand high density of data, it should be emphasizedthat the concentration of suspendedmaterial inferred by this method is only semi-quantitative. When parameterssuch as water color, particle size, composition and morphology are at minimum variance, the estimates may approach a higher degree of absoluteaccuracy.
Discussion
and recommendations
Turbidity as currently defined by regulatory agenciesin the United States is not a very effective measurementof water quality in many cases.Probably the Formazin or Jackson Turbidity Unit can consistently and reliably be related to water and sediment properties only in the laboratory. To relate field measurementsof water quality to light-dependent phenomenasuch as primary productivity, the establishmentof light attenuation standards for environmental protection would seemmore appropriate. To regulate the addition of suspendedmaterials(asin dredging operations)to the environment, the standard parameter should relate better to actual mass concentration to the exclusion of material types and water color. As described above, the same FTU reading could be obtained from a given concentration of kaolinite and twice that concentration of Ca-montmorillonite. Becausetwice asmuch material is in suspension,the resulting siltation problem would be significantly greater for a Ca-montmorillonite discharge than for a kaolinite discharge. We also believe the current turbidity standards are practically and environmentally unrealistic in many situations. As an example, dredge operators in the U.S. are allowed by
Light transmissivity,
suspended sediments and turbidity
297
law to makeadditions of up to 50 FTU above background turbidity. For a background level of IO to 15 FTU in the Anclote study area,this arbitrary numerical standard would allow a three- to five-fold increaseover natural levels (increasein a from 5 to 30). In an areasuch as Crystal River, Florida, the allowable increasesin alpha could be two orders of magnitude over background. In this latter case,the law seemsto imposeno restrictions at all because such levels might be impossibleto attain by even the most carelessand uncontrolled operations. At the other extreme, it may not be realistic to limit turbidity additions to an arbitrary number such as 50 or IOO FTU in areasof very high natural turbidity. TABLE 2. Relations between suspended sediment attenuation coefficient, a, in marine waters Reference
Area
This paper Gordon & Smith (1972) Drake et al. (1972) Jones & Wills
Anclote estuary, Fla Biscayne Bay, Fla Santa Barbara-Oxnard Shelf, Calif. Thames estuary Plymouth Sound Hamoaze
(1956)
11Not
Time
given
by authors,
calculated
Function
August 1971 May-June 1970 February I 970 July May May
1953 1953 1953
from
plotted
concentration,
Cz3.3a C=2,6a
C, (mg/l) Range
and (mg/l)
22'1-47'2
I -15
CE2.7a"
1’5-
Cz7’7a”
7'0-27'3
C2:2.2a' Cz5.8a"
0.6-
5’7 3.5
2.2-24'9
data or tables.
Several approachesshouldbe followed in the future-some simultaneously.We agreewith Austin (1973a) that ‘To perpetuate the useof the JTU or FTU in measurementproblemsof coastal and offshore monitoring would seem . . . to be a path of dubious wisdom’. His suggestion(Austin, npja, 6) that the attenuation coefficient, a well-defined, widely-accepted physical unit, be consideredas a more logical substitute has great merit. However, we feel that turbidity standardsultimately should be defined in terms of the light requirements or silt tolerancesof the organismswe are attempting to protect (e.g. Davis & Nudi, 1971). Field and laboratory measurementsof light received or silt deposited versusplant and animal condition, rather than measurementof tenuously related parameters,should be conducted to determine the appropriatenessof existing water quality standards.At present, it may be more realistic to redefine allowable turbidity increasesin terms of percentage above background rather than an arbitrary numerical increment. Such a standard should, of course, consider the level and duration of the maximum turbidity to which an environment is naturally subjected aswell asthe background or ‘average’ level. However, as geologistswe are not prepared to suggestwhat that percentage increase should be. Until the studies outlined above are conducted, establishmentof such a percentage,though seemingly more logical, would be as arbitrary as the current absolute numerical standards. Obviously, no simple solutions exist, but it is just as obvious that further refinement and standardization of turbidity measurementtechniques is required for environmental impact studiesin estuariesand coastalregions. In addition to agreeingon a replacementfor units of vague origins and dubious merit (JTU, FTU), we must determine more realistic standards for turbidity increases,either in terms of percentage increase or effect upon the biota. Modifications may have to be made to present standards which enjoy political and legal sanctity to the possibledetriment of the environment. Conclusion We have describedan expedient method for satisfying someof the demandsof estuarineand coastalturbidity studieswhile estimating turbidity in legally acceptableunits. We emphasize
J. C. McCarthy,
T. E. Pyle
@ G. M.
Gr~%n
that the specific correlations given here should not be used directly in other areas or with other instruments, but that the strategy outlined should be valuable if individual calibrations are carried out. Such calibrations also serve to emphasize the number of factors affecting turbidity at any one site and the need for additional study of turbidity and siltation effects upon organisms. Acknowledgements This study was supported by the Florida Power Corporation. Assistance in the field and laboratory was provided by Don Eggimann, Phyllis Jones, Jack Dyer and Richard Clingan. David Wallace provided valuable assistance and discussion in all phases of our study. Helpful criticisms and suggestions were given by Larry Doyle, Frank Manheim and Ronald Baird. We are grateful to R. W. Austin for providing very helpful comments as well as copies of unpublished manuscripts which clarified our understanding of the physical aspects of turbidity. References Austin, R. W. r973a Turbidity as a water quality parameter. Proceedings of the Seminar on Methodology for Monitoring the Marine Environment, Seattle, Washington, Oct. 16-18, Environmental Protection Agency (In press). Austin, R. W. r9736. Visibility Laboratory, Scripps Institution of Oceanography, personal communication. Baird, R. C., Carder, K. L., Hopkins, T. L., Pyle, T. E. & Humm, H. J. 1972 Anclote Environmental Project, Annual Report 1971. University of South Florida, Marine Science Institute, Technical Report 12, 251 pp. Baird, R. C., Carder, K. L., Hopkins, T. L., Pyle, T. E. & Humm, H. J. 1973 Anclote Environmental Project, Annual Report 1972. University of South Florida, Department of Marine Science, Contribution 41, 220 pp. Biggs, R. B. 1968 Environmental aspects of overboard spoil disposal. Proceedings of American Society of Civil Engineers, Journal of Sanitary Engineering Division 94 (SA3), 477-487. Biggs, R. B. 1970 Geology and hydrography. In Gross Physical and Biological Eflects of Overboard Spoil Disposal in Upper Chesapeake Bay University of Maryland, Natural Resources Institute Special Report No. 3, 7-15. Briggs, R. 0. & Morris, G. 1966 Instrumentation for the prediction of underwater visibility range as a function of water condition. Proc. Underwater Photo-Optics Seminar, Society of Photographic Instrument Engineers C-XVII-r to C-XVII-7. Davis, C. R. & Nudi, F. A. Jr 1971 Turbidity bioassay method for the development of prediction techniques to assess the possible environmental effects of marine mining. Preprints Oflshore Technology Conference Houston, Texas I, 881-888. Drake. D. E.. Kolaack. R. L. & Fischer. P. 1. 1q72 Sediment transport on the Santa Barbara-Oxnard shelf, Santa Barbara Channel, California. In Shelf Sediment Transport, Process and Pattern. 656 pp. (Swift, D. J. P., Duane, D. B. & Pilkey, 0. H., eds) Dowden, Hutchinson & Ross, Inc., Stroudsberg, Pa. Duchrow, R. M. & Everhart, W. H. 1971 Turbidity measurement. Transactions of American Fisheries Society IO0 (4), 682-690. Duntley, s. Q. 1963 Light in the sea. ~ournol Optical Society of America 53, 2X4-233. F.W.P.C.A. [Federal Water Pollution Control Administration) 1968 Water Oualitv U. S. - Criteria Government Printing Office, Washington, D.C., 234 pp. ’ . Gordon, H. R. & Smith, J, M. 1972 A time series study of beam transmittance in Biscayne Bay. Abstract, EOS 539 4, 400. Hach, C. C. 1972 Understanding turbidity measurement. Industrial Water Engineering February/ March, 18-22. Holmes, R. W. 1970 The Secchi disc in turbid coastal waters. Limnology and Oceanography 15~688-694. Humm, H. J., Baird, R. C., Carder, K. L., Hopkins, T. L. & Pyle, T. E. 1971 Anclote Environmental Project, Annual Report 1970. University of South Florida, Marine Science Institute, Technical Report I, 171 pp. Ingle, R. M. 1952 Studies on the effect of dredging operations upon fish and shellfish. Florida Board of Conservation, Tallahassee, Technical Series No. 5, 26 pp. ,
~I
.
I
-
,,
Ligkt
transmissivity,
suspended
sediments
and turbidity
299
Jerlov, N. G. 1968 Optical Oceanography. 194 pp. Elsevier, London. Johannes, R. E. 1972 Coral reefs and pollution. In Marine Pollution and Sea Life (Ruivo, M., ed.) Fishing New (Books), Ltd., London. Jones, D. & Wills, M. S. 1956 The attenuation of light in sea and estuarine waters in relation to the concentration of suspended solid matter. Journal of Marine Biological Association of the United Kingdom 35,43 I-+++. Manheim, F. T., Meade, R. H. & Bond, G. C. 1970 Suspended matter in surface waters of the Atlantic Continental margin from Cape Cod to the Florida Keys. Science 167, 371-376. Manheim, F. T., Hathaway, J. C. & Uchupi, E. 1972 Suspended matter in surface waters of the northern Gulf of Mexico. Limnology and Oceanography 17 (I), 17-27. Postma, H. 1961 Suspended matter and Secchi disc visibility in coastal waters. NetherlandsJournal Sea Research I (3), 359-390. Pyle, T. E., Baird, R. C., Carder, Ii. L., Hopkins, T. L. & Wallace, D. W. 1972 The making of a power plant I: environment and design. Abstract, QuarterlyJournal of Florida Academy of Sciences 35 (I), 30. Pyle, T. E. & McCarthy, J. C. 1973 The use of historical bathymetric data in assessing environmental impact. Enoironmental Letters 5 (I), 63-69. Robinson, M. 1957 The effects of suspended materials on the reproductive rate of Dapknia magna. University of Texas, Publications of Institute of Marine Science 4 (z), 265-277. Saila, S. B., Pratt, S. D. & Polgar, T. T. 1972 Dredge spoil disposal in Rhode Island Sound. Uniwersity of Rhode Island Marine Technical Report No. 2, 48 pp. Standard Methods 1971 13th edition, 874 pp. American Public Health Association, New York, New York. Tyler, J. 1968 The Secchi disc. Limnology and Oceanography 13, 1-6. Windom, H. L. 1972 Environmental aspects of dredging in estuaries. Proceedings of American Society of Civil Engineers, rournal of Waterways, Harbors and Coastal Engineering Division 98 (WWq), 475-487.