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Tracer applications of ultra-violet absorption measurements in coastal waters
Water Res. Vol. 19. No. 6. pp. 70t-706. 1985 Printed in Great Britain. All rights reserved
00a3.135a 85 $3.00+0.00 Copyright i-?. 1985 Pergamon Press Ltd
TRACER APPLICATIONS OF ULTRA-VIOLET ABSORPTION MEASUREMENTS IN COASTAL WATERS P. FOSTER Marine Science Laboratories, Menai Bridge, Gwynedd LL59 5EY. Wales (Received October 1984)
Abstract--Data on the ultra-violet adsorption characteristics of various natural fresh and marine waters are presented. Differences in both the magnitude and shape of spectra from fiver waters are considered and from them four absorption indices are developed as being of potential tracer value to mixing studies in coastal waters. In this context the indices are assessed with reference to ultra-violet absorption and salinity measurements collected during two winter cruises off the west coast of Great Britain.
INTRODUCTION
An appreciation of the circulation and mixing patterns in coastal waters is prerequisite to successful management of coastal water quality. Valuable information on the direction and speed of the currents can be obtained from drifters, continuously recording current meters attached to fixed buoys or from radio-tracked parachute drogues. The difficulty with such techniques, on anything other than a local scale, is the sparse spatial distribution of data that can be obtained. An alternative approach is to identify intrinsic and unique characteristics of the source waters issuing into the area and to follow their distribution in the marine regime. In a simple coastal water system, influenced by only one river, this natural tracer approach poses no problems as salinity is the most appropriate and effective tracer. In more complex coastal water systems, influenced by a number of rivers, a distinction between the relative contributions can only be made if additional tracers are used. For studies on a small spatial scale, in estuaries for example, artificial tracers such as fluorescein or rhodamine can be used. For coastal water studies, natural tracers are the only practical choice, unless the area under investigation happens to receive nuclear wastes, when radio-tracer techniques may also be used (McKinley et al., 1981). The major problem of the natural tracer approach, when applied to multicomponent systems, is finding the required number of suitable tracers. From the theoretical point of view the most essential criterion that a tracer index must meet is that its values in the various source waters must differ in such a way that, after mixing, significant differences in relative contributions can be detected with existing analytical techniques. It is also advantageous that the analytica! procedures available for the adopted tracers, in addition to the obvious requirement of accuracy, should be rapid and inexpensive. The ultra-violet (u.v.) absorption is one property of natural waters that can be readily determined. More701
over, measurements can be made immediately upon sample collection and filtration on board ship. This work considers the u.v. absorption characteristics of natural waters, develops from them several indices of potential tracer value to mixing studies in coastal waters and finally assesses their value with reference to u.v. data collected from a coastal area off the west coast of Great Britain. RESULTS River water samples were immediately filtered through pre-washed 0.45 ~m pore size filters into small ~ass vials and returned to the laboratory for analysis. Absorption characteristics were determined using a silica cell of appropriate path length and a reference cell containing distilled water which had been irradiated with high energy u.v. light. Absorptions at discrete wavelengths were determined using a Unicam S.P. 500 spectrophotometer. Measurements of the integrated absorption between particular wavelengths were made using a continuously recording Unicam S.P. 800 spectrophotometer and evaluated by integrating the area (ram-') bounded by the sample spectrum, the base line and the extreme values of the chosen wavelength range. Seawater samples were processed in an analogous manner and the u.v. absorption characteristics immediately recorded on board ship. For comparative purposes all reported absorptions have been standardized to an optical path length of 10 cm. Sea water samples were returned to the laboratory for salinity determination using a Bissett Berman Salinometer, Model 6230.
U.V. SPECTRA OF NATURAL WATERS
A number of authors have contributed to a greater understanding of the characteristics of the u.v. absorption spectra of natural waters (Lenoble, 1956; Armstrong and Boalch, 1961; Ogura and Hanya, 1966, 1967; Foster and Morris, 1971, 1974). In general, the spectra of marine as well as non-marine waters show a strong and consistent increase in absorption towards the short wave end (Fig. 1), although rare exceptions have been noted (Sheppard, 1977).
702
P. FOSTER
composite region of the spectrum: the shape or" the spectrum in the organic region; the shape of the spectrum in the transition zone from the organic to the composite region of the spectrum. In this work the intensity factor in the organic region of the spectrum was numerically evaluated as the integrated absorption (ZA) between 250 and 350 nm, while the magnitude of the absorption at 225 nm (A 225) was adopted as the intensity factor for the composite region. The organic and composite shape factors were evaluated as the absorption ratios, A 250/A 275 and A 225/A 250, respectively.
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Fig. 1. Typical u.v. absorption spectrum of natural waters and the potential tracer indices adopted during this work
Conveniently, the form of the spectrum can be considered in two distinct sections. Within the spectral region 250--350nm modifications of the u.v. absorption are direct manifestations of changes in the concentration and/or nature of the dissolved organic matter. Below 250nm inorgamc species, such as nitrate and bromide, in addition to the dissolved organic matter contribute to the absorpuon. These two sections are subsequently referred to as the orgamc and composite regions of the spectrum. It follows from the above consideration of the nature of the u.v. spectrum of natural waters that (1) differences in organic and/or inorganic composition among river waters should be apparent in the magnitude and/or shape of their respective u.v. absorption spectrum and (2) vestiges of these differences should be apparent in the u.v. absorption characteristics of the receiving coastal waters. Consequently, four indices, two intensity factors and two shape factors can be considered as being of potential tracer value to mixing models of coastal waters: the magnitude of the absorpuon in the organic region of the spectrum; the magnitude of the absorption in the
The magnitudes of the potential tracer indices recorded in various river and stream waters draining the Isle of Anglesey and the Welsh mainland are compared in Table 1. Also shown are similar data from coastal and offshore waters. Reference to Table 1 would suggest that the most obvious potential tracer index is the intensity factor in the organic region of the spectrum, EA. The relatively large amount of dissolved organic matter in river waters compared to seawater is reflected in the magnitude of their respective absorptions in the organic region of the spectrum. Consequently, when river waters discharge into the sea the salinity decrease in the receiving waters is accompanied by an increase in the value of EA. This fact has been variously exploited in organic pollution studies in estuarine and coastal waters (Ogura and Hanya, 1968: Foster and Morris, 1971; Foster, 1973; Chapman, 1982). In the tracer context, what is more significant is the fact that there are not only differences in Y-A between marine and freshwaters but often marked differences in the value of IEA among the various fresh waters. Differences in the organic shape factor between one source water and another can be attributed to
Table 1. A comparison of the winter (March) values of the tracer indices recorded in rivers a n d streams draining Anglcsey and the north Wales coast. Also given are values from marine waters. E D T A was used as a reference organic compound. The !:A of 0.01 M EDTA run under identical conditions to those pertaining to the water samples was 670 units Sample Anglesey rivers Braint Cefni Ffraw Crigyll Alaw Wygyr Goch Lligwy
Cadnant North Wales rivers Conwy Anafon O&,wen S¢.iont
A
A 225
A 250/A 275
A 225,'A 250
1938 4240 4173 3655 3935 3408 17,480 3212 1520
3.60 3.55 4.10 4.43 4.33 3.75 10.40 2.80 3.50
1.32 1.26 1.33 1.33 1.34 1.32 1.14 1.32 1.33
4.97 2.33 2.78 3.40 2.32 3.20 2.16 2A9 4.10
2650 1979 1960 3104
2.61 I. 19 1.60 3.12
1.32 1.31 1.29 1.32
2.57 1.78 2.39 3.02
513 311
0.43 0.40
1.30 1.37
2.79 3.65
Seawater
Coastal Offshore
Tracer applications of u.v. absorption measurements in coastal waters differences in relative proportions of the various etissolved organic materials that absorb in the u.v. The value of such an index has been considered previously in relation to the discharge of organic rich effluents to estuaries (Foster and Foster, 1977) and has also been used to some effect in mixing studies in the low salinity waters ( < 8*/00)of the Baltic (Brown, 1977). It is clear from the data in Table 1 that differences in the value of A 250/A 275 among freshwater sources and marine waters are generally small. The one exception in Table 1 is the A 250/A 275 value found in the Afon Goch. The dissolved solutes in this river originate from an old copper mine (Rowlands, 1966) and the waters are highly acidic and metalliferous (Foster et al., 1978). Unless any freshwater source contributing to a multi-component system has a distinct A 250/A275 fingerprint, which on the evidence presented is only likely to occur in highly polluted waters, then the small differences amongst the A 250/A 275 values of the discharging freshwaters will be rapidly lost in the marine regime. The potential value of A250/A275 as a tracer will rapidly decrease with increasing salinity. That inorganic species contribute to the absorption below 250 nm, while detracting from the value of such measurements to organic pollution studies, is advantageous in the tracer context. First, although it is possible for two freshwater sources to have very similar intensity and shape fingerprints in the organic region of the spectrum it is less probable that their composition with respect to u.v. inorganic species will also be identical. This feature is reflected by the values of the tracer indices found in the Afon Anafon and River Ogwen (Table 1). The intensity and shape factors in the organic region of the spectrum were very similar but A225 and A225/A250 values were quite different. Secondly, it is possible that differences apparent in the organic region of the spectrum could be accentuated in the composite region by inorganic inequalities in composition. This aspect is also apparent in the data presented in Table 1. For example, a comparison of the Crigyll and Lligwy data reveals a difference in the organic region of their respective spectrum (IEA values of 3655 and 3212). A much larger relative difference is evident in the composite region of their spectrum with A 225 values of 4.43 and 2.80 respectively. The above considerations suggest that simple measurements of the u.v. characteristics of coastal waters together with salinity could provide five tracer indices (lEA, A 225, A 250/A 275, A 225/A 250 and S°/**), any combination of which could provide diagnostic fingerprints of the various water types present in multicomponent coastal water systems. COASTAL WATER CHARACTERISTICS
Because of the relatively small discharge volumes and the geographical arrangement of the rivers and
703
streams discussed above the coastal waters around Angiesey and along the North Wales coast were considered not to be the most suitable areas to assess the potential of the proposed tracer indices. Cardigan Bay was considered more appropriate and data on the u.v. characteristics and salinities of the waters in this area were collected during two winter cruises in December 1979 and March 1980. CHARACTERIZATION OF WATER TYPES IN DECEMBER
Consideration of the river water data in Table 1 suggested that the most obvious potential tracer index was the intensity factor in the organic region of the spectrum, IEA. The relationship between S%o and lEA recorded during the December cruise is illustrated in Fig. 2(a). To aid subsequent interpretation and discussion in this and other mixing diagrams particular points are identified and their geographical locations are shown in Fig. 2(d). The salinity to IEA data effectively approximate Cardigan Bay to a two component mixing system between high salinity offshore waters of low IEA and low salinity inshore waters of high ZA. This relationship emphasises the influence of freshwater discharges upon the magnitude of the u.v. absorption of seawater in the region 250-350 nm but at the same time demonstrates that, at the time of observation at least, the differences in ZA among the various freshwater sources were not sufficient to be of particular value in tracing the individual sources in the coastal regime. Nevertheless, there is some scatter in the data, the significance of which is discussed later. If a given pair of water properties are to make useful tracers they should exhibit (1) a low correlation coefficient indicating that water types in the area possess distinct intercharacteristic fingerprints and (2) a spatially systematic distribution of the scatter within the study area indicating that the low correlation results from the mixing of the various distinct water types rather than analytical noise. The intercharacteristic correlation matrix from the December cruise is shown in Table 2. In December the highest correlation coefficient was associated with the relationship between S%o and lEA indicating, with respect to the first criterion above, that this pair of water properties was least likely to be of value in the tracer context. The lowest correlation coefficient was associated with the relationship between the shape and intensity factors A225/A250 and A225. The relationship is illustrated in Fig. 2(b) and conforms to that which could be expected from a three component system. With this assumption tentative boundary mixing lines have been inserted in Fig. 2(b). That the mixing of at least three water types were involved in the generation of these data is confirmed by the geographical distribution of the data points. All data points towards apex A [Fig. 2(b)] were associated with the offshore waters of Cardigan Bay [Fig. 2(d)], those
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Fig. 2. Relationships between (a) salinity and gA and (b) A 225/,4 250 and A 225 observed in Cardigan Bay during December. The A 2 2 5 / A 2 5 0 to A225 characteristics of four major water types are shown in (c). In (a) (b) and (c) data points and water types are identified and their geographical locations are shown in (d). towards apex B with coastal waters to the north and those towards apex C with coastal waters to the south. It is clear from Fig. 2(b) that Cardigan Bay must be regarded as a multicomponent system. However, although a mixing triangle can be constructed this does not necessarily imply that only three water types are involved. To interpret mixing diagrams realistically it is first essential to determine the number of water sources contributing to them. The procedures to affect this are as follows. If in December, only three water types were contributing to the mixing pattern in Cardigan Bay then (1) the geographical location of end members derived from mixing diagrams of any pair of the proposed
tracer characteristics would be comparable with those depicted in Fig. 2(b) and (2) changes in water propertles both along the boundary mixing lines and within the mixing frame of reference would be geographically systematic. In the context of these assessments it should be noted that ten different mixing diagrams can be generated from the proposed tracers (any two characteristics from five). The December data conformed to neither criterion. Four major water types could be distinguished. These are shown in Fig. 2(c) and to aid subscqttent discussion are d e s i g n a t e d (A} offshore waters, (B) Tremadoc Bay waters, (C) Southern Coastal waters and (D) Central Coastal waters. Their ~ographica| distribution is illustrated in Fig. 2(d). It could be argued,
Table 2, The intercl~racteristic corrdation matrix for the data r~corded in Cardigan Bay during DecFmber. Number of data points per variable = 41
S%o S%* EA A 225 A250'A275 A 225 'A 250
1.000 -0.890 - 0.532 0.518 0.854
ZA
A225
A250]A275
A225/A250
1.000 0.434 -0.543 - 0.871
1.000 -0.484 - 0.179
1.000 0.469
1.000
Tracer applications of u.v. absorption measurements in coastal waters
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during March. The salinity to A 225 characteristics of four major water types are shown in (c). In (a), (b) and (c) data points and water types are identified and their geographical locations are shown in (d). solely on the evidence of Fig. 2(c), that the properties contributed to by another boundary source relatively of Central Coastal waters arose as a result of mixing enriched in u.v. absorbing organic matter (high ZA). between Tremadoc Bay waters and Southern Coastal The geographical distribution map of the major waters. Pursuing the above criteria showed clearly water types [Fig. 2(d)] shows a small patch of waters that this was not the case. First, it is difficult to within Trcmadoc Bay with tracer characteristics simpropose a realistic mixing pattern of Tremadoc Bay ilar to those in Central Coastal waters. The tracer and Southern Coastal waters which would generate characteristics in the waters contiguous to this patch the geographical distribution of water characteristics and its spatial displacement from the Central Coastal displayed in the Central Coastal waters. Definitive waters would suggest that the patch could correctly evidence for a fourth water type was inherent in other be regarded as a fifth water type, with a different mixing diagrams where the Central Coastal waters origin, but similar characteristics, to water type D. emerged as an end member. Reference to Fig. 2(a) for example, shows a number of data points with low C H A R A C T E R I Z A T I O N OF WATER salinities and T:A values in the range 360--400. Such TYPES IN M A R C H properties characterized the waters in Tremadoc Bay In March salinity was again highly correlated with [Fig. 2(d)]. Other data points, of intermediate salinity and 5"A values >400 were located in the Central EA [r = -0.896, n = 55, Fig. 3(a)]. Offshore waters Coastal waters. It is clear from Fig. 2(a) that no • were characterized by high salinities and low values combination of the salinity to EA characteristics of E;A. Coastal waters of low salinity and high EA associated with Tremadoc Bay waters, Southern formed the other end member and in contrast to Coastal waters and offshore waters could generate the analogous mixing diagram for December [Fig. the salinity to ZA characteristics found in the Central 2(a)] there was no clear distinction between TremCoastal waters. Rather, the waters in this area were adoc Bay and Central Coastal waters with respect to
706
P FOSTER
their Z.4 loading. The relationship ~hich best met the tracer criterion of a low correlation coefficient was that between salinity and the intensity factor A 225. This relationship, illustrated in Fig. 3(b), provided a very effective separation of three water types. Retaining the same symbolism and nomenclature as before then high salinities and low values of A225 [apex A , Fig. 3(b)] characterized the offshore waters [Fig. 3(d)], data points towards apex B, of low salinity and intermediate values of A225, were located in Tremadoc Bay and those towards apex D, of low salinity and high A225, came from the Central Coastal waters. It should be noted that in contrast to December the Central Coastal waters, rather than Southern Coastal waters, emerged as a prominent end member. Reference to Fig. 3(b) shows that the salinity to A 225 characteristics in the coastal waters to the south were intermediate of those of Central Coastal waters and offshore waters and lay close to the mixing line A - D . If a three component mixing system is assumed then it could be argued that the characteristics displayed by the coastal waters to the south simply arose as a result of the gradual dilution of Central Coastal waters with offshore waters. This would imply a southerly advective transport of Central Coastal waters and require a systematic increase in salinity and concomitant decrease in A 225 in a southerly direction along the coast. That this interpretation is unrealistic is clear from the spatial distribution of the characteristics displayed by the coastal waters to the south. Salinities increased and A 225 values decreased in a direction towards Central Coastal waters. If a three component mixing system was a realistic interpretation of the March distributions in Cardigan Bay then the data would conform to the criteria established previously. As in December this was not the case and a consideration of other mixing diagrams and the spatial distributions of intercharacteristic tracer properties rationalised the data to a four component mixing system with Southern Coastal waters as the fouth water type. The salinity to A225 characteristics of the four water types are shown in Fig. 3(c) and their geographical distributions in Fig. 3(d). DISCUSSION The data presented show that a consideration of the u.v. absorption characteristics of natural waters provides a diagnostic tool for identifying and demarcating the spatial distributions of discrete water types in the coastal regime. It is important to note however, that in the two surveys considered here a marked temporal change in the absorption characteristics was associated with the waters in particular areas. In December in the offshore waters, for example, the values of the tracer indices ZA, A225,
A 250 .4 275 and ,4 225 A 250 associated with water~ of salinity 34.09'?~,, ~ere 240, 0.290. 1.410 and 2.340 respectively. In March. in waters of equitable salinit~ (34.077~) the nature of the u.v. absorption spectrum was quite different with tracer index values of 311. 0.405, 1.370 and 3.649. As a consequence, the most effective combination of tracer indices at one time of year may not necessarily be the most effective combination at another. To be able to predict the most effective combination of tracers for a specific coastal area at any particular time would necessitate an appreciation of the temporal changes in the u.v. absorption characteristics of the various source waters. If such information is not available it is recommended that in multicomponent coastal water systems, simple, rapid and inexpensive determinations of the shape and intensity factors in both the organic and composite region of the spectrum should be regarded as potentially valuable tracer indices. REFERENCES
Armstrong F. A. J. and Boalch G. T. (1961) The ultra-violet absorption of sea water. J. mar. biol. Ass. U.K. 41, 591-597. Brown M. (1977) Transmission spectroscopy examination of natural waters, Part C. Estuar. coast. Mar. Sci. 5, 309-317. Chapman P. ([982) Investigations into methods of determining organic pollution in seawater. 3. The use of ultra-violet spectrophotometry. Fish. Bull. S. Afr, 16, 11-19. Foster P. (1973) Ultra-violet absorptio n ,/sa 'hmty " correlation as an index of pollution in inshore sea waters. N. Z. Jl Mar. Freshwat. Res. 7, 369-379. Foster P. and Foster G. M. (1977) Ultra-violet absorption characteristics of waters in an industrialized estuary. Water Res. 11, 351-354. Foster P. and Morris A. W. (1971) The use of ultra-violet absorption measurements for the estimation of organic pollution in inshore sea waters. Water Res. 5, 19-27. Foster P. and Morris A. W. (1974) Ultra-violet absorption characteristics of natural waters. Water Res. 8, 137-142. Foster P., Hunt D. T. E. and Morris A. W. (1978) Metals in an acid mine stream and estuary. Sci. Total Envir. 9, 75-86. Lenoble J. (1956) Sur le role des principaux sels dans l'absorption ultra-violette de 1' eau de mer. C.r. hebd. Seanc. Acad. Sci., Paris 242, 806--808. McKinley I. G., Baxter M. S., Ellett D. J. and Jack W. (1981) Tracer applications of radiocaesium in the Sea of the Hebrides. Estuar. Coast. Shelf Sci. 69-82. Ogura N. and Hanya T. (1966) Nature of ultra-violet absorption of sea water. Nature. Lond. 212, 758. Ogura N. and Hanya T. (1967) Ultra-violet absorption of sea water in relation to organic and inorganic matters. Int. J. Oceanol. Limnol. 1, 91-102. Ogura N. and Hanya T. (1968) Ultra-violet absorbance as an index of the pollution of seawater. J. Wat. Pollut. Control Fed. 40, 464-.-467. Rowtands J. (1966) The Copper Mountain, Vol. 1. Anglesey Antiquarian Society, Studies in Anglesey History. Shepoard C. R. C. (1977) Problems with the use of ultraviolet absorption for measuring carbon compounds in a river system. Water Res. 11, 979-982.
00a3.135a 85 $3.00+0.00 Copyright i-?. 1985 Pergamon Press Ltd
TRACER APPLICATIONS OF ULTRA-VIOLET ABSORPTION MEASUREMENTS IN COASTAL WATERS P. FOSTER Marine Science Laboratories, Menai Bridge, Gwynedd LL59 5EY. Wales (Received October 1984)
Abstract--Data on the ultra-violet adsorption characteristics of various natural fresh and marine waters are presented. Differences in both the magnitude and shape of spectra from fiver waters are considered and from them four absorption indices are developed as being of potential tracer value to mixing studies in coastal waters. In this context the indices are assessed with reference to ultra-violet absorption and salinity measurements collected during two winter cruises off the west coast of Great Britain.
INTRODUCTION
An appreciation of the circulation and mixing patterns in coastal waters is prerequisite to successful management of coastal water quality. Valuable information on the direction and speed of the currents can be obtained from drifters, continuously recording current meters attached to fixed buoys or from radio-tracked parachute drogues. The difficulty with such techniques, on anything other than a local scale, is the sparse spatial distribution of data that can be obtained. An alternative approach is to identify intrinsic and unique characteristics of the source waters issuing into the area and to follow their distribution in the marine regime. In a simple coastal water system, influenced by only one river, this natural tracer approach poses no problems as salinity is the most appropriate and effective tracer. In more complex coastal water systems, influenced by a number of rivers, a distinction between the relative contributions can only be made if additional tracers are used. For studies on a small spatial scale, in estuaries for example, artificial tracers such as fluorescein or rhodamine can be used. For coastal water studies, natural tracers are the only practical choice, unless the area under investigation happens to receive nuclear wastes, when radio-tracer techniques may also be used (McKinley et al., 1981). The major problem of the natural tracer approach, when applied to multicomponent systems, is finding the required number of suitable tracers. From the theoretical point of view the most essential criterion that a tracer index must meet is that its values in the various source waters must differ in such a way that, after mixing, significant differences in relative contributions can be detected with existing analytical techniques. It is also advantageous that the analytica! procedures available for the adopted tracers, in addition to the obvious requirement of accuracy, should be rapid and inexpensive. The ultra-violet (u.v.) absorption is one property of natural waters that can be readily determined. More701
over, measurements can be made immediately upon sample collection and filtration on board ship. This work considers the u.v. absorption characteristics of natural waters, develops from them several indices of potential tracer value to mixing studies in coastal waters and finally assesses their value with reference to u.v. data collected from a coastal area off the west coast of Great Britain. RESULTS River water samples were immediately filtered through pre-washed 0.45 ~m pore size filters into small ~ass vials and returned to the laboratory for analysis. Absorption characteristics were determined using a silica cell of appropriate path length and a reference cell containing distilled water which had been irradiated with high energy u.v. light. Absorptions at discrete wavelengths were determined using a Unicam S.P. 500 spectrophotometer. Measurements of the integrated absorption between particular wavelengths were made using a continuously recording Unicam S.P. 800 spectrophotometer and evaluated by integrating the area (ram-') bounded by the sample spectrum, the base line and the extreme values of the chosen wavelength range. Seawater samples were processed in an analogous manner and the u.v. absorption characteristics immediately recorded on board ship. For comparative purposes all reported absorptions have been standardized to an optical path length of 10 cm. Sea water samples were returned to the laboratory for salinity determination using a Bissett Berman Salinometer, Model 6230.
U.V. SPECTRA OF NATURAL WATERS
A number of authors have contributed to a greater understanding of the characteristics of the u.v. absorption spectra of natural waters (Lenoble, 1956; Armstrong and Boalch, 1961; Ogura and Hanya, 1966, 1967; Foster and Morris, 1971, 1974). In general, the spectra of marine as well as non-marine waters show a strong and consistent increase in absorption towards the short wave end (Fig. 1), although rare exceptions have been noted (Sheppard, 1977).
702
P. FOSTER
composite region of the spectrum: the shape or" the spectrum in the organic region; the shape of the spectrum in the transition zone from the organic to the composite region of the spectrum. In this work the intensity factor in the organic region of the spectrum was numerically evaluated as the integrated absorption (ZA) between 250 and 350 nm, while the magnitude of the absorption at 225 nm (A 225) was adopted as the intensity factor for the composite region. The organic and composite shape factors were evaluated as the absorption ratios, A 250/A 275 and A 225/A 250, respectively.
2C~
° -
~-70
,a2 05
,, 200
V/////////////////A350 Wffvelength
nm
RIVER WATER CHARACTERISTICS
Fig. 1. Typical u.v. absorption spectrum of natural waters and the potential tracer indices adopted during this work
Conveniently, the form of the spectrum can be considered in two distinct sections. Within the spectral region 250--350nm modifications of the u.v. absorption are direct manifestations of changes in the concentration and/or nature of the dissolved organic matter. Below 250nm inorgamc species, such as nitrate and bromide, in addition to the dissolved organic matter contribute to the absorpuon. These two sections are subsequently referred to as the orgamc and composite regions of the spectrum. It follows from the above consideration of the nature of the u.v. spectrum of natural waters that (1) differences in organic and/or inorganic composition among river waters should be apparent in the magnitude and/or shape of their respective u.v. absorption spectrum and (2) vestiges of these differences should be apparent in the u.v. absorption characteristics of the receiving coastal waters. Consequently, four indices, two intensity factors and two shape factors can be considered as being of potential tracer value to mixing models of coastal waters: the magnitude of the absorpuon in the organic region of the spectrum; the magnitude of the absorption in the
The magnitudes of the potential tracer indices recorded in various river and stream waters draining the Isle of Anglesey and the Welsh mainland are compared in Table 1. Also shown are similar data from coastal and offshore waters. Reference to Table 1 would suggest that the most obvious potential tracer index is the intensity factor in the organic region of the spectrum, EA. The relatively large amount of dissolved organic matter in river waters compared to seawater is reflected in the magnitude of their respective absorptions in the organic region of the spectrum. Consequently, when river waters discharge into the sea the salinity decrease in the receiving waters is accompanied by an increase in the value of EA. This fact has been variously exploited in organic pollution studies in estuarine and coastal waters (Ogura and Hanya, 1968: Foster and Morris, 1971; Foster, 1973; Chapman, 1982). In the tracer context, what is more significant is the fact that there are not only differences in Y-A between marine and freshwaters but often marked differences in the value of IEA among the various fresh waters. Differences in the organic shape factor between one source water and another can be attributed to
Table 1. A comparison of the winter (March) values of the tracer indices recorded in rivers a n d streams draining Anglcsey and the north Wales coast. Also given are values from marine waters. E D T A was used as a reference organic compound. The !:A of 0.01 M EDTA run under identical conditions to those pertaining to the water samples was 670 units Sample Anglesey rivers Braint Cefni Ffraw Crigyll Alaw Wygyr Goch Lligwy
Cadnant North Wales rivers Conwy Anafon O&,wen S¢.iont
A
A 225
A 250/A 275
A 225,'A 250
1938 4240 4173 3655 3935 3408 17,480 3212 1520
3.60 3.55 4.10 4.43 4.33 3.75 10.40 2.80 3.50
1.32 1.26 1.33 1.33 1.34 1.32 1.14 1.32 1.33
4.97 2.33 2.78 3.40 2.32 3.20 2.16 2A9 4.10
2650 1979 1960 3104
2.61 I. 19 1.60 3.12
1.32 1.31 1.29 1.32
2.57 1.78 2.39 3.02
513 311
0.43 0.40
1.30 1.37
2.79 3.65
Seawater
Coastal Offshore
Tracer applications of u.v. absorption measurements in coastal waters differences in relative proportions of the various etissolved organic materials that absorb in the u.v. The value of such an index has been considered previously in relation to the discharge of organic rich effluents to estuaries (Foster and Foster, 1977) and has also been used to some effect in mixing studies in the low salinity waters ( < 8*/00)of the Baltic (Brown, 1977). It is clear from the data in Table 1 that differences in the value of A 250/A 275 among freshwater sources and marine waters are generally small. The one exception in Table 1 is the A 250/A 275 value found in the Afon Goch. The dissolved solutes in this river originate from an old copper mine (Rowlands, 1966) and the waters are highly acidic and metalliferous (Foster et al., 1978). Unless any freshwater source contributing to a multi-component system has a distinct A 250/A275 fingerprint, which on the evidence presented is only likely to occur in highly polluted waters, then the small differences amongst the A 250/A 275 values of the discharging freshwaters will be rapidly lost in the marine regime. The potential value of A250/A275 as a tracer will rapidly decrease with increasing salinity. That inorganic species contribute to the absorption below 250 nm, while detracting from the value of such measurements to organic pollution studies, is advantageous in the tracer context. First, although it is possible for two freshwater sources to have very similar intensity and shape fingerprints in the organic region of the spectrum it is less probable that their composition with respect to u.v. inorganic species will also be identical. This feature is reflected by the values of the tracer indices found in the Afon Anafon and River Ogwen (Table 1). The intensity and shape factors in the organic region of the spectrum were very similar but A225 and A225/A250 values were quite different. Secondly, it is possible that differences apparent in the organic region of the spectrum could be accentuated in the composite region by inorganic inequalities in composition. This aspect is also apparent in the data presented in Table 1. For example, a comparison of the Crigyll and Lligwy data reveals a difference in the organic region of their respective spectrum (IEA values of 3655 and 3212). A much larger relative difference is evident in the composite region of their spectrum with A 225 values of 4.43 and 2.80 respectively. The above considerations suggest that simple measurements of the u.v. characteristics of coastal waters together with salinity could provide five tracer indices (lEA, A 225, A 250/A 275, A 225/A 250 and S°/**), any combination of which could provide diagnostic fingerprints of the various water types present in multicomponent coastal water systems. COASTAL WATER CHARACTERISTICS
Because of the relatively small discharge volumes and the geographical arrangement of the rivers and
703
streams discussed above the coastal waters around Angiesey and along the North Wales coast were considered not to be the most suitable areas to assess the potential of the proposed tracer indices. Cardigan Bay was considered more appropriate and data on the u.v. characteristics and salinities of the waters in this area were collected during two winter cruises in December 1979 and March 1980. CHARACTERIZATION OF WATER TYPES IN DECEMBER
Consideration of the river water data in Table 1 suggested that the most obvious potential tracer index was the intensity factor in the organic region of the spectrum, IEA. The relationship between S%o and lEA recorded during the December cruise is illustrated in Fig. 2(a). To aid subsequent interpretation and discussion in this and other mixing diagrams particular points are identified and their geographical locations are shown in Fig. 2(d). The salinity to IEA data effectively approximate Cardigan Bay to a two component mixing system between high salinity offshore waters of low IEA and low salinity inshore waters of high ZA. This relationship emphasises the influence of freshwater discharges upon the magnitude of the u.v. absorption of seawater in the region 250-350 nm but at the same time demonstrates that, at the time of observation at least, the differences in ZA among the various freshwater sources were not sufficient to be of particular value in tracing the individual sources in the coastal regime. Nevertheless, there is some scatter in the data, the significance of which is discussed later. If a given pair of water properties are to make useful tracers they should exhibit (1) a low correlation coefficient indicating that water types in the area possess distinct intercharacteristic fingerprints and (2) a spatially systematic distribution of the scatter within the study area indicating that the low correlation results from the mixing of the various distinct water types rather than analytical noise. The intercharacteristic correlation matrix from the December cruise is shown in Table 2. In December the highest correlation coefficient was associated with the relationship between S%o and lEA indicating, with respect to the first criterion above, that this pair of water properties was least likely to be of value in the tracer context. The lowest correlation coefficient was associated with the relationship between the shape and intensity factors A225/A250 and A225. The relationship is illustrated in Fig. 2(b) and conforms to that which could be expected from a three component system. With this assumption tentative boundary mixing lines have been inserted in Fig. 2(b). That the mixing of at least three water types were involved in the generation of these data is confirmed by the geographical distribution of the data points. All data points towards apex A [Fig. 2(b)] were associated with the offshore waters of Cardigan Bay [Fig. 2(d)], those
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Fig. 2. Relationships between (a) salinity and gA and (b) A 225/,4 250 and A 225 observed in Cardigan Bay during December. The A 2 2 5 / A 2 5 0 to A225 characteristics of four major water types are shown in (c). In (a) (b) and (c) data points and water types are identified and their geographical locations are shown in (d). towards apex B with coastal waters to the north and those towards apex C with coastal waters to the south. It is clear from Fig. 2(b) that Cardigan Bay must be regarded as a multicomponent system. However, although a mixing triangle can be constructed this does not necessarily imply that only three water types are involved. To interpret mixing diagrams realistically it is first essential to determine the number of water sources contributing to them. The procedures to affect this are as follows. If in December, only three water types were contributing to the mixing pattern in Cardigan Bay then (1) the geographical location of end members derived from mixing diagrams of any pair of the proposed
tracer characteristics would be comparable with those depicted in Fig. 2(b) and (2) changes in water propertles both along the boundary mixing lines and within the mixing frame of reference would be geographically systematic. In the context of these assessments it should be noted that ten different mixing diagrams can be generated from the proposed tracers (any two characteristics from five). The December data conformed to neither criterion. Four major water types could be distinguished. These are shown in Fig. 2(c) and to aid subscqttent discussion are d e s i g n a t e d (A} offshore waters, (B) Tremadoc Bay waters, (C) Southern Coastal waters and (D) Central Coastal waters. Their ~ographica| distribution is illustrated in Fig. 2(d). It could be argued,
Table 2, The intercl~racteristic corrdation matrix for the data r~corded in Cardigan Bay during DecFmber. Number of data points per variable = 41
S%o S%* EA A 225 A250'A275 A 225 'A 250
1.000 -0.890 - 0.532 0.518 0.854
ZA
A225
A250]A275
A225/A250
1.000 0.434 -0.543 - 0.871
1.000 -0.484 - 0.179
1.000 0.469
1.000
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during March. The salinity to A 225 characteristics of four major water types are shown in (c). In (a), (b) and (c) data points and water types are identified and their geographical locations are shown in (d). solely on the evidence of Fig. 2(c), that the properties contributed to by another boundary source relatively of Central Coastal waters arose as a result of mixing enriched in u.v. absorbing organic matter (high ZA). between Tremadoc Bay waters and Southern Coastal The geographical distribution map of the major waters. Pursuing the above criteria showed clearly water types [Fig. 2(d)] shows a small patch of waters that this was not the case. First, it is difficult to within Trcmadoc Bay with tracer characteristics simpropose a realistic mixing pattern of Tremadoc Bay ilar to those in Central Coastal waters. The tracer and Southern Coastal waters which would generate characteristics in the waters contiguous to this patch the geographical distribution of water characteristics and its spatial displacement from the Central Coastal displayed in the Central Coastal waters. Definitive waters would suggest that the patch could correctly evidence for a fourth water type was inherent in other be regarded as a fifth water type, with a different mixing diagrams where the Central Coastal waters origin, but similar characteristics, to water type D. emerged as an end member. Reference to Fig. 2(a) for example, shows a number of data points with low C H A R A C T E R I Z A T I O N OF WATER salinities and T:A values in the range 360--400. Such TYPES IN M A R C H properties characterized the waters in Tremadoc Bay In March salinity was again highly correlated with [Fig. 2(d)]. Other data points, of intermediate salinity and 5"A values >400 were located in the Central EA [r = -0.896, n = 55, Fig. 3(a)]. Offshore waters Coastal waters. It is clear from Fig. 2(a) that no • were characterized by high salinities and low values combination of the salinity to EA characteristics of E;A. Coastal waters of low salinity and high EA associated with Tremadoc Bay waters, Southern formed the other end member and in contrast to Coastal waters and offshore waters could generate the analogous mixing diagram for December [Fig. the salinity to ZA characteristics found in the Central 2(a)] there was no clear distinction between TremCoastal waters. Rather, the waters in this area were adoc Bay and Central Coastal waters with respect to
706
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their Z.4 loading. The relationship ~hich best met the tracer criterion of a low correlation coefficient was that between salinity and the intensity factor A 225. This relationship, illustrated in Fig. 3(b), provided a very effective separation of three water types. Retaining the same symbolism and nomenclature as before then high salinities and low values of A225 [apex A , Fig. 3(b)] characterized the offshore waters [Fig. 3(d)], data points towards apex B, of low salinity and intermediate values of A225, were located in Tremadoc Bay and those towards apex D, of low salinity and high A225, came from the Central Coastal waters. It should be noted that in contrast to December the Central Coastal waters, rather than Southern Coastal waters, emerged as a prominent end member. Reference to Fig. 3(b) shows that the salinity to A 225 characteristics in the coastal waters to the south were intermediate of those of Central Coastal waters and offshore waters and lay close to the mixing line A - D . If a three component mixing system is assumed then it could be argued that the characteristics displayed by the coastal waters to the south simply arose as a result of the gradual dilution of Central Coastal waters with offshore waters. This would imply a southerly advective transport of Central Coastal waters and require a systematic increase in salinity and concomitant decrease in A 225 in a southerly direction along the coast. That this interpretation is unrealistic is clear from the spatial distribution of the characteristics displayed by the coastal waters to the south. Salinities increased and A 225 values decreased in a direction towards Central Coastal waters. If a three component mixing system was a realistic interpretation of the March distributions in Cardigan Bay then the data would conform to the criteria established previously. As in December this was not the case and a consideration of other mixing diagrams and the spatial distributions of intercharacteristic tracer properties rationalised the data to a four component mixing system with Southern Coastal waters as the fouth water type. The salinity to A225 characteristics of the four water types are shown in Fig. 3(c) and their geographical distributions in Fig. 3(d). DISCUSSION The data presented show that a consideration of the u.v. absorption characteristics of natural waters provides a diagnostic tool for identifying and demarcating the spatial distributions of discrete water types in the coastal regime. It is important to note however, that in the two surveys considered here a marked temporal change in the absorption characteristics was associated with the waters in particular areas. In December in the offshore waters, for example, the values of the tracer indices ZA, A225,
A 250 .4 275 and ,4 225 A 250 associated with water~ of salinity 34.09'?~,, ~ere 240, 0.290. 1.410 and 2.340 respectively. In March. in waters of equitable salinit~ (34.077~) the nature of the u.v. absorption spectrum was quite different with tracer index values of 311. 0.405, 1.370 and 3.649. As a consequence, the most effective combination of tracer indices at one time of year may not necessarily be the most effective combination at another. To be able to predict the most effective combination of tracers for a specific coastal area at any particular time would necessitate an appreciation of the temporal changes in the u.v. absorption characteristics of the various source waters. If such information is not available it is recommended that in multicomponent coastal water systems, simple, rapid and inexpensive determinations of the shape and intensity factors in both the organic and composite region of the spectrum should be regarded as potentially valuable tracer indices. REFERENCES
Armstrong F. A. J. and Boalch G. T. (1961) The ultra-violet absorption of sea water. J. mar. biol. Ass. U.K. 41, 591-597. Brown M. (1977) Transmission spectroscopy examination of natural waters, Part C. Estuar. coast. Mar. Sci. 5, 309-317. Chapman P. ([982) Investigations into methods of determining organic pollution in seawater. 3. The use of ultra-violet spectrophotometry. Fish. Bull. S. Afr, 16, 11-19. Foster P. (1973) Ultra-violet absorptio n ,/sa 'hmty " correlation as an index of pollution in inshore sea waters. N. Z. Jl Mar. Freshwat. Res. 7, 369-379. Foster P. and Foster G. M. (1977) Ultra-violet absorption characteristics of waters in an industrialized estuary. Water Res. 11, 351-354. Foster P. and Morris A. W. (1971) The use of ultra-violet absorption measurements for the estimation of organic pollution in inshore sea waters. Water Res. 5, 19-27. Foster P. and Morris A. W. (1974) Ultra-violet absorption characteristics of natural waters. Water Res. 8, 137-142. Foster P., Hunt D. T. E. and Morris A. W. (1978) Metals in an acid mine stream and estuary. Sci. Total Envir. 9, 75-86. Lenoble J. (1956) Sur le role des principaux sels dans l'absorption ultra-violette de 1' eau de mer. C.r. hebd. Seanc. Acad. Sci., Paris 242, 806--808. McKinley I. G., Baxter M. S., Ellett D. J. and Jack W. (1981) Tracer applications of radiocaesium in the Sea of the Hebrides. Estuar. Coast. Shelf Sci. 69-82. Ogura N. and Hanya T. (1966) Nature of ultra-violet absorption of sea water. Nature. Lond. 212, 758. Ogura N. and Hanya T. (1967) Ultra-violet absorption of sea water in relation to organic and inorganic matters. Int. J. Oceanol. Limnol. 1, 91-102. Ogura N. and Hanya T. (1968) Ultra-violet absorbance as an index of the pollution of seawater. J. Wat. Pollut. Control Fed. 40, 464-.-467. Rowtands J. (1966) The Copper Mountain, Vol. 1. Anglesey Antiquarian Society, Studies in Anglesey History. Shepoard C. R. C. (1977) Problems with the use of ultraviolet absorption for measuring carbon compounds in a river system. Water Res. 11, 979-982.