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Rain water leachates of heavy metals in road surface sediments
Water Research Vol. 14, pp. 1403 to 1407 Pergamon Press Ltd 1980. Printed in Great Britain
RAIN WATER LEACHATES OF HEAVY METALS IN ROAD SURFACE SEDIMENTS D. M. REvlyr and J. B. ELI,IS Middlesex Polytechnic, Urban Stormwater Pollution Research Group, The Burroughs, Hendon, London NW4 4BT, England
(Received August 1979) Abstract--Samples of street surface and roadside gutter sediments within a separately sewered catchment in N.W. London have been analysed for heavy metal contamination. Particle size distributions are presented and the removal efficiency of metal species demonstrated for various street cleaning procedures. Laboratory sorption and desorption studies are described and solution concentrations are obtained for Pb, Cd and Mn. Solubility curves are typically variable with time with Mn showing the strongest tendency to attain equilibrium. No obvious dependence of metal concentrations on grain size is readily apparent although Cd tends to desorb more rapidly from size fractions greater than 250/an.
INTRODUCTION
A considerable literature now exists confirming the importance of street surface sediments in contributing to heavy metal pollution of stormwater drainage systems. (Lagerwerff & Speeht, 1970; Sartor & Boyd, 1972; Pitt & Amy, 1973; Nightingale, 1975; Ellis, 1976; Wilber & Hunter, 1977; Christensen et al., 1978). However, much of the published work relates specifically to Ph. (Day et al., 1975; Duggan & Williams, 1977; Laxen & Harrison, 1977; Little & Wiffen, 1978). Heavy metal concentrations at the # g g - 1 (ppm) level are typical for dry street surface sediments whilst most stormwater concentrations are at the gg 1-1 (ppb) level. As rainfall leaches and flushes the surface sediment, which includes substantial humic components, some fraction of the metals dissolve in the r u n o f as free or complexed species and additional heavy metal is added on or in the particulates (Hem & Durum, 1973). Urban stormwater sediments are characterized by heterogeneous particulates and always contain a substantial proportion of fine grained sizes transported as suspended solids. Much of the literature refers to the affinity of such fine suspended sediment for metallic pollutants but few investigate the relationships between sediment size and metal leachate concentrations. It is known that conventional street cleaning procedures have low removal efficiencies for small particle sizes and previous work has shown that sizes below 250/an account for some 50% of heavy metals by weight (Sartor & Boyd, 1972; Ellis, 1979). Therefore, this study was carried out to compare the extraction of heavy metals from these finer particles with that from particles larger than 250gin under simulated stormwater flow conditions. Determination of the metal levels over an extended time interval provides information relating to the quality and toxicity
of storm drainage and its dependence on the contact time of sediment with street surface runoff. EXPERIMENTAL
Sample collection was undertaken following a period of several days without rainfall at six roadside sites including motorway, trunk road, estate road and residential side street. The characteristics of these sampling sites are described in Table 1. Samples were collected in plastic con. tainers using nylon brushes and plastic scoops from three specific locations on these road surface__ These were adjacent to gully pots (sample 01~ midway between gnlly pots (samples 03, 04, 05 and 06) and from the road surface (sample 02). Except for sample 01, sediments were collected over a 10 m road distance and the combined samples were consecutively riffled to obtain a representative sub-sample of 100g. For samples 01-05, this 100g sample was sieved using stainless steel mesh into two fractions, greater and less than 250/am and 5 g quantities of each size category were digested with nitric acid for subsequent analysis. Grading curves for sample 06 were determined by dry/ wet sieving techniques using stainless steel mesh and by standard pipette procedures according to BS. 1377 (British Standards Institution, 1967). Leaching experiments on samples 01-05 were carried out using a 1: 10 weight :volume ratio of sediment in previously collected rain water at a pH of 6.5. These conditions together with constant agitation of the suspensions using teflon coated magnetic stirrers were designed to simulate turbulent runoff conditions during storm drainage from the roadside surface. 10 ml aliquots of supernatant liquid were removed at 1 day, 5 day, 11 day, 15 day and 28 day intervals and filtered prior to analysis. Pb, Cd and Mn were determined by atomic absorption spectrophotometry using acetylene-air flame atomisation for the analysis of acid digests and a flameless atomiser for the leachates. STREET CLEANING
AND HEAVY METALS
The sediment removal efficiency of conventional road cleaning procedures on an arterial perimeter road (Table 1) around a recently established housing estate in close proximity to the sampling sites has
1403
1404
l , M. REVrTTand J. B. ELLIS Table I~ Characteristics of sampling sites
Number 01
Site
Traffic details
Sample details
02
MI Motorway, Edgware. N.W. London As 01
4000h -~, HGV, cars. vans, etc.. 55-60 mph. ~s OI
03
As 01
As 0t
04
A4I, Watford Way, Dual Carriageway, Hendon, N.W. London Residential side street, Sunnyside Gardens, Hendon, N.W. London Perimeter road, Grahame Park Estate, Colindale, N.W. London
1750 h- t. Goods, cars and commercial. 40--50 mph.
05 06
450 h - ~. Light commercial, cars, 40 mph.
10 4
I
.
I
Grain Size (microns)
I
I
\
"a\
~
I
2
%
F-1 I
influenced by the removal efficiences for the range of particle sizes indicated by the cleaning runs. The accumulation of such fine polluted fractions in gutter and gully under quiescent, stagnant conditions between storm events inevitably induces toxic levels which impose considerable stress on the stormwater system during eventual storm flushing. Hence, the solution concentration of heavy metals will be of importance when determining the toxicity of road surface runoff and resultant receiving water quality. In addition, as 50Vo of all metals are associated with sizes less than 250/an, the contact time and rates of metal take up and release will be of importance in determining outfall toxicity. The efficiency of jet flushing however, suggests that storm rainfall intensities would be sufficient to wash off much of the residual particulates and thus enhance the potential runoff toxicity. Field observations indicate that 250/am sizes are entrained by turbulent surface runoff generated from storm intensities exceeding 6 mm h - 1 The highly turbulent nature of street sur-
I
-".'%,.".~, oo
Hard shoulder surface, 125 m from kerb Gutter adjacent to kerb. midway between gully pots From gutter adjacent t~ kerb. between parked cars and midway between gully pots Gutter adjacent to kerb between parked cars and midway between gully pots Gutter adjacent to kerb, midway between gully pots
100 h t . Cars, vans, 25--30 mph.
been previously investigated (Ellis, 1979). Grading curves relating to this investigation are shown in Fig. 1 together with sizing alterations and reductions in pollutant loadings after cleaning runs by a mechanical rotary brush sweeper followed by high pressure jet flushing. The relative efficiency of flushing over sweeping on both size and loadings is clearly shown. The reduction in street surface loadings for SS is 30~o following rotary cleaning and a further 45~o following flushing whilst for lead loadings the equivalent reduction in rates is 27 and 55~. This study, as well as the original classic studies of Sartor & Boyd (i 972), demonstrates the importance of small size fractions in contributions to road surface runoff quality. Clearly, considerable amounts of fine sediments, together with their associated metal pollutants remain on the street surface and in roadside gutters after rotary sweeping, which is the normal cleaning practise within this catchment. The choice of the 250/am cut-off for use in the experimental procedure described above was therefore
10 3.
Hard shoulder, adjacent to gully pots
-
~',~
o-II
I.o
I
_1 _. LAF
,c 8.,.. c,-i°, Aec Aft,r eotary
'k
o,..°,°,
- -."':. ARc AF
Io~
l
I
1
5
l
I
10
50
I
I
I
80 gO
99
99-9
"I. Greater Than
Fig. 1. Street cleaning efficiency for particle size and pollutant loadings.
Rain water leachates of heavy metals
L.H.axis Cd Pb
1405 RM. axis
.............. CO
PARTICLESIZE> 2 5 0 / J
~ M r t
?0- -?
3`TX M6
60- -6
..
6W,.1 ].??S 12&0
2&1 2&IS 69¢
S/.2.2 3,772 ?)6
IIIL2 Pb I?.SQ Cd 192
Mn
[
50- -5 &0" -~ Rob 3ROb 30--
t .":.
/~/,1
-i
•°
0--0
0
L.H. axis
1O2O3O
0 1020 30 Time (days)
~°: I I °'% 1" f/
-400
0
10 2 0 3 0
-100 0
10 2 0 3 0
PARTICLE SIZE <2S0p
Pb
70" -?
It04 5,192 O~
2100 L035
60" "6 50" "5 i,O" -l, ROb ROb 30" "3
"'"~.~.~.
20" "2
I
-o,. [
6.192 136.2 192 I
I
0
10 20 30
..
lOl)O
Pb
el6
Mn
I s,1 Cd
:/;
-/
R.H.axis Mn "700 "600
-/,00 ROb
I
-
-300
-2O0
I:
0
-0
-500
-"../
%'!
10- -I
(inset
-500
-200
10" -I
0''0
-600
-300
"
20" "2
Cd
Mn -?00
10 20 30
01
02
Motorway
Notor~ff
gutter sampte
dust samlfle
I" I
0
!
10 20 30 Time (days} 03
0
1020 O(
Mo~ mid-gutter sample
30
-100
-0 0
102030 05
Watfotd
$unnysicle
Way sample
6nrckms sample
values refer to sediment metal levels (opm))
Fig. 2. Leaching curves and original sc~liment concentrations of heavy metals. face runoff is typically exascerbated and maintained by the impact and splash effects of passing traffic. The vigorous agitation of the sediment suspensions maintained throughout the leaching experiments was thus designed to simulate the turbulent hydraulic conditions established on roadside surfaces during stormflow.
original Pb levels in the raw sediment are high, as seen from Fig. 2, ranging from 2100 ppm [sample 01 (<250)'] to 136ppm [sample 04 (<250)']. Hence, a low extraction efficiency of Pb from the sediment by rainwater is indicated in agreement with the results of Ter Haar & Bayard (19711. Inspection of Fig 2 shows 3 distinct patterns of leaching for Pb:
RAINFALLLEACHATESTUDIES
(a) a decrease in leachate concentration within 5 days followed by a constant level or a slight increase; (b) initial enrichment followed by a sharp decrease in concentration; (c) a steady enrichment throughout the time period.
The largest variation in metal level concentrations in the leachates with both time and grain size are observed for Pb (Fig. 2). The upper and lower threshold levels of Pb do not appear to be dependent on panicle size as equally low and high levels are found in both size categories. The minimum and maximum Pb concentrations of 1 and 5 ppb are consistent with the thermodynamic prediction of lead solubility at pH 6.5 in soft water (Hem & Durum, 1973). However the
Pattern (a) c~n be explained by a loss of Pb from the soluble phase occurring at a faster rate than the solubility of metals from the solid state but with these processes approaching equilibrium with time. For (c) the metal is solubilising at a faster rate than the sol-
1406
D M. REVITTand J B. ELLIS
uble form is being lost and within the time period of this experiment an equilibrium situation has not been established. Compared with Pb the Cd leachate levels show less variability in temporal pattern and particle size relations. Particle size has little effect on the leachate Cd concentration levels of the Motorway samples (01, 02, 03) whereas samples 04 and 05 show significant differences between size ranges. In the case of sample 04, the maximum Cd levels vary from 66 ppb for the > 250/~m size to 20 ppb for the <250/xrn size indicating easier removal of this metal from the larger particle size fraction. The Cd levels in the leachate are on average a factor of 10 times high than the corresponding Pb values although the original raw sediment Cd levels are much lower than those of Pb (Fig. 2). Therefore a much more efficient leaching process is indicated for Cd. The rate of leaching will be highly dependent on the chemical speciation of the metal in the original sediment and fo- Cd this favours solubility as opposed to adhesion. The Cd leaching profiles all follow a distinct pattern except for sample 05 (< 250), where the level decreases steeply at first and then more gradually with time. A unimodal distribution with a distinct peak is shown by all other samples. The occurrence of the peak is time dependent with the greatest time lag to peak appearance being found for the smaller size fraction. This indicates a greater resistance on the part of these particles to release Cd to the rainwater. The Mn leachate levels again show variations with regard to grain size and sample location. The original sediment Mn concentrations (718-1240ppm) are of the same order as Pb, however the Mn leachate concentrations attain values up to 300 ppb, showing that Mn like Cd is more readily leached than Pb. Two general leaching patterns with time are observed for Mn: (d) for particles > 250#m there ~s an overall decrease: (el for particles < 250~m an initial increase is followed by steady levels or levels are constant throughout. An exception to pattern (e) is found for sample 05 (<250) where there is a sharp initial decrease followed by a levelling off. It should be noted that sample 05 ( < 250) also gave a unique leaching pattern for Cd levels. Whilst sites 01-O4 are similar in their traffic characteristics and toadings (Table 1), the residential side street site of 05 has distinctly different conditions and this may partly explain the unusual pattern and behaviour of 05 (<250). Comparison of leaching patterns (d) and (e) show that for the larger particle size category there is a tendency for Mn to be removed from the soluble phase with time. However, with the smaller size group, an equilibrium is established immediately or following an initial leaching of Mn from the solid state.
CONCLUSIONS
Time based metal assays of the sediment ieachates show variable trends, with the solubility curves of Mn showing most tendency to attain equilibrium with time. No obvious dependence of metal leachate concentrations on grain size is readily apparent which indicates that greater control of the input of all sediment sizes to stormwater systems is required if toxic levels are to be prevented. However, certain metais such as Cd, tend to desorb more rapidly from the larger size fractions with peak leachate levels occurring within 10 days. Weekly gu!ly cleansing would therefore be required to provide control of metal toxicity levels during the first flush of stormftow. The different traffic and site characteristics of the residential side street location produce temporal leachate patterns which are distinctive for Mn and Cd. The leachate metal levels at this site are not noticeably lower than those with much higher traffic densities, indicating the relative importance of residential street surfaces to the total metal loadings of urban runoff. In the raw sediments, total metal levels do tend to increase with decreasing particle size; this size association being particularly noticeable in the case of Pb for the motorway samples. The overall extraction efficiency of Cd is 10 times that of Mn which in turn is 100 times that of Pb. These values illustrate the inherent dangers of relating metal toxicities directly to raw sediment levels. The experiments have demonstrated the potential significance of metal solubility in determining receiving water quality as welt as the need for improved street cleaning practise. The metal levels recorded could be toxic to tgenthai organisms particularly if receiving waters were "'soft" or if the metal levels were enhanced synergistically by high oxygen demands as during the first l-]ush of a storm event REFERENCES British Standards Institution (1967) Methods ot Testiny Soils for Civil Engineering Purposes, BS1377, London. Christensen E. R., Scherfig J. & Koide M. (1978i Metals from urban runoff in dated sediments of a very shallow estuary. Environ. Sci. Technol. 12. 1168-1 t73 Day J. P., Hart M. & Robinson M. S. (1975) Lead m urban street dust. Nature 253, 343-345. Duggan M. J. & Williams S. (1977! Lead-in-dust mclt? streets. Sci. Total Enriron. 91-97, Ellis J. B. (1976) Sediments and water quality of urban stormwater, Water Serv. 80, 730--734. Ellis J. B. (19791 In Mans Impact on the Hydrotooical Cycle in the U.K, (Edited by Hollis G. E.), pp. 119--216. Geobooks, Norwich. Helmke P. A., Koons R. D, Schomberg P. J. & lskander I. K. (1977) Determination of trace element contamination of sediments. Environ. Sci. Technol. 11,984-989. Hem J. D. & Durum W. D. (1973) Solubility and occurrence of lead in surface waters. J. Am. War. Wks Ass 65, 562-568. LagerwerffJ V. & Specht A. W (1970i Contamination ot
Rain water leachates of heavy metals roadside soil and vegetation with cadmium, nickel, lead and zinc. Environ. Sci. Technol. 4, 583-586. Laxen D. P. H. & Harrison R. M. (1977) The highway as a source of water pollution. Water Res. 11, 1-11. Little P. & Whiffen R. D. (1978) Emission and Deposition of Lead from Motor Exhausts--II. Airborne concentration, particle size and deposition of lead near motorways. Atmos. Environ. 12, 1331-1341. Nightingale H. J. (1975) Lead, zinc and copper in soils of urban storm runoff retention basins. J. Am. War. Wks Ass. 67, 443--446.
w . L 14/1(~-c"
1407
Pitt R. E. & Amy G. (1973) Toxic Materials Analysis of Street Surface Contaminants, EPA R2-73-283. U.S. Govt. Printing Office, Washington, D.C. Sartor J. B. & Boyd G. B. (1972) Water Pollution Aspects of Street Surface Contaminants, EPA R2-72-081. U.S. Govt. Printing Office, Washington, D.C. Ter Haar G. L. & Bayard M. A. (1971) Composition of airborne lead particles. Nature 232, 553-554. Wilber W. G. & Hunter J. V. (1977) Aquatic transport of heavy metals in the urban environment. War. Res. Bull. 13, 721-734.
RAIN WATER LEACHATES OF HEAVY METALS IN ROAD SURFACE SEDIMENTS D. M. REvlyr and J. B. ELI,IS Middlesex Polytechnic, Urban Stormwater Pollution Research Group, The Burroughs, Hendon, London NW4 4BT, England
(Received August 1979) Abstract--Samples of street surface and roadside gutter sediments within a separately sewered catchment in N.W. London have been analysed for heavy metal contamination. Particle size distributions are presented and the removal efficiency of metal species demonstrated for various street cleaning procedures. Laboratory sorption and desorption studies are described and solution concentrations are obtained for Pb, Cd and Mn. Solubility curves are typically variable with time with Mn showing the strongest tendency to attain equilibrium. No obvious dependence of metal concentrations on grain size is readily apparent although Cd tends to desorb more rapidly from size fractions greater than 250/an.
INTRODUCTION
A considerable literature now exists confirming the importance of street surface sediments in contributing to heavy metal pollution of stormwater drainage systems. (Lagerwerff & Speeht, 1970; Sartor & Boyd, 1972; Pitt & Amy, 1973; Nightingale, 1975; Ellis, 1976; Wilber & Hunter, 1977; Christensen et al., 1978). However, much of the published work relates specifically to Ph. (Day et al., 1975; Duggan & Williams, 1977; Laxen & Harrison, 1977; Little & Wiffen, 1978). Heavy metal concentrations at the # g g - 1 (ppm) level are typical for dry street surface sediments whilst most stormwater concentrations are at the gg 1-1 (ppb) level. As rainfall leaches and flushes the surface sediment, which includes substantial humic components, some fraction of the metals dissolve in the r u n o f as free or complexed species and additional heavy metal is added on or in the particulates (Hem & Durum, 1973). Urban stormwater sediments are characterized by heterogeneous particulates and always contain a substantial proportion of fine grained sizes transported as suspended solids. Much of the literature refers to the affinity of such fine suspended sediment for metallic pollutants but few investigate the relationships between sediment size and metal leachate concentrations. It is known that conventional street cleaning procedures have low removal efficiencies for small particle sizes and previous work has shown that sizes below 250/an account for some 50% of heavy metals by weight (Sartor & Boyd, 1972; Ellis, 1979). Therefore, this study was carried out to compare the extraction of heavy metals from these finer particles with that from particles larger than 250gin under simulated stormwater flow conditions. Determination of the metal levels over an extended time interval provides information relating to the quality and toxicity
of storm drainage and its dependence on the contact time of sediment with street surface runoff. EXPERIMENTAL
Sample collection was undertaken following a period of several days without rainfall at six roadside sites including motorway, trunk road, estate road and residential side street. The characteristics of these sampling sites are described in Table 1. Samples were collected in plastic con. tainers using nylon brushes and plastic scoops from three specific locations on these road surface__ These were adjacent to gully pots (sample 01~ midway between gnlly pots (samples 03, 04, 05 and 06) and from the road surface (sample 02). Except for sample 01, sediments were collected over a 10 m road distance and the combined samples were consecutively riffled to obtain a representative sub-sample of 100g. For samples 01-05, this 100g sample was sieved using stainless steel mesh into two fractions, greater and less than 250/am and 5 g quantities of each size category were digested with nitric acid for subsequent analysis. Grading curves for sample 06 were determined by dry/ wet sieving techniques using stainless steel mesh and by standard pipette procedures according to BS. 1377 (British Standards Institution, 1967). Leaching experiments on samples 01-05 were carried out using a 1: 10 weight :volume ratio of sediment in previously collected rain water at a pH of 6.5. These conditions together with constant agitation of the suspensions using teflon coated magnetic stirrers were designed to simulate turbulent runoff conditions during storm drainage from the roadside surface. 10 ml aliquots of supernatant liquid were removed at 1 day, 5 day, 11 day, 15 day and 28 day intervals and filtered prior to analysis. Pb, Cd and Mn were determined by atomic absorption spectrophotometry using acetylene-air flame atomisation for the analysis of acid digests and a flameless atomiser for the leachates. STREET CLEANING
AND HEAVY METALS
The sediment removal efficiency of conventional road cleaning procedures on an arterial perimeter road (Table 1) around a recently established housing estate in close proximity to the sampling sites has
1403
1404
l , M. REVrTTand J. B. ELLIS Table I~ Characteristics of sampling sites
Number 01
Site
Traffic details
Sample details
02
MI Motorway, Edgware. N.W. London As 01
4000h -~, HGV, cars. vans, etc.. 55-60 mph. ~s OI
03
As 01
As 0t
04
A4I, Watford Way, Dual Carriageway, Hendon, N.W. London Residential side street, Sunnyside Gardens, Hendon, N.W. London Perimeter road, Grahame Park Estate, Colindale, N.W. London
1750 h- t. Goods, cars and commercial. 40--50 mph.
05 06
450 h - ~. Light commercial, cars, 40 mph.
10 4
I
.
I
Grain Size (microns)
I
I
\
"a\
~
I
2
%
F-1 I
influenced by the removal efficiences for the range of particle sizes indicated by the cleaning runs. The accumulation of such fine polluted fractions in gutter and gully under quiescent, stagnant conditions between storm events inevitably induces toxic levels which impose considerable stress on the stormwater system during eventual storm flushing. Hence, the solution concentration of heavy metals will be of importance when determining the toxicity of road surface runoff and resultant receiving water quality. In addition, as 50Vo of all metals are associated with sizes less than 250/an, the contact time and rates of metal take up and release will be of importance in determining outfall toxicity. The efficiency of jet flushing however, suggests that storm rainfall intensities would be sufficient to wash off much of the residual particulates and thus enhance the potential runoff toxicity. Field observations indicate that 250/am sizes are entrained by turbulent surface runoff generated from storm intensities exceeding 6 mm h - 1 The highly turbulent nature of street sur-
I
-".'%,.".~, oo
Hard shoulder surface, 125 m from kerb Gutter adjacent to kerb. midway between gully pots From gutter adjacent t~ kerb. between parked cars and midway between gully pots Gutter adjacent to kerb between parked cars and midway between gully pots Gutter adjacent to kerb, midway between gully pots
100 h t . Cars, vans, 25--30 mph.
been previously investigated (Ellis, 1979). Grading curves relating to this investigation are shown in Fig. 1 together with sizing alterations and reductions in pollutant loadings after cleaning runs by a mechanical rotary brush sweeper followed by high pressure jet flushing. The relative efficiency of flushing over sweeping on both size and loadings is clearly shown. The reduction in street surface loadings for SS is 30~o following rotary cleaning and a further 45~o following flushing whilst for lead loadings the equivalent reduction in rates is 27 and 55~. This study, as well as the original classic studies of Sartor & Boyd (i 972), demonstrates the importance of small size fractions in contributions to road surface runoff quality. Clearly, considerable amounts of fine sediments, together with their associated metal pollutants remain on the street surface and in roadside gutters after rotary sweeping, which is the normal cleaning practise within this catchment. The choice of the 250/am cut-off for use in the experimental procedure described above was therefore
10 3.
Hard shoulder, adjacent to gully pots
-
~',~
o-II
I.o
I
_1 _. LAF
,c 8.,.. c,-i°, Aec Aft,r eotary
'k
o,..°,°,
- -."':. ARc AF
Io~
l
I
1
5
l
I
10
50
I
I
I
80 gO
99
99-9
"I. Greater Than
Fig. 1. Street cleaning efficiency for particle size and pollutant loadings.
Rain water leachates of heavy metals
L.H.axis Cd Pb
1405 RM. axis
.............. CO
PARTICLESIZE> 2 5 0 / J
~ M r t
?0- -?
3`TX M6
60- -6
..
6W,.1 ].??S 12&0
2&1 2&IS 69¢
S/.2.2 3,772 ?)6
IIIL2 Pb I?.SQ Cd 192
Mn
[
50- -5 &0" -~ Rob 3ROb 30--
t .":.
/~/,1
-i
•°
0--0
0
L.H. axis
1O2O3O
0 1020 30 Time (days)
~°: I I °'% 1" f/
-400
0
10 2 0 3 0
-100 0
10 2 0 3 0
PARTICLE SIZE <2S0p
Pb
70" -?
It04 5,192 O~
2100 L035
60" "6 50" "5 i,O" -l, ROb ROb 30" "3
"'"~.~.~.
20" "2
I
-o,. [
6.192 136.2 192 I
I
0
10 20 30
..
lOl)O
Pb
el6
Mn
I s,1 Cd
:/;
-/
R.H.axis Mn "700 "600
-/,00 ROb
I
-
-300
-2O0
I:
0
-0
-500
-"../
%'!
10- -I
(inset
-500
-200
10" -I
0''0
-600
-300
"
20" "2
Cd
Mn -?00
10 20 30
01
02
Motorway
Notor~ff
gutter sampte
dust samlfle
I" I
0
!
10 20 30 Time (days} 03
0
1020 O(
Mo~ mid-gutter sample
30
-100
-0 0
102030 05
Watfotd
$unnysicle
Way sample
6nrckms sample
values refer to sediment metal levels (opm))
Fig. 2. Leaching curves and original sc~liment concentrations of heavy metals. face runoff is typically exascerbated and maintained by the impact and splash effects of passing traffic. The vigorous agitation of the sediment suspensions maintained throughout the leaching experiments was thus designed to simulate the turbulent hydraulic conditions established on roadside surfaces during stormflow.
original Pb levels in the raw sediment are high, as seen from Fig. 2, ranging from 2100 ppm [sample 01 (<250)'] to 136ppm [sample 04 (<250)']. Hence, a low extraction efficiency of Pb from the sediment by rainwater is indicated in agreement with the results of Ter Haar & Bayard (19711. Inspection of Fig 2 shows 3 distinct patterns of leaching for Pb:
RAINFALLLEACHATESTUDIES
(a) a decrease in leachate concentration within 5 days followed by a constant level or a slight increase; (b) initial enrichment followed by a sharp decrease in concentration; (c) a steady enrichment throughout the time period.
The largest variation in metal level concentrations in the leachates with both time and grain size are observed for Pb (Fig. 2). The upper and lower threshold levels of Pb do not appear to be dependent on panicle size as equally low and high levels are found in both size categories. The minimum and maximum Pb concentrations of 1 and 5 ppb are consistent with the thermodynamic prediction of lead solubility at pH 6.5 in soft water (Hem & Durum, 1973). However the
Pattern (a) c~n be explained by a loss of Pb from the soluble phase occurring at a faster rate than the solubility of metals from the solid state but with these processes approaching equilibrium with time. For (c) the metal is solubilising at a faster rate than the sol-
1406
D M. REVITTand J B. ELLIS
uble form is being lost and within the time period of this experiment an equilibrium situation has not been established. Compared with Pb the Cd leachate levels show less variability in temporal pattern and particle size relations. Particle size has little effect on the leachate Cd concentration levels of the Motorway samples (01, 02, 03) whereas samples 04 and 05 show significant differences between size ranges. In the case of sample 04, the maximum Cd levels vary from 66 ppb for the > 250/~m size to 20 ppb for the <250/xrn size indicating easier removal of this metal from the larger particle size fraction. The Cd levels in the leachate are on average a factor of 10 times high than the corresponding Pb values although the original raw sediment Cd levels are much lower than those of Pb (Fig. 2). Therefore a much more efficient leaching process is indicated for Cd. The rate of leaching will be highly dependent on the chemical speciation of the metal in the original sediment and fo- Cd this favours solubility as opposed to adhesion. The Cd leaching profiles all follow a distinct pattern except for sample 05 (< 250), where the level decreases steeply at first and then more gradually with time. A unimodal distribution with a distinct peak is shown by all other samples. The occurrence of the peak is time dependent with the greatest time lag to peak appearance being found for the smaller size fraction. This indicates a greater resistance on the part of these particles to release Cd to the rainwater. The Mn leachate levels again show variations with regard to grain size and sample location. The original sediment Mn concentrations (718-1240ppm) are of the same order as Pb, however the Mn leachate concentrations attain values up to 300 ppb, showing that Mn like Cd is more readily leached than Pb. Two general leaching patterns with time are observed for Mn: (d) for particles > 250#m there ~s an overall decrease: (el for particles < 250~m an initial increase is followed by steady levels or levels are constant throughout. An exception to pattern (e) is found for sample 05 (<250) where there is a sharp initial decrease followed by a levelling off. It should be noted that sample 05 ( < 250) also gave a unique leaching pattern for Cd levels. Whilst sites 01-O4 are similar in their traffic characteristics and toadings (Table 1), the residential side street site of 05 has distinctly different conditions and this may partly explain the unusual pattern and behaviour of 05 (<250). Comparison of leaching patterns (d) and (e) show that for the larger particle size category there is a tendency for Mn to be removed from the soluble phase with time. However, with the smaller size group, an equilibrium is established immediately or following an initial leaching of Mn from the solid state.
CONCLUSIONS
Time based metal assays of the sediment ieachates show variable trends, with the solubility curves of Mn showing most tendency to attain equilibrium with time. No obvious dependence of metal leachate concentrations on grain size is readily apparent which indicates that greater control of the input of all sediment sizes to stormwater systems is required if toxic levels are to be prevented. However, certain metais such as Cd, tend to desorb more rapidly from the larger size fractions with peak leachate levels occurring within 10 days. Weekly gu!ly cleansing would therefore be required to provide control of metal toxicity levels during the first flush of stormftow. The different traffic and site characteristics of the residential side street location produce temporal leachate patterns which are distinctive for Mn and Cd. The leachate metal levels at this site are not noticeably lower than those with much higher traffic densities, indicating the relative importance of residential street surfaces to the total metal loadings of urban runoff. In the raw sediments, total metal levels do tend to increase with decreasing particle size; this size association being particularly noticeable in the case of Pb for the motorway samples. The overall extraction efficiency of Cd is 10 times that of Mn which in turn is 100 times that of Pb. These values illustrate the inherent dangers of relating metal toxicities directly to raw sediment levels. The experiments have demonstrated the potential significance of metal solubility in determining receiving water quality as welt as the need for improved street cleaning practise. The metal levels recorded could be toxic to tgenthai organisms particularly if receiving waters were "'soft" or if the metal levels were enhanced synergistically by high oxygen demands as during the first l-]ush of a storm event REFERENCES British Standards Institution (1967) Methods ot Testiny Soils for Civil Engineering Purposes, BS1377, London. Christensen E. R., Scherfig J. & Koide M. (1978i Metals from urban runoff in dated sediments of a very shallow estuary. Environ. Sci. Technol. 12. 1168-1 t73 Day J. P., Hart M. & Robinson M. S. (1975) Lead m urban street dust. Nature 253, 343-345. Duggan M. J. & Williams S. (1977! Lead-in-dust mclt? streets. Sci. Total Enriron. 91-97, Ellis J. B. (1976) Sediments and water quality of urban stormwater, Water Serv. 80, 730--734. Ellis J. B. (19791 In Mans Impact on the Hydrotooical Cycle in the U.K, (Edited by Hollis G. E.), pp. 119--216. Geobooks, Norwich. Helmke P. A., Koons R. D, Schomberg P. J. & lskander I. K. (1977) Determination of trace element contamination of sediments. Environ. Sci. Technol. 11,984-989. Hem J. D. & Durum W. D. (1973) Solubility and occurrence of lead in surface waters. J. Am. War. Wks Ass 65, 562-568. LagerwerffJ V. & Specht A. W (1970i Contamination ot
Rain water leachates of heavy metals roadside soil and vegetation with cadmium, nickel, lead and zinc. Environ. Sci. Technol. 4, 583-586. Laxen D. P. H. & Harrison R. M. (1977) The highway as a source of water pollution. Water Res. 11, 1-11. Little P. & Whiffen R. D. (1978) Emission and Deposition of Lead from Motor Exhausts--II. Airborne concentration, particle size and deposition of lead near motorways. Atmos. Environ. 12, 1331-1341. Nightingale H. J. (1975) Lead, zinc and copper in soils of urban storm runoff retention basins. J. Am. War. Wks Ass. 67, 443--446.
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Pitt R. E. & Amy G. (1973) Toxic Materials Analysis of Street Surface Contaminants, EPA R2-73-283. U.S. Govt. Printing Office, Washington, D.C. Sartor J. B. & Boyd G. B. (1972) Water Pollution Aspects of Street Surface Contaminants, EPA R2-72-081. U.S. Govt. Printing Office, Washington, D.C. Ter Haar G. L. & Bayard M. A. (1971) Composition of airborne lead particles. Nature 232, 553-554. Wilber W. G. & Hunter J. V. (1977) Aquatic transport of heavy metals in the urban environment. War. Res. Bull. 13, 721-734.