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The pollution of effluents from pervious pavements of an experimental highway section: first results
the Science of the Total Environment A~ l a t ~ m l d o n l ll e ~ * l 6v, ~ m l l n c R e ~ h into ¢h¢ E ~ n t .rid i~ m l l t ~ i p .i~ M~
ELSEVIER
The Scienceof the Total Environment 146/147 (1994) 465-470
I
The pollution of effluents from pervious pavements of an experimental highway section: first results G. Stotz*, K. Krauth lnstitut fur Siedlungwasserbau, Wassergiiteund Abfallwirtschaft der Universitiit Stuttgart, Bandtdle I. 70569 Stuttgart, Germany
Abstract
The use of pervious pavements as covering materials for streets has increased. The reason for the use of such pavements is their ability to minimise noise, aquaplaning and water spreading by tyres during rainfall events. Two years ago the province of Baden-Wiirttemberg constructed an experimental highway section to obtain information about the noise-decreasing effects as well as the influence of winter maintenance on the physical behaviour of porous road pavements. In parallel, at the end of 1989 investigations were begun to examine the chemical properties of the drainage water passing through the upper 40 mm of the pervious layer (porous asphalt) with a porosity of up to 20 vol%. Key words." Porous asphalt; Permeability; Properties of the permeate; Pollution loads
1. Introduction
More than three years ago the construction of highways and roads covered with porous surfaces was begun in Germany. Porous asphalts, originally developed for airfields, are expected to be of great advantage in the following respects: Noise reduction. Continually increasing traffic is also combined with a steady increase in noise emissions from tyres and engines. Street covering materials containing a high proportion of air pores are assumed to be sound-absorbing within certain frequency ranges and thus to be noise-reducing. Avoidance of aquaplaning. In the middle of the 1960s the Franklin Institute [1] carried out investigations that showed a very high water perme* Corresponding author.
ability of porous asphalts. The permeability values measured on core samples exceeded the maximum precipitation values of heavy storm intensities. These results justified the assumption that no water films would form on porous asphalts. Avoidance of water-spreading by tyres. If precipitation falling on the street surface is drained off immediately, no water-spreading by vehicles tyres would occur. The elimination of aquaplaning as well as the improvement of visibility for all drivers during rainfall events reduces accident risk. While these advantages are significant for traffic engineers, there are nowadays new concepts in urban storm drainage to be considered for storm run-off reduction and peak flow avoidance [2]. The susceptibility of porous asphalts to becoming clogged is a serious problem [3,4]. The properties of effluents from laboratory-scale unit superstruc-
0048-9697/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0048-9697(93)03500-2
466
tures have been analysed chemically and described elsewhere [4]. The noise reduction efficiency of porous asphalts is well known and is still being investigated, but up to now, detailed and systematic investigations of the chemical properties of the permeates of porous asphalts have not been carried out.
2. Objectives Insufficient knowledge of the permeability behaviour of porous asphalts and the water pollution resulting from precipitation passing through the porous asphalts caused the Landesamt ffir Strassenwesen of the province of Baden-Wiirttemberg to order the Institute ffir Siedlungswasserbau, Wasserg/ite- und Abfallwirtschaft of the University of Stuttgart to carry out investigations on the experimental highway section of federal highway A6. The results of these measurements were also to be compared with those of earlier investigations carried out on sealed highway surfaces [5]. Winter maintenance effects were also to be evaluated.
G. Stotz, K. Krauth/ScL Total Environ. 146/147 (1994) 465-470
Table 1 Data relating to porous asphalt and investigated highway section Porous asphalt age (years) Thickness of covering (mm) Mineral particle size (mm) Porosity (vol%) Length (m) Width (m) Surface area (ha) Cross-slope (%) Vertical slope (%) Average daily traffic (vehicles) Proportion of trucks (%) Investigation period (months)
=2.5-3.5 4 0/8 19.1 50.00 11.25 0.055 1.5 1.2 34 675 25 12
measured at 34 675 vehicles per day, including a heavy vehicle proportion of 25%. The investigations were carried out over twelve months from April 1990 to April 1991. At the beginning of the measuring period the porous asphalt had been in operation for 2.5 years.
3. Description of the experimental highway section
4. Methods of measurement and analytical procedures
The experimental highway section is on the A6 highway from Mannheim to Stuttgart, between the Neckarsulm entrance and the Weinsberg interchange at the Sulmtal parking area. The highway section, originally constructed only for noise reduction studies, is subdivided into two types of porous asphalt paving. The results presented here relate to the measurements carried out on the 0/8 mm porous asphalt section. The highway consists of two lanes and a hard shoulder. For permeate measurement purposes the hard shoulder was equipped with a concrete gully arranged parallel to the right-hand boundary and 50 m in length. The gully was sealed so that the direct penetration of precipitation and surface run-off was impossible. Thus only that portion of precipitation was drained off and measured that had passed through the porous asphalt with a surface area of 0.055 ha. The cross-slope of the road surface is 1.5% and the vertical slope is 1.2%. The thickness of the porous asphalt amounts to 40 mm, and it has a porosity of 19.1 vol% after construction. In 1990 traffic was
Flow depth in a stainless steel measuring channel with a 15° triangular weir at its end together with precipitation data were monitored continuously and recorded by a computer. The sampling procedure was also computer-controlled as well as the data evaluation. All samples taken during a run-off event were combined to give one composite sample (taken so as to be flowproportional), and this was analysed for heavy metals, mineral oil, filterable solids, COD, polycyclic aromatic hydrocarbons (PAH) (as six combined PAHs and fluoranthene), NH4-N, NO2-N, NO3N, pH, acid capacity, chlorine, magnesium, calcium and SO4. All methods and procedures used were identical to those of earlier studies [5], in order to facilitate comparability of the results. The field measurements involved infiltration rate measurements on both the left and right lanes using a double-ring infiltrometer commonly employed in hydrological methods. The infiltration was measured at a constant pressure height of 30-40 mm. Tap water, after addition of a small
G. Stotz, K. Krauth / Sci. Total Environ. 146/147 (1994) 465-470
amount of a surfactant to reduce its surface tension, was used instead of rainwater. To avoid traffic delays, these measurements were carried out only twice a year (in the summer and the winter). 5. Results
5.1. Permeability Volume balance measurements of precipitation and effluent from the porous asphalt for 1 year, including both the summer and winter months, gave the results listed in Table 2. During the summer (from May to October) the precipitation height amounted to 309 mm, and a corresponding run-off of 116 m3/ha was measured. This means that only 3.7% of the precipitation passed through the porous asphalt. In the winter (from November to April) 5.3% of a 237-mm precipitation permeated the porous pavement (i.e. a run-off of 125 m3/ha). Permeation occurred only if the depth of precipitation for single events was >0.6 mm and rain intensity was >0.5 mm/h in the summer and >0.3 mm/h in the winter. The permeability was surprisingly low, hut the permeate percentage values for the summer and winter months clearly show variations depending on the season, The cold and damp weather conditions in winter kept the porous asphalt moist for a fairly long time. Thus losses within the porous structure of the asphalt were small, and in the winter the hydraulic conductivity was higher than in the summer. The greater losses during the summer were caused by prolonged dryness of the asphalt in combination with higher temperatures. Comparisons were also made between the
measured single-effluent events and calculations using a precipitation run-off model with input values corresponding to those of the experimental highway section. The comparison showed that the drainage behaviour of a porous asphalt is quite different from that of impervious asphalt surfaces. Effluents from the previous porous asphalt at Weinsberg occurred after a delay and lasted for a long time. Peak effluent flows were lowered, but the greater the precipitation or the longer the duration of rainfall preceding the main storm, the earlier they occurred (related to the time of peak precipitation). In all cases, permeate volumes were found to be significantly smaller than the run-off volumes from impermeable surface. In situ measurements of infiltration in both ruts and centre lines of the slow and fast lanes were necessary, because of the very poor permeability of the porous asphalt. The experimental method allowed the determination of the lag times between the time of starting the experiment and the time of beginning infiltration and the time at which the highest rate of infiltration was achieved, respectively (Table 3). In summer the infiltration rates for the ruts were 78 mm/h on the fast lane and only 18 mm/h on the slow lane, which is characterised by a high proportion (25%) of heavy vehicles and by a frequent stop-and-go flow of traffic. At the centre lines the rate in the fast lane (28 mm/h) was lower than in the slow lane (42 mm/h). In winter, infiltration rates measured in the ruts were 18 mm/h and 24 mm/h, respectively. Thus, these rates were nearly
Table 3 Time lags and maximun rates of infiltration Station a
Table 2 Balance of permeability
Precipitation (mm) Permeate (m3/ha) Permeate percentages (%) Min. rainfall height (ram) Min. intensity (mm/h) Average permeation duration (h)
467
Summer
Winter
1 year
309 116 3.7 1.4 0.5 6.4
237 125 5.3 0.6 0.3 6.0
546 241 4.4
Wlfm Wlfr Wlfm Wlfr W2fm W2fr W2fm W2fr
Date 4 Dec. 4 Dec. 11 June I 1 June 4 Dec. 4 Dec. 11 June 11 June
1990 1990 1991 1991 1990 1990 1991 1991
Time lag
Rate
(rain)
(ram/h)
8 28 33 35 10 8 23 37
21 22 39 17 17 17 27 78
aWl, slow lane; W2, fast lane; fm, centre line; fr, rut.
468
G. Stotz, K. Krauth / Sci. Total Environ. 146/147 (1994) 465-470
identical to those found for the centre lines. All measured rates were, in comparison, significantly smaller than values given in the literature [1,4,6]. These results therefore confirm the very poor permeability of the porous asphalt of the experimental highway section. The higher permeability results in winter can be explained by the lag times, which in all cases were shorter in winter than in summer.
levels in the winter might be explained by its low resistance to ultra-violet irradiation and easier biochemical degradation in the summer. However, an explanation must be given for the increased concentrations of traffic-related substances, for example heavy metals, in the winter. Obviously, winter maintenance procedures must also be taken into consideration. It is well known that porous asphalts are usually cold and damp for prolonged periods, because they cool down faster. To avoid ice on the road, wet thawing salts are spread on the asphalt surface as a preventive measure, using solid sodium chloride in combination with calcium chloride salt water. Maintenance vehicles usually spread 10 g of wet thawing salt per square metre of street surface area during one cycle of operation, i.e. 7 g of solid salt and 3 g of salt water. Both constituents of the thawing mixture used were analysed chemically to determine their pollutant contents. The results, listed in Table 5, clearly show that the use of wet thawing salt leads to increased loads not only of the main constituents, chlorine and sodium, but also of heavy metals.
5.2. Properties o f the permeate
The run-off weighted averages of the concentrations of all analysed pollutants are listed in Table 4, categorised as summer, winter and yearly concentrations. When comparing the half-yearly values with each other, only concentrations of filterable solid iron and lead were lower in winter than in summer. All other pollutants demonstrated lower concentrations in summer. Extreme differences were found for chlorine, which is used for winter highway maintenance (summer/winter: 36/3921 mg/l), as well as for calcium (summer/winter: 17/205 mg/l). The higher fluoranthene
Table 4 Concentrations of pollutants (weighted averages) Pollutants
Unit
Summer
Winter
Year
Filterable solids COD Mineral oil PAH (6) Fluoranthene Cd Cr Cu Fe Pb Zn CI Ca Mg SO4 NH4-N NO3-N NO2-N pH Acid capacity
mg/1 mg/1 mg/l ng/I ng/l /zg/l /zg/l #g/1 mg/1 #g/l /zg/l mg/l mg/l mg/l mg/l mg/l mg/1 mg/l
64 84 0.23 83 9 0.9 11 49 2.3 137 441 36 17 2.1 30 0.47 0.85 0.14
49 165 0.37 119 79 2.5 17 88 2.1 58 737 3921 205 4.8 136 3.22 3.11 0.29
0.94
1.98
56 126 0.30 106 46 1.7 14 70 2.2 96 596 2061 115 3.5 85 1.90 2.02 0.22 7-8.3 1.48
mmol/l
469
G. Stotz, K. Krauth/Sci. Total Environ. 146/147 (1994) 465-470
Table 5 Composition of wet thawing salt used for winter maintenace Pollutant
Solid salt NaCI (mg/g)
Salt water CaCI 2 (mg/l)
Amount of pollutant contained in 10 g wet thawing salt
Na Ca SO4 Mg Pb Cu Cr Zn Cd
390 1.6 3.04 0.55 0,004 0.002 0.002 <0.01 0.0002
2900 115 000 350 16 0.8 0.5 0.08 1.2 0.052
2757 mg 1095 mg 27 mg 4 mg 35.5 ~g 18.7 #g 14.8/~g 11.3/~g 1.9 t~g
5.3. Pollution loads
For nearly all pollutants the pollution loads related to 1 ha surface area of porous asphalt were found to be greater in the winter than in the summer (Table 6). This conforms to a higher permeability in the winter. Correlations between pollution loads and dryweather periods or rainfall intensities (valid for sealed surface areas in the findings of earlier in-
vestigations [5]) could not be confirmed for the porous asphalt at Weinsberg. It was also impossible to find significant correlations between the loads of filterable solids and heavy metals as well as between the loads of mineral oil and COD. For purposes of comparison, the yearly pollution loads for Pleidelsheim [5] are listed iaa Table 6, calculated on the basis of run-off proportions similar to those of Weinsberg (4.4.% permeability proportion on a yearly average) and assuming the
Table 6 Pollution loads Pollutants
Unit
Summer
Winter
1 year
Pleidelsheim [5] a
Filterable solids COD Mineral oil PAH (6) Fluoranthene Cd Cr Cu Fe Pb Zn CI Ca Mg SO4 NH4-N NO3-N NO2-N Acid capacity
kg/ha kg/ha g/ha mg/ha mg/ha g/ha g/ha g/ha g/ha g/ha g/ha kg/ha kg/ha kg/ha kg/ha g/ha g/ha g/ha mol/ha
7.42 9.71 26.61 9,56 1.07 0.11 1.29 5.68 260.50 15.81 51.08 4.16 2.01 0.24 3.49 54.36 97.83 16.65 109.47
6.12 20.68 46.55 14.88 9.93 0.32 2.18 11.08 272.07 7.32 92.33 491.38 25.64 0.61 17.05 403.74 389.08 36.38 228.28
13.54 30.39 73.16 25.44 10.99 0.43 3.47 16.76 532.57 23.13 143.41 495.54 27.65 0.85 20.54 458.10 486.91 53.03 357.75
34.3 26.4 1700 710 1.50 2.40 24.30 920 52 92 39.7
182
--
aDerived from values given in Ref. 5 on the basis of a run-off proportion of 4.4% of a yearly rainfall of 546 mm.
470 same yearly precipitation depth. A comparison of the yearly pollution loads measured at Weinsberg with the loads calculated for the impervious highway section at Pleidelsheim shows that the porous asphalt can be characterised as a porous structure acting in several ways. The yearly filterable solid loads at Weinsberg are approximately 50% of those at Pleidelsheim. That means that particles are detained by the porous asphalt. Thus the porous structure acts as a filter medium. A description of the interactions occurring within the porous asphalt is difficult, because of the many and diverse substances that are detained in the pores. Closer examination of the deposits in core samples is needed to give full information about the origin of substances involved (abrasion of the road, tyres, mineral structure materials, salts, traffic-related substances). When compared with the findings at Pleideslheim, the yearly loads of PAHs and mineral oil in the permeate of the porous asphalt at Weinsberg indicate that substances having chemical properties and behaviour similar to bitumen are most likely to be detained by the porous asphalt. Penetration of PAHs and mineral oil into the porous structure should not, however, take place on a large scale, for in this case the porous asphalt would lose its stability. The higher NH4-N loads in the winter as well as the higher loads of SO4, chlorine, calcium and magnesium were caused by the use of wet thawing salt. This is also true for all the heavy metals with the exception of lead. Loads of NO3-N and NO:-N seem to be influenced by biochemical processes (nitrification) taking place within the pores of the pervious asphalt. In the summer the percentage of oxidised nitrogen load to total inorganic nitrogen load was nearly 68%, and about 51% in the winter. Since oxygen is required for nitrification, there are
G. Stotz, K. Krauth / Sci. Total Environ. 146/147 (1994) 465-470
reasons for assuming that because of their greater permeability, 'younger' porous asphalts have higher nitrification capacities. The spreading of wet thawing salt onto the porous asphalt at Weinsberg caused a yearly chloride load that was twelve times larger than that calculated for Pleidelsheim. The intention of implanting salts in the porous asphalt could cause serious problems, because of the presence of calcium and SO4 ions, which at sufficient concentrations could combine with other substances to form mortar-like materials. As a result, clogging of the pores could occur, especially within that region of the porous asphalt where the sealing membrane is installed. To confirm these suppositions, the duration of the investigations should be extended to comprise at least two winter periods. In _this way the clogging problems relatedd~to porous asphalts could be studied in d eJail, so that remedial measures could be reco~mlended.
6. Acknowledgements We would like to express our gratitude to the Landesamt fiir Strassenwesen of the province of Baden-Wfirttemberg (LfS) for financial support. We are especially obliged to K. Fischer (LfS) for his engagement within the context of the project.
7. References 1 2 3 4 5 6
E. Thelen and L.-F. Howe. Porous Pavement. Franklin Institute Press, 1967. G. Stotz. Schriftenreihe des Fachgebietes Siedlungswasserwirtschaft Universit~it Gesamthochschule Kassel, Bd. 7 (1991) 225. G. Raimbaultand J.D. Balades. RGRA (1987) 644. W. Hogland, J. Niemczynowiczand T. Wahlmann.Sci. Total Environ. 59 0987) 411. G. Stotz. Sci. Total Environ. 59 0987) 329. H. K6ster. Drainasphalt, Schriftenreihe des IVT ETH Ziirich (1990) 85.
ELSEVIER
The Scienceof the Total Environment 146/147 (1994) 465-470
I
The pollution of effluents from pervious pavements of an experimental highway section: first results G. Stotz*, K. Krauth lnstitut fur Siedlungwasserbau, Wassergiiteund Abfallwirtschaft der Universitiit Stuttgart, Bandtdle I. 70569 Stuttgart, Germany
Abstract
The use of pervious pavements as covering materials for streets has increased. The reason for the use of such pavements is their ability to minimise noise, aquaplaning and water spreading by tyres during rainfall events. Two years ago the province of Baden-Wiirttemberg constructed an experimental highway section to obtain information about the noise-decreasing effects as well as the influence of winter maintenance on the physical behaviour of porous road pavements. In parallel, at the end of 1989 investigations were begun to examine the chemical properties of the drainage water passing through the upper 40 mm of the pervious layer (porous asphalt) with a porosity of up to 20 vol%. Key words." Porous asphalt; Permeability; Properties of the permeate; Pollution loads
1. Introduction
More than three years ago the construction of highways and roads covered with porous surfaces was begun in Germany. Porous asphalts, originally developed for airfields, are expected to be of great advantage in the following respects: Noise reduction. Continually increasing traffic is also combined with a steady increase in noise emissions from tyres and engines. Street covering materials containing a high proportion of air pores are assumed to be sound-absorbing within certain frequency ranges and thus to be noise-reducing. Avoidance of aquaplaning. In the middle of the 1960s the Franklin Institute [1] carried out investigations that showed a very high water perme* Corresponding author.
ability of porous asphalts. The permeability values measured on core samples exceeded the maximum precipitation values of heavy storm intensities. These results justified the assumption that no water films would form on porous asphalts. Avoidance of water-spreading by tyres. If precipitation falling on the street surface is drained off immediately, no water-spreading by vehicles tyres would occur. The elimination of aquaplaning as well as the improvement of visibility for all drivers during rainfall events reduces accident risk. While these advantages are significant for traffic engineers, there are nowadays new concepts in urban storm drainage to be considered for storm run-off reduction and peak flow avoidance [2]. The susceptibility of porous asphalts to becoming clogged is a serious problem [3,4]. The properties of effluents from laboratory-scale unit superstruc-
0048-9697/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0048-9697(93)03500-2
466
tures have been analysed chemically and described elsewhere [4]. The noise reduction efficiency of porous asphalts is well known and is still being investigated, but up to now, detailed and systematic investigations of the chemical properties of the permeates of porous asphalts have not been carried out.
2. Objectives Insufficient knowledge of the permeability behaviour of porous asphalts and the water pollution resulting from precipitation passing through the porous asphalts caused the Landesamt ffir Strassenwesen of the province of Baden-Wiirttemberg to order the Institute ffir Siedlungswasserbau, Wasserg/ite- und Abfallwirtschaft of the University of Stuttgart to carry out investigations on the experimental highway section of federal highway A6. The results of these measurements were also to be compared with those of earlier investigations carried out on sealed highway surfaces [5]. Winter maintenance effects were also to be evaluated.
G. Stotz, K. Krauth/ScL Total Environ. 146/147 (1994) 465-470
Table 1 Data relating to porous asphalt and investigated highway section Porous asphalt age (years) Thickness of covering (mm) Mineral particle size (mm) Porosity (vol%) Length (m) Width (m) Surface area (ha) Cross-slope (%) Vertical slope (%) Average daily traffic (vehicles) Proportion of trucks (%) Investigation period (months)
=2.5-3.5 4 0/8 19.1 50.00 11.25 0.055 1.5 1.2 34 675 25 12
measured at 34 675 vehicles per day, including a heavy vehicle proportion of 25%. The investigations were carried out over twelve months from April 1990 to April 1991. At the beginning of the measuring period the porous asphalt had been in operation for 2.5 years.
3. Description of the experimental highway section
4. Methods of measurement and analytical procedures
The experimental highway section is on the A6 highway from Mannheim to Stuttgart, between the Neckarsulm entrance and the Weinsberg interchange at the Sulmtal parking area. The highway section, originally constructed only for noise reduction studies, is subdivided into two types of porous asphalt paving. The results presented here relate to the measurements carried out on the 0/8 mm porous asphalt section. The highway consists of two lanes and a hard shoulder. For permeate measurement purposes the hard shoulder was equipped with a concrete gully arranged parallel to the right-hand boundary and 50 m in length. The gully was sealed so that the direct penetration of precipitation and surface run-off was impossible. Thus only that portion of precipitation was drained off and measured that had passed through the porous asphalt with a surface area of 0.055 ha. The cross-slope of the road surface is 1.5% and the vertical slope is 1.2%. The thickness of the porous asphalt amounts to 40 mm, and it has a porosity of 19.1 vol% after construction. In 1990 traffic was
Flow depth in a stainless steel measuring channel with a 15° triangular weir at its end together with precipitation data were monitored continuously and recorded by a computer. The sampling procedure was also computer-controlled as well as the data evaluation. All samples taken during a run-off event were combined to give one composite sample (taken so as to be flowproportional), and this was analysed for heavy metals, mineral oil, filterable solids, COD, polycyclic aromatic hydrocarbons (PAH) (as six combined PAHs and fluoranthene), NH4-N, NO2-N, NO3N, pH, acid capacity, chlorine, magnesium, calcium and SO4. All methods and procedures used were identical to those of earlier studies [5], in order to facilitate comparability of the results. The field measurements involved infiltration rate measurements on both the left and right lanes using a double-ring infiltrometer commonly employed in hydrological methods. The infiltration was measured at a constant pressure height of 30-40 mm. Tap water, after addition of a small
G. Stotz, K. Krauth / Sci. Total Environ. 146/147 (1994) 465-470
amount of a surfactant to reduce its surface tension, was used instead of rainwater. To avoid traffic delays, these measurements were carried out only twice a year (in the summer and the winter). 5. Results
5.1. Permeability Volume balance measurements of precipitation and effluent from the porous asphalt for 1 year, including both the summer and winter months, gave the results listed in Table 2. During the summer (from May to October) the precipitation height amounted to 309 mm, and a corresponding run-off of 116 m3/ha was measured. This means that only 3.7% of the precipitation passed through the porous asphalt. In the winter (from November to April) 5.3% of a 237-mm precipitation permeated the porous pavement (i.e. a run-off of 125 m3/ha). Permeation occurred only if the depth of precipitation for single events was >0.6 mm and rain intensity was >0.5 mm/h in the summer and >0.3 mm/h in the winter. The permeability was surprisingly low, hut the permeate percentage values for the summer and winter months clearly show variations depending on the season, The cold and damp weather conditions in winter kept the porous asphalt moist for a fairly long time. Thus losses within the porous structure of the asphalt were small, and in the winter the hydraulic conductivity was higher than in the summer. The greater losses during the summer were caused by prolonged dryness of the asphalt in combination with higher temperatures. Comparisons were also made between the
measured single-effluent events and calculations using a precipitation run-off model with input values corresponding to those of the experimental highway section. The comparison showed that the drainage behaviour of a porous asphalt is quite different from that of impervious asphalt surfaces. Effluents from the previous porous asphalt at Weinsberg occurred after a delay and lasted for a long time. Peak effluent flows were lowered, but the greater the precipitation or the longer the duration of rainfall preceding the main storm, the earlier they occurred (related to the time of peak precipitation). In all cases, permeate volumes were found to be significantly smaller than the run-off volumes from impermeable surface. In situ measurements of infiltration in both ruts and centre lines of the slow and fast lanes were necessary, because of the very poor permeability of the porous asphalt. The experimental method allowed the determination of the lag times between the time of starting the experiment and the time of beginning infiltration and the time at which the highest rate of infiltration was achieved, respectively (Table 3). In summer the infiltration rates for the ruts were 78 mm/h on the fast lane and only 18 mm/h on the slow lane, which is characterised by a high proportion (25%) of heavy vehicles and by a frequent stop-and-go flow of traffic. At the centre lines the rate in the fast lane (28 mm/h) was lower than in the slow lane (42 mm/h). In winter, infiltration rates measured in the ruts were 18 mm/h and 24 mm/h, respectively. Thus, these rates were nearly
Table 3 Time lags and maximun rates of infiltration Station a
Table 2 Balance of permeability
Precipitation (mm) Permeate (m3/ha) Permeate percentages (%) Min. rainfall height (ram) Min. intensity (mm/h) Average permeation duration (h)
467
Summer
Winter
1 year
309 116 3.7 1.4 0.5 6.4
237 125 5.3 0.6 0.3 6.0
546 241 4.4
Wlfm Wlfr Wlfm Wlfr W2fm W2fr W2fm W2fr
Date 4 Dec. 4 Dec. 11 June I 1 June 4 Dec. 4 Dec. 11 June 11 June
1990 1990 1991 1991 1990 1990 1991 1991
Time lag
Rate
(rain)
(ram/h)
8 28 33 35 10 8 23 37
21 22 39 17 17 17 27 78
aWl, slow lane; W2, fast lane; fm, centre line; fr, rut.
468
G. Stotz, K. Krauth / Sci. Total Environ. 146/147 (1994) 465-470
identical to those found for the centre lines. All measured rates were, in comparison, significantly smaller than values given in the literature [1,4,6]. These results therefore confirm the very poor permeability of the porous asphalt of the experimental highway section. The higher permeability results in winter can be explained by the lag times, which in all cases were shorter in winter than in summer.
levels in the winter might be explained by its low resistance to ultra-violet irradiation and easier biochemical degradation in the summer. However, an explanation must be given for the increased concentrations of traffic-related substances, for example heavy metals, in the winter. Obviously, winter maintenance procedures must also be taken into consideration. It is well known that porous asphalts are usually cold and damp for prolonged periods, because they cool down faster. To avoid ice on the road, wet thawing salts are spread on the asphalt surface as a preventive measure, using solid sodium chloride in combination with calcium chloride salt water. Maintenance vehicles usually spread 10 g of wet thawing salt per square metre of street surface area during one cycle of operation, i.e. 7 g of solid salt and 3 g of salt water. Both constituents of the thawing mixture used were analysed chemically to determine their pollutant contents. The results, listed in Table 5, clearly show that the use of wet thawing salt leads to increased loads not only of the main constituents, chlorine and sodium, but also of heavy metals.
5.2. Properties o f the permeate
The run-off weighted averages of the concentrations of all analysed pollutants are listed in Table 4, categorised as summer, winter and yearly concentrations. When comparing the half-yearly values with each other, only concentrations of filterable solid iron and lead were lower in winter than in summer. All other pollutants demonstrated lower concentrations in summer. Extreme differences were found for chlorine, which is used for winter highway maintenance (summer/winter: 36/3921 mg/l), as well as for calcium (summer/winter: 17/205 mg/l). The higher fluoranthene
Table 4 Concentrations of pollutants (weighted averages) Pollutants
Unit
Summer
Winter
Year
Filterable solids COD Mineral oil PAH (6) Fluoranthene Cd Cr Cu Fe Pb Zn CI Ca Mg SO4 NH4-N NO3-N NO2-N pH Acid capacity
mg/1 mg/1 mg/l ng/I ng/l /zg/l /zg/l #g/1 mg/1 #g/l /zg/l mg/l mg/l mg/l mg/l mg/l mg/1 mg/l
64 84 0.23 83 9 0.9 11 49 2.3 137 441 36 17 2.1 30 0.47 0.85 0.14
49 165 0.37 119 79 2.5 17 88 2.1 58 737 3921 205 4.8 136 3.22 3.11 0.29
0.94
1.98
56 126 0.30 106 46 1.7 14 70 2.2 96 596 2061 115 3.5 85 1.90 2.02 0.22 7-8.3 1.48
mmol/l
469
G. Stotz, K. Krauth/Sci. Total Environ. 146/147 (1994) 465-470
Table 5 Composition of wet thawing salt used for winter maintenace Pollutant
Solid salt NaCI (mg/g)
Salt water CaCI 2 (mg/l)
Amount of pollutant contained in 10 g wet thawing salt
Na Ca SO4 Mg Pb Cu Cr Zn Cd
390 1.6 3.04 0.55 0,004 0.002 0.002 <0.01 0.0002
2900 115 000 350 16 0.8 0.5 0.08 1.2 0.052
2757 mg 1095 mg 27 mg 4 mg 35.5 ~g 18.7 #g 14.8/~g 11.3/~g 1.9 t~g
5.3. Pollution loads
For nearly all pollutants the pollution loads related to 1 ha surface area of porous asphalt were found to be greater in the winter than in the summer (Table 6). This conforms to a higher permeability in the winter. Correlations between pollution loads and dryweather periods or rainfall intensities (valid for sealed surface areas in the findings of earlier in-
vestigations [5]) could not be confirmed for the porous asphalt at Weinsberg. It was also impossible to find significant correlations between the loads of filterable solids and heavy metals as well as between the loads of mineral oil and COD. For purposes of comparison, the yearly pollution loads for Pleidelsheim [5] are listed iaa Table 6, calculated on the basis of run-off proportions similar to those of Weinsberg (4.4.% permeability proportion on a yearly average) and assuming the
Table 6 Pollution loads Pollutants
Unit
Summer
Winter
1 year
Pleidelsheim [5] a
Filterable solids COD Mineral oil PAH (6) Fluoranthene Cd Cr Cu Fe Pb Zn CI Ca Mg SO4 NH4-N NO3-N NO2-N Acid capacity
kg/ha kg/ha g/ha mg/ha mg/ha g/ha g/ha g/ha g/ha g/ha g/ha kg/ha kg/ha kg/ha kg/ha g/ha g/ha g/ha mol/ha
7.42 9.71 26.61 9,56 1.07 0.11 1.29 5.68 260.50 15.81 51.08 4.16 2.01 0.24 3.49 54.36 97.83 16.65 109.47
6.12 20.68 46.55 14.88 9.93 0.32 2.18 11.08 272.07 7.32 92.33 491.38 25.64 0.61 17.05 403.74 389.08 36.38 228.28
13.54 30.39 73.16 25.44 10.99 0.43 3.47 16.76 532.57 23.13 143.41 495.54 27.65 0.85 20.54 458.10 486.91 53.03 357.75
34.3 26.4 1700 710 1.50 2.40 24.30 920 52 92 39.7
182
--
aDerived from values given in Ref. 5 on the basis of a run-off proportion of 4.4% of a yearly rainfall of 546 mm.
470 same yearly precipitation depth. A comparison of the yearly pollution loads measured at Weinsberg with the loads calculated for the impervious highway section at Pleidelsheim shows that the porous asphalt can be characterised as a porous structure acting in several ways. The yearly filterable solid loads at Weinsberg are approximately 50% of those at Pleidelsheim. That means that particles are detained by the porous asphalt. Thus the porous structure acts as a filter medium. A description of the interactions occurring within the porous asphalt is difficult, because of the many and diverse substances that are detained in the pores. Closer examination of the deposits in core samples is needed to give full information about the origin of substances involved (abrasion of the road, tyres, mineral structure materials, salts, traffic-related substances). When compared with the findings at Pleideslheim, the yearly loads of PAHs and mineral oil in the permeate of the porous asphalt at Weinsberg indicate that substances having chemical properties and behaviour similar to bitumen are most likely to be detained by the porous asphalt. Penetration of PAHs and mineral oil into the porous structure should not, however, take place on a large scale, for in this case the porous asphalt would lose its stability. The higher NH4-N loads in the winter as well as the higher loads of SO4, chlorine, calcium and magnesium were caused by the use of wet thawing salt. This is also true for all the heavy metals with the exception of lead. Loads of NO3-N and NO:-N seem to be influenced by biochemical processes (nitrification) taking place within the pores of the pervious asphalt. In the summer the percentage of oxidised nitrogen load to total inorganic nitrogen load was nearly 68%, and about 51% in the winter. Since oxygen is required for nitrification, there are
G. Stotz, K. Krauth / Sci. Total Environ. 146/147 (1994) 465-470
reasons for assuming that because of their greater permeability, 'younger' porous asphalts have higher nitrification capacities. The spreading of wet thawing salt onto the porous asphalt at Weinsberg caused a yearly chloride load that was twelve times larger than that calculated for Pleidelsheim. The intention of implanting salts in the porous asphalt could cause serious problems, because of the presence of calcium and SO4 ions, which at sufficient concentrations could combine with other substances to form mortar-like materials. As a result, clogging of the pores could occur, especially within that region of the porous asphalt where the sealing membrane is installed. To confirm these suppositions, the duration of the investigations should be extended to comprise at least two winter periods. In _this way the clogging problems relatedd~to porous asphalts could be studied in d eJail, so that remedial measures could be reco~mlended.
6. Acknowledgements We would like to express our gratitude to the Landesamt fiir Strassenwesen of the province of Baden-Wfirttemberg (LfS) for financial support. We are especially obliged to K. Fischer (LfS) for his engagement within the context of the project.
7. References 1 2 3 4 5 6
E. Thelen and L.-F. Howe. Porous Pavement. Franklin Institute Press, 1967. G. Stotz. Schriftenreihe des Fachgebietes Siedlungswasserwirtschaft Universit~it Gesamthochschule Kassel, Bd. 7 (1991) 225. G. Raimbaultand J.D. Balades. RGRA (1987) 644. W. Hogland, J. Niemczynowiczand T. Wahlmann.Sci. Total Environ. 59 0987) 411. G. Stotz. Sci. Total Environ. 59 0987) 329. H. K6ster. Drainasphalt, Schriftenreihe des IVT ETH Ziirich (1990) 85.