Dynamic Release of Solutes from Roof Bitumen Sheets Used for Rainwater Harvesting

Abstract

Bitumen waterproof sheets are widely used to seal building roofs. Previous works have focused on the mechanical-physical properties of bitumen sheets, as well as their aging and degradation processes, and their impact on sealing properties of the buildings. Due to a growing need over recent years to use rooftops in urban environments for rainwater harvesting purposes, it is highly important to better characterize the quality of the harvested water from the bitumen covered roofs, and to shed more light on the impact of bitumen degradation processes on the release of various components to the harvested roof water. In the present study, the extracted organic and inorganic solutes from bitumen-covered roofs by water flow on the bitumen sheets were examined through a series of experiments, including measurements from the roofs of buildings in the center of Israel during the winter of 2019–2020. The results indicated high levels of organic and inorganic solute loads in the roof water during the first flush of the first rain of the winter, with maximal electric conductivity readings at the order of 4 dS/m. However, it was shown that following the first flush, a ~20 mm of cumulative rainfall was sufficient to wash off all the summers’ accumulated solutes from the roof. After this solute flushing of the roof, harvested rainwater along the winter was of good quality, with electric conductivity readings in the range of 0.04–0.85 dS/m. Moreover, it was shown that bitumen sheets which were exposed to direct sun radiation emitted greater loads of solutes, likely a result of elevated aging and degradation processes. The findings of the present research point to the need to find efficient ways to isolate roof bitumen sheets from direct sun radiation and to design rainwater harvesting systems that will not collect the water drained from the first flush.

Full text

water
Article
Dynamic Release of Solutes from Roof Bitumen Sheets Used
for Rainwater Harvesting
Uri Nachshon 1, * , Meni Ben-Hur 1,2 , Daniel Kurtzman 1, Roee Katzir 1, Lior Netzer 1,2,3, Guy Gusser 3
and Yakov Livshitz 3


Citation: Nachshon, U.; Ben-Hur, M.;
Kurtzman, D.; Katzir, R.; Netzer, L.;
Gusser, G.; Livshitz, Y. Dynamic
Release of Solutes from Roof Bitumen
Sheets Used for Rainwater
Harvesting. Water 2021,13, 3496.
https://doi.org/10.3390/w13243496
Academic Editors: Cristina M.
Monteiro and Cristina Matos Silva
Received: 17 November 2021
Accepted: 6 December 2021
Published: 8 December 2021
Publisher’s Note: MDPI stays neutral
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Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1Institute for Soil, Water and Environmental Sciences, ARO, Volcani Research Center,
Rishon Le-Tsiyon 7505101, Israel; meni@volcani.agri.gov.il (M.B.-H.); daniel@volcani.agri.gov.il (D.K.);
roeek@volcani.agri.gov.il (R.K.); Liorn@water.gov.il (L.N.)
2Department of Geography and Environmental Development, Ben Gurion University of the Negev,
Beer Sheva 8410501, Israel
3Israeli Hydrological Service, Israeli Water Authority, Jerusalem 9195021, Israel; GuyG@water.gov.il (G.G.);
YakovL20@water.gov.il (Y.L.)
*Correspondence: urina@volcani.agri.gov.il
Abstract:
Bitumen waterproof sheets are widely used to seal building roofs. Previous works have
focused on the mechanical-physical properties of bitumen sheets, as well as their aging and degrada-
tion processes, and their impact on sealing properties of the buildings. Due to a growing need over
recent years to use rooftops in urban environments for rainwater harvesting purposes, it is highly
important to better characterize the quality of the harvested water from the bitumen covered roofs,
and to shed more light on the impact of bitumen degradation processes on the release of various
components to the harvested roof water. In the present study, the extracted organic and inorganic
solutes from bitumen-covered roofs by water flow on the bitumen sheets were examined through
a series of experiments, including measurements from the roofs of buildings in the center of Israel
during the winter of 2019–2020. The results indicated high levels of organic and inorganic solute
loads in the roof water during the first flush of the first rain of the winter, with maximal electric
conductivity readings at the order of 4 dS/m. However, it was shown that following the first flush, a
~20 mm of cumulative rainfall was sufficient to wash off all the summers’ accumulated solutes from
the roof. After this solute flushing of the roof, harvested rainwater along the winter was of good
quality, with electric conductivity readings in the range of 0.04–0.85 dS/m. Moreover, it was shown
that bitumen sheets which were exposed to direct sun radiation emitted greater loads of solutes,
likely a result of elevated aging and degradation processes. The findings of the present research
point to the need to find efficient ways to isolate roof bitumen sheets from direct sun radiation and to
design rainwater harvesting systems that will not collect the water drained from the first flush.
Keywords: rainwater harvesting; water quality; bitumen sheets; first flush
1. Introduction
1.1. Hydraulic Sensitivity of the Urban Environment
Recent years, including the summer of 2021, have shown the hazardous and immediate
effect that global climate change has on our society and the natural environment [
1
,
2
]. It is
well known that climate change is already affecting our planet, and a persistent increase
in average temperatures is being observed, alongside increased occurrences of climatic
extremes, such as droughts, extreme rain events, and fatal floods [3,4].
Under the conditions of the global change, urban areas are becoming a sensitive
environment that is highly affected by the changing climate, with many aspects associated
with the hydrological cycle. Cities have a great impact on the hydrology of their region
as the consumption of good quality water is high, and the dense population produces
high volumes of sewage, increasing the risk of natural water resource contamination.
Water 2021,13, 3496. https://doi.org/10.3390/w13243496 https://www.mdpi.com/journal/water

Water 2021,13, 3496 2 of 17
Moreover, in the urban environment, there is a major reduction of rainwater infiltration
and groundwater recharge due to the existence of large impervious areas and changes
in the pattern of surface runoff flows [
5
]. As a result, during rain events, large volumes
of water that, under natural conditions, could have enriched surface water bodies and
below-ground aquifers, are being diverted into the municipal drainage system (MDS) and
lost. As already seen in many places worldwide, these hydrological changes result in
high peak flows and large runoff volumes during rain events [
1
,
4
], which increase the
risk of economic, environmental, and life-threatening hazards, such as city floods and the
overburden of the MDS [6].
The world’s population is becoming concentrated in the cities, with more than 50%
of the total world’s population and more than 75% of the population in North America,
Europe, and Oceania living in cities [
7
,
8
]. Consequently, there is a growing need to move
toward the goal of efficient and appropriate use and management of natural water resources
in urban environments, mainly in arid and semi-arid climates [
9
]. For this purpose, one of
the practices that has gained popularity over recent years in urban environments is the use
of rainwater harvesting (RWH). RWH, which is the collection, storage, transport, and use
of rain water for different purposes [
10
], has been discussed in the scientific literature and
applied in several places worldwide [6,9–15].
1.2. Rain Water Harvesting (RWH) and Water Quality
In the past, RWH consisted of rainwater collection systems where the harvested
water was usually stored in underground cisterns or used directly for irrigation [
6
,
16
,
17
].
Nowadays, RWH, at the single building scale, consists of rainwater collection from large
surfaces, mainly rooftops [
18
]. The collected water can be stored in below- or above-ground
reservoirs [
9
,
19
,
20
]. The collected water can also be injected into the subsurface or used
to recharge groundwater via designated infiltration basins and wells [
6
,
12
,
20
–
22
]. The
quality of the harvested water, which is mainly affected by the quality and state of the
water collection surfaces and delivery systems, determines whether the water can be used
for groundwater recharge (directly to the aquifer or infiltrating through the unsaturated
zone), direct drinking, domestic uses (car wash for example), or irrigation.
Numerous studies have explored the quality of the harvested water from different
roof types and for different types of contaminates, including heavy metals [
23
,
24
], organic
contaminants [
25
,
26
], inorganic substances and suspended solids [
27
–
29
], and the presence
of microorganisms and fecal coliforms [
24
,
30
]. From these works, it appears that different
roof types (e.g., asphalt fiberglass shingle, bitumen sheets, metal, concrete tile, green roof)
have different impacts on the quality of the harvested water. Nevertheless, most studies
have reported low water quality of the first flush in the beginning of the rainy season. It
was explained that the first flush washes off the contaminants that were accumulated over
the roofs during the dry season. These contaminates may include dust and atmospheric
pollutants that settled down, birds and animal feces, and other organic components, such
as leaves and dead rodents [
26
–
28
,
31
]. In addition, roof construction materials may also
undergo various chemo-physical degradation process, which may lead to the release of
various pollutants to the harvested rainwater [32–34].
1.3. Bitumen Sheets and Water Quality
Bitumen is a black adhesive material produced from crude oil. Currently, roof sealing
with bituminous products is a very common practice worldwide, with hundreds of thou-
sands of tons of bitumen being used for this purpose [
35
]. Several works have indicated
the high potential of roofs covered by different types of petroleum bitumen to deteriorate
harvested water quality, including roofs covered with bitumen sheets [
24
–
26
,
29
,
36
,
37
].
Bitumen sheets are flexible layers, typically 4–8 mm thick, composed of bitumen mixed
with different polymers and reinforcing materials, and their use for roof sealing, in mod-
ern buildings is very popular. For example, in Germany, the manufacturing of bitumen
products for roofing membranes and waterproofing sheets represents the second-largest

Water 2021,13, 3496 3 of 17
application for bitumen, following road construction [
38
]. Many bitumen products may
emit polycyclic aromatic hydrocarbons that have mutagenic and carcinogenic proper-
ties [
29
,
37
,
38
]. Bucheli et al. [
25
] detected high levels of the R-mecoprop herbicide and its
S-enantiomer in runoff water from bitumen roofs. It was hypothesized that the herbicide
was added to the bitumen to prevent root penetration and plant growth over the roofs. In
addition, bitumen roofs exhibited high emission rates of inorganic components, including
major ions (e.g., Ca, K, Mg, Na, P, and S) and metals [29,36–38].
The Bitumen sheets (BS) on top of the roofs are exposed to diverse and sometimes
harsh environmental conditions. These conditions include high temperatures during the
summer months, high moisture, and low temperatures during the winter. In addition, many
roofs are exposed to high levels of sun radiation. Several works have studied the impact
of these environmental conditions on the degradation and accelerated aging processes of
the BS. However, these works have focused on the physical and mechanical properties of
the BS [
39
–
42
], ignoring the environmental aspects that these degradation processes may
have, such as the emission of pollutants to the environment. Since the main purpose of
BS is to physically seal the roof surfaces, it is well understood why the physical properties
of the sheets are so important and well-studied. However, as the rooftops start to act as
operational surfaces to harvest rainwater, more aspects of the BS need to be explored.
As detailed above, several works have indicated the high potential of BS to emit
solutes and toxic substances to the environment. However, the impact of the environmental
conditions on the release of these substances and their transport dynamics by harvested
rainwater are still far from being fully understood. Recently, Müller et al. [
43
] discussed
the potential impact that environmental conditions, mainly precipitation and temperatures,
may have on emission of different substances and pollutants from different roof types,
including roofs covered by bituminous products. This study attempts to shed more
light on the dynamics and processes of solutes emission from BS to harvested rainwater
under different environmental conditions, focusing on the impact of sun radiation on
solutes emission.
2. Materials and Methods
This study consists of the following parts: (1) water sampling (survey) of the harvested
rainwater from BS-covered roofs, and (2) a controlled experimental study to characterize
the emission of different solutes from BS to the water phase under various environmental
conditions. Herein, the terms ‘roof water’ and ‘leachate’ are used to denote water that
interacted with BS on rooftops and was surveyed, and water that interacted with the BS in
the experimental section of the work, respectively.
2.1. Roof Water Survey, Sampling and Analysis
The initial evaluation of the quality of harvested water from roofs covered with BS
was conducted by sampling roof water from gutters of four BS-covered roofs in the center
of Israel during the winter of 2019–2020 (Table 1). The average annual precipitation in
the region is 550 mm, and extreme storm events may have high precipitation rates in the
order of 100 mm/d (~once every 10 years). The rainy season usually begins in October
and ends in April. Water samples were collected during the very first flush of the season
at the first rain event (19 October 2019) from all roofs, excluding roof #3. Throughout
the rest of the season, samples were collected in the middle of each rain event, excluding
roof #1, where water samples were taken in the beginning and the end of each event. In
addition, at the first rain event of the season, water samples were collected from roof #1
every 30–60 min to observe the temporal changes of water quality throughout the entire
event. The precipitation depth was measured at the sites of roofs #1 and #3 using a tipping
bucket rain gauge (RainWise RainLogger, Rainfall Data Logging System, Boothwyn, PA,
USA). In addition, rainwater samples were collected in February 2020, and the electric
conductivity (EC) values in the rainwater samples were determined by EC meter. The EC

Water 2021,13, 3496 4 of 17
values indicated the electrolyte concentrations in the water samples, which correlated with
the salinity degree of the samples.
Table 1. Locations of the four roofs used for water sampling.
# Location Description Coordinates Period Roof Age
1 Givat-Brener Private house 31◦52015.500 N
34◦48001.100 E19 October 2019–10 February 2020 13 years
2 Rishon-LeTsiyon-1 Industrial building 31◦59030.600 N
34◦49012.200 E19 October 2019–10 January 2020 >15 years
3 Rishon-LeTsiyon-2 Industrial building 31◦59028.200 N
34◦49013.400 E
30 November 2019–10 January 2020
>15 years
4 Nezer-Sereni Private house 31◦55026.900 N
34◦49026.300 E19 October 2019–10 January 2020 1 year
The collected roof water was filtered through a 0.22
µ
m filter and analyzed for con-
centrations of Na, Ca, Cl, total organic carbon (TOC), and EC. The total dissolved solids
concentration (TDS) was calculated upon EC readings, as detailed by Rhoades [
44
]. The
instruments used for the chemical analyses are detailed in Table 2.
Table 2. Examined elements in the water samples and analytical tools.
Examined Element Analytical Tool
Na
Flame photometer (Model 420) Clinical Flame Photometer,
Sherwood Scientific Ltd., Cambridge, UK
Ca Absorption Spectrometer Aanalyst 400, PerkinElmer Inc.,
Waltham, MA, USA
DOC/TOC Total Organic Carbon Analyzer (TOC-L) + Total Nitrogen
Measuring Unit (TNM-L), Shimadzu, Kyoto, Japan
EC pH, mv, Conductivity/TDS/C◦/F◦, Eutech PC 450,
Thermo-Fisher, Waltham, MA, USA
Turbidity
Turbidimeter, Eutech TN-100, Thermo scientific, Waltham,
MA, USA
Metals 720-ES ICP Optical Emission Spectrometer (ICP-OES),
Varian, CA, USA
2.2. Controlled Experimental Study
An industrial bitumen sheet, which is used worldwide for roof sealing, was used in
this study (BITUMPLAST, 4 mm white, Vyberg, Russia). Pieces of the BS were placed under
different environmental conditions for varied time periods and immersed into distilled
water (DI) to examine the release of different substances from the BS to the water phase.
The four examined environmental conditions included: (i) open air in the laboratory, with
no exposure to direct sun light. Herein, this will be referred to as ‘Lab’; (ii) in a dark oven
at 40
◦
C. Herein, this will be referred to as ‘Oven’; (iii) on the roof of the laboratory (roof #3
in Table 1), shaded by a wooden surface positioned 5 cm above the BS. Herein, this will
be referred to as ‘Shade’; and (iv) in adjacent to the ‘Shade’ samples on the roof, with no
shading. Herein, this will be referred to as ‘Sun’.
The BS pieces were 2 cm wide and 7 cm long. In total, 12 BS pieces were placed in
the different environmental setups, as detailed above. On 9 May 2021 and every week, for
4 consecutive weeks, three pieces (three replicas) were taken to the laboratory for analysis,
as depicted in Table 3. For the analysis, each one of the BS pieces was inserted into a 50 mL
plastic tube, which was filled with DI water, and then shaken for 3 h. Following this, the

Water 2021,13, 3496 5 of 17
leachate water from each tube was filtered through a 0.22
µ
m filter and the EC, turbidity,
and dissolved organic carbon (DOC) concentration were measured, as detailed in Table 2.
Table 3.
Time scheme of the BS experiments. Non-bold and bold checkmarks (
√
/
√
) indicate operations on the small and
large BS pieces, respectively.
Temporal
Information
Date 9 May 2021 16 May 2021 23 May 2021 30 May 2021 6 June 2021 6 July 2021
9 September 2021
days from T0 0 7 14 21 28 58 123
Days from last
exposure to
rain
--------- --------- --------- --------- --------- 30 65
Action
Small pieces
7 cm ×2 cm
12 pieces (for
each setup)
put in place
Three pieces
from each
setup taken
for analysis
Three pieces
from each
setup taken
for analysis
Three pieces
from each
setup taken
for analysis
Three pieces
from each
setup taken
for analysis
--------- ---------
Large pieces
30 cm ×50 cm
All pieces
put in place --------- --------- ---------
All pieces
taken to rain
simulator
and returned
to place
All pieces
taken to rain
simulator
and returned
to place
All pieces taken
to rain simulator
Analyses
EC √ √ √ √ √/√ √ √
Turbidity √ √ √ √ √/√ √ √
DOC √ √ √ √ √/√--------- ---------
Metals --------- --------- --------- --------- √--------- ---------
Major ions √ √ √ √ √/√ √ √
In addition, larger BS, 50 cm long and 30 cm wide each, were positioned next to the
small pieces for a longer period of time and used to study the release of the substances
under conditions of a simulated rain event (details below). These big BS pieces were tested
over three repeated rain events, with dry periods of several weeks in between. Following
each simulated rain event, the BS pieces were returned to their locations at the different
environmental setups. Table 3details the time scheme of the large BS experiment. The
simulated rain events were conducted in a Morin-type rainfall simulator, which enables
the simulation of rainstorms with different rainfall intensities on inclined surfaces [
45
]. The
simulated rain events were of a 43.0 mm cumulative rain, with rain intensity of 48.0 mm/h.
During the simulations, the BS were positioned in slopes of 5
◦
, and the surface runoff water
was sampled every few seconds during the first ~10 min of the simulation, as well as every
few minutes following that. The collected leachate samples were filtered and analyzed for
EC, DOC, and turbidity, as detailed above and in Table 2.
As aforementioned, sun radiation is assumed to have an important role in the degrada-
tion processes of the BS [
40
,
41
,
46
], which may affect the release of different components to
the roof water. Therefore, the ‘Sun’ and ‘Shade’ setups were examined under the conditions
of exposure to direct and indirect sun radiation, which were measured throughout the
period of the study at a meteorological station of the Israeli Meteorological Service, located
~1 km north of roof #3, where the experiment was conducted (data available on line
https://ims.data.gov.il/he/ims/6 (accessed on 13 September 2021)).
3. Results and Discussion
3.1. Roof Water Survey
The sample water from the different roofs indicated a clear pattern of high load of
solutes at the first flush of the first rain event of the season, followed by a sharp reduction
of salinity and concentrations of the different elements in the following rain events of the
season (Figure 1). The EC readings of the first flush samples were two orders of magnitude
higher than sampled rainwater EC (~0.04 dS/m) and the EC reading of the following
sampled roof water during the winter (Figure 1F). The same trend was observed for all

Water 2021,13, 3496 6 of 17
other examined elements (Figure 1). It was also visually observed that the first rain event
generated brownish runoff water from the BS-covered roofs (Figure 2), which was not
observed during the following rain events. This color is likely associated with the release
of organic components from the BS, which correspond to the high measures of TOC at the
first rain event (Figure 1E).
Water 2021, 13, x 7 of 19
Figure 1. Measured concentrations of sodium (A), calcium (B), chlorine (C), TDS (D), TOC (E), and
the electric conductivity (F) of the sampled water from the examined four roofs. The red line in (F)
marks the measured EC of the rainwater sample (~0.04 dS/m). Each data point is of a single sample.
Figure 1.
Measured concentrations of sodium (
A
), calcium (
B
), chlorine (
C
), TDS (
D
), TOC (
E
), and
the electric conductivity (
F
) of the sampled water from the examined four roofs. The red line in (
F
)
marks the measured EC of the rainwater sample (~0.04 dS/m). Each data point is of a single sample.
The high loads of dissolved organic components in the first flush of the rainy season
reflect the buildup of soluble components on the rooftops during the long dry summer,
which were dissolved and washed off the roof by the first flush during the first rain event.
As detailed above, roof #1 was sampled in high temporal resolution so that the temporal
changes of water quality throughout the first rain event could be observed. Figure 3
presents the reduction of sampled water EC during the first rain event of 19 October 2019.
The event length was in the order of 6 h, and the cumulative precipitation was equal to
17.0 mm. This collection of EC measurements indicates the very high solute concentration
of the first flush and the relatively rapid reduction of solute concentration with time. After

Water 2021,13, 3496 7 of 17
~40 min of continuous rain, the EC readings were reduced by more than 60%, and after
~5 h, the measured EC was equal to the measured rainwater EC. In Figure 3, pictures of the
water sampling bottles are also presented. In parallel to the reduction of the sampled water
salinity, the turbidity of the water was also reduced. The first sampled water had a strong
brown color and, with time, the water became cleaner and more transparent.
Water 2021, 13, x 8 of 19
Figure 2. Samples of the brownish roof water obtained at the very first flush. (A) Ponded water at the rooftop, (B) drained
brownish water in the gutter, and (C) collected water at the first flush of the season.
The high loads of dissolved organic components in the first flush of the rainy season
reflect the buildup of soluble components on the rooftops during the long dry summer,
which were dissolved and washed off the roof by the first flush during the first rain event.
As detailed above, roof #1 was sampled in high temporal resolution so that the temporal
changes of water quality throughout the first rain event could be observed. Figure 3 pre-
sents the reduction of sampled water EC during the first rain event of 19 October 2019.
The event length was in the order of 6 h, and the cumulative precipitation was equal to
17.0 mm. This collection of EC measurements indicates the very high solute concentration
of the first flush and the relatively rapid reduction of solute concentration with time. After
~40 min of continuous rain, the EC readings were reduced by more than 60%, and after ~5
h, the measured EC was equal to the measured rainwater EC. In Figure 3, pictures of the
water sampling bottles are also presented. In parallel to the reduction of the sampled wa-
ter salinity, the turbidity of the water was also reduced. The first sampled water had a
strong brown color and, with time, the water became cleaner and more transparent.
Figure 2.
Samples of the brownish roof water obtained at the very first flush. (
A
) Ponded water at the rooftop, (
B
) drained
brownish water in the gutter, and (C) collected water at the first flush of the season.
Water 2021, 13, x 9 of 19
Figure 3. Measured EC in the water collected from roof #1 at the first flush of the season. Time is
from onset of the rain event. The pictured bottles are of the collected water throughout the event to
demonstrate the changes in water turbidity with time.
Monitoring the runoff water from roof #1, over a timespan of 120 days, from October
2019 to January 2020, enabled us to observe that the first flush of each rain event was
slightly more saline than the last flush of the previous event (Figure 4). Similar to the very
first flush of the season, but in a lower magnitude, the short dry periods between the rain
events had a similar effect of buildup of soluble components on the rooftops, which were
dissolved and transported with the drained roof water during rain events.
Figure 3.
Measured EC in the water collected from roof #1 at the first flush of the season. Time is from onset of the rain
event. The pictured bottles are of the collected water throughout the event to demonstrate the changes in water turbidity
with time.

Water 2021,13, 3496 8 of 17
Monitoring the runoff water from roof #1, over a timespan of 120 days, from
October 2019
to January 2020, enabled us to observe that the first flush of each rain event was slightly
more saline than the last flush of the previous event (Figure 4). Similar to the very first flush
of the season, but in a lower magnitude, the short dry periods between the rain events had
a similar effect of buildup of soluble components on the rooftops, which were dissolved
and transported with the drained roof water during rain events.
Water 2021, 13, x 10 of 19
Figure 4. Sampled water EC from roof #1 (A), and associated rain depth data (B).
The components that accumulate on the rooftops, as detailed above, may be a result
of dust and other atmospheric pollutant sedimentation, bird and animal feces, and the
accumulation of other organic components, such as leaves, dead rodents, and birds. In
addition, the degradation processes of the BS used to seal the roofs may also lead to the
release of organic and inorganic solutes from the BS to the liquid water phase.
Whereas the accumulation of dust and atmospheric pollutants, bird and animal feces,
and other organic components is relatively well understood and documented in the liter-
ature [26–28,31], our understanding of the BS degradation processes is more limited.
Therefore, the following experiments were conducted to shed more light on the environ-
mental conditions that accelerate emission of different substances from the BS to the roof
water and to understand the dynamics of removal of these solutes from the roof by the
drained roof water.
3.2. Experimental Study
As detailed above, the experimental study included a set of controlled experiments
with the small BS pieces, which aimed to test the impact of various environmental condi-
tions on the release of different substances from the BS to leachate water. In addition, the
large BS pieces were used with aim to better understand the dynamics of the ‘first flush’,
and the release and removal of the different solutes from the BS by the flowing rainwater.
3.2.1. Small BS Pieces
As detailed above, the BS were exposed to conditions of direct sunlight on the rooftop
(‘Sun’), indirect sunlight on the rooftop (‘Shade’), open air in the laboratory (‘Lab’), and
40 °C in an oven (‘Oven’). The ‘Sun’ and ‘Shade’ BS were exposed to direct and indirect
radiation conditions and air temperatures as presented in Figure 5.
Figure 4. Sampled water EC from roof #1 (A), and associated rain depth data (B).
The components that accumulate on the rooftops, as detailed above, may be a result
of dust and other atmospheric pollutant sedimentation, bird and animal feces, and the
accumulation of other organic components, such as leaves, dead rodents, and birds. In
addition, the degradation processes of the BS used to seal the roofs may also lead to the
release of organic and inorganic solutes from the BS to the liquid water phase.
Whereas the accumulation of dust and atmospheric pollutants, bird and animal fe-
ces, and other organic components is relatively well understood and documented in the
literature [
26
–
28
,
31
], our understanding of the BS degradation processes is more limited.
Therefore, the following experiments were conducted to shed more light on the environ-
mental conditions that accelerate emission of different substances from the BS to the roof
water and to understand the dynamics of removal of these solutes from the roof by the
drained roof water.
3.2. Experimental Study
As detailed above, the experimental study included a set of controlled experiments
with the small BS pieces, which aimed to test the impact of various environmental condi-
tions on the release of different substances from the BS to leachate water. In addition, the
large BS pieces were used with aim to better understand the dynamics of the ‘first flush’,
and the release and removal of the different solutes from the BS by the flowing rainwater.

Water 2021,13, 3496 9 of 17
3.2.1. Small BS Pieces
As detailed above, the BS were exposed to conditions of direct sunlight on the rooftop
(‘Sun’), indirect sunlight on the rooftop (‘Shade’), open air in the laboratory (‘Lab’), and
40
◦
C in an oven (‘Oven’). The ‘Sun’ and ‘Shade’ BS were exposed to direct and indirect
radiation conditions and air temperatures as presented in Figure 5.
Water 2021, 13, x 11 of 19
Figure 5. Environmental conditions during the experiment, measured ~1 km north of the roof loca-
tion, from 9 May 2021 to 9 September 2021. (A) Temperature in shade, (B) direct sun radiation, and
(C) diffuse sky radiation.
The measurements of leachate water EC demonstrated the high impact that the differ-
ent environmental conditions had on the release of solutes from the BS to the liquid water
phase. The highest EC levels were observed in the ‘Sun’ BS, followed by the ‘Shade’ setup
(Figure 6). The ‘Lab’ and ‘Oven’ samples had the lowest levels of solutes released to the
water (Figure 6). In addition, time also affected solute release to the leachate water, as a
constant increase in solute emission from the BS to the water was observed for all treat-
ments, most prominently for the ‘Sun’ treatment. The EC readings of the pieces exposed to
the sun for 4 weeks were almost three-fold higher than the initial conditions, whereas for
the ‘Shade’ setup, the increase over 4 weeks was in the order of 1.6 folds. For the ‘Lab’ and
‘Oven’ setups, it was even lower, in the order of 1.3 folds (Figure 6).
Figure 5.
Environmental conditions during the experiment, measured ~1 km north of the roof
location, from 9 May 2021 to 9 September 2021. (
A
) Temperature in shade, (
B
) direct sun radiation,
and (C) diffuse sky radiation.
The measurements of leachate water EC demonstrated the high impact that the
different environmental conditions had on the release of solutes from the BS to the liquid
water phase. The highest EC levels were observed in the ‘Sun’ BS, followed by the ‘Shade’
setup (Figure 6). The ‘Lab’ and ‘Oven’ samples had the lowest levels of solutes released
to the water (Figure 6). In addition, time also affected solute release to the leachate water,
as a constant increase in solute emission from the BS to the water was observed for all
treatments, most prominently for the ‘Sun’ treatment. The EC readings of the pieces
exposed to the sun for 4 weeks were almost three-fold higher than the initial conditions,
whereas for the ‘Shade’ setup, the increase over 4 weeks was in the order of 1.6 folds. For
the ‘Lab’ and ‘Oven’ setups, it was even lower, in the order of 1.3 folds (Figure 6).
These results indicate that the BS exposed to direct sun released the highest loads of
solutes to the leachate water. It is assumed that this is a result of the accelerated chemical
and physical degradation processes that occur at the BS surface due to exposure to UV
radiation that readily breaks the chemical bonds within the bitumen polymer chains [
46
].
The higher measured EC values of the ‘Shade’ compared to the ‘Lab’ and ‘Oven’ setups
were likely a result of the ‘Shade’ exposure to indirect radiation. However, both ‘Sun’ and
‘Shade’ were exposed to the open atmosphere. Therefore, dust and other atmospheric
pollutants could have settled on these pieces and increased the load of solutes at the

Water 2021,13, 3496 10 of 17
leachate water. To strengthen the notion that the increased EC of these BS was a result
of the bitumen degradation processes and not a result of dust accumulation, changes in
the DOC and metal emissions were also examined. The sources of these components are
less likely to be associated to dust sedimentation, and it is well known from the literature
that BS may emit metals and organic components [
23
–
26
,
47
], as well as other inorganic
components, as a result of the aging and degradation processes [48].
Water 2021, 13, x 12 of 19
Figure 6. Temporal changes in the EC readings of the leachate water from the ‘Lab’, ‘Oven’, ‘Shade’,
and ‘Sun’ bitumen sheets.
These results indicate that the BS exposed to direct sun released the highest loads of
solutes to the leachate water. It is assumed that this is a result of the accelerated chemical
and physical degradation processes that occur at the BS surface due to exposure to UV
radiation that readily breaks the chemical bonds within the bitumen polymer chains [46].
The higher measured EC values of the ‘Shade’ compared to the ‘Lab’ and ‘Oven’ setups
were likely a result of the ‘Shade’ exposure to indirect radiation. However, both ‘Sun’ and
‘Shade’ were exposed to the open atmosphere. Therefore, dust and other atmospheric pol-
lutants could have settled on these pieces and increased the load of solutes at the leachate
water. To strengthen the notion that the increased EC of these BS was a result of the bitu-
men degradation processes and not a result of dust accumulation, changes in the DOC
and metal emissions were also examined. The sources of these components are less likely
to be associated to dust sedimentation, and it is well known from the literature that BS
may emit metals and organic components [23–26,47], as well as other inorganic compo-
nents, as a result of the aging and degradation processes [48].
The DOC levels in the leachate water were examined every week for the ‘Sun’ setup
and at the end of the fourth week for all setups. It is seen that after 4 weeks, the ‘Sun’ setup
DOC levels were greater than 600 ppm, followed by the ‘Shade’ setup, which was in an
order of magnitude lower at the order of 60 ppm. The ‘Lab’ and ‘Oven’ setups had the
lowest DOC reading in the order of 3 and 1.5 ppm, respectively (Figure 7). Similar to the
sampled roof water, the ‘Sun’ leachate water had a yellowish color, which is associated
with high organic loads and elevated DOC levels [23]. In concordance with the EC read-
ings, the DOC levels of the sun-exposed BS pieces increased gradually as the exposure of
the BS to the sun was longer. A linear correlation (R
2
= 0.94) was found between the EC
and DOC concentrations of the ‘Sun’ leachate water (Figure 8), strengthening the assump-
tion that increased EC readings of the ‘Sun’ BS are associated with chemo-physical degra-
dation processes, and not with the dust accumulation on the examined sheets.
Figure 6.
Temporal changes in the EC readings of the leachate water from the ‘Lab’, ‘Oven’, ‘Shade’,
and ‘Sun’ bitumen sheets.
The DOC levels in the leachate water were examined every week for the ‘Sun’ setup
and at the end of the fourth week for all setups. It is seen that after 4 weeks, the ‘Sun’ setup
DOC levels were greater than 600 ppm, followed by the ‘Shade’ setup, which was in an
order of magnitude lower at the order of 60 ppm. The ‘Lab’ and ‘Oven’ setups had the
lowest DOC reading in the order of 3 and 1.5 ppm, respectively (Figure 7). Similar to the
sampled roof water, the ‘Sun’ leachate water had a yellowish color, which is associated with
high organic loads and elevated DOC levels [
23
]. In concordance with the EC readings,
the DOC levels of the sun-exposed BS pieces increased gradually as the exposure of the
BS to the sun was longer. A linear correlation (R
2
= 0.94) was found between the EC and
DOC concentrations of the ‘Sun’ leachate water (Figure 8), strengthening the assumption
that increased EC readings of the ‘Sun’ BS are associated with chemo-physical degradation
processes, and not with the dust accumulation on the examined sheets.
The measured metals concentrations in the leachate water of all examined environmen-
tal setups also indicated the negative effect that direct sun radiation had on the degradation
processes of the BS, which resulted in the release of metals to the water phase (Figure 9).
The emission of Boron (B) was relatively equal for all setups, with observed B levels in
the order of 10–20 ppb in the leachate water. Copper (Cu), iron (Fe), and zinc (Zn) were
not emitted from the ‘Lab’ and ‘Oven’ setups, while for the ‘Shade’ setup, the measured
concentrations of these metals were in the range of 2–10 ppb (Figure 9). For the ‘Sun’
setup, the Cu, Fe, and Zn concentrations were equal to ~10, 100, and 20 ppb, respectively.
Manganese (Mn) was observed to be emitted only from the ‘Sun’ setup with concentrations
in the order of 10 ppb. Even though the overall metal concentrations were relatively low, it

Water 2021,13, 3496 11 of 17
is clearly seen that direct sun radiation, and to some extent the indirect sun radiation, led
to the elevated emission of metals from the BS to the water phase.
Water 2021, 13, x 13 of 19
Figure 7. DOC concentrations for the ‘Sun’ setup throughout the entire length of the experiment
and for the ‘Lab’, ‘Oven’, and ‘Shade’ setups at the fourth week of the experiment.
Figure 8. Linear correlation between the EC and DOC readings of the ‘Sun’ leachate water.
The measured metals concentrations in the leachate water of all examined environ-
mental setups also indicated the negative effect that direct sun radiation had on the deg-
radation processes of the BS, which resulted in the release of metals to the water phase
(Figure 9). The emission of Boron (B) was relatively equal for all setups, with observed B
Figure 7.
DOC concentrations for the ‘Sun’ setup throughout the entire length of the experiment and
for the ‘Lab’, ‘Oven’, and ‘Shade’ setups at the fourth week of the experiment.
Water 2021, 13, x 13 of 19
Figure 7. DOC concentrations for the ‘Sun’ setup throughout the entire length of the experiment
and for the ‘Lab’, ‘Oven’, and ‘Shade’ setups at the fourth week of the experiment.
Figure 8. Linear correlation between the EC and DOC readings of the ‘Sun’ leachate water.
The measured metals concentrations in the leachate water of all examined environ-
mental setups also indicated the negative effect that direct sun radiation had on the deg-
radation processes of the BS, which resulted in the release of metals to the water phase
(Figure 9). The emission of Boron (B) was relatively equal for all setups, with observed B
Figure 8. Linear correlation between the EC and DOC readings of the ‘Sun’ leachate water.

Water 2021,13, 3496 12 of 17
Water 2021, 13, x 14 of 19
levels in the order of 10–20 ppb in the leachate water. Copper (Cu), iron (Fe), and zinc (Zn)
were not emitted from the ‘Lab’ and ‘Oven’ setups, while for the ‘Shade’ setup, the meas-
ured concentrations of these metals were in the range of 2–10 ppb (Figure 9). For the ‘Sun’
setup, the Cu, Fe, and Zn concentrations were equal to ~10, 100, and 20 ppb, respectively.
Manganese (Mn) was observed to be emitted only from the ‘Sun’ setup with concentra-
tions in the order of 10 ppb. Even though the overall metal concentrations were relatively
low, it is clearly seen that direct sun radiation, and to some extent the indirect sun radia-
tion, led to the elevated emission of metals from the BS to the water phase.
Figure 9. Metals concentrations at leachate water from all examined setups in the fourth week of the
experiment. Values of 10
−1
ppb means readings were below the detection limit.
The set of observations detailed above demonstrates the strong effect that direct sun-
light radiation had on the degradation processes of the BS and the resulting emissions of
various organic and inorganic components. However, the observations did not contribute
any information about the dynamics of the release of these substances to rainwater that
flows over the BS, and the removal of these substances from the roof with the drained
water. For this purpose, simulated rain experiments with the larger BS pieces were con-
ducted.
3.2.2. Rain Simulation Experiments
The measured EC values of the drained water from the large BS pieces that were
exposed to simulated rain are in agreement with both the small BS pieces experiments and
the field measurements of the different roofs. The highest EC (1.25 dS/m) was measured
in the very first flush of the first simulated rain event, with a cumulative rain of 1.5 mm
for the ‘Sun’ setup (Figure 10A). This was followed by the ‘Shade’ setup, where the meas-
ured EC was equal to 0.67 dS/m. The ‘Lab’ and ‘Oven’ readings were the lowest, with EC
readings of 0.48 and 0.38 dS/m, respectively. For all setups, a notable reduction in the EC
was observed during the rain event, with the stabilization of drained water EC values, as
well as turbidity (Figure 10D), after a cumulative rainfall of ~20 mm. This is in good agree-
ment with the detailed measurements of roof #1, which showed that the roof water was
Figure 9.
Metals concentrations at leachate water from all examined setups in the fourth week of the
experiment. Values of 10−1ppb means readings were below the detection limit.
The set of observations detailed above demonstrates the strong effect that direct
sunlight radiation had on the degradation processes of the BS and the resulting emissions
of various organic and inorganic components. However, the observations did not contribute
any information about the dynamics of the release of these substances to rainwater that
flows over the BS, and the removal of these substances from the roof with the drained water.
For this purpose, simulated rain experiments with the larger BS pieces were conducted.
3.2.2. Rain Simulation Experiments
The measured EC values of the drained water from the large BS pieces that were
exposed to simulated rain are in agreement with both the small BS pieces experiments and
the field measurements of the different roofs. The highest EC (1.25 dS/m) was measured in
the very first flush of the first simulated rain event, with a cumulative rain of 1.5 mm for
the ‘Sun’ setup (Figure 10A). This was followed by the ‘Shade’ setup, where the measured
EC was equal to 0.67 dS/m. The ‘Lab’ and ‘Oven’ readings were the lowest, with EC
readings of 0.48 and 0.38 dS/m, respectively. For all setups, a notable reduction in the EC
was observed during the rain event, with the stabilization of drained water EC values,
as well as turbidity (Figure 10D), after a cumulative rainfall of ~20 mm. This is in good
agreement with the detailed measurements of roof #1, which showed that the roof water
was solute-free and clear after 4–5 h of precipitation, with a cumulative rain depth of
approximately 20 mm (Figure 4). The first flush salinity of roof #1 was ~50% higher than
the first flush of the ‘Sun’ BS in the first simulated rain event. This is likely a result of
the longer exposure of the roof #1 bitumen to the sun throughout the whole antecedent
Mediterranean summer, during which both degradation processes of the bitumen occurred,
together with the sedimentation of dust and other substances.

Water 2021,13, 3496 13 of 17
Water 2021, 13, x 15 of 19
solute-free and clear after 4–5 h of precipitation, with a cumulative rain depth of approx-
imately 20 mm (Figure 4). The first flush salinity of roof #1 was ~50% higher than the first
flush of the ‘Sun’ BS in the first simulated rain event. This is likely a result of the longer
exposure of the roof #1 bitumen to the sun throughout the whole antecedent Mediterra-
nean summer, during which both degradation processes of the bitumen occurred, to-
gether with the sedimentation of dust and other substances.
Figure 10. Measured EC (A–C) and turbidity (D–F) of drained water obtained for all examined setups at the first rain
simulation (A,D), second rain simulation (B,E), and third rain simulation (C,F).
At the first flush of the second simulated rain event, which occurred 30 days after the
first event (Table 3), a cumulative precipitation of 2 mm generated EC levels of 0.3, 0.2,
0.2, and 0.15 for the ‘Shade’, ‘Sun’, ‘Lab’, and ‘Oven’ setups, respectively (Figure 10B).
Even though first flush salinity of the ‘Shade’ setup was higher than the ‘Sun’ setup, the
reduction of the ‘Shade’ salinity with the proceeding of the rain event was faster than the
‘Sun’ setup, which maintained relatively high and constant EC levels up to a cumulative
rainfall of ~10 mm. All other setups, including ‘Shade’, presented a relatively rapid reduc-
tion in salinity of the drained water, with a notable reduction measured after a cumulative
rainfall of 4 mm. The ‘Oven’ and ‘Shade’ setups reached and stabilized at the lowered
salinity levels, of ~0.05 dS/m, after about 10 mm of rain, whereas the ‘Lab’ and ‘Sun’
reached these EC levels after a cumulative rainfall of 17 and 22 mm, respectively. In re-
spect to drainage turbidity, the ‘Sun’ setup drainage was an order of magnitude higher
than all other setups up to a cumulative rainfall of 4 mm. It was only after 10 mm of cu-
mulative rain when the turbidity of the ‘Sun’ setup decreased to the turbidity levels of
other setups.
A third simulated rain event occurred after an additional 65 days of dry conditions
under the different environmental conditions. The EC levels of all treatments were higher
than their counterparts in the second simulated rain event but lower compared to the first
event. This is likely a result of the longer dry period between the second and third simu-
lated rain events compared with the 30-day dry period between the first and second rain
events.
Figure 10.
Measured EC (
A
–
C
) and turbidity (
D
–
F
) of drained water obtained for all examined setups at the first rain
simulation (A,D), second rain simulation (B,E), and third rain simulation (C,F).
At the first flush of the second simulated rain event, which occurred 30 days after the
first event (Table 3), a cumulative precipitation of 2 mm generated EC levels of 0.3, 0.2, 0.2,
and 0.15 for the ‘Shade’, ‘Sun’, ‘Lab’, and ‘Oven’ setups, respectively (Figure 10B). Even
though first flush salinity of the ‘Shade’ setup was higher than the ‘Sun’ setup, the reduction
of the ‘Shade’ salinity with the proceeding of the rain event was faster than the ‘Sun’ setup,
which maintained relatively high and constant EC levels up to a cumulative rainfall of
~10 mm. All other setups, including ‘Shade’, presented a relatively rapid reduction in
salinity of the drained water, with a notable reduction measured after a cumulative rainfall
of 4 mm. The ‘Oven’ and ‘Shade’ setups reached and stabilized at the lowered salinity
levels, of ~0.05 dS/m, after about 10 mm of rain, whereas the ‘Lab’ and ‘Sun’ reached these
EC levels after a cumulative rainfall of 17 and 22 mm, respectively. In respect to drainage
turbidity, the ‘Sun’ setup drainage was an order of magnitude higher than all other setups
up to a cumulative rainfall of 4 mm. It was only after 10 mm of cumulative rain when the
turbidity of the ‘Sun’ setup decreased to the turbidity levels of other setups.
A third simulated rain event occurred after an additional 65 days of dry conditions
under the different environmental conditions. The EC levels of all treatments were higher
than their counterparts in the second simulated rain event but lower compared to the
first event. This is likely a result of the longer dry period between the second and third
simulated rain events compared with the 30-day dry period between the first and second
rain events.
The DOC levels were measured in the drained water from all BS setups during the
first simulated rain event at the first flush, after about 2 mm of cumulative rain where
salinity and turbidity were maximal, and after a cumulative rainfall of ~30 mm, when
salinity and turbidity were low and stable. Figure 11 presents the measured DOC levels,
and it is clearly seen that the sun-exposed BS released the highest levels of DOC to the
drained water. In agreement with the above-mentioned observations, this again indicates
the major effect that sun radiation has on chemo-physical processes at the BS surface, which
led to enhanced aging and degradation processes of the SB. In turn, this led to the elevated
release of organic and inorganic solutes to the liquid water that flows over the BS.

Water 2021,13, 3496 14 of 17
Water 2021, 13, x 16 of 19
The DOC levels were measured in the drained water from all BS setups during the
first simulated rain event at the first flush, after about 2 mm of cumulative rain where
salinity and turbidity were maximal, and after a cumulative rainfall of ~30 mm, when
salinity and turbidity were low and stable. Figure 11 presents the measured DOC levels,
and it is clearly seen that the sun-exposed BS released the highest levels of DOC to the
drained water. In agreement with the above-mentioned observations, this again indicates
the major effect that sun radiation has on chemo-physical processes at the BS surface,
which led to enhanced aging and degradation processes of the SB. In turn, this led to the
elevated release of organic and inorganic solutes to the liquid water that flows over the
BS.
Figure 11. DOC concentrations in drained water from the different BS setups at the beginning of the
rain event (first flush) and toward the end.
3.3. Results Integration
Both the field survey and experiments indicated the high loads of solutes that were
leached from the BS at the first flush of the rainy season (Figures 1, 3, 4 and 10). It was
shown that sun-exposed sheets emitted the highest loads of organic and inorganic solutes
and metals compared to all other examined environmental conditions (Figures 6, 7 and 9–
11), likely a result of the accelerated aging and degradation processes of the sun-exposed
bitumen sheets [46]. As detailed above, BS may emit toxic substances, such as polycyclic
aromatic hydrocarbons and volatile organic compounds [24], which were not examined
in this work. It should be validated by future studies, but it is believed that the release
dynamics of these toxic substances would be in agreement with the findings of this work,
i.e., high concentrations at the first flush, negligible emission of the toxic components at
the following rain events, and the highest emission from BS exposed to direct sunlight.
As aforementioned, the high loads of solutes in the first flush were a result of the
accumulation of soluble components during the summer on the rooftops, a result of deg-
radation processes of the BS, and the sedimentation of dust and other pollutants. More
studies are needed to better characterize the runoff properties from different roofs as a
Figure 11.
DOC concentrations in drained water from the different BS setups at the beginning of the
rain event (first flush) and toward the end.
3.3. Results Integration
Both the field survey and experiments indicated the high loads of solutes that were
leached from the BS at the first flush of the rainy season (Figures 1,3,4and 10). It was shown
that sun-exposed sheets emitted the highest loads of organic and inorganic solutes and
metals compared to all other examined environmental conditions (
Figures 6,7and 9–11
),
likely a result of the accelerated aging and degradation processes of the sun-exposed
bitumen sheets [
46
]. As detailed above, BS may emit toxic substances, such as polycyclic
aromatic hydrocarbons and volatile organic compounds [
24
], which were not examined
in this work. It should be validated by future studies, but it is believed that the release
dynamics of these toxic substances would be in agreement with the findings of this work,
i.e., high concentrations at the first flush, negligible emission of the toxic components at
the following rain events, and the highest emission from BS exposed to direct sunlight.
As aforementioned, the high loads of solutes in the first flush were a result of the
accumulation of soluble components during the summer on the rooftops, a result of
degradation processes of the BS, and the sedimentation of dust and other pollutants.
More studies are needed to better characterize the runoff properties from different roofs
as a function of roof cover, slope, and other environmental conditions. However, for
the conditions examined here, of a flat or low-slope roof covered with BS, both the field
measurements and rain simulator experiments showed that a cumulative rainfall of about
20 mm was sufficient to wash off the solutes that accumulated over the roof during the
summer (Figures 1and 10). The rain simulator experiments conducted herein (
Figure 10
)
showed that a dry period of about 30 days is sufficient to result in a notable release of
solutes from the bitumen to the roof water, mainly for the conditions of roofs that are
exposed to direct sunlight. However, good quality roof water was measured for shorter
dry periods between the rain events, at the order of 20 days, as observed for the surveyed
roofs (Figures 1and 4).

Water 2021,13, 3496 15 of 17
The sedimentation of the different pollutants over the rooftops and the aging and
degradation processes of the BS are processes that occur at the upper surface of the BS.
These processes arise every long dry period, regardless of the roof washing processes that
may have occurred during the preceding winters and rain events. Therefore, a reduction of
pollutants emission from BS-covered roofs over the years is not expected. This assumption
is supported by the high loads of solutes that were measured at the four roofs monitored
in the survey, which are all older than 10 years, excluding roof #4 (Table 1). Future studies
should clarify this point, but old BS may emit higher loads of solutes and pollutants due
to the natural aging of the BS, which accelerates the chemo-physical degradation process
that leads to the release of various pollutants to the harvested rainwater [
32
–
34
]. This
may be another explanation for the higher EC values measured at the old roofs that were
surveyed compared to the EC readings of the first flushes generated in the rain simulator
experiments from a new BS (Figures 1F and 10A–C).
4. Summary and Conclusions
This work aimed to shed more light on the processes of emission and transport of
organic and inorganic solutes from bitumen sheets used for roof sealing. Studying these
processes is important, as rainwater harvesting from rooftops in urban environments is
becoming a common practice in many cities worldwide, and the characterization of the
harvested water quality has great importance. Even though bitumen sheets are widely
used to seal rooftops in modern cities, our understanding of the impact of these surfaces
on the quality of harvested water from the roofs is still limited.
In this study, four bitumen-covered roofs in the center of Israel were monitored over
the winter of 2019–2020 to characterize concentrations of organic and inorganic solutes in
the roof water throughout a series of consecutive rain events. Following this, two series
of controlled experiments were conducted to better understand the impact of different
environmental conditions on the emission of solutes and metals from the bitumen sheets to
the water phase that flows over them during rain events. The examined environmental
conditions were of: (i) open air in the laboratory with no direct sun radiation; (ii) 40
◦
C in a
dark oven; (iii) shaded conditions on a rooftop; and (iv) sun exposed bitumen sheets on the
same roof.
Both the field survey and experiments indicated the high loads of solutes being flushed
off the bitumen sheets at the first flush of the rainy season. It was shown that sun-exposed
bitumen sheets emitted the highest loads of organic and inorganic solutes and metals
compared to all other examined environmental conditions.
These research findings may suggest that for water quality consideration, rooftops
that are covered with bitumen sheets and used for rainwater harvesting should be shaded
by different means to reduce the release of the different pollutants. Shading methods may
include the spreading of a porous medium such as gravel at a thickness of a few centimeters
on top of the bitumen sheets. Other options may be the use of more environmental and
comprehensive solutions, such as green roofs, or shading of the bitumen sheets by solar
panels. In addition, engineers and designers of rainwater harvesting systems may consider
different ways to divert the first flush water to the municipal drainage system and not to
harvest this water, which may be highly contaminated. Another solution could be to replace
bituminous products by other materials that do not emit pollutants to the environment, or
to improve the formulation of the bitumen sheets to be more environmentally friendly.
Author Contributions:
Writing—original draft, U.N. and L.N.; Writing—review & editing, M.B.-H.,
D.K., R.K., G.G. and Y.L. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by The Israel Water Authority, grant #: 4501847430.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Water 2021,13, 3496 16 of 17
Data Availability Statement:
Raw data + matlab codes used for analyses can be found at: Nachshon,
Uri (2021): BITUMEN DATA. figshare. Dataset. https://doi.org/10.6084/m9.figshare.17134787.v1,
(accessed on 17 November 2021).
Conflicts of Interest: The authors declare no conflict of interest.
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