Geoscience Reference
In-Depth Information
Case study 6.4 Study of road-deposited sediment management: Tampa, Florida, USA
The management of road-deposited sediments (RDS) is an important component of urban
pollution control. The most effective method of control is the removal of RDS through street
sweeping, and disposal. There have been very few systematic studies of the effectiveness of RDS
sweeping on improving urban runoff water quality. An exception is a study undertaken for the
city of Tampa in Florida, reported in Brinkmann & Tobin (2003). Generally, street-sweeping
frequency and policy for individual cities is not regulated by scientific information, and this
study set out to address this.
Tampa is a mid-sized city, containing sectors of industry, commerce and medium-density
residential housing. To assess the compositional variability of RDS within the city, street-
sweeping samples were taken from residential, commercial and industrial areas of the city
and analysed. The RDS from industrial areas exhibited the highest levels of zinc, copper and
barium, and was linked to industrial activity (Case Table 6.4). Levels of strontium, nickel,
chromium and vanadium were highest in commercial areas (Case Table 6.4) and this was
linked to the increased vehicular activity in these areas. Overall, RDS element levels were
mostly considered to be non-harmful, but concern over levels of copper, lead and zinc were
raised. The fine-grained fraction of the RDS was greater in industrial and commercial areas
than in residential areas, and this was linked to increased combustion of fossil fuels and
industrial emissions. Organic matter was higher in residential areas, due to increased vegeta-
tion, and available phosphorus was also higher in these areas, probably due to increased garden
fertilizer use.
Compositional analysis showed that the RDS sweepings removed from the city were com-
posed predominantly of inert sands, cement and organic matter. Combined with a high nutrient
content it was concluded that in many cases these street sweepings could be recycled as topsoil
and in soil amendment. For those RDS containing high metal levels however, such a use was not
appropriate. If the sweepings were treated by removing the fine fraction (
<
63
μ
m), however,
then metal levels would be reduced enough to allow recycling.
A further aspect of this study was to look at the impacts of street-sweeping removal of RDS
on sediment and water contamination levels. Sectors of the city were swept either every two
weeks, weekly, twice weekly, or not swept at all. Three conclusions were reached. First, street
sweeping was most effective at reducing RDS accumulation when carried out on a weekly basis.
Lower levels of sediment and contamination were found on frequently swept surfaces. Second,
street sweeping does result in a marked reduction in contaminant levels in runoff, with the most
significant improvements seen with sweeping on a twice-weekly basis. Finally, maintaining a
weekly street sweeping schedule is much more effective at reducing sediment and water pollu-
tion levels than other factors, such as weather conditions. The conclusion that can be drawn
Case Table 6.4 Compositional variation in road-deposited sediment in the city of Tampa. (From Brinkmann & Tobin 2003.)
Zone
Copper
Zinc
Lead
Mass per cent
Organic matter
Available
(ppm)
(ppm)
(ppm)
< 63 μ m
(%)
phosphorus
Industrial
125
96
65
27.3
5.57
353
Commercial
23
79
61
24.0
2.18
346
Residential
24
59
72
19.0
6.62
520
 
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