Environmental Engineering Reference
In-Depth Information
Table 2
Stream data
Sample Au Pt Pd
1 10 nd 5
2 20 40 25
3 5 nd nd
4 5 nd 10
5 15 nd nd
Locations of samples 1-5 are shown
in Fig. 2.
The maximum values in the stream samples occur in sediment collected just
downstream of the bridge carrying the main road. They are lower than in road dust
or gully samples, with maximum values of 40 ppb Pt, 25 ppb Pd, 20 ppb Au and
undetected Rh, Ru, Os and Ir (Table
2
).
Discussion
The increase of average Pt, Pd and Rh with traffic flow, and the remarkable similar-
ity of the pattern of distributions of high, medium and low concentrations of each
of these elements at sites A-K (Fig.
2
), suggests one source from catalytic convert-
ers fitted to vehicles. Hathersage has no point sources for PGE emission as it is
dominantly residential, lacking a hospital or industry, and therefore is a particularly
clear example of a distribution of Pt, Pd and Rh predominantly from one source
type, although there may be minor contributions from people with jewellery and
dental fillings. Therefore the ratios of Pt/Pd and Pt/Rh for road dust can be taken as
broadly characteristic of UK vehicle emission ratios for 2005-2006. Individual
values of Pt, Pd and Rh and ratios in samples collected in winter or summer vary
from the average at each site. This could be due to PGE removal by intermittent
rainfall or variable traffic flow or variable emissions, with occasional loss of a
larger particle of PGE from a catalytic converter. This could give unusually high
values for an individual PGE such as the highest value of 305 ppb Pd collected in
winter 2006 at high traffic flow site F.
Pt/Pd ratios for samples collected during rainfall in winter 2006 have higher Pt/
Pd ratios in gully waste than in road dust, suggesting removal of Pd in the gullies,
perhaps facilitated by the wet conditions during sample collection (Fig.
4
). Similar
increased mobility of Pd over Pt has been observed both in environmental
[9]
and
geological samples
[17-20]
.
Dispersal of Pt, Pd, Rh and Au from roads to gullies and then streams has been
documented
[9, 21-25]
. This is the case for these samples with decreasing values
for all six PGE and Au from road dust, to gully waste and into streams. The low
PGE values in the River Noe are to be expected as the PGE concentrations will be
diluted by the river sediment.
