Having entered the soil environment near roads, contaminants will either be retained in the soil or transported through the soil. Depending on soil characteristics and other environmental conditions, different contaminants are transported with the soil
water through the soil at varying rate. Mobile compounds (such as chloride) move rapidly whereas many heavy metals and organic contaminants move much slower. Often, contaminant concentrations are much higher in the upper soil layers than further down the soil profile.
Sampling of soil water gives a picture of the rate of transportation of contaminants down a soil profile whereas sampling of soil gives a picture of the contaminant quantities having accumulated in the various soil layers over a long period of time.
Seepage and soil water (pore-retained water) can be sampled, although with less ease than groundwater below the water table. There, suction lysimeters (also called tension lysimeters) may be used. In principle, the soil water is sucked out of the soil through a lysimeter body that acts as a membrane or filter (Fig. 7.7). These devices include a high air-entry porous tip inside which a partial vacuum may be applied via a flexible pipe connected to an external vacuum pump. By this means water is pulled into the tip and, after collection, is sampled by gravity when possible or by gas displacement. For heavy-metal sampling, lysimeters should be made of Teflon, glass, PET or other material unable to sorb the metals. The flux of contaminants down the soil profile is often of interest. Since tension lysimeters give information on concentrations only, water volumes have to be measured separately or modelled so as to make calculations of pollutant fluxes possible.
Besides the more classical methods of soil water sampling, alternative road surface infiltration samplers have been designed in which water seeps from the road surface down and goes through separate layers of pavement and embankment towards a circular “funnel” (Sytchev, 1988). The device is installed during pavement construction with the layers of pavement being placed over the top of the sampling inlet. Two layers of siliceous sand of different grain sizes are situated there on fine — mesh screens to prevent entrance of solids to a sampling bottle where the seeping water is collected. The water amount in the sampling bottle is detected by measuring resistance (conduction sensor). There are two small metal pieces in the bottle; resistance between them is different when there is air or water. When a sufficient
amount of water has been collected in the sampling bottle, a gas (commonly N2) is injected into the bottle so as to close the valve under the funnel. The water specimen is then forced back to the surface and runs into the sampling bottle (Fig. 7.8a, b).
I = bore hole in road pavement and embankment
2=cement bed 3=sampling bottle (glass)
4 = plug (plastic)
5 = pipe for gas
6 = pipe for water sample 7=sand filter
8=electrical conduction sensor 9=quick-acting coupling (blue for water)
10 = quick-acting coupling (black for gas)
II = connector for conduction sensor
Soil sampling can be performed in any season, except in periods of frost. Sampling from consecutive soil depths gives information on the displacement of accumulated pollutants down the soil profile. Natural upper soil profiles are usually richer in organic matter content favouring the retention of several pollutants, namely heavy metals and organic pollutants. This pattern should be analysed in order to observe differences in contaminant content and behaviour across a soil profile. Usually, soil samples are taken out using a steel cylinder of a given volume so as to allow volume-related physical and chemical analyses. Just as for water, soil samples should be transported without delay and kept cool.
To collect samples beneath pavements, a core hole will usually be required in the pavement surface. Drilling conventionally uses a water-cooled core cutter, but these should not be used when abstracting samples for chemical assessments as the water will be likely to change the chemical conditions in the underlying ground by introducing contaminants and/or diluting what was already there.
