Methods in the Exploratory Risk Assessment of Trace Elements in the Soil-Groundwater Pathway

Abstract

Groundwater protection starts with soil protection. When soils are contaminated there is always a chance that the groundwater is or will be affected by this contamination as well. Analytical risk assessment is often very time-consuming and expensive. The goal of exploratory investigations is to investigate the risk of groundwater contamination with the minimum possible effort. Exploratory investigations concerning trace elements can be carried out by analyzing soil material, percolation water or groundwater (LABO 2003). While percolation water and groundwater analysis can only confirm a contamination, information gathered by soil analysis permits the current and future contamination risk for the groundwater to be estimated and additionally allows precautionary measures to be planned in order to protect the groundwater. The risk of contamination can only be judged when standards are available for a good quality of soil or percolation water. Local or regional background values of trace elements in the soil or percolation water can serve as adequate standards. This chapter will present and compare the analytical methods applied in exploratory investigations for assessing risks to the soil-groundwater pathway. We start with a little material science regarding trace element analysis in soil science and continue by listing some common methods used to analyze the total content of trace elements in the soil, presenting the conversion functions which allow results from different digestion agents to be translated. Subsequently an aqueous batch extraction method is presented which is routinely used to estimate the soluble fraction of trace elements in the soil for exploratory purposes, and its results are compared with results gained from direct percolation water sampling and a method which adjusts one parameter to in situ conditions.

Appendix

1.1 Construction Manual for Low Sorption Suction Cup Lysimeter with Internal Sample Collection Vessel

The suction cup probe is built for trace element analysis. Thus, carefully deburring and cleaning each item after mechanical manufacturing and before assembly is of vital importance for good results.

The numbers after each item refer to Figs. 5, 6, 7, 8, 9 and Tables 4 and 5 in the appendix.

Fig. 5

Electronic conductivity (EC) in extracts compared to EC in situ

Fig. 6

Suction cup and probe shaft

Fig. 7

Suction cup probe and extension shaft

Fig. 8

Separator

Fig. 9

Components of sample collection vessel (a–c) and of recovery device (d)

Table 4 Material list for low sorption suction cup lysimeter with internal sample collection vessel (items no. 1–12)

Table 5 Material list for low sorption suction cup lysimeter with internal sample collection vessel (items no. 13–28)

1.2 Suction Cup and Probe Shaft

First a female thread is cut into the suction cup (1) to accommodate the suction cup filling (4). The sealing ring (2) is then placed around the male thread of the suction cup (1). Next the male thread is cut into the filling (4) and a silicone sealing ring (3) is placed over the male thread of the filling (4). Afterwards the filling (3–4) and cup (1–2) are bolted together. Then a female thread is cut into the lower end of the Plexiglas tube for the probe shaft (5) to accommodate the male thread of the suction cup (1). Subsequently the probe shaft (5) is joined to the suction cup (1–4). It is very important that the sealing is perfect. Sharp edges on the components might cut into the sealing rings and cause leaks. To avoid leakages the edges need to be smoothed. A leaching test is mandatory in every case.

1.3 Probe Extension

The probe shaft can be extended with additional Plexiglas extension tubes (7). The connection is made with a brass sleeve (6). In order to connect the probe and extension tubes the diameters of the tubes are reduced by dipping the end of the probe shaft (5) in liquid nitrogen for approximately 30 s. The tube is then inserted halfway into the brass sleeve (6). Subsequently the diameter of the extension tube (7) is reduced in the same manner and then the tube is inserted into the other side of the brass sleeve. This joint does not need any further sealing. To add further extensions the above procedure is repeated.

To separate the extension tubes an empty, bottomless bottle whose neck fits snugly onto the tube (“separator”, Fig. 8) is placed around the brass sleeve and filled with liquid nitrogen. The nitrogen reduces the diameter of the extension tube and the extension tube can be pulled out of the brass sleeve. Next the brass sleeve can be removed from the lower (extension) tube.

1.4 Finishing Plug

To assemble the finishing plug (8–12) a Plexiglas capillary (8) is inserted through a hole into the plug (9). As shown in Fig. 7, 10 mm of the capillary (8) extend beyond each end of the plug (9). Then the silicone hose (10) is connected with the outer end of the Plexiglas capillary (8). Next the hose barb (12) is inserted into the end of the silicone hose. The pinchcock (11) is then adjusted to the hose. Finally the assembled finishing plug is plugged into the far end of the (extended) probe. To test for leakages 100 mbar of negative pressure are applied to the system. A change in pressure, e.g. measured with a high-sensitivity manometer, is a sign of a leakage.

1.5 Sample Collection Vessel

First, the following parts need to be manufactured by drilling bores and cutting threads into them so that they resemble the items in Fig. 9: valve (14), base plug (15), finishing plug (19), fixation screw and connecting tube (22). All the manufactured components are then carefully deburred and cleaned. Next the uptake tube (17) is prepared by cutting the material into appropriate lengths. The tubes are then straightened by heating the material to release stress from the material. To do so, some pieces of the uptake tube are threaded into a glass tube with a suitable inner diameter (approximately 2.2 mm) and heated for 2 h at 180 °C in an oven. Heating and cooling the material causes the molecules in the material to rearrange. Stress is thereby released and the material is thus stabilized. To prepare the vessel (18) the FEP tubes are cut to appropriate lengths. All the manufactured items are cleaned so they are ready to use for trace element analysis.

Assembly starts by inserting the trunk (13) approx. 20 mm deep into the valve (14) as shown in Fig. 9a. Then the reducing nozzle (16) is applied at full length onto the uptake tube (17). Subsequently the nozzle (16) is inserted into the bore of the base plug (15). Next the far end of the uptake tube (17) is heated to approx. 180 °C to make it flexible and then bent with the smallest possible inner radius. The thin wall capillaries are easily buckled, so it should be bent very carefully. Next the FEP tube (18) is applied by inserting the uptake tube (17) into the FEP tube (18) and then pressing the FEP tube (18) over the assembled basis stopper (13–17). After that, the finishing plug (19) is inserted into the opposite end of the sample collection FEP tube (18). The diameters of the components are calculated and manufactured with as little play as possible to ensure that the sample collection vessel is mechanically resilient and sealed tight.

1.6 Flow Barrier

The bolt of the male thread of the connecting tube (22) is made a few mm longer than the female thread of the fixation screw (20). First the PTFE filter membrane (21) is gently pulled taut over the bolt of the connecting tube (22). Next the fixation screw (20) is softly screwed onto the connecting tube and then immediately released. Two or three layers of PTFE tape are wrapped around it to fix the membrane (21) onto the male thread of the connecting tube (22). Finally, the connection tube and the fixation screw are screwed back together. The PTFE membrane is permeable to air, but impermeable to water. This way the flow barrier prevents the sample collection vessel from overflowing.

To mount the flow barrier (20–22) with the sample collection vessel the male thread of the fixation screw (20) is screwed into the female thread in the finishing plug. Finally, the valve (14) is screwed into the base plug (15) to finish assembling the sample collection device. If new components are used, the connection between the valve and base plug is leakproof. Frequently opening and closing the valve, however, can widen the play of the thread, possibly creating a leak. A few layers of PTFE tape wrapped around the male thread usually stop the leak.

1.7 Recovery Device

To assemble the ballast of the recovery device a steel rod (24) is inserted into a 40 cm long piece of FEP tube (25) and lower (23) and upper (26) stoppers are plugged into each end. The steel rod should be locked tight by the stoppers. A female thread links the lower stopper (23) to the connection tube (22) of the flow barrier. The additional ballast of the steel rod is needed to pull the nylon rope (which is otherwise too inflexible) straight when the recovery device is lowered through the extension shaft. The nylon rope (28) is connected with the ballast by a union nut. To fasten the rope the nylon rope is inserted through the bore into the union nut (27) and subsequently knotted and pulled tight so the knot is hidden in the union nut (Fig. 9d). To prevent the knot from bursting when the device is used to recover a full sample collection vessel, the knot is heated with a heat gun. Finally, the union nut (27) is tightly screwed onto the upper stopper (26) of the ballast.

To recover a sample collection vessel with the recovery device, the device is slowly lowered through the probe shaft until the lower stopper (23) hits the connection tube (22). The recovery device is the screwed onto the sampling device and pulled up to collect the sample collection vessel.

To install the sampling collection vessel in the suction cup probe, the sampling device is linked with the recovery device and lowered through the probe shaft. The vessel is dropped very lightly for the last few centimetres. If all components are manufactured precisely enough, the trunk (13) should find the bore of the suction cup filling (4) and the valve (14) makes a tight connection with the filling (Fig. 9a). Check whether the sample collection vessel has properly connected with the suction cup by softly pulling on the rope. If it has properly connected some resistance can be felt.

1.8 Preparing a Newly Built Device for Sampling

Before using the sampling device in the field, carefully clean it. A blank check is conducted for every device with ultrapure water. Before using the device all parts that may have contact with the percolation water sample are conditioned with 2 % HNO₃ for at least 2 h and subsequently rinsed twice with ultrapure water. These parts are then stored in ultrapure water until use. The suction cup and the sample collection vessel are protected during transportation against contamination with a plastic cap (e.g. PE bottle with 100 ml wide neck) or PE plastic foil. The protective caps are discarded after each sampling event.

1.9 Cleaning After Sampling

After sampling the device is disassembled into the suction cup (1 and 2), cup filling (3 and 4), valve (13 and 14), finishing plug (19), basic shaft (5) and sample collection vessel (15–19). All components are rinsed with distilled water until no particles are left. The bore of the valve is cleaned mechanically e.g. with a pipe cleaner (non-metallic!). Then each part is thoroughly rinsed with ultrapure water. Next the suction cup, the cup filling, the valve and the finishing plug are stored in containers with 2 % HNO₃. The sample collection vessel is rinsed twice with ultrapure water and subsequently filled completely with 2 % HNO₃. The basic shaft does not need to be stored in nitric acid since it does not come into contact with the sample. If spare parts need to be applied they are cleaned and prepared in the same manner before installing. A blank check is conducted for the entire system before every use by sampling ultrapure water with the assembled device and storing it for at least 12 h in the device before analyzing it.