GB2303449A

GB2303449A - Testing soil samples for petroleum hydrocarbon presence

Info

Publication number: GB2303449A
Application number: GB9514905A
Authority: GB
Inventors: Robert Gittins
Original assignee: ELE International Ltd
Current assignee: ELE International Ltd
Priority date: 1995-07-20
Filing date: 1995-07-20
Publication date: 1997-02-19
Also published as: GB9514905D0

Description

METHOD FOR TESTING SOIL FOR THE PRESENCE OF PETROLEUM HYDROCARBONS WHICH GIVE OFF VAPOUR.

This invention relates to testing of soil samples for the presence of petroleum hydrocarbons which give off vapour at ambient temperatures. This includes petrol, diesel and crude oil. It excludes hydrocarbons which give off no vapour such as bitumen, greases, lubricating oil and some heavy fuel oils.

Testing of soil samples for the presence of petroleum hydrocarbons is in many cases carried out by extracting the hydrocarbon from a soil sample with a solvent. The soil sample is taken from a relevant place and stored in a sealed container so that volatile components are not lost. An analysis of the extracted material is then carried out either by a colorimetric method in the field or laboratory or by infra-red absorbtion method in a laboratory. The use of a weighed quantity of soil for the analysis and a reading from the instrument used for the final measurement of concentration enables a quantitative estimation to be made of the degree of contamination of the soil. The ability of the solvent used to extract the sample is not the same for all petroleum fractions and correction factors need to be used.The same solvent is not used for the colorimetric method and the infra-red method which also requires correction factors. A further variation in the methods is that the colorimetric method relies on detecting aromatic hydrocarbons and the infra-red method primarily detects aliphatic hydrocarbons. The colorimetric method detects tars and bitumens and the infra-red method does not. A further problem is that naturally occurring organic matter of a polar nature can give a response to the tests giving an overestimation of the contaminant levels. This is overcome by treating the solvent extract with a material which removes polar organic matter.

The use of these methods are however universally accepted as a measurement of soil contamination because of the quantitative nature of the result.

In other methods of testing the soil, bore holes are made in the ground and the concentration of volatile petroleum hydrocarbon vapour accumulating in the hole is measured by using a photo-ionisation detector or a flame ionisation detector. This method gives an indication of the presence of volatile organic materials in the soil. It is not quantitative as the vapour concentration in the hole depends upon the the age of the hole, the porosity of the soil to vapour, and the way the hole has been sealed before the test. In addition, the reading obtained in this way cannot be quantitative as it is not a steady reading. It falls in an indeterminate way as the air is drawn from the hole.

A further problem of making the reading with a photo-ionisation detector or a flame ionisation detector is that they both respond to the naturally occurrring polar organic matter in soil such as terpenes derived from vegetation.

In an attempt to make the use of the photo-ionisation detector or flame ionisation detector quantitative, sometimes quantities of soil sample are placed in sealed sample jars and measurement made of the volatile petroleum hydrocarbon vapour in the headspace. This method also cannot be quantitative as the vapour is rapidly pumped away and a steady reading is not obtained.

According to the present invention a method is provided to take a fixed quantity of soil and prepare it for testing for volatile petroleum hydrocarbons using an ionisation detector in such a way that quantitative reproducible results are obtained for the concentration of the petroleum hydrocarbon in the soil.

A further embodiment of the invention is the use of a selective membrane device to eliminate the response of the ionisation detector to naturally occurring polar hydrocarbons such as terpenes. This selective membrane is the subject of UK Patent Application No GB 9401102.0 A specific embodiment of the invention will now be described.

The equipment used is a blender and a photo-ionisation detector fitted with a selective membrane inlet system as shown in Figure 1. Referring to Figure 1, the pump 1 draws air through a carbon trap 2 from one side of the membrane housing 3 past the coated porous PTFE membrane 4. The other side of the housing 3 is connected to a PTFE inlet probe 5 with a filter. When taking a measurement, the probe 5 is inserted into the sand 6 in the blender 7. As the air from the sample in the blender is drawn past the membrane, the petroleum hydrocarbon vapour in the air diffuses through the membrane. The output from the pump 1 passes through a second carbon trap 8 to the other side of the membrane housing 9 and carries the vapour which has diffused through the membrane to the photo-ionisation detector 10.

The diffusion process takes place through the liquid absorbed in the porous PTFE membrane. The liquid is squalane which is a non-polar hydrocarbon. The naturally occurring polar organic materials do not pass through the membrane.

An example of a procedure for carrying out a test is as follows: a. 240 grams of dry silver sand are placed in a blender.

b. 60 grams of the soil sample are added.

c. The blender is operated for 10 seconds to break up the soil sample and distribute it evenly in the silver sand.

d. The probe is inserted into the silver sand in the blender.

e. The reading of the photo-ionisation detector rises to a steady maximum level within two minutes or more rapidly if the hydrocarbon contamination is petrol.

f. This reading is taken as the quantitative value of the contamination. The value of the reading in terms of mg petroleum hydrocarbon contamination per Kg soil will depend on previously carried out calibration with the material present on the site.

g. The probe is removed from the blender and the silver sand tipped away with the sample. The low ratio of soil sample to silver sand prevents clogging of the blender.

It will be clear to a skilled person that the exact amounts of soil and silver sand can be varied and alternate calibrations carried out. It will also be clear that the calibration for a volatile petroleum hydrocarbon such as petrol will be different to that for a less volatile petroleum hydrocarbon such a diesel.

Claims

1. A method of testing soil for the presence of petroleum hydrocarbons which give off vapour comprising the following steps: a. Putting an inert filler such as silver sand into a blender.

b. Adding an amount of soil sample to be tested.

c. Operating the blender for a short period to distribute the soil sample in the inert filler.

d. Drawing a sample of air from the inert filler via a selective membrane into an ionisation detector.

e. Quantifying the presence of the hydrocarbon by calibration.

2. The method as claimed in claim 1 where the inert filler is any other type of free flowing mineral filler.

3. The method as claimed in claim 1 or 2 where the selective membrane is impregnated with squalane.

4. The method as claimed in claim 1 or 2 where the selective membrane is impregnated with any liquid material used as a stationary phase in gas chromatography.

5. The method as claimed in claim 3 or 4 where the ionisation detector is a photo-ionisation detector or flame ionisation detector.