This article throws light upon the top three methods used for collecting soil samples. The methods are: 1. Soil Samples for Volatile Organic Compounds (VOC) Analysis 2. Sample for Metals 3. Sample for Biological Examinations.
Method # 1. Soil Samples for Volatile Organic Compounds (VOC) Analysis:
If samples are to be analyzed for volatile organic compounds, they should be collected in manner that minimizes disturbance of the sample.
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(A) Sampling Method for low concentration (<200 ug/kg) of VOC:
When the total VOC concentration in the soil is expected to be less than 200 pg/kg, the samples may be collected directly with the En Core® Sampler or syringe. If using the syringes, the sample must be in the sample container (40 ml pre-prepared vial) immediately to reduce volatilization losses.
The 40 ml vials should contain 10 ml of organic-free water for an un-preserved sample or approximately 10 ml of organic-free water for an unpreserved sample or approximate 10 ml of organic free water and a preservative. It is recommended that the 40 ml vials be prepared and weighted by the laboratory. When sampling directly with the En Core® Sampler, the vial must be immediately capped and locked.
A soil sample for VOC analysis may also be collected with conventional sampling equipment. A sample collected in this fashion must either be placed in the final sample container (En Core® Sampler or 40 ml pre-prepared vial) immediately or the sample may be immediately placed into an intermediate sample container with no head space.
If an intermediate container (usually 2-oz. soil jar) is used, the sample must be transferred to the final sample container (En Core® Sampler or 40 ml pre-prepared vial) as soon as possible, not to exceed 30 minutes. After collection of the sample into either the En Core Sampler or other container, the sample must immediately be stored in an ice chest and cooled.
En Core Sampler:
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The sample shall be capped, locked, and secured in a plastic bag.
(B) Sampling Method for High Concentrations (> 200/µg/kg) of VOC:
The sample may be packed into a single 2-oz. glass container with a screw cap and septum seal. The sample container must be filled quickly and completely to eliminate head space. Soils sediments containing high total VOC concentrations may also be collected as described in sampling method for low concentrations, and preserved using 10 ml methanol.
Method used is primarily to collect surface and shallow subsurface soil samples. Surface soils are generally classified as soils between the ground surface and 6 to 12 inches below ground surface.
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Stainless steel spoons may be used for surface soil sampling to depths of approximately 6-inches below ground surface where conditions are generally soft and non-indurate and there is no problematic vegetative layer to penetrate.
Hand augers may be used to advance boreholes and collect soil samples in the surface and shallow subsurface intervals. Typically, 4-inch stainless steel auger buckets with cutting heads are used; the bucket is advanced by simultaneously pushing and turning using an attached handle.
1. Surface Soil Sampling:
When conducting surface soil sampling with hand augers, the auger buckets may be used with a handle alone or with a handle and extensions. The bucket is advanced to the appropriate depth and the contents are transferred to the homogenization container for processing.
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2. Subsurface Soil Sampling:
Hand augers are the most common equipment used to collect shallow subsurface soil samples. Auger holes are advanced one bucket at a time until the sample depth is achieved. When the sample depth is reached, the bucket used to advance the hole is removed. The clean auger bucket is then placed in the hole and filled with soil to make up the sample and removed.
(i) Direct push soil sampling methods:
These methods are used primarily to collect shallow and deep soil samples. Three methods are available for use with either the Geoprobe or the drill rig adapted with a hydraulic hammer. All methods involve the collection and retrieval of the soil sample within a thin-walled liner. Following sections describe each of the specific sampling methods.
(a) Large Bore Soil Sampler:
The Large Bore sampler is a solid barrel direct push sampler. It is equipped with a piston-rod point, which is primarily used for collection of depth-discrete subsurface soil samples. The sample barrel is approximately 30-inches (762 mm) long and has a 1.5-inches (38 mm) outside diameter.
The sampler is capable of recovering a discrete sample core 22 inches x 1.0 inch (559 mm x 25 mm) contained inside a removable liner. The resultant sample volume is a maximum of 283 ml.
After the LB sample barrel is equipped with the cutting shoe and liner, the piston-rod point assembly is inserted, along with the drive head and pistons top assembly. The assembled sampler is driven to the desired sampling depth, at which time the piston stop pin is removed, freeing the push point.
The LB sampler is then pushed into the soil a distance equal to the length of the LB sample barrel. The probe rod string, with the LB sampler attached, is then removed from the subsurface the LB sampler is removed. After retrieval, from the probe rod string. The drive head is then removed to allow removal of the liner and soil sample.
(b) Macro-Core Soil Sampler:
The Macro-Core sampler is a solid barrel push sampler equipped with a piston-rod point. It is primarily used for collection of either continuous or depth- discrete subsurface soil samples. The standard MC sampler has an assembled length of approximately 52 inches (1321 mm) with an outside diameter of 2.2 inches (56 mm).
The MC sampler is capable of recovering a discrete sample core 45 inches x 1.5 inches (1143 mm x 38 mm) contained inside a removable liner. The resultant sample volume is a maximum of 1300 ml. The MC sampler may be used in either an open-tube or closed-point configuration.
(c) Dual Tube Soil Sampler:
The Dual Tube soil sampling system is a direct push system for collecting continuous materials from within a sealed outer casing of 2.125-inch (54mm) OD probe rod. The sample is collected within a liner that is threaded onto the leading end of a string of 1.0-inch diameter probe rod. Collected samples have a volume of up to 800 ml in the form of a 1.125-inch x 48-inch (29 mm x 1219 mm) core.
(ii) Split Spoon or Drill Ring Methods:
Split spoon sampling method is used primarily to collect shallow and deep soil samples. All split spoon samplers are basically split cylindrical barrels that are threaded on each end. The leading end is held together with in beveled threaded collar that serves as a cutting shoe.
The other end is held together the sub used to attach the spoon to the string of drill rod. Two basic methods are available for use, including the smaller diameter standard split spoon, driven with the drill rig safety hammer, and the larger diameter continuous split spoon, advanced inside and slightly ahead of the lead auger during hollow stem auger drilling.
(a) Standard Split Spoon:
A drill rig is used to advance a borehole to the target depth. The drill string is then removed and a standard split spoon is attached to a string of drill rod. Split spoons used or soil sampling must be constructed of stainless steel and are typically 2.0-inches OD and 18-inches to 24-inches in length.
After the spoon is attached to the string of drill rod it is lowered into the borehole. The drill rig safety hammer is then used to drive the split spoon into the soil at the bottom of the borehole. After the split spoon has been driven into the soil, filling the spoon, it is retrieved to the surface, where it is removed from the drill rod string and opened for sample acquisition.
(b) Continuous Split Spoon:
The continuous split spoon is a large diameter split spoon inside a hollow stem auger. Continuous split spoons are typically 3-inches to 5-inches in diameter and either 5-feet or 10 feet in length.
After the auger has been advanced into the soil column a distance equal to the length of the sampler being used it is returned to the surface. The sampler is removed from inside the hollow stem auger and the threaded collars are removed. The split spoon is then opened for sampling.
(iii) Shelby Tube Sampling Method:
Shelby tubes also referred as thin-walled push tubes or Acker thin-walled samplers. They are used to collect subsurface soil samples in cohesive soil and clays during drilling activities. In addition to samples for chemical analyses, Shelby tubes are also used to collect relatively undisturbed soil samples for geotechnical analysis, such as hydraulic conductivity and permeability to support hydrogeologic characterizations at hazardous waste and other sites.
A typical Shelby tube is 30-inches in length and has a 3.0-inch OD (2.875 ID) and may be constructed of steel, stainless steel, galvanized steel, or brass. They are attached to push heads that are constructed with a ball-check to aid in holding the contained sample during retrievel.
If used for collecting samples for chemical analyses, it must be constructed or stainless steel. If used for collecting samples for standard geotechnical parameters, any material is acceptable.
To collect a sample, the tube is attached to a string of drill and is lowered into the borehole, where the sampler is then pressed into the undisturbed clay or silts by hydraulic force. After retrieval to the surface, the tube containing the sample is then removed from the sampler head.
(iv) Backhoe Sampling Method:
Backhoes may be used in the collection of surface and shallow subsurface soil samples. The backhoe offers the capability of collecting samples from very specific interval. If possible, the sample should be collected without entering the trench.
Samples may be obtained from the trench wall or they may be obtained directly from the bucket at the surface. Various techniques used for safety collecting soil samples with the aid of a backhoe are as follows.
(a) Scoop and bracket method:
If a sample interval is targeted from the surface, it can be sampled using a stainless steel scoop and bracket. First a scoop and bracket are affixed to a length of conduit and is lowered into the backhoe pit. Then the scoop and scrape is taken away from the soil.
This material likely represents soil that has been smeared by the backhoe bucket from adjacent material. After the smeared material has been scraped off, the scoop is removed.
(b) Direct-From-Bucket Method:
It is also possible to collect soil samples directly from the backhoe bucket at the surface. Some precision with respect to actual depth or location may be lost with this method but if the soil to be sampled is uniquely distinguishable from the adjacent or nearby soils, it may be possible to characterize the material as to location and depth.
3. Sampling of soil for Bulk Density:
For the determination of bulk density, five samples with a minimal volume of 100 cm3 have to be taken from the mineral topsoil (0-10 cm) of non-stony soils using the core or excavation method bulk density is a mandatory parameter it can be estimated using pedo-transfer functions. A typical example of a pedotransfer functions is the Adams (1973) equations:
Where %OC is the percentage total organic carbon and MBD is the mineral bulk density.
Reference method for some parameters of soils is given in table 1. Physical & chemical parameters of soil and their estimation is shown in table 2.
Method # 2. Sample for Metals:
Composite Sample for detection of metals should be taken in a 2 litre polythene bottle & immediately digested with nitric acid & sulphuric acids as follows:
Take the sample in a evaporating dish, acidify to methyl orange with conc. H2SO4 & add 5 ml Conc HNO3 & 2 ml of H2O2 to reduce chromate by placing over a water both or hot plate to about 10 ml. Now transfer this liquid into conical flask with the help of 5 ml conc HNO3. Add 10 ml Conc. H2SO4 and a few glass beads. Now evaporate the contents on hot plate till dens fumes of SO2 just appear in the Flask.
Cool to room temperature & 50 ml distilled water & boil if solids are present & filter through a sintered glass crucible. Transfer the filtrate with 100 ml volumetric flask & make it upto the make by addition of distilled water. Now use this solution for the determination of various metals by using different techniques.
Method # 3. Sample for Biological Examination:
For biological examination, the sample is taken in a dry & pure pyrex bottle. There should be no gap between stopper and effluent. Any gap of air should be avoided.
When sample contains suspended matter:
Take the sample in 500 ml bottle. Add 10 gm alum & 2 ml conc. ammonia solution, stopper & mix thoroughly for about 3-4 minutes & keep it for 5 minutes. Take supernatant liquid into BOD bottle & use until it overflows. Stopper it & now use it for D.O. & BOD. & other parameters.
When sample contains chlorine water:
Neutralize the chlorine by taking this solution & then place the liquid in BOD bottle & use it for D.O. & B.O.D.
When sample contains ferrous ions:
Take 300 ml sample in BOD bottle. Add 0.7 ml Conc H2SO4 & add KMnO4 until it acquires red- violet tinge for about 20 minutes. Add potassium oxalate solution until it discharges. Now add 2ml mananous sulphate solution (0.1N) & 2 ml alkaliazide reagent (0.1N) & proceed for titration for D.O.
Time between Collection & analysis of sample:
As far as possible all samples should be analysed immediately without any delay. If any how sample is to be kept for some time due to one reason or the other then it should be kept after adding proper reagents a 4°C. The pH, temperature, pressure, oxygen CO2 etc. should be determined immediately.
Preservation of sample:
There are some standards for the preservation of the sample for different constituents. The recommended methods are given in the following table.
Results:
The result should be tabulated as follows:
1. Sample No.
2. Date on which sample taken.
3. Source of sample & Name of industry.
4. Sampling station or place.
5. Name of collector.
A. Physico chemical Characteristics:
1. Colour
2. Odour
3. Temp °C.
4. pH.
5. Solids.
(i) Total, mg/l
(ii) Suspended mgl
(iii) Settleable, mg/l
(iv) Dissolved, mg/l
B. Inorganic Pollution (metallic & metalloids):
1. Copper (as Cu), mg/l
2. Iron (as Fe), mg/l
3. Lead (as Pb), mg/l
4. Manganese (as Mn), mg/l
5. Zinc (as Zn), mg/l
6. Nickel (as Ni), mg/l
7. Mercury (as Hg), mg/l
8. Cadmium (as Cd), mg/l
9. Chromium (trivalent) (as Cr), mg/l
10. Chromium (Hexavalent) (as Cr), mg/l
11. Arsenic (as As), mg/l
12. Antimony (as Sb), mg/l
13. Boron (as B), mg/l Non-metals
14. Acidity/Alkalinity (as CaCO30, mg/l
15. Phosphate (as PO4), mg/l
16. Sulphide (as S), mg/l
17. Sulphite (as SO3), mg/l
18. Sulphate (as SO4), mg/l
19. Chloride (as CI), mg/l
20. Fluoride (as F), mg/l
Organic Pollution
21. Dissolved oxygen, mg/l.
22. BOD (5 days at 20°C), mg/l
23. COD, mg/l
24. Total nitrogen kjeldahl, mg/l
25. Nitrite (as Nitrogen), mg/l
26. Nitrate (as Nitrogen), mg/l
27. Total organic carbon, mg/l
28. Ammonical nitrogen, mg/l
Oil & grease
29. Oil and grease, mg/l.
N.B.- The errors should be calculated for each parameter & at least mean of 15 readings should be given in the final table.