Site Investigation is a term used to describe the process by which a determination is made regarding the environmental condition of a site. This process is undertaken prior to any Site Remediation activity at a site. The process that is used can be by way of soil borings, ground water monitor wells, or a combination of the two. This type of investigation is commonly done by a subsurface evaluator (usually a Geologist or Hydrogeologist). Listed below is a summary of some of the current methods most commonly used in the environmental field to investigate a site.

GeoProbe Unit - This type of drilling technique allows the continuous retrieval of soil samples from the surface to the total depth of the boring. To collect soil samples, Macro Core (MC) open samplers are used. These samplers are of an open tube design and measure approximately 2" in outside diameter by 44" long. The samplers are fitted with a removable cutting shoe and a clear acetate liner. Each of the samplers are fitted with a new acetate liner prior to each use. The acetate liner assists in the removal of the soil sample from the tube and helps insure sample integrity. In order to obtain ground water samples by this method, soil sample GeoProbe points can be converted to temporary well points. The ground water sample points are first cored with the MC samplers and then 1" screened PVC is inserted into the open bore hole. The screened PVC usually consists of .02 slot screen followed by 1" PVC riser to the surface. After the well point fills with ground water the sample is retrieved with a disposable bailer. One of the main advantages of this type of equipment is that the unit is small and can often access places that a larger, more conventional drilling rig cannot. This type of equipment works best in unconsolidated formations and has a maximum depth range of 30-40 feet.

Auger Drill Rig ­ This type of drilling rig is used to obtain both soil and ground water samples in a similar method to the GeoProbe Unit described above. Instead of "Macro Core's" samples are obtained with a "Split Spoon Sampler", which is approximately the same dimensions as the Macro Core, samples are retrieved from the split spoon sampler by "breaking" open the spoon and obtaining the soil sample. However, one of the main differences between these two types of sampling equipment is their size. The auger rig is a much larger piece of equipment, which can limit access to certain areas, but the auger rig has greater drilling power and can convert a soil boring into a standard 4" monitor well. This type of equipment works best in unconsolidated formations or rock formations that are weathered.

Air Rotary Drill Rig ­ This type of drill rig is one of the largest mobile drill rigs and is used to advance borings or ground water wells into bedrock formations or to drill deep wells. This type of rig can drill into any type of formation (unconsolidated or bedrock). One of the biggest problems with this type of rig is the large size and large working area required.

Laboratory Analysis

After the subsurface evaluator collects all of the samples from a site, they are delivered to a third party laboratory for analysis. The type of analysis required varies depending on the contaminate that is being investigated. In the case of underground storage tanks (UST's) used for residences, the most common material stored is #2 heating oil. Soil samples obtained from an excavation that has had a discharge of #2 heating oil would be analyzed for Total Petroleum Hydrocarbons (TPHC). If the material stored in the UST is gasoline the samples would be analyzed for volatile organics. A farm property may require analysis for herbicides and/or pesticides. An industrial type of operation may require analysis for TPHC, volatile organics, or metals contamination, depending on the operational history of the site.

All samples must be accompanied by a chain of custody document when transferred from one individual to the next. More importantly, the laboratory that is used must meet all State requirements for quality assurance/quality control and must document all aspects of the analytical process.

Field Observations & Field Screening Techniques

While the site investigation is underway, the subsurface evaluator keeps detailed notes on the observed conditions. Notation is made of the types of soil that are encountered, the strata thickness, and color and consistency of the soils. Depth to bedrock is also recorded along with depth to ground water, if it is encountered. Also recorded are any variations from the native soils such as staining and/or odor to the soils. All of the field observations are recorded in a field book that is kept by the evaluator.

A common piece of equipment used by the subsurface evaluator is an organic vapor meter, commonly referred to as an OVM. This piece of equipment is used to "screen" any impacted soils and aid the evaluator in determining when a "clean zone" has been reached. The OVM works best with contaminates that have a relatively high volatility such as gasoline of light heating and fuel oils.

Ground Water Investigation Via Monitor Wells

Ground water is defined as water found below the surface that completely occupies or saturates the interstitial pore space between the individual grains of an unconsolidated formation or the fractures and crevices of a consolidated formation.

When conducting an investigation of the ground water, it is necessary to install monitoring points known as monitor wells. Monitor wells allow the acquisition of ground water samples and aid in determining ground water flow characteristics such as flow direction and velocity.

Monitor wells can be drilled and constructed in a variety of sizes (depth and diameter) with a variety of drill equipment. Well diameters can be 2", 4", 6", 8", or 10". The most popular diameter is probably the 4" well. This is a commonly used diameter since a 4" well can be used to monitor and sample ground water and, if necessary, be easily converted into a recovery well.

The equipment used to drill a monitor well is commonly known in the industry as a "drill rig". Drill rigs, like monitor wells, come in different sizes. Drill rigs can drill, or "advance", a well using the hollow or solid stem auger method, the air rotary or mud rotary method, or the cable rig method. Most of these methods were described above.

In most cases, a drill rig using a hollow stem auger will be sufficient to drill and construct a monitor well in an unconsolidated formation. A hollow stem auger looks and works like a corkscrew. While drilling with the auger, the auger pulls itself downward while pushing dirt upward. The dirt that's pushed upward is known as the "drill cuttings". If drilling is being done in a very contaminated area these cuttings must be drummed and transported off site. After drilling is completed the resultant hole in the ground is known as the "borehole". With the borehole completed, the monitor well can now be constructed.

Typical unconsolidated well construction is always done in basically the same fashion. The drill rig advances the augers downward until the water table is reached. After the water table is reached, drilling continues for approximately another 5 to 10 feet. A 4-inch diameter, threaded PVC (PVC is the accepted construction material of choice, unless you are investigating solvents and need a more heavy duty material like stainless steel) screen is installed in the borehole. The screen is installed to ensure screening across the water table and the entrance of ground water into the well (through the slots) from the surrounding formation. Solid, 4-inch diameter, threaded PVC casing is attached to the screen to complete the well to ground surface. The annular space surrounding the screen is filled with #2 grade Morie sand to a level approximately 1 foot above the top of the well screen. A bentonite seal is placed on top of the sand pack. The remainder of the annular space in the well is filled with cement/bentonite grout to the surface. The well is then finished with a flush mounted locking protective cover or a stick-up. Flush mounts are used in parking lots or in areas where the monitor wells are designed to be inconspicuous. Stick-ups are used in areas where it may be difficult to find the wells at a later date such as wooded areas or overgrown, abandoned properties.

Monitor wells that are installed in consolidated formations are constructed in basically the same manner, with the addition of one step and a different type of drill rig. Consolidated formations often require drilling using the air rotary or mud rotary method. Instead of using an auger to push the drill cuttings to the surface, air or drilling mud is used to bring the cuttings to the surface. After advancing the borehole at least 10 feet into the consolidated formation, drilling is stopped and solid steel casing is installed in the borehole. This casing is cemented into the borehole and the cement must be left to dry overnight. The purpose of the casing and cement is to block any ground water or contamination that might be above the consolidated formation from traveling downward and impacting a deeper formation. After the casing is set into place and the cement has set, the casing shoe (cement at the very bottom of the casing) is drilled out. The well in drilled down to the desired depth and the well screen and inner casing is installed in the same fashion as the unconsolidated well.

If you require additional information on any of the items presented above, please contact our office at 908-851-0057 and our staff will be happy to assist you.

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