|
continuous coring and/or drilling with tri-cone roller or fixed-
<br />blade drag bits. Generally, rotary drilling is used when more
<br />conventional hollow-stem auger drilling either is or becomes
<br />infeasible. Various drilling fluids (mud or air), used to keep
<br />the borehole from caving and to remove drill cuttings, are
<br />chosen according to the nature of the soils and/or geologic
<br />formations expected to be encountered as well as the
<br />monitoring program. Samples may be collected directly from
<br />cores. A geologist or engineer from Apex Envirotech, Inc.,
<br />continuously logs each boring during drilling and checks
<br />returned drill cuttings for indications of both the first
<br />recognizable occurrence of groundwater and volatile
<br />hydrocarbons, using either a portable PID, FID, or
<br />explosimeter. All drilling equipment is either steam cleaned
<br />or washed between borings to minimize the potential for
<br />cross-contamination.
<br />Frequently, hollow-stem augers are used to drill and sample
<br />to either a minimum depth or auger refusal. In such cases,
<br />the augers may be left in place as temporary surface casing,
<br />with the center plug removed and drilling/coring carried out
<br />through the augers. Alternatively, a shallow conductor
<br />casing, or surface casing, may be set by drilling to a desired
<br />depth with a large-diameter bit, then setting the casing and
<br />proceeding with the drilling/coring. After total drill depth (TD)
<br />is reached, the borehole may be logged by geophysical
<br />means or hydraulically tested. If casing is not set to the
<br />bottom of the borehole, the lower portion of the hole may be
<br />grouted or backfilled accordingly. The borehole may be
<br />drilled out (reamed) as necessary with a large-diameter bit.
<br />Upon reaching TD, drilling fluid is circulated to remove
<br />cuttings. Selected casing is then run into the borehole and
<br />set to the desired depth. Monitoring wells are cased with
<br />clean, threaded, factory-perforated and blank casing. The
<br />perforated interval consists of slotted casing, generally with
<br />0.020-inch-wide by 1.5 inch-long slots, with 42 slots per foot.
<br />Centering devices may be fastened to the casing the ensure
<br />even distribution of filter material and grout within the
<br />borehole annulus. The well casing is thoroughly washed
<br />and/or steam cleaned, or may be purchased as pre-cleaned,
<br />prior to installation. All recoverable drilling fluid and/or
<br />cuttings are collected for temporary storage and disposed of
<br />properly pending analytical results.
<br />After setting the casing, sand or gravel filter material is
<br />poured into the annular space to fill from boring bottom to
<br />generally 1 foot above the perforated interval. A one to two
<br />foot-thick bentonite plug is set above this filter material to
<br />prevent grout from infiltrating the filter pack. Either neat
<br />cement, containing about five percent bentonite, or sand-
<br />cement grout is then tremmied into the annular space from
<br />the top of the bentonite plug to near surface. A traffic-rated
<br />vault is installed around each wellhead for wells located in
<br />parking lots or driveways, while steel "stovepipes" are
<br />usually set over wellheads in landscaped areas.
<br />After installation, the wells are thoroughly developed to
<br />remove residual drilling materials from the wellbore, and to
<br />improve well performance by removing fine material from the
<br />filter pack that may pass into the well. Well development
<br />techniques used may include pumping, surging, bailing,
<br />swabbing, jetting, flushing, and air-lifting. All development
<br />water is collected either in drums or tanks for temporary
<br />storage, and properly disposed of pending laboratory
<br />analytical results. To minimize the potential for cross-
<br />contamination between wells, all development equipment is
<br />either steam cleaned or properly washed prior to use.
<br />Following development, the well is allowed to stand
<br />undisturbed for a minimum of 24 hours before its first
<br />sampling.
<br />SOP-9
<br />GROUNDWATER PURGING AND SAMPLING
<br />Prior to water sampling, each well is purged by evacuating a
<br />minimum of three wetted well-casing volumes of
<br />groundwater. When required, purging will continue until
<br />either the discharge water temperature, conductivity, or pH
<br />stabilize, a maximum of ten wetted-casing volumes of
<br />groundwater have been recovered, or the well is bailed dry.
<br />When practical, the groundwater sample should be collected
<br />when the water level in the well recovers to at least 80
<br />percent of its static level.
<br />The sampling equipment consists of either a "Teflon" bailer,
<br />PVC bailer, or stainless steel bladder pump with a "Teflon"
<br />bladder. If the sampling system is dedicated to the well,
<br />then the bailer is usually "Teflon," but the bladder pump is
<br />PVC with a polypropylene bladder. In general and
<br />depending on the intended laboratory analysis, 40-milliliter
<br />glass, volatile organic analysis (VOA) vials, with "Teflon"
<br />septa, are used as sample containers.
<br />The groundwater sample is decanted into each VOA vial in
<br />such a manner that there is no meniscus at the top of the
<br />vial. A cap is quickly secured to the top of the vial. The vial
<br />is then inverted and gently tapped to see if air bubbles are
<br />present. If none are present, the vial is labeled and
<br />refrigerated for delivery, under strict chain-of-custody, to the
<br />analytical laboratory. Label information should include a
<br />unique sample identification number, job identification
<br />number, date, time, type of analysis requested, and the
<br />sampler's name.
<br />For quality control purposes, a duplicate water sample is
<br />collected from each well. This sample may also be analyzed
<br />or put on hold at the laboratory. When required, a trip blank,
<br />prepared at the laboratory, is placed in the transport cooler.
<br />It is labeled similar to the well samples, remains in the cooler
<br />during transport, and is analyzed by the laboratory along
<br />with the groundwater samples. In addition, a field blank may
<br />be prepared in the field when sampling equipment is not
<br />dedicated. The field blank is prepared after a pump or bailer
<br />has been either steam cleaned or properly washed, prior to
<br />use in the next well, and is analyzed along with the other
<br />samples. The field blank analysis demonstrates the
<br />effectiveness of the in-field cleaning procedures to prevent
<br />cross-contamination.
<br />To minimize the potential for cross-contamination between
<br />wells, all well development and water sampling equipment
<br />not dedicated to a well is either steam cleaned or properly
<br />washed between uses. As a secondary precautionary
<br />measure, wells are sampled in order of least to highest
<br />concentrations as established by available previous
<br />analytical data.
<br />In the event the water samples cannot be submitted to the
<br />analytical laboratory on the same day they are collected
<br />(e.g., due to weekends or holidays), the samples are
<br />temporarily stored until the first opportunity for submittal
<br />either on water ice in a cooler, such as when in the field, or
<br />in refrigerator at Apex's office.
<br />SOP-10
<br />MEASURING LIQUID LEVELS USING WATER LEVEL
<br />METER OR INTERFACE PROBE
<br />Field equipment used for liquid-level gauging typically
<br />includes the measuring instrument (water-level meter or
<br />interface probe) and product bailer(s). The field kit also
<br />includes cleaning supplies (buckets, solution, spray bottles,
|