In most wells, this is the inner edge of the well's top of casing. If there is an inner and outer casing, use the casing that is highest. If the top of the casing is not horizontal, use the edge that is highest above the ground and mark it with an arrow either scratched into the surface or drawn in waterproof ink. With a decontaminated tape or ruler, measure and record the measuring point's vertical height above the land surface to the nearest 3 cm (0.1 foot). This is called the well's Æstickup'.

If there are no existing wells, then a well should be constructed according to the appropriate guidelines. Contact the Arizona Department of Water Resources (ADWR).

b. Check for Trapped Vapors

Hold the probe of a photo-ionization meter or explosimeter at nose level and take vapor readings around and over the well. If volatile levels in the breathing zone are more than 5 ppm, at least Level C protection is required. This level of personal protection requires wearing a Tyvek suit, air-purifying mask, niton or Viton&reg (DuPont Dow Elastomers) gloves, and overboots. Consult your personal safety officer before testing or sampling this well.

If the reading in the breathing zone is less than 5 parts per million (ppm) volatiles (Modified Level D) then insert a photo-ionization meter or explosimeter probe into the well casing to measure the degree of personal hazard from trapped vapors. Monitoring wells at landfills and fuel-spill sites are especially susceptible to the buildup of methane and other gases.

c. Measure Water Level

Measure the depth to water below the measuring point to the nearest 3 mm (or 0.01 foot). The objectives of this task are to determine: 1) the altitude of the water table for flow direction studies; and 2) the correct pump size and length of tubing for well purging and water-quality sampling. Make sure the sampling pump is turned off before measuring the water level and that pumping from adjacent wells is not influencing the water level at the sampling site. In most cases, a graduated steel or fiberglass tape or electric sounder is recommended. Cover the bottom of the tape with a water indicator paste, gel or non-ferrocyanide chalk that changes color when wet, or attach a metal cup, or "popper" that makes a "pop" sound when it hits the water's surface.

Electric sounders indicate water contact with either a light, meter or buzzer. After the first water contact, gently raise and lower the probe until the light, meter or buzzer activates. Record the reading at the measuring point. Electric sounders usually are ineffective if there is floating product in the well. Other methods such as a tape, interface probe or clear bailer must be used. If you have any doubt about the accuracy of your water-depth measurement, repeat it as many times as necessary until you get an unambiguous and consistent reading.

Before you put an expensive probe into an unfamiliar well, use a non-leaded weighted line the same size as the probe to verify that you have sufficient access.

d. Plumb Well Depth

If you use an electric sounder, make sure the switch is turned off. Lower the weighted tape or sounding cable all the way to the bottom of the well. Continue reeling out until it feels limp in your hands. Raise and lower the tape or cable several times to ensure that the weight or probe has not become Æhung up' on the casing or screen joints. After you are confident that the maximum depth has been reached, slowly take up the slack. Record the depth below the measuring point to the nearest 3 mm (0.01 foot).

Reel up the tape or cable, and decontaminate it and the reel before its reuse in the next well. Some electric depth sounders allow the complete removal of their electronic components before cleaning so that the entire reel and cable can be safely immersed in decontaminating fluids.

Determining the finished well depth enables you to compare it to driller's log information, to assess the extent of in-screen filling by fine sediments (and therefore the efficiency of well development and the actual open length of the screen interval), to help determine the well's condition for sampling, and to measure the length of cable and tubing necessary to place the pump intake at the optimum depth.

e. Measure and Sample the Oil/Solvent Interface

If volatile vapors or oil-coated tape or cables from depth sounding indicates that fuels or solvents may be inside the well, decontaminate the equipment, and leave this well for last. (Garrett, 1988, p. 16). When you return later, measure the level and thickness of the hydrocarbon layer with a decontaminated Æinterface probe'. Modern devices are accurate to within 1.5 mm (0.005 foot), and feature visual and audible indicators.

Lower the interface probe down the well until the alarm sounds continuously, indicating hydrocarbons have been detected. Record that depth to the nearest 3 mm (0.01 foot). Now continue to slowly lower the probe until the alarm oscillates (changes in intensity), indicating that water has been detected. Record that depth. The difference is the thickness of the hydrocarbon layer. Reel in the probe and decontaminate it thoroughly before reuse.

Use the same procedure for detecting and measuring heavier- than-water solvents at the bottom of the well.

To obtain a sample of the hydrocarbon compounds with a clear, check-valve bailer, first lower the bailer into the well until you can hear it make contact with the oil, or use the depth measured with the interface probe. Now quickly lower the bailer another 30 to 50 cm (1 to 1.5 feet), withdraw it from the well, measure the thickness of the hydrocarbon, and record the results. (Garrett, 1988, p. 16) If you have a camera, photograph the bailer and its contents.

Pour the hydrocarbon into a clean glass container, such as a 40-ml vial with a teflon septum used for VOC samples. Carefully seal, wipe, label, and store the sample container in a cooler. Preserve with sulfuric acid to pH <2 and cool to 4 C.

If other potential sources of oil or fuels exist in the area, such as tanks, contaminated soils, pipelines or hoses, obtain samples from them so that residues can be compared with the floating product in the well. (Garrett, 1988, p. 16)

Remove the rest of the hydrocarbon with a bailer, and record the volume. If complete removal is impossible due to large volumes in the aquifer, consult with your project manager.

Thoroughly decontaminate the bailer and line before reuse. Use the same procedures, equipment and precautions for detecting and sampling heavy solvents in the bottom of the well.

f. Purge and Sample the Monitoring Well

Removing or Æpurging' stale water from the well, pump and tubing to ensure that the sample is representative of aquifer water at the monitored depth is one of the most important aspects of sample collection. Select the method of purging and sampling based on site-specific conditions and goals described in the Sampling Plan under ÆField Methods and Procedures' and submitted to federal and/or state agencies for approval before sample collection takes place.

The low-flow purging method is an effective approach when compared to other methods. However, it was not officially approved by the EPA at the time this manual went to press. Recent studies by the EPA and others regarding sampling for metals and colloidal-associated contaminants recommend both purging and sampling with a single pump at rates of 0.2 to 0.5 liters per minute. (Puls and Barcelona, 1989, Puls and Powell, 1992, Backhus et al., 1993) The low-flow purge and sample method causes less agitation in the well, and reduces excess turbidity that can affect water quality. It also greatly reduces the volume of purge water, which may be difficult or expensive to dispose of.

To use the low-flow purging method, set the pump intake approximately in the middle of the well screen or open interval. Measure the water level in the well to the nearest 3 mm (0.01 ft) before turning on the pump, and record it in your notebook or form. Purge the well at a low flow rate until field parameters, such as turbidity and dissolved oxygen stabilize. Generally, stabilization criteria for parameter trend analysis are established by the QA/QC officer before ground water purging takes place. Recommended purging criteria are less than 10% change for turbidity or 5% for dissolved oxygen over 3 successive readings 3 to 5 minutes apart. The ADEQ has written guidelines for operating and calibrating different turbidimeters in the field.

It is also important to monitor drawdown in the well during purging because excess drawdown from pumping may create locally high entrance velocities of ground water around the well, and increase turbidity. If drawdown exceeds about 0.15m (0.5 ft), the purge rate is too high, and should be gradually reduced. Record the pumping rate and water levels in your field notes.

Once the field parameters have stabilized, fill the sample bottles directly from the end of the tubing at a flow-rate of about 0.1 liters/min. Allow the discharge to flow gently down the inside of the bottle, and cap immediately. However, when sampling for radon, VOCs and other analytes that quickly degrade by aeration, an alternative method is to insert the tubing into the sample bottle and draw the tubing up as the surface of the water rises (see section E.1 below).

After sampling, carefully remove the pump assembly from the well. The teflon-lined polyethylene tubing can be dedicated to each well by sealing it in a plastic garbage bag and labeling it with the monitoring well number. If possible, store the tubing on-site until the next round of sampling. Decontaminate the pump according to procedures in STEP 6.

Purge volumes, times and impacts from floating hydrocarbons are reduced significantly if an inflatable packer is set above the pump just above the top of the well screen or open interval. (Decontaminate the inflatable packer before use, and be certain that it is made of a material that will not contaminate the samples.) Also, in order to increase resistance to gas transfer during sampling, teflon-lined polyethylene thick-walled tubing is recommended.

Various physical parameters, measured before and during well purging and sampling, provide both scientific and legal evidence that the sample is representative of aquifer conditions. Measurements are made with sensitive probes placed in a flow-through cell that are either above or below the pump intake or on the ground surface. These parameters include temperature, electrical conductivity, pH, dissolved oxygen, redox and turbidity. Others (such as specific contaminants) may be measured according to program objectives. Several manufacturers offer probes and meters that are connected to data loggers for easy downloading into computer databases.

Pumps most commonly used and recommended for both purging and sampling are electric submersible, bladder, and gas piston types that are constructed with stainless steel and teflon components. Many types of pumps are available for dedicated sampling programs, where the pump, wiring and tubing remains in the well over the life of the sampling program. Initial equipment decontamination is necessary.

The photograph shown on the previous page depicts a field sampling scene at a monitoring well. The electric depth sounder (next to the well) monitors drawdown during purging. The table supports the flow-through cell unit for measuring temperature, electrical conductivity, pH, and dissolved oxygen, and a turbidimeter for turbidity measurements. The sample cooler is pictured at the lower right and a 55-gallon drum for storing purge water is at the far right.

Bailers are not recommended for purging or sample collection because they artificially induce chemical changes in the sample and water column by agitation, mixing and aeration. The use of bailers may give you non-conservative water-quality parameters, high turbidity, and de-gassing of VOCs. If bailers must be used for purging and sampling, measure the turbidity and dissolved oxygen of bailed water at 3 to 5 minute intervals and clearly describe the collection method.

2. Sampling Production Wells

Production wells are public-supply, irrigation, stock or industrial wells that already are equipped with pumps. Unlike open monitoring wells, which have an accessible water column for in situ measurements, supply wells have installed pumps, wiring and pipes that may prevent the use of sounding tapes or probes to measure water depth or quality. If there is sufficient access for in situ measurements, turn off the pump and wait for the aquifer to stabilize before taking these measurements.

The ADEQ QAPP (1991, p. 148) recommends the following procedure for sampling production wells:

Select the sampling point. Collect the sample as close to the well head as possible, prior to any storage tanks, treatment systems, and distribution supply lines. Samples generally are collected from faucets, plugs, and/or valves in the discharge pipe or in open well systems, at the end of the discharging pipe.

It may be necessary, although less desirable, to install a temporary sampling point in the discharge pipe to obtain a sample. This can be accomplished by drilling and threading a hole in the discharge pipe and installing a faucet or valve. Make sure the sample point you select or install does not introduce any additional turbulence or contamination into the sample. In some situations, the only possible sample point is out of the storage tanks or supply line faucets. If so, record this in the field notes.

When possible, run water at least five minutes through the faucet or valve prior to sampling. Measure field parameters (such as temperature, pH, electrical conductivity, D.O., turbidity) until they stabilize. As with purging monitoring wells, measure and record physical parameters to verify adequate purging has occurred.

3. Sampling Domestic Wells

Obtain as much information as possible from the homeowner or tenant. This includes well screen depth and length, yield, construction material, diameter, treatment, location, aquifer type, and the name of the well driller. In Arizona wells are registered with the Arizona Department of Water Resources (ADWR), which maintains well characteristic information on all registered wells.

Run the water at the maximum rate for at least five minutes to evacuate the water-storage tank and other plumbing. Record initial and final temperatures, pH and electrical conductivity values. If possible, measure physical parameters every minute until they stabilize. Collect samples as close to the well as possible and before any treatment system. If the end of the faucet has an aerator, carefully remove it (with owner's permission) before obtaining a sample, especially samples for VOC or bacteriological analyses. Record sampling point location, fill sample bottles at a slow rate, and preserve according to program and laboratory requirements.

IMPORTANT: Phase III continues to Step 11-sections D & E and Steps 12, 13, and 14.