Sampling Techniques for Arsenic in Water

Arsenic is a naturally occurring, toxic metalloid. A metalloid is an element that has some properties of a metal and some properties of a nonmetal. Industries often use arsenic in a variety of products such as wood preservatives, paints, fertilizers and dyes. Due to the presence of arsenic in a natural state and from industrial applications, arsenic is often a concern in groundwater. According to the United States Environmental Protection Agency, arsenic has a maximum allowable contaminant level of 10 parts per billion in groundwater. Laboratories may analyze groundwater samples to determine the presence or absence of arsenic.

Things You'll Need

• Groundwater monitoring well
• Low-flow pump
• Tubing
• Plastic sample bottle preserved with nitric acid, 250mL size
• Electronic water level indicator
• Meter(s) - turbidity, pH, specific conductance, and temperature

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Instructions

• 1

  Open the groundwater monitoring well and allow the water level to equilibrate. While the well is open, set up your sampling equipment.

• 2

  Measure the depth to the top of the water table and the total depth of the well using the electronic water level indicator. The water level indicator should measure the water level to the nearest 0.01 feet. The measuring tape attached the probe will provide the total well depth measurement.

• 3

  Calculate the purge volume for the well. Subtract the depth to groundwater from the total well depth to determine the length of the water column inside the well. Calculate the volume of the water in the well using the equation: V = 0.041*d2*h where V is the volume in gallons, d is the diameter of the well in inches, and h is the length of the water column in feet. Multiply this well volume by three and five to determine the typical purge volume of the well. Therefore, if you calculate that the volume of water in the well is 2 gallons, the purge volume would be between six and 10 gallons.

• 4

  Place the pump tubing in the well and begin to purge the groundwater from the well casing using a low flow rate. Low flow rates generally produce a less turbid sample, which is more representative of groundwater quality. Collect groundwater samples periodically during the purging process and analyze the samples for the field parameters pH, specific conductance, temperature and turbidity.

• 5

  Continue to purge the well until at least three well volumes are removed and the field parameters are stable. The field parameters are stable when turbidity measurements are similar and preferably below 10 Nephelometric Turbidity Units, pH is consistent within 0.1 Standard Units, specific conductance measurements are within 10 percent and the temperature is constant for at least three or more measurements.

• 6

  Collect the groundwater sample directly from the pump tubing into the pre-preserved sample bottle. Close the bottle tightly when it is full and complete any information requested by the laboratory such as a bottle label and chain of custody form. Place the sample in a cooler with ice and transport to the laboratory for analysis.