GROUNDWATER SAMPLING

The groundwater sampling method is shown in Figure 1. The hole is sealed by the pressurized liner (usually overfilled with water). The portion of the hole to be sampled is defined by the spacer attached to the exterior of the liner. The water from the formation flows into the spacer interstitial space and thereby to the port through the liner. The port is connected to a tube that carries the water to the bottom of the hole and then up to the pumping system. The water flows through the first check valve into the U shaped tube. Both legs of the U tube are filled to the level of the water in the formation, but the second check valve depresses the water level in the slender side of the U tube. A lightweight ball (Teflon) and no spring are used in the first check valve so that the water level in the 1/2"tube of the U is not affected by the valve.

 

The water is pumped from the U shaped tube by the downward displacement of the water surface in the large tube. (See the drawing) The displacement is effected by applying a gas pressure to top end of the large tube. The pressure is usually applied from a compressed nitrogen bottle. The water in the large tube is displaced downward through the bottom of the U and upward through the second check valve (steel ball with spring). The water rises in the slender tube to the surface.

During the purge cycle, the water is completely displaced from the tubing and then allowed to refill. After the purge, the gas pressure is reduced to a value which will not force the free water surface to the depth of the U. The first portion of the sample water stroke is discarded (see Water FLUTe Sampling Procedure for the details).

Attributes of the sampling procedure:

• The bottom of the U tube is usually located near the bottom of the hole. This provides a very large volume of water for pumping of each port regardless of the elevation of the port in the hole.
• Because the liner occupies such a small fraction of the hole volume, there is space for relatively large and long pump tubing.
• Each port has its own dedicated pumping system.
• The U tubes are all located near the bottom of the hole and can be pumped with the same pumping pressure, simultaneously, for a great savings of the cost of sampling labor. Some 15 port systems are purged and sampled in less than one hour.
• The head for each port can be tagged from the surface with a slender electric tag line, if the water table is not below about 150 ft. from the surface.
• Pressure gauges are often located just below the first check valve for each port to monitor the discrete head response. This is useful for aquifer characterization during seasonal and forced head variations. For deep (>200ft) water tables, this is the only FLUTe method for monitoring the water level in the large tube, other than the measurement of the collected purge volume from each port.

For more information, contact us at info@flut.com.