Methods for Measuring Groundwater Elevation and Vacuum Pressure During Soil Vapor Extraction

Transcription

1 Technical Note Methods for Measuring Groundwater Elevation and Vacuum Pressure During Soil Vapor Extraction Introduction Soil vapor extraction (SVE) is an easy and low-cost remediation method for removing volatile hydrocarbons, typically petroleum by-products, from contaminated soil and groundwater. Designs of SVE systems vary, but the concept involves: 1) pumping air through the vadose zone to volatilize hydrocarbons to a gas and 2) pumping these vapors through an extraction well. As a result, these gases introduce new pressure fluctuations into an aquifer that affect groundwater elevations (Wong, 1997). Pressure fluctuations caused by SVE systems, in conjunction with barometric pressure fluctuations, pose a unique challenge to accurately measuring groundwater elevations in monitoring wells. By setting up the monitoring well using one of the methods described below, groundwater level fluctuations in monitoring wells can be accurately measured in sealed-well SVE systems. If you are recording vacuum measurements to establish the radius of influence in the vadose zone, check with your regulator to determine the barometric sensor accuracy requirements before you select instruments for your project. Method 1 Absolute Level TROLL and BaroTROLL with Suspension Cable In this method, an absolute Level TROLL Data Logger and a BaroTROLL Data Logger are set up to log data. They are then sealed in the SVE well. The Level TROLL records pressure changes due to water level elevation changes and borehole pressure changes. The BaroTROLL collects borehole pressure changes. The two data sets are merged using Win-Situ Baro Merge Software in order to determine water level elevations in the well. Equipment Required Level TROLL 400 or 700 Data Logger, or Rugged TROLL Data Logger BaroTROLL or Rugged BaroTROLL Data Logger Information subject to change without notice. In-Situ, In-Situ logo, Baro Merge, BaroTROLL, HERMIT, isitu, Pocket-Situ, RDO, RuggedCable, RuggedReader, TROLL, and Win-Situ are trademarks or registered trademarks of In-Situ Inc All rights reserved. Tech Note Rev /2014

2 Suspension cable (2) Twist-lock hanger or Rugged TROLL backshell hanger (2) Direct-connect TROLL Com or docking station, or Rugged TROLL Com Well dock, 2-in. Win-Situ 5 Software Instrument Setup Figure 1.1 Instruments are placed inside a sealed well to collect data during the SVE process using Method Synchronize the internal clocks on the Level TROLL and BaroTROLL devices. 2. Set up a scheduled log for the Level TROLL instrument using Win-Situ 5 Software. For instructions on setting up a scheduled log, consult the Help menu in Win-Situ 5 Software. 3. Set up a scheduled log for the BaroTROLL that is identical in start time to the scheduled log for the Level TROLL. Instrument Deployment 1. Place the Level TROLL instrument into the borehole using a twist-lock hanger and a suspension cable anchored by a well dock. 2. Place the BaroTROLL at the top of the monitoring well (out of the water) to monitor pressure fluctuations during the SVE test. 3. Seal the top of the well during the SVE test. 2

3 Method 1 Advantages The BaroTROLL Data Logger captures the borehole pressure fluctuations over the entire SVE test. Sealing the well is simplified with this method. Method 1 Disadvantages Method 2 Two instruments per monitoring well will increase costs. The field engineer cannot communicate with either data logger during the SVE application. This prevents viewing the well data in real time. The data from the Level TROLL must be post corrected using the BaroTROLL data to reflect the true groundwater elevation. Refer to the In-Situ technical note, Manual Level Mode Correction for Non-Vented Sensors, or use the Win-Situ Baro Merge Software utility. The merged data set has a compounded potential error of two instruments, making it less accurate than Method 2. Vented Level TROLL and TROLL Com with Well Cap In this setup, a gauged (vented) Level TROLL Data Logger, vented cable, and part of the TROLL Com are sealed inside the SVE well using a well cap cable holder. The borehole pressure fluctuations are compensated through the vent tube that runs from the TROLL Com, through the cable, and to the back of the Level TROLL pressure sensor. The vent tube allows the Level TROLL device to capture accurate water level elevations with varying pressure fluctuations within the borehole. The field engineer can also communicate with the Level TROLL to view data, download data, and set up additional logs without the need to post correct the data or unseal the well to retrieve the data. Equipment Required Gauged Level TROLL 500 or 700 Data Logger Cable Connect TROLL Com Communication Device Vented (Poly or Tefzel ) RuggedCable System Well dock, 2-in. Well cap cable holder Desiccant pack Bushings, one 2 x 1.5 in. male and one 2-in. female Win-Situ 5 Software (with PC) or Win-Situ Mobile Software (with RuggedReader Handheld PC) 3

4 Instrument Setup Figure 1.2 Instrument is placed inside a sealed well to collect data during the SVE process using Method 2. The instruments are programmed using a PC and Win-Situ 5 Software or with a RuggedReader Handheld PC and Win-Situ Mobile Software. Figure 1.3 RuggedReader Handheld PC 1. Connect the TROLL Com to the RuggedReader or PC. 2. Synchronize the instrument clock to the computer. 3. Program a data log. Define the parameters to include units of measurement and the logging rate. 4. Disconnect the TROLL Com from the computer. 4

5 Instrument Deployment 1. Thread the well cap cable holder over the USB cable. Attach the rubber stopper. When using the serial TROLL Com, the well cap cable holder components must be separated and individually threaded over the TROLL Com body. They will not fit over the DB9 connector. The gray rubber stopper will fit tightly over the DB9 TROLL Com body. After the components of the well cap cable holder are on the DB9 TROLL Com, reassemble the gray rubber stopper and the bushing. 5

6 2. Connect the instrument and cable, and thread them through the bushings. 3. Connect to the TROLL Com. 4. Orient the service grip on the cable 1ft below the surface connector. 5. Attach the 2-in. well dock, making sure that the cable runs through the well dock. 6. Fold the TROLL Com cable in half and fit the cable and the TROLL Com through the 2-in. well dock as shown below. This will allow the service grip to support the cable on the 2-in. well dock. 6

7 7. Insert the TROLL Com and cable into the well, allowing the TROLL Com cable to run out of the well through the bushings and well cap cable holder. 8. Thread the bushings together. Thread the bottom of the well cap cable holder to the bushing. 9. Epoxy the top 1.25 in. of the 2-in. well to seal the bushing assembly in place. 10. Insert the rubber stopper into the well cap cable holder. Apply vacuum grease to the inside and outside of the rubber stopper to help ensure a tight seal after the cable holder is tightened around the compression fitting. 11. Attach the bushings and the well cap cable holder to a 2-in. well. 7

8 12. Protect the TROLL Com connector from damage by putting a sealed zip-lock bag containing a package of desiccant over the connector. Method 2 Advantages Only one Level TROLL per monitoring well is required. The field engineer can view real-time data during the test and set up or download additional logs without removing equipment from the well. Data does not need to be post corrected for barometric fluctuations. Method 2 Disadvantages The field engineer must precisely follow the setup instructions to deploy the Level TROLL into each monitoring well. The gauged Level TROLL device doesn t capture pressure fluctuations within the borehole during the course of the SVE application. 8

9 Method 3 Absolute Level TROLL and BaroTROLL with Two TROLL Com Devices and Two Well Caps In this method, an absolute Level TROLL Data Logger and a BaroTROLL Data Logger are set up to log data that can also be viewed in real time using two TROLL Com devices. The real-time water level data will not be compensated for barometric pressure when it is viewed during the test. The instruments are sealed in the SVE well using a PVC T fitting and two well cap cable holders. The Level TROLL records pressure changes due to water level elevation changes and borehole pressure changes. The BaroTROLL collects borehole pressure changes. The two data sets are merged using Win-Situ Baro Merge Software in order to determine water level elevations in the well. Equipment Required Level TROLL 400 or 700 Data Logger, or (Rugged TROLL Data Logger) Cable-connect TROLL Com (2), or Rugged TROLL Com (2) Non-vented RuggedCable System (2), or Rugged TROLL Cable System (2) Well dock, 2-in. (2) Well cap cable holder (2) Bushings, one 2 x 1.5 in. male and one 2-in. female PVC T-fitting Win-Situ 5 Software (with PC) or Win-Situ Mobile Software (with RuggedReader Handheld PC) 9

10 Instrument Setup Figure 2.1 Instruments are placed inside a sealed well to collect real-time data during the SVE process using Method Synchronize the internal clocks on the Level TROLL and BaroTROLL devices. 2. Set up a log for the Level TROLL instrument using Win-Situ 5 Software. For instructions on setting up a log, consult the Help menu in Win-Situ 5 Software. 3. Set up a log for the BaroTROLL that is identical in start time to the log for the Level TROLL. Instrument Deployment See the Instrument Deployment section for Method 2. Modify the setup to include a PVC T fitting that will accommodate two TROLL Com devices and two well cap cable holders. Method 3 Advantages The BaroTROLL Data Logger captures the borehole pressure fluctuations over the entire SVE test. The field engineer can access uncompensated real-time data during the SVE test. Method 3 Disadvantages Two instruments, two direct communication cables, and two TROLL Com devices per monitoring well will increase cost. 10

11 Method 4 The data from the Level TROLL must be post corrected using the BaroTROLL data to reflect the true groundwater elevation. Refer to the In-Situ technical note, Manual Level Mode Correction for Non-Vented Sensors, or use the Win-Situ Baro Merge Software utility. The merged data set has a compounded potential error of two instruments, making it less accurate than Method 2. TROLL 9500 with Internal Barometric Sensor, TROLL Com, and Well Cap In this setup, a gauged (vented) TROLL 9500 Data Logger, vented cable, and part of the TROLL Com are sealed inside the SVE well using a well cap cable holder. The borehole pressure fluctuations are compensated through the vent tube that runs from the TROLL Com, through the cable, and to the back of the TROLL 9500 pressure sensor. The vent tube allows the TROLL 9500 to capture accurate water level elevations with varying pressure fluctuations within the borehole. The field engineer can also communicate with the TROLL 9500 to view data, download data, and set up additional logs without the need to post correct the data. Additionally, the TROLL 9500 includes an internal barometric sensor that can measure the pressure inside the borehole. Keep in mind that the barometric measurement is not needed to compensate level measurements because the level sensor is vented to the atmosphere. The TROLL 9500 Instrument includes an optical dissolved oxygen sensor that is necessary for air sparging with SVE. Consult an In-Situ Inc. representative for more information. Equipment Required Gauged TROLL 9500 Instrument Cable Connect TROLL Com Communication Device Vented (Poly or Tefzel) RuggedCable System Well dock, 2-in. Well cap cable holder Desiccant pack Bushings, one 2 x 1.5 in. male and one 2-in. female Win-Situ 5 Software (with PC) or Win-Situ Mobile Software (with RuggedReader Handheld PC) 11

12 Figure 2.2 TROLL 9500 is placed in a sealed well to gather real-time water level and barometric data in Method 4. To set up and deploy the TROLL 9500 follow the Method 2 Instrument Setup and Instrument Deployment steps. Method 4 Advantages Only one instrument per monitoring well is required. The field engineer can view real-time data during the test and set up or download additional logs without removing equipment from the well. Data does not need to be post corrected for barometric pressure fluctuations. The engineer can use the instrument's internal barometric sensor to monitor borehole pressure fluctuations inside the monitoring well independent of water level measurements. Method 4 Disadvantages The TROLL 9500 cannot be used in sub one-inch wells. This option is more expensive than using a vented Level TROLL. 12

13 Optional Non-destructive Installation with PVC Extension and Strain Relief It is optional to install the well cap cable holder in a temporary, non-destructive manner using parts purchased from a hardware store. This configuration includes a well dock to provide strain relief for the TROLL Com cable. It also includes a length of PVC pipe that can be drilled to accommodate connection to a differential pressure gauge if appropriate for the application. This is an example of how you might approach an installation to a flush-mount well, but the configuration has not been extensively tested in. x 2 in. rubber boot and 2 hose clamps 4. Threaded coupler 2 in. female NPT 2. 2 in. schedule 40 PVC 2 in. to 4 in. long 5. 2 in. male NPT to 1.5 in.female 3. Well dock (purchase from In-Situ Inc.) Method Accuracy Comparison Example Example for comparison* Level Sensor 35 ft Usable Depth Method 1 or 3 Non-Vented Level TROLL & Baro TROLL 6. Well cap cable holder with liquid/airtight passthrough assembly (purchase from In-Situ Inc.) Method 2 Vented Level TROLL Method 4 TROLL psia = ± 0.07 ft 15 psig = ± ft 15 psig = ± ft Barometric Sensor ± 0.07 ft NA ± 0.11 ft Water Level Total Accuracy Barometric Pressure Accuracy ± 0.14 ft (The accuracy of the Level and Barometric sensors added together.) ± ft ± ± 0.07 ft NA ± 0.11 ft * All accuracy examples are for the full temperature range of the sensor. ** The example for method 1 and 3 calculations are for the BaroTROLL Instrument. Use of a Rugged BaroTROLL will decrease the accuracy. References Wong, J Design of Remediation Systems, Boca Raton. CRC/Lewis Publishers, Boca Raton, Florida. 13