Frequently Asked Questions

	What is MDPE Technology? MDPE is a modification of traditional soil vapor extraction (SVE) that utilizes the extraction of both groundwater, soil vapor, and/or phase separated and dissolved phase contaminants simultaneously. MDPE was developed primarily for the remediation of volatile organic contaminants (VOCs) and the contaminants in low to moderate permeability subsurface soil/rock formations. Normally, the groundwater table is lowered slightly in order to dewater the "saturated" zone, so that the SVE process can be applied to the newly exposed soil. Volatile compounds sorbed onto the previously saturated soil are stripped by a combination of vacuum and induced vapor flow to the extraction wells.
	Is MDPE appropriate for my site? *Lithology with High Permeability* Chances are, unless your site has very high hydraulic conductivity (gravels and sands), MDPE can be successful at remediating significant PSH and VOC's from the subsurface. Even at sites with high hydraulic conductivities, the MDPE process has been quite successful at remediating hydrocarbon PSH and to a lesser extent, vapor phases. A simple 8-hr test will quickly determine the feasibility at a particular site. Proper management of the rate of liquid drawdown by the field engineer/geologist is critical at these sites. *Deep water Table* If a project site has a water table greater than 60 feet below grade, it’s possible that EnVac will utilize a submersible pump in conjunction with its large liquid ring surface pump to ensure that acceptable and sustainable vacuums and induced air flow rates are achieved. Although EnVac has performed extraction of liquids and vapors from greater depths than 60 feet using only the drop tube method without the assistance of a submersible pump, the possibility of reduced efficiency is taken into account. EnVac's application of MDPE (using high vacuum and high induced air flow rates) lends itself to a broader range of applications (i.e. greater depths and higher liquid flow rates) than the lower vacuum and induced air flow rates applied by “*EnVac imitators*”
	What can I expect to pay for quality MDPE services? A primary consideration is the location and distance for mobilization of equipment and whether vapor emission treatment is required in the state you're in. Most jobs are bid in a lump sum arrangement with a sliding scale to take duration into account. Contact EnVac for a site specific bid price and to answer any questions. In Texas, EnVac follows the specific RCG’s (recommended cost guidelines) provided by the TCEQ.
	Does EnVac provide pilot testing services as well as MDPE? Yes, EnVac provides a range of pilot testing services specifically designed to meet its customers' needs. Please contact EnVac directly by email or telephone to get specific assistance.
	How much vacuum does EnVac apply to my monitor/recovery wells? Anywhere from 1 to 2 inches of Hg and up to as much as 20 inches of Hg may be applied to an individual well depending upon how many wells are extracted from simultaneously and how permeable the screened subsurface material is. On average, the vacuum pressure applied to each extraction well is between about 5-inches Hg and 15-inches Hg. The higher the permeability of subsurface soil and rock in the screened interval, the lower the effective vacuum will be at a particular extraction well. Additionally, monitor/extraction well construction may influence effective vacuum (i.e., length of screened interval, poor completion). In extraction wells where the water table is deeper than approximately 30 feet below grade, and a drop tube is utilized with vacuum and air flow provided by the Kaiser liquid ring pump, vacuum pressure in the extraction well will begin to diminish slightly. This is simply because there is some diminution of vacuum pressure in the process of introducing ambient air flow at the wellhead in order to lift fluids to the surface.
	What kind of air flow rates can I expect at my site(s)? Air flow rates from extraction wells will be highly dependent upon factors such as soil permeability, screened interval, and potential well construction issues (i.e. poor surface seal).
	How much groundwater will EnVac generate at my site? Groundwater production will again be a function of soil/rock permeability and proximity of well screen to the water table. The goal of MDPE is not groundwater production, but rather the hydrocarbons that are trapped in the interstitial pore spaces in or near the saturated zone of an aquifer.
	How is the reclamation of recovered groundwater handled? During MDPE, EnVac stores recovered fluids in its tank (up to 2,600 gallons) until the end of the event, at which time the client's reclamation facility or disposal handler wil meet us at the site. All liquids are gauged and offloaded to the appropriate reclamation/disposal facility or transporter .
	Can EnVac remove phase separated hydrocarbons from my site(s)? Yes. Please contact EnVac to discuss specific site characteristics.
	Can EnVac remove dissolved phase separated hydrocarbons from my site(s)? Yes. Please contact EnVac to discuss specific site characteristics.
	What diameter wells can EnVac utilize to perform MDPE? EnVac can work with 2-inch, 4-inch, 6-inch or larger well casings
	At what depths can EnVac effectively perform MDPE? EnVac routinely works at depths up to 60 feet below grade using a drop tube only. If necessary, EnVac may utilize a submersible pump in conjunction with the powerful liquid ring surface pump (i.e., water table > 60 feet, pilot testing, etc.).
	What sort of air permit issues will be encountered if I chose to use MDPE to remediate my site(s)? These issues will depend upon the state that the site is located in. Contact EnVac for further assistance. In Texas, a PI-7 air permit exemption will be required and should be applied for through the “New Source Review Board” of the TCEQ. EnVac will be happy to be of assistance with this issue when a proposal is presented to the TCEQ and a Corrective Action Response Form (CARF) is issued .