Fluctuations in the groundwater table should also be considered when designing an SVE system. This is most important for horizontal extraction wells, where the screen is parallel to the water table surface. These results are accomplished because surface seals force fresh air to be drawn from a greater distance from the extraction well. If a surface seal is used, the lower pressure gradients result in decreased flow velocities. This condition may require a higher vacuum to be applied to the extraction well.
Pilot studies are an extremely important part of the design phase.
Data provided by pilot studies is necessary to properly design the full-scale SVE system. A pilot test is recommended for evaluating SVE effectiveness and design parameters for any site, especially where SVE is expected to be only marginally to moderately effective. Different extraction rates and wellhead vacuums are applied to the extraction wells to determine the optimal operating conditions. Vapor concentrations are also measured at two or more intervals during the pilot study to estimate initial vapor concentrations of a full-scale system. The vapor concentration, vapor extraction rate and vacuum data are also used in the design process to select extraction and treatment equipment.