On capture width and capture zone gaps in multiple-well systems

A barrier well line's capture width along its alignment is shown to depend only on total system flow rate Q(T) and aquifer background flow rate per unit width Q(o) (prepumping gradient times transmissivity) under idealized horizontal flow: Irrespective of number of wells, the capture width is Q(T)/2Q(o), as for a hypothetical single well with flow rate Q(T). This result applies even to barrier lines with irregular well spacings and flow rates-and, in fact, to the capture width through the center of any pumping configuration that is symmetrical respecting a line perpendicular to uniform background pow A flow-perpendicular line of regularly spaced, equal wells ("regular barrier line") has the same capture width along its alignment as an identical well line parallel to background pow has through its center. In the general symmetrical case, the center capture width remains Q(T)/2Q(o) whether transverse well spacings are large (allowing contaminated ground water to escape through capture zone gaps), critical (with incipient gaps), or safely small, For design reference, critical spacings derived for regular barrier lines of N wells with N through 100 and N-->infinity vary from Q/pi Q(o) (N = 2, previously known) to Q/2Q(o) (N-->infinity), where Q is each well's flow rate. Incipient gaps at critical spacing occur always between the two center wells, or the center well and two adjacent wells. Spacings substantially smaller than critical are recommended for contaminant capture. Designs are illustrated that preclude capture zone gaps.