How much more water will a 4-inch well produce over a 2-inch well? Before you answer, the exposed surface area is twice as much area. Therefore, it seems like it should be twice as much, i.e. 1 foot of 2-inch screen has 75 square inches exposed, whereas 1 foot of 4-inch screen has 150 square inches of screen exposed. So what's your answer? Twice? One and one-half? How about 1¿ (10%) more? By doubling the diameter of a water well, it only increases the yield by about 10 percent. In clean medium-to-coarse sand formations it even may be less.
The hydraulics of ground water flow - you know, all the calculus and differential equations - say that if two wells are of the same well efficiency (rarely is a well 100%) and the flow to the well is laminar, not turbid (which is usually the case), then the amount of water flowing into the well is controlled by the hydraulics of the formation and not so much the size of the well. Some 2-inch wells are capable of pumping in excess of 50 gpm, however, no 2-inch pumps are capable of 50 gpm. Therefore, wells often are drilled larger - not for more volume, but to accommodate the larger pump sizes. Larger wells also have lower ground water entrance velocities through the screen, which can cause excessive drawdowns in high-capacity wells.
Although increasing the well diameter is not very effective in increasing the well yield, other well modifications greatly can increase well yield without increasing the diameter. Since very few wells are 100 percent efficient (and many wells are less than 50 percent efficient), if the efficiency could be increased, the well yield also would be increased proportionately.
The best way to increase efficiency of a screened well is to have a screen with a large percentage of open slot area. The slots need to be as large as possible, but not so large that would allow for sand to pass through the well screen. Slotted screens often used in monitoring wells serve their intended purpose of allowing water to enter the well to measure water levels. They rarely are adequate to produce the volumes of water needed for remediation purposes or water supply. Wire wrapped screens (PVC or stainless steel) offer many times the percentage of open area to the well, which allows the well to be much more efficient. Longer lengths of screen also can increase yield without increasing the well diameter.