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In addition to Ian's comment. The sanitary seal is also to stop faecal contaminants draining either straight into the well or straight into the groundwater source close to the well. The latter is relevant as the action of pumping water from the well, even at a low rate, draws groundwater from the surrounding strata back into the well. So, the shorter the distance from the well, the greater the chance of faecal contaminants surviving the journey and being pumped back up the well. Therefore, if the well is tapping an unconfined groundwater source (it sounds as if this is the case), it is important to inhibit the deposition of faecal matter around the well. This means that the bigger the apron, the better the protection because the longer will be the time for the faecal material to reach the groundwater and therefore the greater the chance that harmful elements will have died off. Even if you only have a small apron, it is important to ensure that water is channelled away and cannot pool around the well head or around the edge of the apron because that will attract animals, who may well deposit faecal matter while they stop for a drink.