地下水与社区健康的关系。
几乎每个社区都在所有国家生活在一个低于一定深度的土壤上，饱和水。这有时非常靠近表面，有时非常深;有时几乎静态，有时在主动运动中;有时受到雨水和干旱的影响，有时在雨水和干旱的影响下造成的升级，有时在其海拔地区均匀。有时，即使它的水平高于潮水，距离潮水的水平和遥远的距离，有时它会受到上升和下降的行动。
我曾说过这种水有时几乎静态;它永远不会完全如此。在某种形式或其他形式中，它是接受添加，并且通过蒸发或泄漏正在遭受废物。在其维持其水平的趋势之后，必须通过其他点的流量提供任何点的废物，并且在任何点的加法必须通过向其他要点的流量均衡。有时，隆粘土或岩石的边缘保持在土壤中接近由磨机产生的条件的土壤，在给定的仰角处保持饱和线，然而，在这种情况下很少是水平的，因为损失的倾向蒸发或泄漏超过另一部分，并且由于偏远来源流入。通常，河流主要通过地下流动进给。在大雨期间，一部分垮台在表面上流出，直接进入河流。它的大多数都进入了地面，提高了其饱和水的水平，迫使它向前的排水线，无论是溪，溪还是河流。在这些排水线的流动相当大的情况下，土壤中的水总是较高，有时比河流的水分更高，只有当后者被表面水流入暂时抬起时才节省。 It requires a certain amount of extra elevation or head to force the water forward toward the river, in order to overcome the friction of the particles of earth among which it is flowing. In coarse gravel this difference of head may not amount to much, but as the obstructing particles become finer, increasing the friction surface and reducing the size of the space through which the water must run, more and more head is required to produce the discharge. If the discharge is into water of permanent level, the depth of the water from the surface at any given point may remain approximately the same, but if the discharge is into a contiguous body of water, subject to a strong tidal rise and fall, the elevation of the ground-water for some distance back from the shore will be subject to corresponding fluctations. Although the level of the ground-water be some feet higher than the highest high tide, it will fall somewhat as the tide falls, and will rise somewhat as the tide rises, even although its lowest surface elevation during low tides may never run so low as the level of the highest tides.
