Intrusive Bodies Influencing Groundwater Flow

Igneous intrusive bodies occasionally are observed to act as subterranean barriers or dams. Good examples are dikes (Fig. 3.11), sills (Fig. 3.12), and stocks. Fresh igneous rocks are nonpermeable, but with time they become fractured and may be somewhat conducting. Clay-rich weathering products may fill such fractures and improve their sealing properties. 

In the case of dikes, water gets dammed on the upstream side (Fig. 3.11). When placing a well near a dike it is essential to recognize the existence of the dike, although it is often concealed by alluvium. 

Lack of such information may result in a dry borehole drilled on the downflow side of the dike. 

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