Commercial coal deposits of the San Juan basin are associated with intertonguing Cretaceous marine and nonmarine deposits. Sandstone units in this stratigraphic sequence act as ground-water aquifers that may be affected by mining operations. Sandstone aquifers can be classified as areally extensive with significant potential for ground-water development, or as discontinuous aquifers of limited potential.

Coal deposits in the lower part of the stratigraphic sequence generally are associated with areally extensive sandstones.

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These include the Dilco coal which rests on the Dalton Sandstone, and the Cleary coal which overlies the widely distributed Point Look Sandstone. Menefee and Kirtland-Fruitland coals are commonly associated with discontinuous sandstones of limited ground-water potential.

Much of the natural ground water associated with the coal deposits of the San Juan basin does not meet the water-quality standards of the Public Health Service or the Office of Surface Mining. However, to prevent further degradation, stripping and reclamation practices should be based on the regional hydrologic system and on the hydrologic characteristics of the coal-associated aquifers.

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