INTERPRETATIONS OF STRUCTURAL AND STRATIGRAPHIC CONTROLS ON GROUNDWATER QUALITY IN CHIMAYÓ, ESPAÑOLA BASIN, NEW MEXICO

In the east-central part of Chimayó, a town in the Rio Grande rift of north-central New Mexico, many of the domestic wells screened in Miocene basin fill (Tesuque Formation) have elevated levels of sodium, bicarbonate, sulfate, and chloride. The problem is so severe that this part of Chimayó needs to procure additional sources of drinking water. Comparison of a groundwater quality map (John Shomaker & Assoc.) with recent geologic maps (STATEMAP program) indicates that geologic features may control the spatial distribution of the poor-quality groundwater. Wells producing poor-quality groundwater from the Tesuque Fm are generally found west of the Chimayó structure, a west-down normal fault and monocline along the eastern side of Chimayó. The solutes in the poor-quality groundwater may be derived from Paleozoic strata interpreted to underlie the Tesuque Fm west of the Chimayó structure. We interpret that the fault of the Chimayó structure and an east-down conjugate fault to the west facilitate upwelling of the poor-quality groundwater. Springs sourced in the Tesuque Fm appear to be more abundant to the east of the Chimayó structure, indicating that this structure also locally acts as a barrier to westward groundwater flow. In the Tesuque Fm, the poor-quality groundwater appears to be restricted within or below an informal lithologic unit called lower-lithosome B, which consists of muddy floodplain deposits with 5-15% gravelly sand channels that trend south-southwest. We interpret that the predominately muddy texture of lower-lithosome B may form a local boundary which largely confines poor-quality groundwater to progressively greater depths in a westward (down-dip) direction. Furthermore, poor-quality groundwater that does enter lower-lithosome B likely flows preferentially in its coarse channels to the southwest. A potential source for additional drinking water is saturated basin fill or fractured crystalline bedrock east of the Chimayó structure. Any new supply wells in the Tesuque Fm west of the Chimayó structure should be screened adequately above and west of the lower-lithosome B unit.