Well and spring waters were collected and analyzed in Valle Trinidad in order to assess a relationship between the geology and valley hot water system. The chemistry of the thermal waters is distinct from that of the normal groundwater system such that the extent of the thermal component was determined to be limited in the alluvium. The hot waters are restricted below a clay layer or series of layers functioning as a semipermeable membrane that prevents interaction of the water on either side of this barrier. The presence of the thermal water can be recognized only where it penetrates into the alluvium above the clay layer. The thermal water emanates at the hot spring, S1, where the clay seam outcrops at the surface. The chemistry identifies a thermal component at the town spring, S3, probably percolating up along a permeable normal fault-associated zone. The well data also support a north- south fault through well locations PW10, PW11, PW12, which serves as a zone of leakage for the thermal waters. Geothermometers were utilized from the geochemical data and indicate the reservoir temperature for the thermal water to be 90° to 103°C. The extent of the thermal water below the clay cannot be predicted. The remainder of the valley groundwater system follows a normal groundwater circulation pattern in the alluvium flowing from the perimeter recharge areas to the center of the valley, yielding an increase in the total dissolved solids and the major ions, Cl and SO4 , in the central areas.