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! nitrate is probably the result of heavy applications of chemical fertilizer during the last decade. J. W. Powell (1958, Ground- Water Resources of the San Luis Valley, Colorado, U. S. Geological Survey Water Supply Paper 1379) makes no mention of high nitrate concentrations in this area. Nitrate concentration is variable in waters of the unconfined aquifer. Concentration is primarily a function of depth--a well 45 feet (13.7 m) deep will have a higher nitrate concentration than a well 90 feet (27.4 m) deep. Water from any well less than 100 feet (30.5 m) deep within the area of nitrate concentration of more than 10 mg/l should be tested for nitrate content before it is used for dr i nk i ng. The quality of water in the confined aquifer near the edge of the basin is excellent; the specific conductance (an index of dissolved- sol ids concentration) is generally less than 200 umhos/cm. The chemical quality of the water in the aquifer near the basin perimeter reflects the quality of the recharge water. As the confined water moves down-gradient, the presence of clay beds, lava flows, and minerals in the basin fill exert their influence, both chemically and physically to change the general composition of the water. Compositional changes take place at a faster rate than concen- tration. Ion exchange with clay in the aquifer accounts for a loss In calcium and an increase in sodium. Silicate hydrolysis contributes to the alkaline pH, and as the water moves toward the center of the basin, it becomes a sodium bicarbonate type. Medium- and high-sodium (alkali) hazard water exists in a much larger area in the confined system than in the unconfined. Inasmuch as the high-alkali-hazard water underlies arable land in some areas, it is utilized for irrigation. However, the soil in these areas is clayey, and the danger of reducing the soil permeability exists. In the central part of the closed basin between the depths of 100 and 1,000 feet, (30.5 and 305 m) the water of the confined aquifer has a brownish color and contains gas. Part of the gas is flammable and contains hydrogen sulfide. Although the color agent in the water is itself generally considered harmless, it is commonly associated with high-flouride concentrations, high- to very high sal inity hazard, medium- to high-alkal i hazard, and the above mentioned gases. Fossil peat beds in buried lake deposits are believed to account for the organic derivatives that impart the brown color to the sodium bicarbonate water. Flouride concentrations ranges from O. I to 13 mg/l in water from the confined aquifer. c:' :1.: 0-7