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Ground water was sampled from wells completed in the Gallup, Dakota, and Morrison aquifers in the San Juan Basin, New Mexico, to examine controls on solute concentrations. Samples were collected from 38 wells primarily from the Morrison aquifer (25 wells) in the northwestern part of the basin. A series of samples was collected along ground-water flow paths; dissolved constituents varied horizontally and vertically. The understanding of the flow system changed as a result of the geochemical analyses. The conceptual model of the flow system in the Morrison aquifer prior to the study reported here assumed the Westwater Canyon Member of the Morrison aquifer as the only significant regional aquifer; flow was assumed to be two dimensional; and vertical leakage was assumed to be negligible. The geochemical results indicate that the Westwater Canyon Member is not the only major water-yielding zone and that the flow system is three dimensional. The data presented in this report suggest an upward component of flow into the Morrison aquifer. The entire section above and below the Morrison aquifer appears to be controlled by a three-dimensional flow regime where saline brine leaks near the San Juan River discharge area. Predominant ions in the Gallup aquifer were calcium bicarbonate in recharge areas and sodium sulfate in discharge areas. In the Dakota aquifer, predominant ions were sodium bicarbonate and sodium sulfate. Water in the Morrison aquifer was predominantly sodium bicarbonate in the recharge area, changing to sodium sulfate downgradient. Chemical and radioisotopic data indicate that water from overlying and underlying units mixes with recharge water in the Morrison aquifer. Recharge water contained a large ratio of chlorine-36 to chlorine and a small ratio of bromide to chloride. Approximately 10 miles downgradient, samples from four wells completed in the Morrison aquifer were considerably different in composition compared to recharge samples. Oxygen stable isotopes decreased by 2.8 per mil and deuterium decreased 26 per mil, relative to recharge. Carbon-14 radioisotope activities were not detectable. Chloride-36 radioisotope ratios were small and bromide to chloride concentration ratios were large. These results suggest two potentially viable processes: ion filtration or trapping of ancient dilute water recharged under a humid climate. For water samples near the San Juan River, pH decreased to about 8.0, chloride concentrations increased to more than 100 milligrams per liter, and ratios of chlorine-36 to chlorine and bromide to chloride were small. Leakage of deep basin brine into the fresher water of the Morrison aquifer appears to control ion concentrations. INTRODUCTION In October 1984, the U.S. Geological Survey (USGS) began a regional assessment of the San Juan structural basin aquifer systems in New Mexico, Colorado, Arizona, and Utah as part of its national Regional Aquifer-System Analysis (RASA) program (Bennett, 1979). The San Juan Basin is located in New Mexico, Colorado, Arizona, and Utah, covering an area of approximately 21,600 mi 2 (fig. 1). The basin, a structural depression in the eastern part of the Colorado Plateau, is approximately 140 mi wide by 200 mi long, and land-surface altitudes range from about 4,500 ft in the northwest to about 11,000 ft in the southeast. The San Juan Basin is an arid region where development of energy and water resources is essential to the economy. Surface-water resources are fully allocated, so ground-water resources are vital to industries, municipalities, and ranchers. Purpose and Scope This report presents geochemical and isotopic data used in examining sources of solutes and hydrologic and chemical controls that affect the concentration and distribution of solutes in aquifers in the San Juan Basin. The Gallup, Dakota, and Morrison aquifers were chosen for detailed geochemical analysis because of available water wells having known completion data, ground-water modeling results, and mineralogical analyses. These aquifers are equivalent stratigraphically to the Gallup Sandstone, Dakota Sandstone, and Morrison Formation. These aquifers are used extensively as water supplies for industry, communities, and livestock. The report examines in detail the geochemistry of the three aquifers. The scope is constrained primarily to the northwestern part of the basin due to limited areal distribution of wells completed in a single aquifer and disturbances to the natural ground-water system by mining and petroleum industries in other parts of the basin. The main focus of this report is on the Morrison aquifer from the communities of Sanostee to Shiprock. Data for the Gallup and Dakota aquifers are provided for purposes of comparison and examination of vertical changes in flow and water quality. Hydrologic and water-quality data for the underlying Entrada Sandstone also are evaluated for the effects that water from this unit may have had on the Morrison aquifer. Previous Studies The hydrogeology of the San Juan Basin was described comprehensively by Stone and others (1983). USGS Hydrologic Investigations Atlases (HA 720-A through 720-J) have been published for 10 major aquifers in the basin (Craigg and others, 1989,1990; Kernodle and others, 1989,1990; Dam and others, 1990a, b; Levings and others, 1990a, b; and Thorn and others, 1990a, b). Data are presented on maps and in tables to describe the geology, hydrology, and water quality for each aquifer. The only previous study that examined the regional San Juan Basin ground-water geochemistry of Jurassic and Cretaceous aquifers was performed by Berry (1959). He collected physical and chemical data from water wells, producing oil or gas wells, and drill-stem tests. BOUNDARY OF THE COLORADO PLATEAU 105"

Geochemistry of ground water in the Gallup, Dakota, and Morrison aquifers, San Juan Basin, New Mexico