The Arbuckle aquifer is an extensive aquifer that contains mostly saline water and that immediately overlies Precambrian ' basement ' rocks throughout Kansas, except for major uplift areas where it has been removed by erosion. In the southeast part of the state, it is a major freshwater aquifer. The upper part of the Arbuckle contains significant oil and gas reservoirs in central and south-central Kansas. During the last 40 years the Arbuckle also has become the major zone of fluid disposal in the state. Most of the fluids disposed into the Arbuckle were produced from oil and gas wells in other formations. However, in recent years, state water agencies have become increasingly concerned about injection of fluids into the subsurface and the effects of injection on the hydrologic systems involved. An investigation of the geohydrology of the Arbuckle aquifer and of the hydrologic aspects of fluid disposal into the Arbuckle was conducted to evaluate these effects. Hydraulic characteristics obtained from drill stem tests, injection tests, and numerical modeling have indicated a range of permeability in the Arbuckle from 1 millidarcy to 30 darcys. Analysis of injection tests indicated that average permeability in the basin areas probably is in the 50-300 millidarcy range. Analyses of 76 geophysical logs indicate an average porosity of about 12%. An evaluation of the geohydrology of the Arbuckle shows that it is a large regional flow system that is in hydraulic connection with several other major aquifers. Groundwater flow within the Arbuckle is principally from the west-northwest to the east-southeast. Brine disposal in the Arbuckle has been increasing over the years. Rates of injection average about 60 gal/min. Model analysis, using aquifer properties similar to those expected in the basin areas and under selected conditions of well injection into the Arbuckle, indicates that, even with an injection rate of only 100 gal/min, pressure increases equivalent to fluid-level rises of up to 100 ft are expected as far as 500 ft away from the injection well. The model analysis indicates that the effects of transmission of fluid through the confining layer on overlying units are minor. (Lantz-PTT)