The primary objective of the present study was to elucidate the relative contributions of exchangeable cations and surface forces to the swelling pressure, Π, of montmorillonite. To achieve this objective, measurements were made of the amount of water imbibed by Na‐saturated montmorillonites having different cation exchange capacities, b dimensions, and specific surface areas under applied pressures of 0, 1, 3, and 5 atm. The resulting data were used to develop the empirical equation (Π + 1) = exp [α/(mw/mm)] in which mw/mm is the mass ratio of water to montmorillonite and α is a linear function of the specific surface area and cation exchange capacity. Analysis of double‐layer theory in the light of this equation led to the conclusion that Π is affected less by exchangeable cations than by surface forces. It was suggested that exchangeable cations have a secondary effect because few of them dissociate from the layer surfaces.

Irrigation was introduced to provide more stability and profit to Great Plains agriculture and the High Plains aquifer is a primary source of irrigation water. Irrigation pumping began in the 1920's and by the 1980's had transformed 6.5 million ha of dryland crop production and rangeland into highly productive farmland (Supalla et al., 1982). The High Plains aquifer underlies 445 million ha in parts of Colorado,

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