SUBSURFACE DRAINAGE SYSTEM-DESIGN AND DRAIN WATER-QUALITY

Subsurface drainage systems in regions with poor-quality, shallow ground water collect water that potentially may have adverse affects on the environment. In semiarid regions, the poor-quality ground water may result from evaporative salinization, leaching of the soil root zone from excess irrigation, or some combination of these two processes. Traditional subsurface drainage system design procedures do not consider the water-quality aspects associated with the depth and spacing of drains in a particular hydrogeologic setting. Typically, deep drains at relatively wide spacings tend to collect a larger fraction of deep ground water as compared to shallow ground water. In many cases, the deep ground water is of poorer quality. The purpose of this study is to illustrate and qualitatively describe the results of numerical simulations that examine the influence of drain spacing and depth on drain water quality. This paper considers steady and transient recharge to the water table in a layered anisotropic soil. Results of this and subsequent analyses can be used to propose conceptual modifications of traditional drainage system design for water quality control in irrigated regions.