Groundwater interacts with surface water in a wide variety of physiographic and climatic landscapes from the mountains to the sea. The interaction of these two hydrologic systems has been studied from the scale of a few centimeters to many kilometers. For decades, studies of the interaction of groundwater and surface water concentrated primarily on the interaction of groundwater with streams in alluvial systems. Numerous analytical solutions were developed over the years to consider a wide variety of theoretical and field conditions, and analytical solutions are still being developed. In addition to the analytical studies, the use of numerical models to evaluate the interaction of groundwater and surface water has become commonplace over the past 25 years. Recently, improved numerical methods for simulating the interconnection of groundwater and surface water have been developed, especially with respect to simulating the surface water component.
An example of the benefits of past studies of the interaction of groundwater and surface water is evidenced by the increasing amount of research devoted to managing the conjunctive use of these resources. Conjunctive use studies included those concerned with optimum development of the water resource, with water quality ramifications, and with economic analyses.
In recent years, studies of the interaction of groundwater and surface water have expanded in scope to include studies of headwaters streams, lakes, wetlands, and estuaries. Interest in the relation of groundwater to headwaters streams increased greatly in the past 15 years because of concerns related to acid precipitation. The relation of groundwater to lakes has been of growing interest since the 1960's because of concerns related to eutrophication as well as to acid precipitation. Interest in the relationship of groundwater to wetlands and to coastal areas has accelerated in the past 10 to 15 years because of the great loss of these ecosystems to development. Although studies of these systems have increased in recent years, this effort is minimal compared to what is needed.
To evaluate the interaction of groundwater and surface water in all environments and at all scales, analytical and numerical methods will need to be continually improved. For example, to effectively manage resources it will be necessary to simulate systems as realistically as possible, that is, realistic system geometries and transient conditions. This is needed to evaluate the effects of complex areal and temporal distribution of recharge, as well as the effects of transpiration directly from groundwater, on the interaction of groundwater and surface water. Both of these processes have been shown to be most active near boundaries of all types of surface water, affecting flow rates, flow direction, water chemistry, and deposition of minerals.
Future studies of the interaction of groundwater and surface water would benefit from, and indeed should emphasize, interdisciplinary approaches. Physical hydrologists, geochemists, and biologists have a great deal to learn from each other, and contribute to each other, from joint studies of the interface between groundwater and surface water. The biogeochemical processes within the upper few decimeters of sediment beneath nearly all surface water bodies can have a profound effect on the chemistry of groundwater entering surface water, as well as on the chemistry of surface water entering groundwater.