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In deeper ground-water flow systems, the contact time between water and minerals is much longer than it is in shallow flow systems, As a result, the initial importance of reactions relating to microbes in the soil zone may be superseded over time by chemical reactions between minerals and water (geochemical weathering), As weath- ering progresses, the concentration of dissolved solids increases, Depending on the chemical composition of the minerals that are weathered, the relative abundance of the major inorganic chemicals dissolved in the water changes (see Box E), Surface water in streams, lakes, and wetlands can repeatedly interchange with nearby ground water, Thus, the length of time water is in contact with mineral surfaces in its drainage basin can continue after the water first enters a stream, lake, or wetland, An important consequence of these continued interchanges between surface water and ground water is their potential to further increase the contacltime between water and chemically reactive geologic materials, CHEMICAL INTERACTIONS OF GROUND WATER AND SURFACE WATER IN STREAMS, LAKES, AND WETLANDS Ground-water chemistry and surface-water chemistry cannot be dealt with separately where surface and subsurface flow systems interact. The movement of water between ground water and surface water provides a major pathway for chemical transfer between terrestrial and aquatic systems (see Box F), This transfer of chemicals affects the supply of carbon, oxygen, nutrients such as nitrogen and phosphorus, and other chemical constituents that enhance biogeo- chemical processes on both sides of the interface, This transfer can ultimately affect the biological and chemical characteristics of aquatic systems do\vnstream. "The movement of water between ground water and surface water provides a major pathway for chemical transfer between terrestrial and aquatic systems" 23