NGWA Event and Education Recordings Archive

Surface water/groundwater (SW/GW) interaction is a hotly debated water management topic. Surface-water managers voice concern that pumping from alluvial aquifers may decrease stream flow. Contrastingly, alluvial groundwater managers point to decreased stream flow as a potential cause for lower aquifer levels. Research conducted in the northern segment of the Brazos River Alluvium Aquifer revealed complex SW/GW interactions in a highly heterogeneous system under dynamic flow conditions. Ionic chemistry combined with isotopes (δ²H and δ¹⁸O), core data, bank observations, and hydrographs indicated current thoughts may be based on erroneous assumptions and data may be inadequate for many management decisions. We found a gaining stream but with a compartmentalized alluvial aquifer only partially connected to the river. Although the river showed the greatest influence on aquifer levels, the river seldom contributed water directly to the aquifer. Data on aquifer characteristics extrapolated from existing wells appeared to have inherent bias and less than adequate spatial coverage. The ionic chemistry and isotope data indicated the aquifer contained stored groundwater recharged directly by local precipitation and was distinct from the river water. Hydrograph analysis showed flow reversals during high river stages creating short-term losing stream conditions but with a hysteresis effect. We discovered we knew little about bedrock contributions to aquifer recharge, little about groundwater interactions with old gravel-mine lakes, and almost nothing about floodplain tributaries interacting with the aquifer. Streams and aquifers in most alluvial valleys are hydrologically connected. However, we may not understand the spatial and temporal complexity required to make effective policy decisions to manage these systems conjunctively and sustainably.