THE IMPORTANCE OF DETAILED STRATIGRAPHY IN ASSESSING ALLUVIAL AQUIFERS

Alluvial aquifers contain locally significant amounts of water that may provide needed irrigation and domestic supplies. Due to their shallow, unconfined nature alluvial aquifers are easily accessible and directly recharged, making them sustainable under effective management. However, they also can be affected rapidly by production, drought, and contamination. The location of alluvial aquifers often provides a direct connection with adjacent streams or underlying bedrock aquifers. Therefore, modeling the sediment geometry and boundaries of an alluvial aquifer can develop a stronger understanding of aquifer framework and help interpret flow dynamics such as groundwater/stream interactions and cross-formational flows more accurately. The sediments that comprise alluvial aquifers are heterogeneous and typically fine upward from channel gravels to sands, silts and clays. Laterally, sediments are discontinuous, as coarse channel deposits may abruptly end adjacent to finer overbank deposits. This heterogeneous discontinuity of sediment texture affects production, recharge, and water connectivity. This project develops detailed stratigraphic models in several portions of the northern segment of the Brazos River Alluvium Aquifer. Special interests in this project are the stratigraphy of lateral boundaries and the overlying vadose zone. Sediment cores collected with a GeoProbe 6620DT rig are used to construct a stratigraphic model of the heterogeneous sediments. The detailed stratigraphic model can then be correlated to hydrogeological data to better understand responses to aquifer recharge, river level fluctuations and well productivity. This model will allow us to better understand aquifer framework and paleo stream characteristics to hopefully aid groundwater management decisions.