the Lower Trinity Aquifer

Staff Report & Agenda Item Details

4. the Lower Trinity Aquifer Water budget information for the Edwards-Trinity fPlateau) and Trinity aquifers were extracted from active model cells within the respective aquifer footprints. o a a a The General-Head Boundary (GHB) package of MODFLOW was used to represent flow out of the study area between the Hill Country portion of the Trinity Aquifer and the Edwards [Balcones Fault Zone) Aquifer or the confined parts of the Trinity Aquifer underlying the Edwards [Balcones Fault Zone) Aquifer. The groundwater availability model includes some portions of the Edwards Group outside the official boundary of the Edwards-Trinity (Plateau) Aquifer. Though flow for these areas is not explicitly reported, the interaction between the Edwards Group (outside the Edwards-Trinity Plateau Aquifer) and the underlying Trinity Aquifer would be shown in the "flow between aquifers" segment of Table L, if Layer L was present in the district. Only the outcrop area of the Hill County portion of the Trinity Aquifer was modeled, and the down-dip extent that underlies the Edwards (Balcones Fault Zone) Aquifer is not included. The model was run with MODFLOW-96 [Harbaugh and McDonald, 1996). a GAM Run 19-011: Cow Creek Groundwater Conservation District Groundwater Management Plan March 5,2019 Page 6 of 17 Marble Falls, Ellenburger-San Saba, and Hickory Aquifers We used version 1.01 of the groundwater availability model for the minor aquifers in the Llano Uplift area. See Shi and others (201,6) for assumptions and limitations of the model. a The groundwater availability model for the minor aquifers in Llano Uplift area contains eight layers: Layer L (the Trinity Aquifer, Edwards-Trinity [Plateau) Aquifer, and younger alluvium deposits), Layer 2 [confining units), Layer 3 [the Marble Falls Aquifer and equivalent unit), Layer 4 (confining units), Layer 5 (Ellenburger-San Saba Aquifer and equivalent unitJ, Layer 6 [confining units), Layer 7 [the Hickory Aquifer and equivalent unit), and Layer B [Precambrian units]. a Perennial rivers and reservoirs were simulated using MODFLOW-USG river package Springs were simulated using MODFLOW-USG drain package. For this management plan, groundwater discharge to surface water includes groundwater leakage to the river and drain boundaries. a The model was run with MODFLOW-USG beta version [Panday and others,2013) a RESULTS: A groundwater budget summarizes the amount of water entering and leaving the aquifers according to the groundwater availability model. Selected groundwater budget components listed below were extracted from the groundwater availability model results for the Trinity, Edwards-Trinity [Plateau), Ellenburger-San Saba, and Hickory aquifers, located within Cow Creek Groundwater Conservation District and averaged over the historical calibration periods, as shown in Tables 1" through 4. 1,. Precipitation recharge-the areally distributed recharge sourced from precipitation falling on the outcrop areas of the aquifers (where the aq