Designing a New Well Field for Long-Term Sustainable Yield

The City of Amarillo, Texas is developing a new well field in the Ogallala aquifer. The Potter County well field will offload production from the existing Carson County well field where historical pumping has generated water table declines of over 100 feet since the 1950’s. The City is planning to balance pumping demands between the well fields in order to limit aquifer impacts. Initial plans called for a 6 square mile footprint with 4 wells per square mile. MODFLOW simulations using the Texas Water Development Board’s Northern Ogallala GAM predicted that a significant area would be dewatered within a 17 year time frame under this scenario, prompting the recommendation that production be spread out over a larger area. During Phase 1 activities, logs from an area of about 40 square miles were evaluated and used to develop a 3D stratigraphic model using C Tech Development Corporation’s MVS software. The model was used to select favorable locations for 4 prototype test wells and 11 exploratory borings completed between July 2008 and March 2009. Pump tests were performed and data collected during the study were used to update the GAM. Several predictive simulations were run to develop the final well field design which will consist of 21 wells averaging about 600 feet in depth spread out over an area of about 18 square miles. Under the most likely pumping scenarios, annual production will grow from 7.5 million gpd in 2011 to between 14 million and 17 million gpd in 2051. Simulation results show that drawdowns will remain below regulatory limits through the year 2049 in all but the southernmost extremity of the well field. Substantial water rights acreage remains undeveloped north of the currently planned well field footprint so as depletion approaches regulatory limits in the south, production can be shifted northward.