INTEGRATION OF INFORMATION ON GROUNDWATER AGE DATING, RECHARGE RATES, AND RESULTS OF GROUNDWATER MODELING OF THE HIGH PLAINS (OGALLALA) AQUIFER IN THE TEXAS PANHANDLE

Ronit Nativ studied the hydrogeology of the Texas part of the High Plains (Ogallala) aquifer during her post-doctoral research at the Bureau of Economic Geology during 1984-1986. Research on the mainly unconfined aquifer has since made advances on several topics of interest to Nativ.

Nativ was an early proponent of the playa-focused recharge model for the Ogallala aquifer in the southern High Plains. Recent groundwater models have mimicked playa-focused recharge with areally distributed recharge rates of <0.2 to ~6 cm/yr. Groundwaters of varying ages occur along flow paths since recharge is superposed on lateral flow. Early 1990s studies showed that Ogallala ground waters in the southern, central, and northern High Plains have statistically similar ¹⁴C activities averaging ~55 ± 24 pmc. Nativ interpreted 0 to 73 TU (1984-85 samples) in Ogallala groundwater in the southern High Plains aquifer as reflecting variations in vadose-zone thickness and vertical travel time. Additional sampling in 1992 showed above background tritium (6.4 to 32.2 TU) in half of the samples. Residence time of groundwater in the aquifer beneath the northern part of the Texas Panhandle averages ~1700 yr, given the distribution of recharge rate, saturated thickness, and specific yield used in a recent calibrated flow model. Backtracking particles from a municipal well from which one of the lowest ¹⁴C activities (20.8 pmc) was measured, however, gives an average groundwater age of ~3400 yr. These results qualitatively suggest that average age of unconfined ground water in the High Plains aquifer is generally 1000 to 2000 yr and locally younger than 50 yr. Groundwater at the end of flow paths deep in the aquifer could be >10,000 yr old.

Irrigation has pumped ~65 km³ of groundwater in the northern part of the Texas Panhandle; irrigation withdrawal was greatest during the 1980s at ~1.85 km³/yr. Return flow of some of the applied water to the water table can be estimated from rate and efficiency of irrigation, water-table depth, and drainage rate. Return flow doesn't reach the water table where drainage rate is less than drawdown rate. The modeled difference in hydraulic head between assuming no return flow versus the maximum feasible rate of return flow, however, accounts for less than 6 m of drawdown, about the same as flow-model calibration error.