Investigation of the source of regional salinization of the Ogallala aquifer, Southern High Plains, Texas, U.S.A.

摘要

Two extensive plumes (combined area >1000 km2) within the regionally important unconfined Ogallala aquifer overlie the Panhandle oil and gas field in the Southern High Plains, Texas, U.S.A. Salinity varies within the plumes spatially and increases with depth. Vertically-averaged total dissolved solids concentrations range from 400 to >2000 mg L –1.; Relative to upgradient Ogallala water, the plume waters have δ 18O (–6.7 to –8.8‰) and δD (–42 to –88‰) values that tend to be depleted and have higher chloride (>150 mg L–1) and sulfate (>75 mg L–1) concentrations. Various end-member-mixing models suggest that the plume composition reflects the presence of paleowaters recharged during Middle to Late Wisconsinan time rather than salinization associated with petroleum production. Paleowaters mix with salt-dissolution zone waters from the underlying Upper Permian formations before discharging upward into the Ogallala Formation as a result of natural, topographically induced flow.; Modeling of variable-density flow and solute transport with the program SUTRA has identified three broad regions of upward cross-formational discharge, including the area of observed salinization. Distinct flow paths, on the order of tens of km to >100 km in length, occur under steady-state conditions. Horizontal flow velocities range from 10–5 to 10–8 m day–1 and vertical velocities range from 10 –9 to 10–11 m day–1 for various units. The upward discharge within the plume area is in the range of 10–4 to 10–5 m3 day–1, and the TDS concentrations are typically >3000 mg L–1. Cross-formational discharge is controlled primarily by the geometry of the underlying units, as influenced by the Amarillo uplift, pinch-out of the laterally adjoining confined aquifer in the Triassic Dockum Group, variations in the saturated thickness of the Ogallala aquifer, and the presence of potential pathways related to salt dissolution.; Transient-state simulations indicate that ongoing pumping has had negligible impact on the salinity distribution in the Ogallala aquifer, but the velocity distribution in the underlying units may have been affected to depths of 150 m below the base of the Ogallala aquifer after 30 years. As the water table declines with continued pumpage for irrigation, salinity could increase in the vicinity of the plume. Careful areal and vertical characterization is warranted prior to future well-field development.