Hydrogeochemistry and Water Resources of the Triassic Lower Dockum Group in the Texas Panhandle and Eastern New Mexico

Abstract
Topographically controlled ground-water-basin divides, established during the Pleistocene by erosion of the Pecos and Canadian River valleys, prevent ground-water recharge in the outcrop of the Triassic Dockum Group from entering the confined part of the aquifer in the Dockum Group beneath the Southern High Plains in the Texas Panhandle and eastern New Mexico. Ground water in the confined aquifer was probably recharged from precipitation at higher elevations in eastern New Mexico before thick deposits of Dockum Group sandstones were eroded. The confined aquifer in the Lower Dockum Group is separated from the overlying, heavily pumped High Plains aquifer in the Neogene Ogallala Formation and Cretaceous Edwards/ Trinity Groups by thick mudstones in the Upper Dockum Group. Hydrologic separation beneath most of the Southern High Plains is indicated by (1) hydraulic heads of Lower Dockum Group ground water that are 300 to 700 ft (90 to 200 m)lower than the water table of the High Plains aquifer, (2) δ18O and δD values of Lower Dockum Group ground water that are 2 to 3 ‰ and 15 to 25 ‰ lighter, respectively, than ground water in the Ogallala Formation, and (3) different associations of hydrochemical facies in the two aquifer systems. The significant differences in hydraulic head between the two aquifer systems reflect the Pleistocene cutoff of recharge and the continuation of discharge during the Holocene, which have caused a decrease in ground-water storage. The δ18O and δD values suggest that Dockum Group ground water was recharged in a cool climate at elevations above approximately 5,900 ft (1,800 rn) in eastern New Mexico. Chemical composition of Dockum Group ground water is controlled by reactions with Dockum Group minerals, including calcite, chalcedony, dolomite, feldspar, kaolinite, opal, pyrite, and smectite. Accounting for salinity greater than 20,000 mg/L in Na-CI facies are (I) discharge of ground water that dissolved halite in underlying Permian strata or (2) Cretaceous seawater that was incompletely flushed from low-permeability mudstones in the Dockum Group. Spatially variable water quality occurs in unconfined parts of the Dockum Croup where ground water from adjacent formations mixes with Dockum Group ground water. Ground water in the Lower Dockum Group beneath the Southern High Plains in the Texas Panhandle and eastern New Mexico has not proved to be a substitute for Ogallala Formation ground water everywhere. Development of Dockum Group water resources for irrigation and municipal supply will continue to be limited to areas having thick sandstone deposits, moderate head lift of less than 500 ft (150 m), and fresh to slightly brackish salinity as great as 3,000 mg/L. Production of Dockum Group ground water beneath the Southern High Plains will deplete stored water because the recharge rate to the confined Dockum Group is negligible.
Authors
Alan R. Dutton
William W. Simpkins
Citation

Dutton, A. R., and Simpkins, W. W., 1986, Hydrogeochemistry and Water Resources of the Triassic Lower Dockum Group in the Texas Panhandle and Eastern New Mexico: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 161, 51 p.

Code
RI161
ISSN
2475-367X
Number
161
Number of figures
19
Number of pages
51
Publisher
The University of Texas at Austin, Bureau of Economic Geology
Series
Report of Investigation
Year
1986