Meteorologic and isotopic characteristics of precipitation events with implications for groundwater recharge, Southern High Plains

Abstract

Recharge estimates for the Ogallala aquifer in the Southern High Plains vary by two orders of magnitude (0.01 to 1.6 inches/yr or 0.25 to 41 mm/yr) because of differing interpretations of the contribution of the area's numerous playa lakes to the recharge of the aquifer. Higher rates are suggested by studies in which the playa lakes are assumed to be a major source of recharge. To reevaluate the recharge potential of precipitation, we measured meteorologic and isotopic properties of single precipitation events at five stations for one year. Values of precipitation for δ 18O (−22.7‰ to −4.7‰) and δD (−162% to +35%) plot along the world meteoric water line. Mean annual weighted values of δ 18O and δD are −7.5‰ and −48‰, respectively. Air masses from the Gulf of Mexico account for most summer precipitation, which is more isotopically enriched than winter precipitation, mainly derived from air masses from the eastern Pacific Ocean. Precipitation is more depleted in δ 18O and δD toward the northwestern part of the Southern High Plains, becoming isotopically enriched toward the southeast. These isotopic differences reflect temperature, continental, and altitude effects. Isotopic composition of Ogalla ground water has a spatial distribution similar to that of precipitation. Ground water, however, is consistently more enriched than precipitation sampled in the same area, suggesting evaporation during recharge. Evaporation can occur either in playa lakes prior to infiltration or in the vadose zone. Tritium in precipitation ranges from 1.1 to 14.7 TU, and in groundwater it ranges from zero where the vadose zone is thick, to 73 TU, where the zone is thin. The calculated recharge rate (using tritium as a tracer) ranges from 0.5 to 3.24 inches/yr, or 12.7 to 82 mm/yr. Such high recharge rates were only measured below playa lakes. Because of the fast recharge rates and the slightly enriched values of δ 18O and δD in ground water, the Ogalla a aquifer is most likely recharged by focused percolation of partly evaporated playa lake water, rather than by slow regional diffusive percolation of precipitation.