HYDROGEOLOGY OF AN ALLUVIAL AQUIFER SYSTEM WITH HIGH LEVELS OF NITRATE AND AMMONIA

Elevated levels of ammonia in three wells at the Hornsby Bend Center for Environmental Research (CER) near Austin, Texas, have renewed interest in the local groundwater system and chemistry of Hornsby Bend. Projected population growth in both Hornsby Bend and neighboring Hunters Bend intensifies the need for careful water planning. We take a multi-method approach for characterizing the hydrogeology and geochemistry of this alluvial aquifer system. Water levels from a spatially extensive collection of wells are used to construct the current water table map for the study area. Chemical measurements from these wells along with electrical resistivity and electromagnetic geophysical datasets guide interpretation of groundwater flowpaths and geochemical reactions.

The water table map shows the Colorado River gains water from the unconfined, alluvial aquifer system, but dam-induced stage changes may cause the river to lose water over a restricted area beyond the expected hyporheic zone. Geochemical data show elevated ammonia concentrations in five wells and elevated nitrate concentrations in seven wells with respect to background conditions. The ammonia levels for the five wells range from 36 to 159 ppm over a background of 2 ppm, while the nitrate levels for the same wells range from .9 to 1.10 ppm, and this ammonia to nitrate ratio is indicative of anoxic conditions in at least discrete zones of the aquifer. The electromagnetic results suggest buried pipes are present near the high ammonia well that may be related to the necessary highly reducing conditions. Both ammonia and nitrate contamination at Hornsby Bend are potentially related to the application of biosolids, leakage from water treatment ponds, and/or legacy contamination from past agricultural activities. Electrical resistivity surveys reveal large channels of sands and gravels incised into the underlying Taylor Clay that provide high permeability conduits for groundwater flow.