USING HEAT AND SALINITY AS SIMULTANEOUS NATURAL TRACERS TO QUANTIFY RECHARGE IN THE TRINITY RIVER ALLUVIAL AQUIFER, TEXAS, USA

Increasing water demand and scarcity is raising the importance of alluvial aquifers, which are heavily influenced by groundwater-surface water interaction. Successful management of water supply and quality depends on accurate knowledge of the water balance, particularly given the frequent exchange of water and potential contaminants between surface and subsurface environments. More crucial for future management is understanding likely effects of climate change and urbanization on that balance.

The Trinity River in Texas supplies drinking water for over 15 million people in the two largest metropolitan areas in the state. Its underlying alluvial aquifer (TRAA), consisting of ≤50 m thick Quaternary alluvial silt, sand and gravel capped by ≤10 m of black vertisol soil, holds 20 times the river's total annual discharge.

Monitoring at a shallow borehole in the upper reaches of the TRAA reveals episodic variation in groundwater salinity and temperature. The borehole is open to 2 meters of TRAA at 6 meters depth, 25 meters from a stream, 150 m down gradient from a periodically desiccated wetland. During a recent overbank flood, stream and borehole water levels rose 0.5 and 2.5 hours, respectively after the onset of precipitation. Within 8 hours borehole electrical conductivity rose 5\% , and from 23-56 hours borehole temperature rose by 0.5°C while salinity continued a steady decline. These variations can be related to simultaneous lateral infiltration from the upstream wetland and vertical infiltration driven by the flood. Initial analysis indicates a saline plume from the desiccated wetland traveled to the borehole via combined surface/subsurface pathways at 75-440 m/day. The temperature rise reflects an infiltration front with initial velocity 10-30 cm/day. Both transport phenomena likely reflect discrete transport pathways, with buried sand channels enhancing lateral transport and deep vertisol soil cracks enhancing vertical infiltration. Quantitative analysis of recharge volumes via these two pathways at this site awaits completion of detailed numerical models.

Such detailed observations of complex recharge processes provide an important constraint for regional models of water supply and quantity, and are crucial for integrated water resource management in these settings.