SCALE DEPENDENCE OF HYDRAULIC CONDUCTIVITY IN TEXAS HYPORHEIC ZONES

The hyporheic zone is the region of porous and permeable sediments below and alongside streams and rivers where interactions between surface water and groundwater occur. Sediment permeability and hydraulic gradients control fluxes of water and solutes into, out of, and through the hyporheic zone. These physical properties control residence times and related biotic processes in the hyporheic zone which, in turn, affect hyporheic invertebrate community structure, water quality, and ecosystem health in both the hyporheic zone and surface waters.

Hydraulic conductivity was measured at 60 sites in nine 1^st to 7^th-order rivers across Texas, USA. Sites span large precipitation and temperature gradients (east, central, and north TX), and diverse geology and land uses. At each site, slug tests were used to directly measure hydraulic conductivity at 3 – 5 shallow wells spanning the length of a riffle. Substrates were extremely heterogeneous within and across sites and usually contained large gravel and cobbles across the streambed. Measured hydraulic conductivity at most sites fell within ranges associated with clean or silty sand to gravel (0.009 – 0.260 cm/s).

To better parameterize hydrologic models of hyporheic zone exchanges, and to test the scale-dependence of hydraulic conductivity measurements, cross-riffle tracer tests will be conducted at several of the sites in the Colorado River Basin, TX At each site, sodium chloride will be injected into the hyporheic zone at the top of the riffle and specific conductance will be continuously monitored throughout a grid of small wells across the riffle. Calculations of hydraulic conductivity determined from tracer breakthrough curves and concurrent slug tests will be compared to reveal any systematic differences between the two methods.

There is a positive relationship between hydraulic conductivity and invertebrate abundance (p<0.001, R²=0.52) and richness (p<0.001, R²=0.41) at our sites. Climate change can influence stream flow variability, which has a direct effect on sediment transport, colmation, and sediment reworking; all of which effect hydraulic conductivity and suitability of the hyporheic zone as habitat and refuge for invertebrates. Specifically, increased frequency and duration of low-flow conditions will likely lead to decreased hyporheic zone permeability, invertebrate abundance, and species richness.