|
In this multiyear study funded by Texas Commission on Environmental Quality, the current geomorphic method
for assessment of sensitive features is quantitatively evaluated using hydrologic methods. Typical upland karst
features in the Edwards aquifer recharge zone, specifically sinkholes, are studied to determine their
connectedness to the subsurface. Because sinkholes may allow water or contaminants to be transported rapidly to
the subsurface via fractures or conduits, they have the potential to be sensitive features during development.
Eight pairs of constant-head, single-ring infiltrometer experiments at sinkholes and control plots, as well as
another excavated solution cavity, have been conducted at two sites located in the Edwards aquifer recharge zone
in southern Travis and northern Hays Counties. Initial results indicate that infiltration rates at soil-lined
sinkholes are not significantly higher, in fact somewhat lower, than the background. Both rates are within the
range of infiltration for soils in the area, as described by county soil surveys. Although these features have
slightly lower infiltration rates than average, owing to the 30 to 40 percent clay in the soil, these features
most likely recharge volumetrically more than the background as a result of microtopography. The excavated
solution cavity and a sinkhole with a cobble-filled drain had infiltration rates as much as 30 times that of the
background. More tests are planned for the San Antonio area to determine possible differences between Barton
Springs and San Antonio segments of the Edwards aquifer. Future testing using GPR will explore geometry of the
soil/rock contact, and ponding tests with dyed water will allow us to compare the area of wetting and preferential
flowpaths within sinkholes and control plots. Subsequent excavation of features will enable us to determine the
hydrologic function of epikarst.
Guidance documents for best engineering practices for protecting water quality during development near
features found to be sensitive were revised and updated and are in review. |
