Characterizing Upland Recharge to the Edwards Aquifer Using Physical and Chemical Response of Cave Drips

Cave drips are useful for characterizing recharge and transport through soils, particularly in upland karst settings. Discrepancies between previous and recent studies of recharge in the Barton Springs Segment (BSS) of the Edwards Aquifer highlight the importance of upland recharge and how little is known about it in the BSS. We outline a methodology for using cave drips to characterize upland recharge and present initial findings from a study of drips in four BSS caves. 

Soils in the BSS are heterogeneous, making it difficult to characterize their hydraulic properties over larger areas, particularly with methods that only yield information about a discrete location (i.e., infiltration tests, moisture sensors). This is particularly true in the BSS where thin, clay-rich soils often contain macropores (i.e., desiccation cracks, roots, burrows) that act as preferential flowpaths for rapid recharge through the soil zone. Cave drips are well suited for characterizing recharge in upland areas as they often have large source areas.

Drip responses to storm events were monitored at drips in four BSS caves. Hydrograph separation and chemical analyses allowed distinction of fast flow, through macropores and conduits, from slow flow drainage of the soil column. Natural and artificial soil tracers indicate that surface water reaches many of the drips within a few hours of the onset of storm events, even though reported soil Ksat values (0.06-0.57 in/hr) are relatively low and no discrete recharge was observed within the subsurface drainage basin of three of the caves.  These results indicate that upland recharge may contribute a greater portion of total recharge in the BSS than previously estimated, and that rapid recharge can occur in the absence or discrete recharge features via macropore flow.