VARIABILITY IN GROUNDWATER FLOW AND CHEMISTRY IN THE HOUZHAI KARST BASIN, GUIZHOU PROVINCE, CHINA

Studying the movement of introduced and ambient tracers in karst basins can provide insights into groundwater flow that would otherwise be unobtainable. The 73.5-km² Houzhai basin in Guizhou province of southern China has been studied for more than 30 yr and the conduit network has been mapped. However, results of dye tracing within the basin were unavailable. This study provides information on catchment connectivity and a snapshot of groundwater chemistry during the summer monsoon season, which are important for assessing susceptibility to pollution and managing water resources.

In June 2018, 11 springs and karst windows across the lower basin were sampled at baseflow conditions for field parameters, major solutes, and stable isotopes including δ¹³C, δ²H, and δ¹⁸O. Two fluorescent dyes (fluorescein and eosine) and a solute (~900 kg of NaCl dissolved in ~4000 L of water) were injected as tracers. Electrical conductivity (EC), temperature, and stage were logged at 5-min intervals at 3 springs over periods of 4–7 days. After several days of rain, water chemistry was monitored at 1- to 2-hr intervals for 46 hr on the falling limb of the hydrograph at the outlet spring (Maoshuikeng).

EC spiked at a spring 1.75 km downgradient of the injection point after ~24 hr, which coincided with visual confirmation of dye arrival. Dye detection along a tributary conduit farther downgradient suggests hydraulic-gradient reversals during periods of increased flow. Dye tracing also confirmed that flowpaths diverge within the conduit network and take at least two separate paths before reconnecting upgradient of Maoshuikeng. A spatial comparison of karst spring Mg/Ca ratios suggests a general increase along catchment flowpaths, with some exceptions. These exceptions point towards locations of anomalous groundwater flow. At the outlet, EC, T, pH, and several solutes (K, Na, Sr, Si, Cl, SO₄) reached minimum values ~24 hr after the stage peak, indicating arrival of the stormflow pulse, then rebounded. In contrast, NO₃ fluctuated but broadly declined during recession, which suggests gradual flushing of inputs.