Tracer test on karst springs in deeply-incised valley area in western Hubei, China

Abstract

Tracer test was undertaken from Tianshen Wan depression feeding the lower Triassic, a fractured limestone aquifer deeply incised by Wujia River in western Hubei. The objective was to confirm the hydraulic connection between two springs of the both banks of the Wujia River. Generally, springs on different bank of deeply-incised river are always recharged by the water from the respective sides of the valley. In this study, result of tracer test has proven the two springs are directly connected under the riverbed and both feeded by groundwater from the left bank of Wujia River, suggesting that groundwater in the karst aquifer mainly discharged through White-fish spring on the left bank and some groundwater moves downward deeply and then upward through subvertical karst conduit under the river bed and discharged to the right bank of Wujia River by Red-fish spring. A proposed conceptual model was brought forward to explain the formation of the unique karst springs. In the first state, groundwater discharged to the river only through the karst cave on the south of White-fish spring. In the second state, as the crust uplift and river incision, both White-fish spring and Red-fish spring formed, and the White-fish spring became the main output point of the aquifer. However, the cave formed earlier turned into a cave filled by water but now only discharge after rainstorm when the two springs can't discharge groundwater in aquifer in time. In the last phase, the conduits between the two springs under the riverbed of Wujia River were continuously enlarged by karstification, inducing more groundwater discharged by Red-fish spring. The evolvement model of the two springs may be used for reference on karst springs in deeply-incised valley in the similar area.