Conduit roughness and dye-trace breakthrough curves: why slow velocity and high dispersivity may not reflect flow in distributed systems

Dye-trace breakthrough curves (BTCs) that increase in velocity and decrease in dispersivity through a melt season have been interpreted as indicating a switch from a distributed to a conduit subglacial drainage system, but this interpretation has not been validated in glaciers where the drainage system configuration was independently known. To test if processes other than a change in the configuration of the subglacial drainage system could produce similar BTCs, we measured BTCs from a persistent, mapped subglacial conduit beneath Rieperbreen, Svalbard, which lacks a distributed system because it is frozen to its bed. This conduit produced slow and highly dispersed BTCs early in the melt season when meltwater delivery rates were low, and fast and sharply peaked BTCs after the snowpack had retreated past the injection moulin. At Rieperbreen, the seasonal evolution of BTCs was controlled by decreases in conduit roughness as increased rates of meltwater delivery increased the relative submergence depths of rocks on the conduit floor. Because seasonal changes in roughness can produce slow and highly dispersed BTCs, dye-tracing studies may not be capable of uniquely identifying subglacial drainage system configurations. As a result, conduits may form earlier in melt seasons than previously recognized.