Step-like rising and falling of a breakthrough curve observed at a karst spring

Spring breakthrough curves induced by conventional advection and dispersion often present a gentle (smooth) change of concentration with time. We investigated an electrical-conductivity breakthrough curve which originated from a chemical release into the aquifer matrix and was measured in June 2007 at Qinglondong Spring, southwest China. The curve presented abrupt rising and falling limbs. Modeling efforts were focused on the migration of the contaminants from the conduit wall to the spring. The conduit production segment was defined to be the conduit section where the chemical plume entered the conduit from aquifer fractures. The first model assumed that the time variance of the plume front reaching the conduit wall was much shorter than transport time over the conduit production segment. The model yielded a decrease of spring discharge which was inconsistent with the observed increase with time. In the second model, the above assumption was relaxed, and wall contaminant flux was inverted as function of downstream distance and time. The observed abrupt rising and falling in the breakthrough curve were interpreted as: (1) Taylor dispersion was too small to identify for a short conduit; (2) seepage from the fractures to the conduit was monodirectional and diluting.