Characterizing snowmelt anomalies in hydrochemographs of a karst spring, Cumberland Valley, Pennsylvania (USA): evidence for multiple recharge pathways

Conductivity, water level, air temperature, and depth of snowpack were monitored during a 26-day melt period of 88-cm-deep snowpack at a karst spring to characterize internal runoff and diffuse infiltration. Chloride from road salt provided a tracer and the snowpack a recharge source during the melt period. The melt period was divided into phases based on air temperature and chemograph pattern. For the first and third phases, mean air temperatures were below freezing, but above freezing during the second and fourth phases. During the first phase when the temperature peaked above freezing, conductivity typically spiked 10–50 μS/cm, suggesting input of road salt from conduits. When the snowpack continuously melted, conductivity and water-level trends were upward with smaller daily spikes in conductivity indicating infiltration from the dilute snowpack. This pattern suggests that road salt input continued when snowmelt recharged through the epikarst, but at lower concentrations than the conduit input. Refreezing of the snowpack and shallow subsurface for a brief period interrupted the recharge, and there was no longer a sawtooth pattern of conductivity. It is apparent that frozen conditions did not cease recharge because a dual recharge process was evident. While dual recharge from internal runoff and diffuse infiltration occurred, the portions varied because of changing melting rates. Observed patterns indicated internal runoff dominated during frozen periods because recharge water moved as overland flow across a frozen surface to focused pathways. Diffuse infiltration became available during warmer periods because subsurface thawing allowed the snowmelt to penetrate the epikarst. Results of snowmelt monitoring in spring discharge indicated the transient nature of karst recharge.