SEASONAL AND STORM RESPONSE OF SEDIMENT TRANSPORT IN A KARST SYSTEM

We sampled sediment in a seep and in a well drilled into a karst conduit near Easton, PA. The sediment concentrations in the well were low (around 10 mg/L) in the late spring and summer, and increased in the fall and early winter to 600-900 mg/L. The high sediment concentrations were associated with seasons that received less rainfall, suggesting that lower sediment occurs when repeated rain flushes the karst system. In contrast, the ion concentrations varied throughout the year with no seasonal pattern, showing a high coefficient of variation (29%) indicative of conduit flow. The seep had lower sediment concentrations than the well, less than 15 mg/L through the year. The seep was monitored hourly during a storm event, and sediment concentrations followed a pattern similar to the storm intensity. The highest sediment yield (13 mg/L) matched the time the ion concentration was most diluted. The travel time from storm peak to sediment peak was approximately 5 hours. Travel time was also monitored through the year using temperature spikes after storm events. For 11 storms, the average travel time was 4.5 hours, but the range was 1 to 15 hours. These data show the potential variation in sediment transport due to both storms and seasonality. This variation implies sampling for microbes or contaminants that can bind to sediment in karst must consider the temporal heterogeneity of the system.