Water and Material Sources and Pathways During Extreme and Non-Extreme Events in White Clay Creek, Pennsylvania, USA

Abstract:

Extreme events produce water quality responses that are both unique to the event and systematic for the watershed regarding water and material flows to the stream channel and out of the catchment. We present hydrologic, solute, and suspended sediment data collected from a third-order mixed-landuse watershed in the mid-Atlantic USA collected over multiple years that includes Hurricane Irene, Tropical Storm Lee, and Hurricane Sandy. Concentration-discharge relationships in solutes and suspended sediment indicate an increase in hydrologic connectivity during extreme events which connects material sources to the stream. Water sources to the stream differ between the hurricanes. Isotope hydrograph separation indicates discharge during Hurricane Irene was dominated by runoff and event water, with peak discharge comprised of only 35% pre-event water. Conversely, Hurricane Sandy peak discharge was comprised of 75% pre-event water, which further chemical separation indicates is mostly soil water. Total suspended export increases by up to an order of magnitude in extreme events compared to high flows during non-extreme events, but also differs between hurricanes – 64 metric tons during Irene, 32 metric tons during Sandy, and 6 metric tons during a seasonally high storm in April 2012. Sediment chemical data indicates different sources activate during extreme events when compared with non-extreme events. For example, short term fallout radionuclide (Beryllium-7 and excess Lead-210) event-scale mass balances indicate watershed landscape erosion contributes to suspended particulate material during extreme events, but not during non-extreme events. Furthermore, particulate organic material (POM) showed significant declines in mineral (BET) surface area, POC, and PON content with increasing discharge (p < 0.0001 in all cases). The greatest declines in BET surface area and organic matter content occurred during Hurricanes Sandy, Irene, and Tropical Storm Lee. Relationships among POM carbon, nitrogen, and metal-oxide compositions suggest that at lower flows, POM can be both formed and altered within the stream channel while at higher flows, POM additions to the stream come primarily from landscape source areas.