Hydrogeochemical processes controlling subsurface transport from an upper subcatchment of Walker Branch watershed during storm events. 2. Solute transport processes

Geochemical studies of transport from watersheds to date have focused on stream chemistry and/or soil lysimeter data to interpret soil processes. Direct measurements of subsurface transport of solutes from hillslopes during storm events and integration with hydrometric analyses is seriously lacking. This study presents subsurface transport of solutes (K, Ca, Mg, Mn, Fe, Al, Na, SO4, and Si) from a 0.47-ha hillslope subcatchment at the uppermost reach of the west fork of Walker Branch watershed for six storm events. Transport of Fe and Al was primarily (> 55%) as suspended particles > 0.4 micrometers which resulted in a very rapid increase in their concentrations at the initiation of subsurface flow. Weathering via particle detachment (i.e. suspended particles) was believed to be due to rapid infiltration of new water of low ionic strength which increases the diffuse double layer facilitating detachment of particles. The concentration of Fe and Al in filtered solutions (< 0.4 micrometers) increased as flow increased and decreased during the recession limb of the hydrograph. The mechanisms for the increase then decrease is believed to be electrostatic exchange from surfaces of conducting pores followed by a kinetically controlled release via mineral decomposition. The most commonly observed solute transport pattern during storm events was an increase in concentration as flow rate increased, followed by a decrease in concentration during the recession limb of the hydrograph. This response was due to flushing of solutes primarily from water-filled mesopores with macropores serving as conduits for solutes with limited attenuation. Soil matrix pores (> 6 micrometers diameter) exhibited an unlimited supply of solutes for transport through larger pores in that soil solution concentrations showed no depletion of elements during storm events. A mass balance of solutes was computed for the subcatchment for 1988, and subsurface storm flow through the upper soil profile (< 3m) resulted in only a minor export of deposited solutes. However, subsurface saturated flow during individual storms were shown to be significant exporters of nutrients.