Diffusion and seepage-driven element fluxes from the hyporheic zone of a karst river

The hyporheic zone (HZ) can be an important source of solutes to streams. Hyporheic solute fluxes are commonly dominated by advective exchange. However, fluxes from the HZ also may include diffusion and upward advection of ground water from underlying aquifers. We compared the relative importance of these transport mechanisms on solute budgets of a large, spring-fed river in north-central Florida using measurements of spring, river, and porewater chemistry, hydraulic gradients, and sediment hydraulic conductivity, and dilution of an injected dye (Rhodamine WT). Downstream increases in Fe, soluble reactive P (SRP), Ca²⁺, and Cl^- concentrations of the river water suggest solute sources in addition to the major source springs. Shallow porewater concentrations of Fe, Mn, Ca²⁺, SRP, and Cl^- were elevated relative to the river. Calculations of Fickian diffusion based on concentration gradients of these solutes indicate diffusion could account for the downstream increase in Fe concentration but only 5% of the downstream increase in SRP and <0.1% of the increases in Ca²⁺ and Cl^-. Downstream decreases in Mn concentrations reflect in-stream retention despite predicted diffusion. Dye-trace results indicate that ∼13% of the river discharge originates from sources other than the major springs. Measured head gradients and low sediment hydraulic conductivity suggest vertical groundwater flow through the HZ is small. We used the SRP budget to partition the additional groundwater inputs between seepage through the HZ (∼3% of river discharge) and flow paths that bypass the HZ (∼10% of total river discharge). Flow paths that bypass the HZ dominated additional water delivery to the river, but diffusion, resulting from steep chemical gradients and lowpermeability sediments, is an important mechanism for transporting solutes from the HZ to the river.