Surface and hyporheic transient storage dynamics throughout a coastal stream network

Abstract

Transient storage of stream water and associated solutes is expected to vary along stream networks in response to related changes in stream hydraulic conditions and morphologic gradients. These spatial changes are relevant to a wide variety of processes (e.g., biogeochemical cycling), yet data regarding these dynamics are limited and almost exclusively confined to the general storage terms of transient storage models with a single-storage zone (1-SZ). We used a transient storage model with two-storage zones (2-SZ) to simulate field data from conservative solute injections conducted in a coastal stream network in Massachusetts to separately quantify surface transient storage (STS) and hyporheic transient storage (HTS). Solute tracer additions were performed at basin-wide, low-flow conditions, and results were compared with respect to stream size. Strong positive relationships with reach contributing area indicated that the size of the main channel and the size and residence time in surface and hyporheic storage zones all increased from small to large streams. Conversely, longitudinal dispersion and the storage zone exchange coefficients had no consistent trends downstream. The influence of storage exchange on median transport time ( $F_{MED}^{200}$) was consistently large for STS and negligible for HTS. When compared to 1-SZ model estimates, we found that the general 1-SZ model storage terms did not consistently describe either STS or HTS exchange. Overall our results indicated that many zone-specific (STS and HTS) storage dynamics were sensitive to the combination of hydraulic and morphologic gradients along the stream network and followed positive trends with stream size.