DETERMINING THE RELATIONSHIP BETWEEN MEASURED RESIDENCE TIME DISTRIBUTIONS IN LATERAL SURFACE TRANSIENT STORAGE ZONES IN STREAMS AND CORRESPONDING PHYSICAL CHARACTERISTICS

Transient storage (TS) in stream ecosystems (consisting of surface transient storage (STS) and hyporheic transient storage) provides ecosystem services such as nutrient retention and refugia for aquatic communities. However, they can retain contaminants as well. Therefore, it is critical to determine the residence time distribution (RTD) of transient storage. A RTD includes the length of time for the first pulse of water to leave TS, and the mean residence time of water in TS.

Currently, there is no definitive method of determining the RTD of any component of TS. RTDs have been determined with tracer injection alone, though this is time consuming and lacks a theoretical foundation. Therefore, a concrete relationship between TS physical characteristics and RTDs is desirable. This has been observed in flume experiments with TS. However, direct application of these relationships to natural TS leads to errors due to simplified geometries.

The focus of this study determines RTDs in lateral STS, which is adjacent to the main channel of a stream, and its relationship to physically measurable parameters of lateral STS. Twenty sites on small- to medium-sized streams throughout Oregon were each injected with NaCl to determine four residence timescales: Langmuir time (τ_L), negative inverse slope of the normalized concentration curves of the primary gyre (τ₁) and of the entire STS zone (τ₂), and the mean residence time (τ_STS). The RTDs of these sites were compared to the measured length, width, and depth of each lateral STS zone studied, and to the velocity of the adjacent main channel. The data were used to calculate dimensionless parameters submergence, a measure of bed roughness, and k, a measure of exchange that relates τ_STS to lateral STS and associated parameters.

Relationships with τ₁ and τ₂ could not be uniquely defined. τ_STS was found to be a scalar of τ_L, and their ratio (τ_L /τ_STS) was positively correlated with lateral STS submergence. τ_L and τ_STS were positively correlated with lateral STS parameters, and inversely correlated with main channel velocity. The value of k from this study was comparable to the value of k from other studies in flumes. The relationship between RTDs and lateral STS parameters in the streams studied is strong enough for useful application in other natural streams.