Flow stratification due to groundwater inflow in a small urban stream

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732Conservative solute tracer experiments were conducted in Indian Creek, a small urban stream located in Philadelphia, Pennsylvania, USA with estimated low flow rates between 50 L s-1 and 70 L s-1. The stream was typically 5.5 m wide and 0.2 m deep. Streams of this size are usually modeled as one-dimensional, assuming that downstream of some initial ""mixing zone"", the stream is completely mixed both vertically and laterally, and remains completely mixed throughout the remainder of the reach of interest. However, we observed vertical stratification of a tracer cloud in a 0.95 m deep by 30 m long pool. The tracer cloud was initially completely mixed both laterally and vertically across the stream prior to entering the pool. We suggest that the cause of limited mixing is due to a balance between groundwater inflow and transverse dispersion at the cross section. We show that the unsupported assumption of complete mixing may result in a wide range, and thus increased uncertainty, of the values of streamflow parameters, and in particular, the longitudinal dispersion coefficient estimated from these data. We conclude that the assumption of complete mixing and one-dimensional modeling must be checked against actual field conditions, even in small streams

Influence of experiment design parameters on the shape of the observed breakthrough curve

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732Multiple conservative tracer (NaBr) injection experiments were conducted in Indian Creek, a small urban stream located in Philadelphia, Pennsylvania, USA to compare the effects of various experiment design factors on the shape of the observed breakthrough curve. Three experiments were conducted in which instream background solute concentration was increased by 2 orders of magnitude. The injection duration of these experiments ranged from less than 1 hour to 24 hours. A fourth experiment was conducted in which the instream background solute concentration was increased by 3 orders of magnitude. The injection duration of this experiment was approximately one hour. All experiments were conducted during baseflow conditions (53 L s-1 to 65 L s-1). The initial falling limb of the breakthrough curve was similar for all experiments, regardless of injection duration or magnitude of the increase in background solute concentration. The slope of the midrange of the falling limb was most influenced by the maximum observed tracer concentration and the slope of the late tail of the falling limb was most influenced by the duration of the experiment

Transport of solute plumes in beaches

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732Improvements on the traditional convection-dispersion equation for simulating solute transport in aquifers were made over the years. These include the usage of scale-dependent dispersivities and fractional integration. These improvements, however, were based on situations where the flow is unidirectional. There are no theories for dealing with the transport in highly variable (in space and time) environment, such as in tidally-influenced beaches. In this work, we argue that using velocity-dependent dispersion coefficients is not suitable or at least not necessary for beach solute transport where the pore water saturation, and the flow velocity direction and magnitude vary periodically due to the sea tides. We compared the two numerical solutions of (1) constant and (2) velocity-dependent dispersion coefficients in a typical tide-influenced beach. The two-dimensional numerical code MARUN (Boufadel et al., 1999a, J. Contaminant Hydrology, 37, 1-20) was used to obtain the numerical solutions. The two numerical solutions are very close to each other

Transport of solute plumes in beaches

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732Improvements on the traditional convection-dispersion equation for simulating solute transport in aquifers were made over the years. These include the usage of scale-dependent dispersivities and fractional integration. These improvements, however, were based on situations where the flow is unidirectional. There are no theories for dealing with the transport in highly variable (in space and time) environment, such as in tidally-influenced beaches. In this work, we argue that using velocity-dependent dispersion coefficients is not suitable or at least not necessary for beach solute transport where the pore water saturation, and the flow velocity direction and magnitude vary periodically due to the sea tides. We compared the two numerical solutions of (1) constant and (2) velocity-dependent dispersion coefficients in a typical tide-influenced beach. The two-dimensional numerical code MARUN (Boufadel et al., 1999a, J. Contaminant Hydrology, 37, 1-20) was used to obtain the numerical solutions. The two numerical solutions are very close to each other