Effect of Passive Surface Water Flux Meter Design on Water and Solute Mass Flux Estimates

Standard methods for determining pollutant loads in streams typically require the collection of separate instantaneous measurements of water velocities and solute concentrations at discrete points in space and time. A recently developed device, the passive surface water flux meter (PSFM), has been introduced as an alternate method for the measurement of time-integrated surface water flux (velocity) and solute mass flux. This paper extends PSFM development by evaluating and comparing two PSFM designs in laboratory flumes, as well as reporting on initial steady-state field testing. The shape of the PSFM body determines the velocity with which water passes through the device, and different designs may thus be preferred for different applications. Experiments compared the accuracy of flux measurement by the previously introduced hydrofoil-shaped PSFM and nitrate-sorbing cartridge with that of a newly designed cylindrical-shaped PSFM and phosphate-sorbing cartridges. Testing was performed in a laboratory flume at steady water velocities between 0.2-0.58 m/s and results verified the ability of the new PSFM designs to accurately measure solute and water flux under steady-state conditions. Water fluxes were measured to be within 4 and 7% of true water fluxes for the hydrofoil PSFM and cylindrical PSFM, respectively. The accuracy of solute flux estimates was similar to those found in the water flux experiments. While both PSFM designs exhibited similar accuracy, the cylindrical-shaped PSFM with phosphate-sorbing cartridges was chosen for field testing because of the relative ease of construction compared to the hydrofoil-shaped device. Field experiments performed under natural steady-flow stream velocities in Sweetwater Branch, Gainesville, Florida indicated that in relatively stable field conditions, the cylindrical PSFM was able to accurately measure water and phosphate mass fluxes to within 3 and 7% of true fluxes, respectively.