ASSESSING SPATIAL VARIABILITY IN SEEPAGE FLUX USING SMALL VOLUME ELASTIC MEASUREMENT BAGS

A small perennial stream in southern Ontario is displaying many symptoms of eutrophication. A watershed characterization, integrating hydrology, geochemistry, and microbial and community ecology was undertaken to determine the nature and cause of the ecological shift. Aquatic plant species coverage varied throughout the channel due to differences in microenvironments that were linked to variations in ground water baseflow, hydrostratigraphy, and water chemistry. Key to assessing the variability in baseflow was a large network of 10.2 cm diameter seepage meters. Seepage measurements were taken in both concentrated clusters (up to 24 seepage meters along a 50 m length of channel), and moved in small groups over several months along the entire 14 km channel length. In total, 232 seepage-meters were deployed at 45 locations during two separate monitoring periods.

Overall, a reduction in ground water discharge was found to coincide with an upstream decrease in aquifer thickness and a corresponding increase in aquatic biomass. However, the seepage measurements displayed large local spatial and temporal variability. To a degree this was expected given the observed variability of channel sediments and vegetation types and coverage. While field-testing determined the volume at which elastic effects occur with the measurement bags (condoms) the mechanical relaxation effect was not initially assessed. The mechanical relaxation of the latex creates a hydraulic potential that results in an initial short-term influx of water into the condom. Subsequent laboratory tests showed that the condoms, with respect to their workable volume, have a significant mechanical relaxation effect. Furthermore, the relaxation volume was found to be fairly variable. While the small inexpensive seepage meters are easy to transport and install, thus allowing an enhanced characterization of seepage variance, the use of small volume elastic bags (condom) for seepage measurement presents significant problems.