STREAM-AQUIFER INTERACTIONS IN URBAN WATERSHEDS ALONG LINED AND UNLINED CHANNELS

Lined and unlined urban channels and lined storm drains are used worldwide to convey storm runoff away from urban areas. They also convey dry weather flows such as car wash runoff, lawn runoff, and other “nuisance flows” in urban centers. Areas in southern California and elsewhere have shallow water tables where dry weather flows in concrete lined storm drains consist primarily of perennial groundwater baseflows. In these areas, groundwater leaks through cracks, joints, weepholes, and intentional dewatering structures into storm drains, which eventually flows into natural channels, coastal wetlands, and estuaries. The “lined” sections of these channels are hardly impervious; in fact, the lining of the channels often affords excellent opportunities to collect groundwater samples at discrete points that are usually not present along unlined channels where well control is sparse or where technical or legal issues prevent installation of piezometers. Frequently, as much as 95% of the nutrient and trace element loading to urban catchments during dry weather is from seepage of groundwater that is laden with pollutants. Unlined channels mitigate several contaminants more effectively than lined channels due to biologically active hyporheic zone and riparian vegetation. This project presents several examples of our experiences in conducting stream/aquifer studies in lined and unlined channels in Southern California. Sub-projects include evolution of groundwater quality along flowpaths, detection of unwanted recharge mounds in shallow groundwater systems due to leaky water main pipes, and geologic factors in water quantity and water quality fluxes in urban catchments . Tools that are utilized and described include isotopic tracers (O-H-S-N stable isotopes); trace element speciation analysis; and innovative sampling methods.