COMPARISON OF WATER CHEMISTRY ISSUING FROM SPRINGS IN LINED AND UNLINED CHANNELS IN URBAN AREAS, IMPLICATIONS FOR STREAM RESTORATION

Many areas in Coastal California have shallow water tables where discharge into urban channels during dry weather consists partly or mostly of spring flows and groundwater seeps. In concrete lined channels artificial springs and seeps are created by cracks, joints, weepholes, and passive dewatering structures. Where urban channels are unlined, dynamic redox processes occur due to the presence of hyporheic zones where flows issue from spring and seep orifice. Hyporheic zones are vegetation/carbon rich saturated areas occupying streambeds and streambanks adjacent to unlined open channels. Biogeochemical processes are operative in hyporheic zones due to plant and animal respiration and organic matter decay. Such dynamic processes are much more restricted along concrete-lined channels. With respect to the main thrust of this paper, the water chemistry of flows of springs and seeps into channels with and without concrete linings are compared. Our studies in the Malibu Creek Watershed of Southern California are emphasized. Groundwater seeps along lined segments of channels emit much higher nitrate and selenium with concentrations of ammonia, manganese, and iron usually below detection limits. This suite of parameters is indicative of oxidizing conditions in the aquifer because dissolved oxyanions of selenium and nitrate are stable in oxic waters, while iron and manganese remain stable as oxide cements that are mostly insoluble in oxidizing waters. Groundwater issuing from springs and seeps along unlined sections of creeks move through a hyporheic zone leading to relatively low concentrations of nitrate and selenium near and below detection limits, and much higher concentrations of ammonia, manganese, and iron. This suite of parameters is demonstrably associated with suboxic to reducing conditions. The reducing environment in hyporheic zone removes nitrate due to denitrification and dissimilatory nitrate reduction, while reducing selenium to low solubility forms. The iron and manganese oxides in the sediments in the hyporheic zone are mobilized into their soluble forms. Iron and manganese ions in solution are quickly oxidized and precipitate out when they enter oxidizing surface waters through springs. These findings are directly relevant to water quality benefits associated with stream restoration projects that are planned or underway in Coastal California.