Description
Studies of wetland hydrology are important for use in environmental impact and wetlands restoration assessment. Most detailed studies of wetland hydrology are limited to the Northern and Eastern United States. This study of a riparian wetland in the semi-arid Southwest US has characterized the hydrogeology, water fluxes, and chemistry over a one-year time period. The San Diego State University Riparian Wetland Field Site is located along the Santa Margarita River in Camp Pendleton, California. The site was instrumented with 10 piezometers for groundwater level measurement, groundwater chemistry sample points, and hydraulic testing. Piezometers were installed using a jackhammer along a transect approximately perpendicular to the Santa Margarita River. Water level measurements were taken every other week for over one year's time. Water table fluctuations of 1.5 ft were observed during the year and were highest farther from the river channel. The highest groundwater elevations occur during the wet season in response to precipitation. The wetland hydroperiod is described as having high groundwater elevations, gradients, and discharge during the wet season followed by a decrease during the dry summer months. Groundwater samples were taken once during the wet and the dry season, using a peristaltic pump. Groundwater and surface water chemistry were analyzed for common ions. Concentrations for common ions in river water samples were greater than groundwater concentrations in both the wet and dry season. The horizontal hydraulic conductivity of the site, determined by slug testing the piezometers, ranged from 0.55 ft/day to 139.28 ft/day. Soil cores were taken using a drive point piston sampler. The cores were used in falling head permeameters, core logging, porosity estimates, and a grain size analysis. Vertical hydraulic conductivity values from the falling bead permeameters ranged from 0.07 ft/day to 541.42 ft/day. The porosity of the sediment is approximately 38%. Results from the core logs and grain size analysis show the shallow aquifer to be comprised of silty sands with pebbly to coarse layers interspersed with mud laminae characteristic of a braided channel fluvial environment. Hydrologic data for river flow, precipitation, and pan evaporation were collected from the United States Geologic Survey (USGS) and Camp Pendleton's Office of Water Resources. Evapotranspiration (ET) was estimated using pan evaporation data, the Blaney-Criddle method, and diurnal groundwater elevation variation data. Blaney-Criddle ET estimates ranged from 3.68 in/month in the wet season to 6.81 in/month in the dry season. Over 903 of recorded precipitation occurred from November 1996 to January 1997. The Santa Margarita River near the site is a gaining stream whose discharge varies with seasonal groundwater elevation due to precipitation. The primary water source to the wetland is groundwater and wet season precipitation. The primary outflow of water is groundwater outflow and evapotranspiration. A vegetation survey was performed to characterize the vegetation in relation to hydrogeology and hydrology. Three zones of vegetation, consisting of coastal and valley freshwater marsh vegetation, southern willow scrub vegetation, and southern cottonwood-willow riparian forest vegetation were observed within the riparian wetland that grade from the river as a function of groundwater depth and elevation variation.
