Groundwater Baseflow Sourced from Miocene Rocks and Residuals Carries Elevated Selenium into Southern California Streams

The geochemical patterns of nitrate and selenium exhibit spatial variability within the shallow aquifer of San Diego Creek Watershed, Orange County California. The surface streams in the watershed are fed primarily by groundwater baseflow from the shallow aquifer. A new area was investigated in the watershed in 2004 and 2005 near "Santa Ana Delhi Channel." A covariate relationship is shown in the groundwater data collected near Santa Ana Delhi Channel for selenium, nitrate, and salinity. Despite wide scatter of data, there is a clear positive correlation shown between selenium and nitrate concentrations, and a negative correlation between nitrate/specific conductance and selenium/specific conductance. A possible set of hypotheses for the correlative relationships between selenium, nitrate, and salinity are as follows: (1) Geologic history plays an important role in controlling selenium content in the shallow aquifer, and selenium may originate from upland marine shales of Miocene age. (2) A historic marsh in low lying swampy areas of the watershed acted as a sink for selenium in runoff waters. (3) Draining the marsh in the late 1800s changed prevailing hydrologic and geochemical structure of the land, essentially converting this selenium sink to a selenium source whereby infiltrating precipitation and shallow groundwater leaches selenium from the historic marsh deposits. Selenium-rich groundwater contributes substantial flow to surface streams via groundwater seepage and springs. (4) Oxidation of selenium by nitrate produces a positive correlation between high nitrate and high selenium. (5) Groundwaters with highest salinity in the local areas near Santa Ana Delhi Channel are sourced from lower permeability alkali soils that were never planted to crops, and nitrogen fertilizer was not used on these soils. These fine textured soils were not leached as effectively historically, and retained more salt during leaching, so crops were not planted there. As a result, low selenium, high salinity, low nitrate correlations are observed in areas with saline soils; and high selenium, low salinity, high nitrate correlations are observed in the lower salinity areas where agriculture was practiced.