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

摘要

The geochemical patterns of nitrate and selenium exhibit spatialvariability within the shallow aquifer of San Diego Creek Watershed, OrangeCounty California. The surface streams in the watershed are fed primarily bygroundwater baseflow from the shallow aquifer. A new area was investigated inthe watershed in 2004 and 2005 near “Santa Ana Delhi Channel.” A covariaterelationship is shown in the groundwater data collected near Santa Ana DelhiChannel for selenium, nitrate, and salinity. Despite wide scatter of data, there isa clear positive correlation shown between selenium and nitrate concentrations,and a negative correlation between nitrate/specific conductance andselenium/specific conductance.A possible set of hypotheses for the correlative relationships betweenselenium, nitrate, and salinity are as follows: (1) Geologic history plays animportant role in controlling selenium content in the shallow aquifer, andselenium may originate from upland marine shales of Miocene age. (2) Ahistoric marsh in low lying swampy areas of the watershed acted as a sink forselenium in runoff waters. (3) Draining the marsh in the late 1800s changedprevailing hydrologic and geochemical structure of the land, essentiallyconverting this selenium sink to a selenium source whereby infiltratingprecipitation and shallow groundwater leaches selenium from the historic marshdeposits. Selenium-rich groundwater contributes substantial flow to surfacestreams via groundwater seepage and springs. (4) Oxidation of selenium bynitrate produces a positive correlation between high nitrate and high selenium.(5) Groundwaters with highest salinity in the local areas near Santa Ana DelhiChannel are sourced from lower permeability alkali soils that were neverplanted to crops, and nitrogen fertilizer was not used on these soils. These finetextured soils were not leached as effectively historically, and retained more salt