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Relationships between Water Quality Constituents in the Delta and the Influence of Different Sources of Water

Bay-Delta salinity is generally measured in the field as specific conductance, loosely referred to as electrical conductivity (EC). However, drinking water and other beneficial uses are often best measured in terms of other constituents such as chloride, bromide, sodium, and total dissolved solids. Computer simulations of Delta water quality typically report Delta salinity predictions as EC or practical salinity units, which then requires conversion to the water quality constituent of concern. Water quality grab samples have been used to develop conversion relationships between the key indicators of salinity. These conversion relationships vary spatially throughout the Delta depending on the site-specific contributions from seawater intrusion, San Joaquin inflow or other sources of water. The site-specific contributions to EC from these different water sources vary by water year type and season, but can be quantified as a function of salinity in the western Delta which is strongly dependent on Delta outflow.

A general conversion approach has been developed that first calculates the contribution to EC from seawater using measured or simulated EC at Jersey Point. The seawater EC is then converted to other constituents using conversion relationships developed from Chipps Island grab sample data. EC originating from non-seawater sources is converted using a conversion relationship representative of agricultural drainage water quality. This new approach allows accurate estimation of salinity constituent concentrations from both continuous field EC measurements and computer simulations of EC. The grab sample analysis also gives insights into the influences of seawater intrusion and agricultural drainage within the Delta and how these influences have changed historically and might change in the future. These findings will help scientists and engineers better understand how flows and water quality influence beneficial uses of Delta water.