Using Cross-Sectional Models to Develop Measurable Objectives for Saltwater Intrusion

Sean Culkin gave this presentation at the California Water and Environmental Modeling Forum 2016 Annual Meeting is a session called Managing Groundwater According to SGMA – Developing Approaches moderated by Rob Gailey. Download the presentation here.

Using Cross-Sectional Models to Develop Measurable Objectives for Saltwater Intrusion
Sean Culkin, Cameron Tana and Derrik Williams, HydroMetrics Water Resources Inc.

The Sustainable Groundwater Management Act defines saltwater intrusion as an undesirable result that must be avoided to achieve basin sustainability. Groundwater Sustainability Plans need to define measurable objectives that prevent undesirable results such as saltwater intrusion. A water quality standard for salt concentrations is certainly necessary as a measurable objective, but measurable objectives based on groundwater elevations can also facilitate management of the basin to prevent saltwater intrusion for two main reasons:

1. If the saltwater/freshwater interface is offshore, water quality may not indicate risk of saltwater intrusion resulting from depressed groundwater levels until the interface comes onshore. Measurable objectives based on groundwater elevations can be designed to prevent intrusion over the long term.
2. If a groundwater model is used to manage a basin, modeling the density dependent flow of the saltwater/freshwater interface can be numerically intensive and may produce errant results without a fine vertical model grid. By developing measurable objectives based on groundwater elevations, basinwide models do not need to model density dependent flow and groundwater elevation results from the basinwide models can be used for comparison to measurable objectives when evaluating groundwater management alternatives.

Measurable objectives based on groundwater elevations can be developed using density dependent cross-sectional models. The use of these models accounts for basin-specific geology as opposed to using more generalized approaches like the Ghyben-Herzberg relationship. Cross-sectional models were developed for the critically overdrafted Santa Cruz Mid-County Basin and the adjudicated Seaside Basin on the central coast of California. The cross-sectional models extend offshore to identify onshore groundwater elevations that allow the interface to equilibrate at a known location offshore, regardless of the interface’s current position. This approach is a time independent approach that has the advantage of not requiring time series model inputs. Model layers were extended offshore based on understood geology and the aquifer parameters were varied based on a Monte Carlo approach. Results from the Monte Carlo approach were used to assess the probability that a protective groundwater elevation will prevent saltwater intrusion. Based on these probabilities, measurable objectives for groundwater levels were established for coastal locations. These measurable objectives can be compared to groundwater elevation data to assess whether the basin will avoid the undesirable result of saltwater intrusion over the long term. Groundwater flow model results can also be compared to the measurable objectives to evaluate groundwater management alternatives.

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