Groundwater Model Studying Effects of Existing Recharge Basin and Proposed Subsurface Barrier for a Ranch in Santa Rosa Creek Watershed

• Main Author: Young, Stefan J
• Format: Others
• Published: DigitalCommons@CalPoly 2021
• Subjects: Groundwater; Recharge; GMS; Dam; Basin; Cambria; Civil and Environmental Engineering
• Online Access: https://digitalcommons.calpoly.edu/theses/2294; https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=3837&context=theses

A groundwater model of a 126.2-acre ranch in Cambria, California was expanded upon to analyze the effects of artificial recharge and a subsurface barrier. The ranch lies within the 48mi2 Santa Rosa Creek Watershed along the Central Coast of California. The mainly agricultural watershed outfalls to the Pacific Ocean to its west. Creek Lands Conservation, a non-profit that aims to conserve and restore habitat along the Central Coast, plans to identify projects to restore stream flow during dry seasons in the creek that runs through the Santa Rosa Creek Watershed and to increase artificial groundwater recharge. This study focuses on two of those projects. One project is an existing recharge basin and the other is a subsurface barrier. The objective of this numerical model is to improve upon an existing model by using a longer duration of data to calibrate the model, calibrating the model to hydraulic properties of soil samples that were obtained from the site at various depths, refining elevations of layers through integration of new borehole exploration data, and adding updated and new data such as mountain front recharge and pumping rates. The modeling program used was GMS which allows calculation and determination of heads and flow directions. Within the model, there are three separate layers based on hydrogeological characterization from previous studies. There is an upper unconfined zone, a confining clay layer, and a confined zone. A package within GMS (Groundwater Modeling System) called PEST (i.e., Parameter ESTimation) was used to calibrate the model to known water surface elevations throughout the site. Data such as elevations, head boundaries, stream flow, pumping rates, recharge, evapotranspiration, well locations, and hydraulic properties of the subsurface was processed and incorporated into the overall model in GMS. Recharge rates from the basin were estimated to be 0.1 m/day roughly starting in February and ending in May for each year. The model showed that the confining layer slows down the flow of water from the recharge basin, but it does eventually percolate into the underlying groundwater aquifer before reaching Santa Rosa Creek after a time period of 5 years. The proposed subsurface barrier does reduce travel times of groundwater by roughly a year and helps percolation of water into the confined layer. With the subsurface barrier it was seen that the water held within the confined aquifer increased on average 5,200 m3 each year.