Abstract
This project evaluates the feasibility of groundwater banking in the Central Sacramento County Basin (Central Basin) using a Three Dimensional (3-D) model. The 3-D model was developed using Groundwater Modeling System (GMS) with MODFLOW. The Central Basin is located in Sacramento County in northern California. The Central Basin under study in the current project is bounded by three rivers (on the west by the Sacramento River, on the north by the American River, on the south by the Cosumnes and Mokelumne rivers), and on the east by the Sierra-Nevada mountain range. The surface area of the Central Basin is approximately 388 square miles (248,000 acres), and receives an average annual precipitation of 14 to 20 inches. Sacramento County’s Sacramento Central Groundwater Authority (SCGA) manages the basin, provides guidelines and recommendations for the long-term sustainability of the Central Basin. According to SCGA, the annual average sustainable groundwater extraction rate from the basin could be as high as 273,000 acre-feet per year (AF/year). The current extraction rate stands at 250,000 AF/year. The current and future water demands will increase due the population increase and extended periods of droughts. The increase on water demands will most likely cause the extraction rate from the Central Basin to be reaching the upper extraction limit of 273,000 AF/year. With growing needs for groundwater, sustainable approaches utilizing groundwater system are to be considered as an integral part of a water resources management plan. One approach to groundwater sustainable solution is to artificially-recharge groundwater aquifers. This study focuses on the potential for groundwater recharge in the Central Basin for three water years (wet year, above normal year and critical year). In order to accomplish the objective of this study, a conceptual groundwater model was created. The model assumptions, input parameters and boundary conditions are discussed later in this report. The simulation analysis of Central Basin model involved developing a conceptual model of the hydrogeology and surface water/groundwater interactions that exist within the project site. A conceptual model is a general description of the site, including the groundwater formations, geologic layers, subsurface hydrogeology, and inflow/outflow to the aquifer system. The Central Basin model was developed in multiple phases, using various tools available within the GMS computer software. First, the stratigraphy and geology of the Central Basin including the top soils formation were developed. Groundwater and soil properties were assigned to each of the layers created based on theoretical values obtained for literature. Three different water year types were selected: critical (2008), above normal (2005) and wet (2006). Data for precipitation/percolation, rivers/seepage, and the groundwater usage through the extraction wells were applied to the model for each of the selected years. The data included used in this report was obtained from several sources such as California Department of Water Resources (DWR) and SCGA monitoring stations. The model was ran and calibrated through 200 iterations, to re-produces the groundwater conditions for each of the selected year types. During the calibration process, the hydraulic conductivity (K) values of foot per day (ft/day) were found to be the key element in the movement and formation of the groundwater in the Central Basin. The initial values for the hydraulic conductivity were set to ranging from 0.1-to-10 ft/day, and eventually values ranging from 8-to-59 ft/day were attained. In addition, the results show the formation of cone of depression in the central portion of the basin, which is consistent with historic trends of groundwater elevation contours in the region. The only exception was that the location of the cone of depression formulated throughout these results seemed shifted further to the north than the historic records. After finding the reasonable hydraulic conductivity values for the model and calibrating the GMS model, the Central Basin GMS model was ran with a water banking scheme utilized by adding three recharging wells in the vicinity of the cone of depression. The three wells that were added to the calibrated model will add 10,000 AF of groundwater to the Central Basin. Another recharge volume of 20,000 AF (through six recharging wells) was investigated to evaluate the impact of groundwater recharge in the Central Basin and determine the maximum volume of recharge that can be used for this basin. The banking of 10,000 AF during the critical and wet years, were effective in raising the water table elevation in the cone of depression area, without causing any negative impact elsewhere in the basin. The minimum groundwater contour elevations was raised from -123 feet to -112 feet and from -99 feet to -92 feet for the critical and wet years, respectively. There was no change in the maximum contour elevation in either case. In all scenarios evaluated, there was a change in the inflow/outflow to and from the rivers to the aquifer. The water banking scheme of 20,000 AF during the wet year results were quite different from the banking of 10,000 AF during the same year. The additional inflow further raised water table around the cone of depression area. However, it also caused a significant rise in the water table in the south-west corner of the Central Basin, which is close to the ground surface elevation. Groundwater elevation near the ground surface may cause problems such as liquefaction. According to the findings of the Central Basin model, banking up to 10,000 AF of groundwater during any year type is feasible. However, more than 10,000 AF of groundwater banking would require further analysis and consideration.