Keywords:flood management, infrastructure, storage
California remains significantly dependent upon surface water. A review of the California Water Balance Summary, 2001-2010 (California Water Plan Update 2013, Volume 1,...
California remains significantly dependent upon surface water. A review of the California Water Balance Summary, 2001-2010 (California Water Plan Update 2013, Volume 1, Chapter 3, Table 3-2), indicates that in an average year like 2010, about 65 maf (million acre-feet) (more than 80 percent) of 80 maf total dedicated and developed water supply is associated with surface water. Surface storage is an essential element of managing the state’s surface water resources.
The naturally arid conditions found in much of California, coupled with seasonal variations of too much or too little water prompted water planners of the past to implement conveyance and storage projects to support land development, population, and economic growth. After construction, these dams captured seasonal runoff and stored it for beneficial uses during drier times. Today, these projects facilitate a larger set of water management objectives including reliable water supplies, water quality and ecosystem maintenance, flood management, and hydropower generation.
In many areas of the state, surface water and groundwater are used conjunctively. Coordinated surface water and groundwater management can be either formal or informal. For example, a managed groundwater recharge program where surface water is infiltrated to an aquifer for later use is formal; excess applied surface water in agricultural areas during wetter years that increases the availability of groundwater in drier years is often more informal.
Dams and surface water storage continue to be a critical tool for providing water management flexibility in California. The amount of surface water in California, as noted above, often make it a foundational integration element of more diverse local and regional water management portfolios. In addition to storing water for use by residents, businesses, and industries, these facilities provide vital supplies during warm and dry periods for growing crops and maintaining the state’s managed wildlife refuges.
Water management in California has always been challenging. The state’s variable climate is marked by long droughts and severe floods, with stark regional...
Water management in California has always been challenging. The state’s variable climate is marked by long droughts and severe floods, with stark regional differences in water availability and demand. California has adapted by building a vast network of storage and conveyance facilities to deliver water from the wetter parts of the state to population and farming centers in the Bay Area, the San Joaquin Valley, and Southern California. The Sacramento–San Joaquin Delta is a fragile link in the state’s water supply network, and a plan to address supply reliability, known as California WaterFix, faces many hurdles. California’s extensive network of dams is aging. Agricultural demand is becoming less flexible, as farmers are increasing tree crops (especially nuts), which must be watered every year. Conflicts are growing between human water use and water needed to support fish and other wildlife. And the latest cycle of droughts and floods provides a glimpse of an uncertain future under climate change. California’s water management challenges are complex, but they can be addressed. Solutions will involve difficult and sometimes costly trade-offs, as well as contentious legal and political changes.
INTERA Incorporated (INTERA) was retained by the Calleguas Municipal Water District (CMWD) to develop a numerical groundwater model of the East Las Posas...
INTERA Incorporated (INTERA) was retained by the Calleguas Municipal Water District (CMWD) to develop a numerical groundwater model of the East Las Posas Management Area (ELPMA), which includes the locally-recognized east and south sub-basins of the Las Posas Valley Basin (LPVB). Groundwater in the ELPMA is found in a multiple-aquifer system characterized by intense faulting and folding, which is known to exert structural controls on groundwater flow and movement. The ELPMA is known to receive recharge from surface water flows in Arroyo Las Posas/Simi that runs east to west along the southern edge of the basin. Flows in the Arroyo have become perennial as a result of discharges from wastewater treatment plants and dewatering wells within and upstream of the ELPMA. Hence, understanding and modeling the surface-water/groundwater interaction along the Arroyo is an important component of the numerical model development. CMWD also owns and operates the Las Posas Basin Aquifer Storage and Recovery (ASR) Project, consisting of eighteen high capacity ASR wells and associated facilities located in the ELPMA that are used to inject and recover potable water purchased from Metropolitan Water District of Southern California (Metropolitan). Basin response to injection/extractions at the ASR well fields and the evaluation of storage capacity of the ASR well field are key considerations for this modeling project.
The Santa Clara Valley Water District (District) is the primary water resources agency in Santa Clara County. This Groundwater Vulnerability Study was conducted...
The Santa Clara Valley Water District (District) is the primary water resources agency in Santa Clara County. This Groundwater Vulnerability Study was conducted to predict the vulnerability of groundwater to potentially contaminating land use activities to aid the District in its management and protection activities. Groundwater vulnerability is comprised of two key components: 1) groundwater sensitivity and 2) potentially contaminating activities. Groundwater sensitivity is generally defined as the relative ease with which a contaminant on or near the land surface can migrate to the aquifer of interest based on the intrinsic characteristics of the aquifer and the overlying unsaturated materials. Groundwater sensitivity is combined with the potentially contaminating activities risk to characterize overall groundwater vulnerability.
The Study Area is comprised of three groundwater subbasins: the Santa Clara, the Coyote, and the Llagas. Each has unique hydrogeologic characteristics as well as unique current and historic land uses. Water-bearing units in the Study Area have been grouped into two major aquifer systems, the Shallow Aquifer and Principal Aquifer. Groundwater vulnerability was assessed separately for each aquifer. Generally, the Shallow Aquifer occurs above regional confining layers. The Principal Aquifer lies beneath the Shallow Aquifer and supplies most of the groundwater produced for beneficial uses in the Study Area. The Principal Aquifer occurs under semi-confined to confined conditions. In areas where confining layers do not exist or are not laterally and vertically extensive, only the Principal Aquifer occurs. Accordingly, in the recharge zones, depth to water was characterized as the first encountered groundwater for both the Shallow and Principal aquifers. In contrast, the Shallow and Principal aquifers were uniquely defined in the confined areas with the Shallow Aquifer characterized by the first encountered groundwater and the Principal Aquifer defined by the top screened interval of wells tapping the primary groundwater production zone.
The southern area and lateral margins of the Santa Clara Subbasin are unconfined recharge areas. An extensive regional aquitard occurs in the northern interior portion of the subbasin. The Santa Clara Subbasin is currently highly developed with residential, commercial, and industrial areas. Due to the high density of urban land uses including major industrial manufacturing and processing facilities, point-source contamination is prevalent but generally contained in the Shallow Aquifer. This is due, in part, to the protection offered by the significant confining layers found in the northern portion of the subbasin. The Santa Clara Subbasin is currently undergoing continued urban expansion and redevelopment of formerly industrial areas to residential use.
No significant laterally extensive confining layers exist in the Coyote Subbasin, and groundwater generally occurs under unconfined conditions. Compared with the Santa Clara Subbasin, the Coyote Subbasin is relatively rural, undeveloped, and mostly unincorporated with far fewer industrial/commercial contaminant release sites. Due to generally unconfined conditions, the Coyote Subbasin is hydrogeologically sensitive to groundwater contamination. Existing water quality impacts are related to agricultural practices and rural (e.g., septic) land use. Coyote Valley has a high potential for future residential and commercial development.
Groundwater in the Llagas Subbasin occurs under unconfined to confined conditions. Generally unconfined recharge areas are found in the northern portion of the subbasin, while confining layers become more frequent and laterally and vertically extensive in the southern areas. Accordingly, the northern portion of the subbasin is relatively more hydrogeologically sensitive to groundwater contamination compared with the southern subbasin. The Llagas Subbasin has urban development focused in the north and south with the central portion of the subbasin comprised predominantly of agricultural development and large residential parcels. The Llagas Subbasin is currently impacted by high levels of nitrate associated with rural land use and agriculture and perchlorate from historic releases from a flare manufacturer.
A comprehensive literature review was conducted to identify established methods to conduct vulnerability assessments. Based on the evaluation of available groundwater sensitivity assessment methods and an understanding of available hydrogeologic and water quality information for the Study Area, a statistical method was selected to quantify the sensitivity to contamination of the Shallow Aquifer and Principal Aquifer. Water quality data (i.e., nitrate concentrations and distribution) were used for calibration and verification purposes to identify and rank groundwater sensitivity factors. Based on the logistic regression statistical analysis, four factors were found to be the most important in characterizing groundwater sensitivity. These include 1) soil media characteristics in the vadose zone, 2) groundwater recharge, 3) depth to top of screen, and 4) annual groundwater production. Groundwater recharge in the Study Area includes a significant amount of artificial recharge conducted by the District. Traditionally, artificial recharge and groundwater production would not be considered intrinsic aquifer characteristics. However, for this Study, groundwater sensitivity did include consideration of quasiintrinsic characteristics such as artificial recharge and production. In addition to the objective statistical analysis, subjective refinements relative to artificial recharge areas and characterization of depth to water/aquifer were applied based on observation, technical judgment, and limitations in the nitrate data set. These refinements included subjectively ranking areas near artificial recharge areas as highly sensitive and using depth to first encountered groundwater to represent the depth to water factor for the Principal Aquifer in the recharge zones. This results in higher sensitivity in the recharge areas for the Principal Aquifer.
Figures ES-1 and ES-2 show the sensitivity of the Shallow and Principal aquifers, respectively. Not surprisingly, the sensitivity assessment found that the Shallow Aquifer is more sensitive than the Principal Aquifer.
In addition, the analysis indicates that the sensitivity of the Shallow and Principal aquifers is generally highest in the Llagas Subbasin, followed by the Coyote Subbasin, and Santa Clara Subbasin. Despite the protection afforded by the regional confining layer in the southern portion of the Llagas Subbasin, both the Shallow and Principal aquifers are highly sensitive to contamination due to high recharge rates and permeable soils. The sensitivity of the Shallow and Principal aquifers in the Coyote Subbasin are also relatively high due primarily to shallow aquifer conditions, high recharge rates, and large amounts of groundwater production. Although the confined zone in the Santa Clara Subbasin affords relatively good protection from surface contamination, the outer western confined zone appears to be highly sensitive to contamination due to the significant groundwater production in this area.
Unlike the sensitivity assessment, it is not possible to statistically calibrate potentially contaminating activities risk. Accordingly, potentially contaminating activities are commonly identified and ranked based on subjective observation and experience (e.g., California Drinking Water Source Assessment and Protection Program). For the potentially contaminating activities risk ranking, emphasis was placed on accurately characterizing the maximum risks so as not to underestimate potential risks.
The potentially contaminating activities risk analysis included four main risk factors or categories—general plan land use, potentially contaminating business activities, known contaminated sites, and supplemental data. Twenty-nine land use categories were selected and ranked with respect to groundwater impacts. Relative risk ranks were developed for 48 categories of business activities. Known and open contamination sites were given the highest ranking with a lower ranking for closed sites. Additional supplemental data for irrigated agriculture, septic system density, mines, landfills, and petroleum pipelines were also ranked and included in the analysis. These various risk factors were combined to generate the overall potentially contaminating activities risk distribution map. The potentially contaminating activities risk analysis found large portions of the Santa Clara Subbasin are at high risk due to its high level of development with commercial and industrial areas and many associated industrial and commercial contaminant release sites along with the lingering impacts of agricultural releases. Relatively lower overall risks are associated with the Coyote Subbasin, which is relatively rural, undeveloped, and mostly unincorporated with far fewer industrial/commercial contaminant release sites. Nonetheless, most of the subbasin shows a moderate level of risk associated with irrigated agriculture. It is important to note that the Coyote Valley has the most potential for future development and thus the most potential for an increase in risk in the future. Relatively lower overall potentially contaminating activities risk (compared with the Santa Clara Subbasin) is found in the Llagas Subbasin due to its more rural nature. Areas of relatively higher risk are associated with commercial and industrial development in the vicinity of the cities of Morgan Hill and Gilroy. Moderate risk is found in the central portion of the subbasin associated with irrigated agriculture. While continued conversion of rural to urban land use in the vicinity of the cities of Morgan Hill and Gilroy in the future will likely increase risk in these areas, the central portion of the subbasin is expected to remain relatively unchanged with respect to risk, given the established zoning.
The sensitivity assessment and potentially contaminating activities risk were combined to create the overall vulnerability maps for the Shallow and Principal aquifers. Figures ES-3 and ES-4 show the vulnerability of the Shallow and Principal aquifers, respectively. As might be expected, the vulnerability of the Shallow Aquifer is greater than the Principal Aquifer in areas of confinement in the Santa Clara Subbasin. The density of commercial/industrial sites and known contamination release sites in the northern Santa Clara Subbasin make the Shallow Aquifer highly vulnerable to contamination. The Principal Aquifer has fewer areas of very high vulnerability compared with the Shallow Aquifer due to the relative lower sensitivity of the Principal Aquifer.
The Llagas and Coyote subbasins exhibit high to very high groundwater vulnerability in both the Shallow and Principal aquifers. The high vulnerability is driven primarily by the high sensitivity in these two subbasins. Given the potential for future development in the Coyote Subbasin, the high degree of vulnerability of the subbasin requires the highest level of effort directed toward protection.
Following completion of the vulnerability assessment, a web-based geographical information system tool was developed, which incorporated the sensitivity, potentially contaminating activities risk, and vulnerability maps. Additional maps are also provided to enhance the usefulness of the tool. The tool enables District staff to work interactively with the vulnerability study analysis. The tool enables District staff to evaluate potential impacts of land use changes, prioritize basin management activities, prioritize review of known contamination sites, update existing data layers, update or modify risk factors, and add supplemental data layers.