California Water Science Center

Shallow Aquifer Assessment Study for the Monterey Bay and Salinas Valley

Map of Salinas Valley Study Area and target wells.

Map of Salinas Valley Study Area and target wells.

High nitrate and trace element concentrations in groundwater represent a health threat to domestic well users. The concentrations and distributions of these constituents in shallow groundwater in the Salinas Valley of the central coast of California are poorly understood, and communities are concerned about the quality of their drinking water. A comprehensive assessment of water quality in this region is needed to provide essential data for water resource management.

For a comprehensive assessment, statistical methods (grid based and spatially weighted approaches) to assess water quality provide higher resolution of constituents present at high concentrations, as well as a better understanding of the spatial distribution of these constituents, when more data are available. Data collection (sampling and analyses) is expensive, often limiting the amount of data available for statistical analyses. Datasets collected by methods other than established USGS protocols are often available, but questions about the comparability of the data often exclude these external data from USGS studies. Water-quality data for the shallow aquifers in the Salinas Valley are needed, and more data provide a better statistical assessment of the shallow aquifers.

This project will complement the concurrent USGS GAMA Program assessment of water quality in the shallow aquifers of the Salinas-Monterey Bay region by collecting additional water-quality data across the study area. The GAMA Program is sampling 40 grid wells in the Salinas Valley, while this project will sample 80 household taps spatially distributed across the Salinas Valley, 20 of which will co-occur with the GAMA well samples.

This project aims to provide essential water-quality data of the shallow aquifers in the Salinas Valley for the Central Coast Regional Water Board, while also providing an evaluation of water-quality data collected using established USGS protocols compared to less constrained voluntary well-sampling methods, in order to assess whether these data are suitable for incorporation into a regional, comprehensive groundwater assessment. The project aims to (1) assess water-quality conditions in the shallow aquifers of the Salinas Valley, and (2) determine if sample collection protocols affect water-quality data. More groundwater samples provide greater statistical mass, which results in an improved ability to identify small targets, and to identify the spatial distribution of the small targets.

Groundwater samples from 80 household taps spatially distributed across the Salinas Valley will be collected by the USGS and analyzed for major and minor ions, trace elements, field parameters, and nutrients, as outlined by the Regional Water Quality Control Board.

Groundwater samples from 20 domestic wells spatially distributed across the Salinas Valley will be collected by the USGS and analyzed for VOCs, pesticides, nutrients, major and minor ions, and trace elements. Two samples will be collected at each site, one at the well head following standard USGS sampling protocols (Koterba and others, 1995; U.S. Geological Survey, 2006; 2009), and one sample from an outside household tap, simulating voluntary well-sampling program methods (e.g., State Water Resources Control Board, 2011).

Water-quality data (VOCs, pesticides, nutrients, major and minor ions, and trace elements) collected by the two different sampling methods will be analyzed in order to assess the comparability of data, and by corollary, data collected for other voluntary well-sampling programs.

Data collected for this study will provide essential water-quality data of the shallow aquifers in the Salinas Valley of the central coast of California, in accordance with USGS Science Strategy (USGS Circular 1309) for conducting a "water census" for quantifying, forecasting, and securing freshwater for America's future. This data will provide the Central Coast Regional Water Board with a better understanding of the constituents present at high concentrations, and their distribution in the region. Data collected for this project will provide the GAMA Program additional data for the comprehensive assessment of shallow aquifers in the Salinas-Monterey region.

In addition, the question of the comparability of data has broad implications. An assessment of the comparability, and thus utility, of the data will potentially allow the USGS GAMA Program to incorporate additional data sets (state and regional voluntary well sampling data) into comprehensive groundwater assessments of the shallow aquifers, in the central coast region and statewide, and thereby provide an improved understanding of constituents present at high concentrations and their distribution in shallow groundwater, in line with the USGS Science Strategy. This project supports the USGS’s Water Resource Mission Area priorities including characterizing the quality of water resources, groundwater, aquifers and wells, and water data for the Nation.

Cooperating Agency: Central Coast Regional Water Board
Project Chief: Justin Kulongoski
Phone: 619-225-6122
Email: kulongos@usgs.gov

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