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Fiscal Year 2006 Progress Report for the USGS/SCWA Cooperative Study to Evaluate the Ground-Water Resources of the Santa Rosa Plain

The primary objectives of the Santa Rosa Plain study are to: 1) develop an updated assessment of the geohydrology and geochemistry of the Santa Rosa Plain; 2) develop a multi-aquifer ground-water flow model for Santa Rosa Plain; and 3) evaluate the hydrologic impacts of alternative ground-water management strategies for the basin.

The study has four principal tasks: 1) data compilation - a geographic information system (GIS) will be the primary means of organizing data for archiving, searching, interrelating, and displaying hydrologic and related information; 2) collection of new data, with a focus of water-quality sampling; 3) data interpretation and geohydrologic characterization - including refining hydrologic budgets, and updating conceptual models of the ground-water flow system based on the new data and the results of ongoing United States Geological Survey (USGS) geologic studies in the basin; and 4) the simulation of ground-water flow in the Santa Rosa Plain. This document presents the progress made toward the completion of this study by USGS scientists in Federal Fiscal Year 2006 (FFY06).

Task 1

Data compilation efforts included producing maps of the study area, identifying the locations of public and private wells, compiling existing water-quality data from other USGS projects and from stakeholders, compiling historic water-level data, compiling and digitizing historical land-use data, compiling pumpage data, and compiling well-construction data. The maps include maps of the general study area and preliminary well locations. The well locations provided to date have been entered into the GIS database; however, some locations must still be provided by stakeholders. Available water-quality and water-level data have been digitized. Existing land-use data in digital and hard-copy formats were compiled; the hard-copy data are being digitized by SCWA personnel. The USGS is still awaiting data from several of the co-funding agencies.

In support of developing a precipitation-runoff model (described in Task 4), GIS-based spatially distributed input data for characterizing watershed properties that are required as inputs for the precipitation-runoff model were developed. The primary input data consists of a Digital Elevation Model (DEM) and parameters derived from the elevation data (i.e., topographic parameters used for modeling potential evapotranspiration and flow routing parameters used for modeling streamflow). Additional GIS-based input data include spatially-distributed estimates of 1971-2000 average, monthly precipitation and average monthly maximum and minimum air temperature; hydrographic data (location of drainage lines, water bodies, drainage divides); soils data (STATSGO and SSURGO), vegetation data (vegetation type, canopy cover); surface geology (consolidated and unconsolidated surficial material); land use; and estimates of imperviousness. FFY06 accomplishments include the compilation of all required GIS data and the completion of GIS data processing needed to developed inputs for the precipitation-runoff modeling. Additional data compilation completed in FFY06 included the identification of a regionally distributed network of daily and hourly climate monitoring sites (including current and historical records) needed for model input and regionally distributed USGS stream-gage records were collected for model calibration. The average daily and hourly records were compiled from federal and state sources and were processed into the formats required for model input.

Hydrographs showing ground-water level fluctuations for about 90 wells located in the Santa Rosa Plain have been constructed. The ground-water level data were obtained from California Department of Water Resources (DWR) computer database and hardcopy records in office files. A search was made for well construction and lithologic information for each well. Ground-water level data for most of the wells are ten years or less in length and cover variable time periods. Generally, the DWR data are only semiannual and emphasize seasonal water-level highs and lows. However, variations in the timing of the wet and dry seasons between years and changes in the months in which measurements were made partly obscure correlations between precipitation amounts and water levels. Over half of the wells are shallow, here defined as less than 200 ft deep. The earliest ground-water level records date from the late 1940s; the most recent records, including 2006, were collected from a small network of about 20 wells.

Analysis of the ground-water level data, thus far, has focused on the differences in seasonal and long-term changes recorded in deep versus shallow wells and how the changes relate to differences in annual precipitation amounts. The variable geographic distribution, non-uniform time periods, and incomplete well construction information present challenges for developing a detailed history of ground-water level changes throughout the Santa Rosa Plain.

Task 2

Water-chemistry data has been compiled for approximately 250 wells and a dozen surface-water sites located within the Santa Rosa drainage basin. These data were obtained from the California Department of Water Resources (DWR), several co-funders for the Santa Rosa Plain study, and the USGS Ground water Ambient Monitoring and Assessment (GAMA) project. The majority of the data represents one-time sampling of major inorganic constituents (sodium, chloride, etc). Exceptions include multiple (5 or more) analyses from about three dozen wells periodically sampled by DWR as far back as 1950, and wells sampled as part of the recently published GAMA project which emphasized anthropogenic constituents such as volatile organic compounds, pesticides, and wastewater compounds.

Interpretation of the existing water-chemistry data is in progress to identify possible spatial and temporal patterns. As part of spatial analysis of historic data discontinued DWR ground-water quality sites are in the process of being located.

Task 3

A team from the Geologic Division of the USGS (GD) is working to define the hydrostratigraphy of the Santa Rosa Plain and is developing a three-dimensional geologic model of the plain that will be incorporated into the ground-water flow model. The team leader is Dr. Victoria Langenheim and its members include USGS scientists from the Menlo Park, California and Denver, Colorado offices.

Well driller's information DWR water-well data for the 15 township/ranges in the Santa Rosa Plain (R7W, 8W and 9W; T 5N, 6N 7N, 8N and 9N) were compiled. Data were collected for 200–300 wells per T/R, compiling and locating almost 2,600 wells in all. The myriad of driller's descriptions (45,000 records of data) were simplified into about 20 lithologic terms (sand, clay+gravel, basalt, etc) and have a preliminary version that covers the entire Santa Rosa Plain has been completed. These interpreted lithologic units were used to generate a preliminary three-dimensional lithology model by extrapolating lithology away from individual boreholes to fill the intervening volume. The interpolated data in the resulting solid model have the appearance of stratigraphic units, with aspect ratios that emphasize the horizontal dimension over the vertical. Horizontal bedding was assumed and faults were not explicitly included in the model; these generalizations will be modified in the future.

Three reports have been published as part of this study. The first report is a preliminary version of the gravity map that covers most of the Santa Rosa Plain (USGS Open-File Report 2006-1056) this February. Since February, about 250 additional stations have been collected to better characterize the basin margins. This updated gravity with conjunction with information gleaned from the well compilation effort forms the foundation for the basement surface (bottom of the ground-water flow model).

The second report is a digital dataset for water well locations and well data compiled from USGS Water Supply Paper 1427, originally published by G.T. Cardwell in 1958.

The third report presents the results of lithostratigraphic and biostratigraphic analyses of well cuttings from two deep wells drilled on the Santa Rosa Plain for Sonoma County Water Agency (USGS Open-File Report 2006-1196). The report discusses the significance of fossil occurrences in the wells to the stratigraphy beneath Santa Rosa Plain and correlative units exposed around the margin of the plain.

Lithologic samples and suspension seismic velocity logs were collected at two City of Santa Rosa water-well drill sites. One well is on the Trenton Ridge (a bedrock high) the other in the Cotati basin. The team continues to interact with Cal American Water Company and geohydrologic consultants for stakeholder cities of Sebastopol, Santa Rosa, Larkfield and Windsor, where new wells will be drilled in late 2006 and in 2007.

In the three-dimensional geologic model, a preliminary surface (based on the better-quality logs and cuttings) on the top of the Petaluma and Wilson Grove Formations on either side of the Trenton Ridge has been delineated. Current efforts focus on defining a top surface for the Plio-Pleistocene "Glen Ellen" Formation and related gravels, and refining the three-dimensional framework model with the integration of the newly compiled extensive DWR well dataset discussed previously.

Task 4

As part of developing a ground-water flow model of the Santa Rosa Plain, a precipitation-runoff model is being developed to characterize the spatially and temporally variable water balances, including estimates of ground-water recharge. To develop the precipitation-runoff model, the climate and water balance for the study area in response to spatially and temporally distributed precipitation, air temperature, and potential evapotranspiration estimates must be characterized using a combination of data and modeling. Input data consists of available climate and meteorological records (precipitation, air temperature, solar radiation, wind speed, relative humidity, potential evapotranspiration) from a regional network of monitoring sites as described in Task 1. Climate modeling consists of developing multi-year daily estimates of spatially distributed precipitation and air temperature using average monthly estimates of precipitation and air temperature and spatial interpolation models that utilize the available daily and hourly data (both historical and current) from the regional monitoring network. The daily results are developed for the precipitation-runoff modeling used to help characterize the water balance and to develop estimates of recharge. During FFY06, historical records from federal and state climate stations were compiled and processed into the climate database and the format needed for the precipitation-runoff model. Preliminary model runs were completed in FFY06 for an initial modeling period of 1950–2005. Model calibration based on a comparison of simulated and measured daily streamflow records is continuing. Finalized recharge estimates will be based on the calibrated precipitation-runoff model.