Analysis of the Goleta Groundwater Basin Using ArcHydro Groundwater

Dylan Berry

Mike Herrman

Bruce Stevenson

Erik Young

UC Santa Barbara

Kryzstozf Janowicz

Geography 176C

Spring 2013

Abstract

Local groundwater supplies are becoming an increasingly important addition to

many water portfolios, especially considering long-term sustainability, climate change and

increasing energy prices. When properly managed, groundwater aquifers act to store and

clean the local water supply, and eliminate the negative side effects of traditional water

supply management systems. In many regions today, groundwater supplies are set aside

as reserves to be used in times of drought. Spatial analysis of the Goleta Groundwater

Basin was conducted using ArcHydro Groundwater to provide a better understanding of

the aquifers and their potential as a freshwater resource. The results yielded an estimated

groundwater storage of 209,000 acre-feet. Further analysis suggests that agricultural sites

in the Goleta area were primarily responsible for the large drawdown that occurred

between 1941 and 1955. In addition, the existence of the Goleta Slough is most likely due

to the combination of low ground surface elevation and high groundwater elevation.

1. Background

1.1 Study Area

The Goleta Basin is located at roughly 34°26′26″N, 119°48′49″W, just northwest

of Santa Barbara on the central coast of California. The basin consists of mostly gradually

rolling terrain, with elevation ranging from 5 to 90 feet above mean sea level. It sits

between the Santa Ynez Mountains and the ocean, and is relatively flat, so that surface

water travelling down the mountains often percolates through the ground surface,

occupying subterraneous aquifers. The mountain terrain is most dominated by

sandstones and shales, which erode to produce sediment. The sediment is then

distributed throughout the valley by fluvial and colluvial processes. The area has

historically been dominated by both military and agricultural land use, and was an

unincorporated part of Santa Barbara County until Goleta became its own city in 2002.

1.2 Motivation

Local groundwater supplies are becoming increasingly important due to a large

variety of factors. Global population is growing exponentially, straining supplies of limited

resources. Climate change has led to an overall decrease in freshwater supplies, and has

altered freshwater distribution. Reliance on fossil fuels and increased energy prices has

made interbasin transfers of freshwater more expensive and less desirable. In addition,

the historic overuse of freshwater resources has threatened the stability of many

California aquifers and native riparian species, so more water must be conserved for

these systems. These, in addition to other factors, have led populations looking to

localized resources for freshwater needs. When properly managed, groundwater aquifers

act to store and clean the local water supply, and eliminate the negative side effects of

traditional water supply management systems. In many regions today, groundwater

supplies are set aside as reserves to be used in times of drought.

Throughout much of the late 20th century, the Goleta basin was in a state of

unstable overdraft. The Wright Judgment in 1989 served to adjudicate the water

resources of this basin, and ordered the restoration of groundwater levels to a steady

state (Gibbs 2011). While these protections have effectively revived the aquifers, the

Goleta Water District released a Groundwater Management Plan report on the state of the

basin and its role in the local water supply. While quantitative models are useful in many

regards, these types of reports could be supplemented by spatial analysis to confirm

and/or visualize these findings.

Study of groundwater resources is especially important in coastal regions, as

saltwater intrusion could potentially contaminate the freshwater aquifer and prevent its

further use. The study of saltwater intrusion is slightly outside the scope of this project,

but lends credibility to the importance of groundwater studies in this region.

1.3 Purpose

The purpose of this project is to use geographic information science to analyze the

groundwater supply and infrastructure of the Goleta Basin, in order to gain a better

understanding of its potential for harvesting groundwater.

2. Data

Data for this study was retrieved from 3 main sources: Evenson 1962, Upson 1951,

and Bachman 2011. Stratigraphy data was obtained from the Evenson 1961 and Upson

1951 studies, which were both conducted with help from the US Geological Survey. These

studies detailed geologic stratigraphy for 48 wells in the Goleta Basin. The geologic strata

elevations were not explicitly listed in Evenson 1962 study, so elevations were estimated

from two cross-section diagrams which spanned the basin. Borehole well locations were

also obtained from the Evenson 1962 study, in the form of a borehole map. This map also

included the detailed wells from the Upson 1951 study. Historic water levels from 1941

and 1955 were also included in the Evenson study. The boundary of the Goleta Basin was

obtained from the Bachman 2011 Groundwater Management Plan, along with an

estimation for the average specific yield of basin.

3. Methods

3.2 ArcHydro Geodatabase

The framework of the ArcHydro Groundwater extension is the ArcHydro

geodatabase, which relates water-related datasets and feature classes in a way that can be

used for further spatial analysis. Using the Create AHGW Database tool, a geodatabase

template was created and included feature classes such as Well, Boreline, Borepoint, and

Georaster. These features were then populated using Microsoft Access and the ArcHydro

Groundwater text import tool.

3.3 Borelog

Information about each well was included the Borelog, which started as a Microsoft

Excel document, and was uploaded into the geodatabase via Microsoft Access. The WellID

attribute field in the Borelog table was designed to match the HydroID of the Well feature

class (Figure 1). The Borehole log included multiple entries attached to a single well, each

single entry corresponding to a geologic strata. Attribute fields such as reference

elevation (RefElevation), elevation at the top of the strata (TopElevation), and elevation at

the bottom of the strata (BottomElevation), were detailed for each entry. Soil type in the

form of text was also included, and later converted into a hydrogeologic unit classification

integer (HGUID).

Figure 1: Borelog Table (Excel version)

3.4 Wells: Georeferencing and digitizing

The well map was georeferenced onto the NAD 1927 datum and using a State

Plane V FIPS 0405 projection (Figure 2). These specifications were chosen to match those

of the original datasets. The wells were then digitized to their respective locations. Each

well was then assigned a unique identifier called WellID, which is equivalent to HydroID in

other parts of the study.

Figure 2: Georeferenced well map with digitized well point features (La Rocque, 1950)

3.4 Borelines

Borelines are line features that represent a well column, which can include

representations of individual strata. The Boreline feature class was tied to the Borelog via

WellID. They were then visualized in ArcScene, by importing the Boreline feature class of

the geodatabase and changing the symbology to display the hydrogeologic unit IDs

(HGUID). These were then projected over a georeferenced map (1-meter LIDAR) to

represent the aquifer as a whole (Figure 3).

Figure 3: Boreline visualization using ArcScene

3.5 Borepoints and Horizon IDs

Borepoints are the point features located at the top elevation of a single strata.

These are created and used to define horizons. Horizons are used to define a single

individual layer as it changes throughout the aquifer. The topmost surface is defined with

an integer value of 1, the next with a integer value of 2, etc. The manual assigning of

HorizonID is required because a single layer can be covered by different numbers of

strata at different well points. Borepoints of no interest for a particular analysis were

assigned a HorizonID of 9999.

3.6 Kriging

The kriging interpolation tool was used to interpolate a surface based on the

spatial distribution and elevation of borepoints, defined by HorizonIDs (Figure 4). The

kriging interpolation was chosen over the IDW interpolation because the krig offers more

accurate smoothing (no peaks and pits), which is more realistic in considering

groundwater aquifers. The kriging interpolation was limited to 5 points or 20,000 units

(about 1.5 miles), in order to eliminate the effect of very distant values.

Figure 4: Kriging interpolation raster (this example shows ground surface elevation).

3.7 Georasters

The georaster is a visualization of a horizon by projecting a kriging raster onto a

custom surface in ArcScene (Figure 5). This creates a near-3D representation, which can

be manipulated to gain a more intuitive and in-depth understanding of the basin. Many

trends are more readily seen in visualizations than in number tables. Georasters were

created for defined geologic formations (Figure 5), simplified geologic strata (Figure 6),

and historic water levels (Figure 7).

Figure 5: Georaster visualization of the Young Alluvium and Santa Barbara geologic formations

3.8 Simplification of the Strata

Simplification of the geologic strata was required in order to visualize the

groundwater aquifers (Figure 6). While many large formations were clearly defined the in

original detailed borelines, it would be impossible to create a single layer horizon due to

the various smaller layers which often intertwine throughout the basin. in addition, single

outlier borelines would have to be eliminated to prevent skewing of the model. In this

simplification process, the 8 original HGUID classifications were simplified into just 2

classes: Aquifers and aquitards (layers with high and low hydraulic conductivity,

respectively). Hydraulic conductivity is a property of soils or rocks that that describes the

ease with which water can move through them. Higher values denote more ease of

movement and lower values denote less ease of movement. Layers less than 10 feet in

thickness were typically ignored. In addition, larger trends of layers were considered, so

that many grouped formations would be considered as a single, larger formation. The

result of this process was a simplified boreline array, which yielded a simplified borehole

array, which allowed for the assignment of HorizonIDs into layers. These were then

interpolated using Kriging tool and used to make georasters as detailed above.

Figure 6: Simplification of boreline strata

3.9 Extraction of georaster values

The average thickness of each aquifer had to be calculated in order to estimate

total basin storage. The interpolated elevations were extracted from the kriging

interpolations of the upper/lower aquifer model using the Extract Values to Points tool in

the Spatial Analyst toolbox of ArcGIS. A mean average of these values was taken and used

to compute an average elevation, which was then used to compute an average thickness of

each.

Figure 7: Georaster visualization of historic water levels throughout the basin

3.10 Basin Area

The area of the basin was found by georeferencing a basin map included in the

Bachman 2011 Groundwater Management Plan. A polygon shapefile was created of this

area, and then the calculate geometry tool was utilized to compute the area of the

polygon.

3.11 Basin Volume Calculations

After extracting the values of the top and bottom of the upper and lower aquifers

the group had to simplify the data to use in our estimation of basin storage (Figure 8). In

order to accomplish this the group took the extracted values of depth and subtracted the

top from the bottom value to obtain the thickness of the aquifer layer at each well. Once

the thickness of the layers for all 48 wells were obtained the group averaged them

together to get the average thickness across the entirety of the aquifer layer. Once

average thickness for the upper and lower aquifer layers was determined we could use

the area of the Goleta Groundwater Basin that we had calculated with the Calculate

Geometry tool. The average thickness of each aquifer was multiplied by the total area of

the basin. The sum of these values was the estimated total volume of water bearing

formations. To calculate accessible groundwater storage, the total volume was multiplied

by the average specific yield of the basin, obtained from the Bachman 2011 report.

The volume represented by the total amount of space present within the combined

aquifer layers but an aquifer layer is not a section of the subsurface that is filled entirely

with water. These aquifer layers contain soil and rock which account for most of the

volume. What the soil and rock do not occupy is known as the void space and this is

where the water can be found. Even within the void space only a certain amount of that

water can be easily extracted.

The percentage of space that is easily accessible (think economically accessible),

can be represented with the specific yield. The specific yield ca vary drastically

throughout the subsurface so we used an average specific yield value that was estimated

in the Goleta Groundwater Management Plan. The estimated average specific yield was

multiplied by the total volume of the two aquifer layers we craeted to give the usable

groundwater storage estimate.

Accessible groundwater storage is defined as the volume of water that is

economically feasible to be harvested as a freshwater resource. It accounts for the

porosity of the medium and the ease of extraction. This value can vary drastically

throughout the subsurface so an average was used in calculation.

Figure 8: Georaster visualization of the upper and lower aquifers.

3.12 Water Level contours

Water level contours were created using the contour tool on the water level

kriging interpolations from 1941 and 1955. The contour intervals were selected at 4 feet

for the 1941 water levels and 5 feet for the 1955 water levels as this allowed for the most

accessible visualization of drawdown.

Figure 9: contour map of water table

4. Results and Analysis

4.1 Volume estimation

The estimated accessible groundwater storage in the Goleta Groundwater Basin

was estimated to be 209,000 acre feet. This is comparable to the the estimation published

by the Goleta Water District of 200,000 acre feet (Toups 1974). The total volume is

distributed between two aquifers. The upper confine of the upper aquifer roughly follows

the land surface. The lower confine of the upper aquifer is roughly coinciding with the

topmost layer of the Santa Barbara Formation, which mainly consists of unconsolidated

clay, silt and sand of marine origin. While these finer grained particles are typically less

permeable, pumping tests show their permeability to be about 100 gallons per day per

square foot (Upson 1952). This Santa Barbara Formation is the location of the basin’s

major lower aquifer, which provides almost all of Goleta’s groundwater (Bachman 2011).

In fact, many of the wells in the basin are only designed to pull from this lower

aquifer, leaving the upper aquifer almost untouched (Upson 1952). While the upper

aquifer is less readily accessible to groundwater pumping due to its stronger

consolidation of particles, in the future this could prove an important secondary source to

offset harvesting of the lower basin.

4.2 Drawdown wells between 1941 and 1955

A significant change in groundwater level occurred between 1941 and 1955. The

1955 level was at a lower elevation throughout the entire basin, and showed specific

areas that had experienced especially dramatic drawdown. By creating a contour map and

overlaying this on the well map, we could hypothesize which wells, or groups of wells,

were most responsible for drawdown during this time frame. These are most likely 10P3,

15G4, and/or 15G3 in the eastern basin, and 8K7, 8J1, and/or 8H1 in the central basin.

These two areas coincide with historical agricultural land use, and were most likely used

to irrigate farmland.

4.3 Artesian Situation

By overlaying the ground surface georaster with the 1941 water level georaster,

an artesian formation is visualized (Figure 10). An artesian situation occurs when the

groundwater level exceeds that of the ground surface, forming a natural spring or

wetland. In this case, the location of the artesian situation coincided with that of Goleta

Slough, a local wetland. This lends confidence to our analysis.

Figure 10: Potential artesian well in Goleta Slough.

5. Conclusion

Spatial and visual analysis of the Goleta Groundwater basin yielded an estimated

groundwater storage of 209,000 acre-feet. This was found by simplifying the geologic

stratigraphy, using kriging interpolations to create surfaces, and then extracting the krig

values to calculate a mean thickness of each aquifer. Multiplying by the area of the basin,

and again by the specific yield concluded the estimation. Further, groundwater contour

analysis suggests that agricultural sites in the Goleta area were primarily responsible for

the large drawdown that occurred between 1941 and 1955. Lastly, the existence of the

Goleta Slough is shown as the overlapping of the ground surface and water level surfaces

(artesian), and its existence is likely due to the combination of low ground surface

elevation and high groundwater elevation.

6. Discussion

6.1 Sources of error

In consideration of time, financial, legal and physical constraints, the calculations in

this report utilized some some values that were assumed to be accurate and true. Many

were taken from studies used by the Goleta Water District, the main water regulating

entity in the region, for accuracy and comparability.

First, the average specific yield of the basin was taken to be 0.1, as was published in

the Goleta Water District’s 2011 Groundwater Management Report (Bachman 2011). We

utilized this number in calculating the groundwater storage from total aquifer volume.

Because this factor is a direct multiplier, it has a strong effect on the calculated volume. A

separate study calculated the average basin specific yield to range from 0.1 to 0.15 (DWR

2003). Again, we feel justified in using the value provided by the GWD for purposes of

comparability. In reality, the specific yield varies strongly throughout different geologic

strata and locations. For this reason, our analysis may constitute an oversimplification of

reality.

Another assumption utilized in this study was the consistency of physical

properties among geologic layers. For instance, it was assumed that all sand formations

would be permeable to groundwater, and all clay formation would be impermeable.

However, as is documented in the Upson 1952 study, the main water-bearing formation in

the basin contains some unconsolidated clay layers (It was for this reason that the

simplification process was required to estimate the location of water bearing formations).

In reality, physical geologic properties exist within a continuous scale, not distinct

categories.

A smaller source of error would include the ambiguity in the stratigraphy data

provided in the Evenson study used for roughly half of the wells. The data in this study

was provided in the form of a 2D cross-section diagram, with layers drawn onto an

elevation scale. While the illustration allowed for reasonable estimation of elevation

within 2 meters, it may have caused some systematic skewing of the results. This error

could have been eliminated if another source of stratigraphy data were available in

numerical form.

6.2 Challenges

Our earliest challenges were related to acquiring data of geologic stratigraphy for

the Goleta basin. Much of the research done within the last 30 years has been in the hands

of private companies, such as Goleta Water District, that were unwilling or unable to share

the raw data. This challenge is representative of the current age of privately funded

research, in which data is often withheld from public knowledge.

Another challenge we faced as a team, was attempting to limit the scope of the

project. Once the stratigraphy data was digitized, many types of hydrologic analysis and

visualization were possible. We decided to focus on the most relevant analyses for

assessing the harvest potential of the basin (and also creating the coolest visuals).

Actually learning how to use the ArcHydro tools proved to be a fourth challenge.

Fortunately, a wide range of tutorials are available for free on the Aquaveo support

website. However, these were slow-paced and used different examples that were not

always directly comparable to our study. Therefore, trial and error methods were

sometimes required to complete the desired analyses.

Toward the end of the project we felt the challenge of data organization. With

multiple versions of the project, borehole tables, geodatabases, visualizations, layers,

rasters, and images, it became apparent that a more formal organization would be

beneficial. For group projects utilizing many versions of the same data, metadata would be

helpful as a reminder of the specifications of the specific set. In addition, separating

folders by user proved to be useful.

6.3 Further analysis

Further spatial analysis of the basin could include a huge variety of visualizations

and calculations. Some potentially beneficial visualizations would be 2D well cross-

sections, geovolumes of the aquifers, and cutaway models of the basin. These would lend

an even more thorough understanding to the structure of the water bearing formations.

MODFLOW analysis could be learned and utilized to obtain quantifiable calculations of

flow rates, flow directions, and groundwater storage. With additional data describing

infiltration rates in the basin, sustainable yield could be calculated. This would be the most

accurate assessment of the Goleta groundwater basin’s potential to be utilized as a

groundwater resource.

6.4 Goleta groundwater as a potential resource

While the volume estimation of the basin would suggest a strong potential for use

as a freshwater resource, further analysis would be required to make a valuable

assessment. In the context of a larger freshwater supply system, a changing climate, and

an uncertain future, it could be reasonably decided that the groundwater basin be

reserved for emergency use. While a local freshwater resource would beneficially offset

energy consumption and more effectively provide a smaller local freshwater supply, more

information would be required to make an informed analysis.

References

Bachman, Steven. “2010 Groundwater Management Plan: Goleta Groundwater Basin.”

Goleta Water District. 11 May 2011.

California Department of Water Resources. “California’s Groundwater Bulletin 118:

Central Coast Hydrologic Region, Goleta Groundwater Basin.” 2003.

Evenson, R.E., Wilson, H.D., & Muir, K.S.”Yield of the Carpinteria and Goleta Ground-Water

Basins, Santa Barbara County, California, 1941-58: A Survey and restudy of the

groundwater hydrology of two coastal basins.” US Geological Survey & Santa Barbara

County Water Agency.Santa Barbara, CA. 1962.

Gibbs, D. Public Works Department, Water Resources Division. (2011). Santa barbara

county 2011 groundwater report. Retrieved from website:

http://www.countyofsb.org/uploadedFiles/pwd/Water/WaterAgency/Report Document

FINAL.pdf

La Rocque, GA. “Wells and Water Levels in Principal Ground-Water Basins in Santa

Barbara County, California.” County of Santa Barbara. US Geologic Survey. Washington DC.

1950.

Toups Corporation. “Water resources management study: South Coast – Santa

Barbara County, a report prepared for the ad hoc committee on water supply.”

Santa Ana, California. 1974.

Upson, J.E. “Geology and Ground-Water Resources of the South-Coast Basins of Santa

Barbara, California.” US Geological Survey & Santa Barbara County. US Department of the

Interior. Washington DC. 1951.

Special Thanks to

Krzysztof Janowicz. UCSB Geography

Erin Wetherley. UCSB Geography

Grant McKenzie. UCSB Geography

Jon Jablonski. UCSB Map & Imagery Library

Jon Harvey UCSB Earth Science

Keith Clark. UCSB Geography

Hugo Loaiciga. UCSB Geography

Dylan Parenti. UCSB Geography

Aquaveo Corporation