HomeMy WebLinkAbout08/15/2000, 5 - GROUNDWATER EVALUATION STUDY council Au a Dat
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C I T Y OF SAN LUIS OBISPO
FROM: John Moss, Utilities Directopf-
Prepared By: Gary Henderson, Water Division Manager 4Q..J'k
SUBJECT: Groundwater Evaluation Study
CAO RECOMMENDATION
1. Receive report and direct staff to prepare contract amendment with Team Engineering and
Management, Inc. for additional analysis relative to potential impacts to riparian, stream, and
subsidence issues.
2. Direct staff to develop a detailed strategy and implementation schedule to pursue
development of additional water resources through the coordinated operation of both surface
water and groundwater resources.
DISCUSSION
Background
In the fall of 1998 staff identified the need to provide an analysis of the City's local groundwater
to determine the feasibility of increasing the groundwater production and its contribution to the
City's potable supplies. Staff had originally intended that this work be completed in-house,
however competing priorities and a greater understanding of the complexity of the issues to be
analyzed revealed that the study would require the services of a consulting hydrologist. As a
component of the Council's 1999-01 Major City Goal for Long Term Water Supply
Development, it was identified that the groundwater basin analysis would be completed and
presented to Council in the summer of 2000.
The evaluation of increasing groundwater production has entailed two main components. One,
an analysis by a consulting hydrologist was performed using the City's computer model for
estimating safe annual yield, historical data from previous City groundwater pumping during the
1987-91 drought,previous reports on the local groundwater basin, and current pumping data and
information, to determine the potential additional safe yield that may be obtained from the basin
under a number of conjunctive operating scenarios with our surface water resources. And two,
an analysis completed by staff from both the Water and Wastewater Divisions of the available
water treatment technology to remove the nitrates and PCE known to be currently contaminating
the groundwater basin, the by-products or waste from those treatment processes in terms of both
quantity and quality, and the corresponding impacts of discharge of those wastes to the City's
sewer system, the water reclamation facility (WRF), WRF permit compliance, and possible
affects on the reusability of the WRF effluent. The primary constituents of concern were Total
Dissolved Solids (TDS), Sodium, and Chloride.
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Council Agenda Report—Groundwater Evaluation Study
Page 2
This report provides preliminary information relative to the potential yield, and the technical/cost
feasibility of producing additional groundwater resources for the City. The report also identifies
areas where additional studies will be required before proceeding with actual development of any
additional groundwater resources. The report does indicate that the City could feasibly develop
an additional 890 afy of safe annual yield overall (which is in addition to the current 500 acre
feet of groundwater which is part of the adopted safe annual yield) by implementing a
groundwater development and treatment program used conjunctively with surface water supplies.
Coordinated Operations and Potential Yield
The City contracted with TEAM Engineering and Management, Inc. to prepare an analysis of
potential management strategies for increasing the City's safe annual yield by use of
groundwater resources. Bill Hutchison, Professional Hydrologist, modified the City's computer
model, which is used to estimate the safe annual yield from coordinated operations of Salinas and
Whale Rock Reservoir, to evaluate various coordinated operations strategies for groundwater
resources in conjunction with surface water supplies.
Various operations strategies were evaluated to determine which alternative could most
effectively increase the City's safe annual yield while taking into account the limitations of the
small groundwater basin. While the City extracted almost 2,000 acre feet for several years
during the drought, sustained pumping at these levels would not be achievable over the long-term
and could result in additional subsidence or other environmental impacts. The computer model
was used to run a number of scenarios which included pumping more during wet years, constant
groundwater pumping,and pumping more during dry years.
The analysis revealed that the conjunctive operation of the reservoirs and the groundwater
resources is improved by extracting more groundwater during dry years when reservoir levels are
down. The main factors supporting this operating strategy have to do with the fact that the
Salinas Reservoir spills often (i.e. use it during wet periods or it spills and flows downstream)
and leaving large amounts of water in the reservoirs increases evaporation losses. Based on
information from historical pumping and operation of the groundwater wells, Scenario 43 in
Table 6-1 and 6-2 of the Groundwater Yield Study (Attachment 1) would be achievable from a
pumping standpoint and would not result in water levels being drawn below historic low levels.
This is an important consideration to minimize the potential for additional subsidence in the
areas overlying the groundwater basin.
Scenario #3 proposes to operate the wells to pump 1,500 acre feet per year (afy) following dry
years or periods, 1,000 afy following normal rainfall years/periods, and 500 afy following wet
years/periods. Table 6-1 uses a 4-year drought index which takes into account the previous four
years of rainfall to determine what pumping rate should be used for the year. Table 6-2 only uses
the previous year to determine whether it was a dry, normal or wet year to determine
groundwater pumping. The analysis reveals that operations based on a 4-year index would
increase the safe annual yield for Scenario#3 over that calculated based on a 1-year index. Table
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Council Agenda Report—Groundwater Evaluation Study
Page 3
6-1 reveals that the City could increase our safe annual yield by approximately 890 afy by
operating the wells in a coordinated fashion with our surface water supplies.
Groundwater Quality and Treatment Options
The groundwater production has been restricted for many years due to nitrate and PCE
contamination. The PCE contamination was being removed during the drought period by using
carbon filtration tanks which were located at Auto Parkway and near Embassy Suites on the
Dalidio property. Carbon filtration is an effective means of treating the PCE contamination but
will not remove the nitrates.
The Auto Parkway and Denny's wells were shut down in 1992/93 due to nitrate contamination.
The Auto Parkway well was the largest production well and accounted for approximately 1,000
afy of the total groundwater production. To increase groundwater production, it will be
necessary to construct a new well or wells to allow groundwater productions similar to Auto
Parkway well. This will also require treatment for nitrates and PCE.
The most viable alternatives for nitrate treatment are ion exchange or reverse osmosis processes.
The capital and operating costs can vary significantly between these two alternatives and
additional studies will be necessary to determine the best treatment alternative. A preliminary
analysis of the treatment options was prepared in 1994 by Dan Gilmore, Utilities Engineer. At
that time, capital costs were estimated between $1 and 2 million dollars. With site acquisition,
treatment facilities, and other site improvements the capital costs could potentially increase to
$3-5 million. Even at the high range of $5 million, the cost per acre foot would be
approximately half of other water supply alternatives currently being pursued (i.e. Salinas
Reservoir Expansion Project and Nacimiento). The treatment of the groundwater is feasible and
would likely be significantly less expensive than other supplemental water sources.
Regulatory and Water Quality Impacts
The State Department of Health Services will have to review the treatment process used to
remove nitrates and PCE as well as the location and construction of new wells. The treatment
facilities will be designed to meet all State and Federal regulations for water quality.
Groundwater is typically "harder"than our surface water sources which caused some complaints
during the drought when large quantities of groundwater were being used. Either treatment
alternative will result in a "softening" of the water and water quality is not anticipated to be a
concern for customers. The Department of Health Services should not have a problem with the
City's treatment processes as long as they meet all state and federal standards. Nitrate removal
systems are a common practice for groundwater treatment in California and DHS has approved
proposed processes previously. Grover Beach has a nitrate removal facility that uses the ion
exchange process for some of their groundwater.
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Council Agenda Report—Groundwater Evaluation Study
Page 4
Wastewater Impacts
The treatment processes used to remove nitrate from the groundwater will require a discharge of
the reject water to the wastewater collection system for treatment at the Water Reclamation
Facility (WRF). Staff have evaluated the potential impacts to the water quality of the effluent
leaving the WRF. The analysis indicates that the one area of potential problem would be an
increase in the levels of Total Dissolved Solids (TDS) which would cause the effluent TDS
levels to exceed our current National Pollutant Discharge Elimination System (NPDES) permit
limit. The City will be discussing modifications to the permit limits with the Regional Water
Quality Control Board (RWQCB). If TDS limits were modified to allow TDS levels up to
drinking water standards of 1000 mg/l, no treatment modifications at the WRF would be
required. Staff believes that these modifications to our permit could be obtained but will require
further discussions and negotiations with the RWQCB staff.
Water Reuse Impacts
Increases in TDS levels in the effluent could potentially impact the uses for reclaimed water.
While any increase in TDS of water used for irrigation is not desirable, studies have shown that
plants will continue to thrive with irrigation water TDS values far exceeding those proposed
here. As an example, prior to receiving State Water, the City of Morro Bay consistently
exceeded 1,000 mg/1 TDS in their potable water system.
Conclusion
Increasing water supplies to the City of San Luis Obispo through the coordinated operation of
both surface and groundwater resources has the potential to increase the safe annual yield by 890
acre feet per year. Treatment alternatives will require further evaluation and modifications to the
City's NPDES permit for wastewater effluent discharge to the creek. The project for increasing
groundwater production appears feasible and no"fatal flaws"have been identified at this point.
Next Steps
There are a number of issues which will require further studies or additional staff work which are
bulleted below:
1. Impacts of additional groundwater production on stream flows, riparian areas, and
subsidence.
2. Initial environmental review.
3. Well and treatment facility location selection and acquisition.
4. Preliminary design and engineering level cost estimates.
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Council Agenda Report—Groundwater Evaluation Study
Page 5
5. NPDES permit resolution.
6. Final design and construction.
Based on Council concurrence, staff will develop the scope of work and cost for preparing the
additional analysis associated with impacts of additional groundwater production on stream
flows, riparian areas, and subsidence. Staff will return to Council with an amendment to the
contract with TEAM Engineering and Management, Inc. to prepare the analysis and studies. In
addition, staff would prepare a strategy and estimated timeline for completion of the steps that
are identified above for discussion by Council.
FISCAL IMPACT
There is no fiscal impact associated with this issue at this time. Staff will return to Council for
approval of the contract for consultant services relative to evaluation of impacts to stream flow,
riparian areas and subsidence. The current preliminary estimate for the amendment to the
consultant contract is $15,000 to $30,000.
ALTERNATIVES
The Council could decide not to pursue the continued investigation of the potential of increasing
safe annual yield of the City's water resources through the coordinated operations of our surface
water and groundwater resources. Staff does not recommend this alternative since the City must
secure additional water resources to meet the goals adopted in the General Plan and this project
may prove to be significantly less expensive on a cost per acre foot basis than other alternatives
currently being pursued.
ATTACHMENTS
1. "Groundwater Yield Analysis" (in part),July 25, 2000
On-File in Council Office:
1. Full copy of"Groundwater Yield Analysis", William Hutchinson, July 25, 2000
5-5
MEETING AGENDA
DATE
Groundwater Yield Analysis
FIAGMT
❑CDD DIR
❑FIN DIR
❑FIRE CH0❑PW DIR❑POLICE CHF❑REC DIRL <-RL DIR F ❑PERS DIR
Prepared For
City of San Luis Obispo
San Luis Obispo, California
Prepared By
TEAM Engineering& Management
Bishop, California
Phoenix, Arizona
William R. Hutchison
Professional Hydrologist
P.H. 808
July 25, 2000
5-6
Groundwater Yield Analysis
TABLE OF CONTENTS
Section Page
Executive Summary
1.0 INTRODUCTION 1
2.0 REVIEW OF PREVIOUS STUDIES 1
3.0 EXISTING DATA REVIEW 2
3.1- Precipitation 2
3.2 Groundwater Pumping 2
3.3 Groundwater Levels 3
4.0 REGRESSION MODELS OF GROUNDWATER LEVELS 4
5.0 RESERVOIR OPERATIONS MODEL 5
6.0 MANAGEMENT SCENARIOS 5
6.1 Scenario Description 6
6.2 Safe Annual Yield 6
6.3 Groundwater Level Changes 7
7.0 CONCLUSIONS AND RECOMMENDATIONS 8
Appendix A—Updates and Enhancements to Reservoir Operations Spreadsheet Model
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TABLE OF CONTENTS (continued)
LIST OF TABLES
Table Title
Table 3-1 Rainfall at Cal Poly
Table 3-2 Summary of City Pumping
Table 3-3 Auto Parkway Well Data
Table 3-4 Denny's Well Data
Table 3-5 Fire Station#4 Well Data
Table 3-6 Mitchell Park Well Data
Table 34 Pacific Beach No. 1 Well Data
Table 3-8 31S/12E-03P02—Medina by Border Patrol
Table 3-9 31S/12E-1OF03—Kundert—LOVR/Higuera
Table 3-10 31S/12E-10G02—Ball,So Higuera Irrig
Table 3-11 30S/12E-32J01 —Devaul—Los Osos Valley
Table 3-12 31S/13E-16N01 —Lindsey-Edna
Table 3-13 31S/13E-17Q04-Righetti-Orcutt
Table 4-1 Auto Parkway Model
Table 4-2 Denny's Model
Table 4-3 Fire Station#4 Model
Table 4-4 Mitchell Park Model
Table 4-5 Pacific Beach No.1 Model
Table 4-6 31S/12E-03P02 Model—Medina by Border Patrol
Table 4-7 31S/12E-10F03 Model—Kundert—LOVR/Higuera
Table 4-8 31S/12E-1OG02 Model—Ball, So Higuera Irrig
Table 4-9 30S/12E-32J01 —Devaul—Los Osos Valley
Table 6-1 Summary of Operational Scenarios Involving Increased Groundwater
Pumping—4-Year Drought Index
Table 6-2 Summary of Operational Scenarios Involving Increased Groundwater
Pumping—1-Year Drought Index
5-8
TABLE OF CONTENTS (continued)
LIST OF FIGURES
Figure Title
Figure 3-1 Annual Precipitation at Cal Poly
Figure 3-2 Total City Pumping
Figure 3-3 Auto Parkway Well Hydrograph
Figure 34 Denny's Well Hydrograph
Figure 3-5 Fire Station#4 Hydrograph
Figure 3-6 Mitchell Park Well Hydrograph
Figure 3-7 Pacific Beach No. 1 Hydrograph
Figure 3-8 31S/12E-031`02—Medina by Border Patrol
Figure 3-9 31S/12E-10F03—Kundert—LOVR/Higuera
Figure 3-10 31S/12E-1OG02—Ball, So Higuera Irrig
Figure 3-11 30S/12E-32J01 —Devaul—Los Osos Valley
Figure 3-12 31S/13E-16N01 —Lindsey-Edna
Figure 3-13 31S/13E-17Q04—Righetti-Orcutt
Figure 3-14 Well Location Map—USGS Quad Base
Figure 3-15 Well Location Map—Aerial Photo Base
Figure 4-1 Auto Parkway Model Hydrograph
Figure 4-2 Denny's Model Hydrograph
Figure 4-3 Fire Station #4 Hydrograph
Figure 4-4 Mitchell Park Model Hydrograph
Figure 4-5 Pacific Beach No. 1 Model Hydrograph
Figure 4-6 31S/12E-03P02—Medina by Border Patrol
Figure 4-7 31S/12E-1OF03—Kundert—LOVR/Higuera
Figure 4-8 31S/12E-1OG02—Ball, So Higuera Irrig
Figure 4-9 30S/12E-32J01 —Devaul—Los Osos Valley
Figure 6-1 Safe Annual Yield
Figure 6-2 Pumping vs. Safe Annual Yield
Figure 6-3 Pumping vs. Increases in Safe Annual Yield
Figure 6-4 Auto Parkway Minimum Groundwater Elevations
Figure 6-5 Denny's Minimum Groundwater Elevations
Figure 6-6 Fire Station No. 4 Minimum Groundwater Elevations
Figure 6-7 Mitchell Park Minimum Groundwater Elevations
Figure 6-8 Pacific Beach No. 1 Minimum Groundwater Elevations
Figure 6-9 Medina by Border Patrol (3P2) Minimum Groundwater Elevations
Figure 6-10 Kundert—LOVR/Higuera (10F3) Minimum Groundwater Elevations
Figure 6-11 Ball So Higuera Irrig(10G2) Minimum Groundwater Elevations
Figure 6-12 Devaul—Los Osos Valley Road (32J1) Minimum Groundwater
Elevations
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Executive Summary
TEAM Engineering and Management, Inc. has completed an analysis of the groundwater
basin that underlies the City of San Luis Obispo. The objectives of the investigation
were:
• Identify opportunities for increased groundwater pumping
• Identify opportunities to manage groundwater pumping in conjunction with
reservoir supplies.
• Develop estimates of safe annual yield of all water supply sources with increased
groundwater pumping
Groundwater pumping by the City of San Luis Obispo in the early 1990s provided much
needed water, but may have resulted in impacts. Pumping amounts as high as 1900
AF/yr caused declines in groundwater levels in nearby wells. In recent years,
groundwater pumping has been reduced due to nitrate contamination of the basin.
Safe annual yield from Salinas Reservoir and Whale Rock Reservoir (i.e. without
groundwater pumping) was estimated to be 7286 AF/yr. If 500 AF/yr of groundwater is
pumped (current city assumption for planning purposes), the safe annual yield increases
to 7787 AF/yr. Safe annual yield estimates were made using an updated and enhanced
version of the spreadsheet model used by city staff for the last several years. The model
now includes an expanded ability to simulate groundwater pumping and track
groundwater level changes as a result of that pumping
Based on simulations, groundwater pumping in dry years causes a significant increase in
safe annual yield. If pumping 1500 AF/yr in dry years, 1000 AF/yr in average years, and
500 AF/yr in wet years, safe annual yields increases to 8679 AF/yr without causing
subsidence impacts. Additional detailed analyses are needed to refine the evaluation of
potential subsidence, and address the potential for streamflow, and riparian vegetation
impacts.
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1.0 INTRODUCTION
TEAM Engineering and Management, Inc. has completed an analysis of the groundwater
basin that underlies the City of San Luis Obispo. The City has relied on groundwater to
meet some of its water demand over the years. During the drought of the late 1980s and
early 1990s, the groundwater basin was a major source of supply. Nitrate contamination
of the groundwater basin caused a reduction in pumping. Other concerns of groundwater
pumping include impacts to riparian areas and subsidence.
The work completed by TEAM consisted of the following:
• Reviewed previous studies
• Compiled and reviewed existing data related to the groundwater basin (e.g.
groundwater pumping, groundwater levels, etc.)
• Analyzed the groundwater level data and developed empirical models of
groundwater levels using multiple regression techniques
• Updated and enhanced the spreadsheet model of reservoir operations by including
the regression models of groundwater levels
• Developed and ran management scenarios to evaluate opportunities and
constraints of expanded use of the local groundwater basin
• Developed estimates of safe annual yield for each of the scenarios
Overall, this study represents a preliminary effort to integrate groundwater basin use with
reservoir supplies. The objectives of the investigation were:
• Identify opportunities for increased groundwater pumping
• Identify opportunities to manage groundwater pumping in conjunction with
reservoir supplies.
• Develop estimates of safe annual yield of all water supply sources with increased
groundwater pumping
Issues related to identifying potential impacts of increased pumping to environmental
resources and subsidence as well as the treatment systems that may be required will be
part of subsequent analyses. The conclusions in this report should be used to guide this
future work regarding the potential for increased groundwater pumping and the
groundwater level changes that would result.
2.0 REVIEW OF PREVIOUS STUDIES
Previous studies of the local groundwater basin include:
• John L. Wallace & Associates completed a groundwater study in 1988. This
report described the hydrology of the area and focused on locating sites for new
well construction.
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• John L. Wallace & Associates and Timothy S. Cleath completed an exploratory
drilling and testing program as well as an analysis of water quality information in
1988.
• John L. Wallace & Associates completed a "Groundwater Development Plan and
Environmental Impact Review" in 1989. This report presented a general
operations plan for integrating groundwater into the City's water distribution
system.
• Boyle Engineering Corporation completed an evaluation of the groundwater basin
in 1991. This report provided descriptions of the geology and hydrology of the
area, presented a water budget that quantified the inflows and outflows to the
groundwater basin, and presented recommendations regarding the location and
operation of new wells.
• JR Associates completed a geophysical investigation of the South Higuera Street
site in 1991. The investigation included resistivity and seismic refraction
methods.
• The California Department of Water Resources completed a draft study of the San
Luis-Edna Valley groundwater basin in 1997. This report also details the
development of a numerical groundwater model of the area.
These reports provided an overview of the area as well as information related to potential
impacts that may be caused by groundwater pumping.
3.0 EXISTING DATA REVIEW
Existing data on the groundwater basin are contained in the reports listed above and in
files of the City of San Luis Obispo Utilities Department and the San Luis Obispo County
Engineering Department. These data were provided by the City and reviewed as part of
this effort.
3.1 Precipitation
Precipitation data at Cal Poly from July 1869 to December 1999 were provided and are
summarized in Table 3-1 and Figure 3-1. The length of the record is outstanding and
provides a good foundation on which to base water management decisions related to the
frequency and duration of historic droughts.
3.2 Groundwater Pumping
Groundwater pumping records for City wells from April 1989 to February 2000 were
provided and are summarized on Table 3-2 and Figure 3-2. Note that pumping in 1990,
1991 and 1992 were high in response to drought conditions. Pumping during this period
was between 1500 and 1900 acre-feet per year (AFY). After the drought, pumping was
reduced to an annual amount of between 160 and 550 AFY.
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The reservoir operations model that has been used by the City for planning purposes
includes groundwater pumping as a single annual amount. During the environmental
review of the Salinas Reservoir Expansion Project, an annual pumping amount of 500
AFY was assumed. Note that in recent years, pumping has been significantly less than
500 AFY.
3.3 Groundwater Levels
Groundwater level data for City wells from July 1988 to March 2000 were provided and
are summarized in Tables 3-2 through 3-7 and as hydrographs in Figures 3-3 through 3-7.
Note that the groundwater levels responded to both pumping and rainfall.
Characteristically, groundwater levels declined during the drought in the early 1990s and
then recovered in response to a combination of above normal rainfall years and reduced
pumping. Problems with subsidence associated with increased pumping were reported
during the drought of the late 1980s and early 1990s. The groundwater level data provide
a convenient management tool in that groundwater levels in the future should not be
lowered to levels below the historic minimum groundwater levels to prevent potential
additional subsidence.
Groundwater level data for wells outside the City limits and monitored by the County are
summarized in Tables 3-8 through 3-13, and as hydrographs in Figures 3-8 through 3-13.
Two of these wells (31S/13E-16N01 Lindsey-Edna and 31S/13E-17Q04 Righetti-Orcutt)
are located in Edna Valley. Based on an inspection of the hydrographs in comparison to
other wells, it is clear that City pumping does not affect these wells. The other wells
monitored by the County appear to be influenced by City pumping. These wells are:
State Well Number Common Name
31S/12E-1OG02 Ball, So Higurea Irrig
31 S/12E-10F03 Kundert- LOVR/Higuera
31S/12E-03P02 Medina by Border Patrol
30S/12E-32J01 Devaul-Los Osos Valley
The locations of the wells that are the focus of this investigation are presented in Figures
3-14 and 3-15.
More detailed analysis would be necessary to evaluate specific impacts to riparian and
wetland vegetation as well as threshold levels that may result in subsidence. For
purposes of this preliminary analysis, results of scenarios are presented in terns of how
the estimated groundwater levels that would result from a particular pumping pattern
compare to these historic minima. Note that these minima, as well as the maxima and
averages are presented in Tables 3-2 through 3-7.
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4.0 REGRESSION MODELS OF GROUNDWATER LEVELS
As seen in the hydrographs of groundwater levels (Figures 3-3 through 3-11),
groundwater levels respond to pumping and rainfall. This qualitative observation can be
tested quantitatively through a variety of techniques, including the development of
regression models of groundwater levels. A regression model is one in which the
expected value of one variable, for example groundwater elevation in a specific well, is
related to the values of other variables, such as rainfall and groundwater pumping. The
resulting model takes the form of an equation:
GWE=bo+ (bl*earl)+ (b2*var2) +(b3*var3)+ .... +(b„*var„)
Where:
GWE=groundwater elevation (ft)
bo.b i ,b2,b3,bo =regression coefficients
earl,varZ vara,var„ =variables such as rainfall or pumping from individual wells
By applying multiple regression techniques to the groundwater level, groundwater
pumping, and rainfall data, regression coefficients can be estimated. The resulting
models can then be used to develop predictions of groundwater elevations under a
reasonable range of pumping scenarios.
Variables for each model included rainfall at Cal Poly and pumping in nearby wells.
Each model included rainfall in each of the previous four months and pumping in nearby
wells for each of the previous four months. An automated technique known as stepwise
regression was then employed to select which variables best describe the variation in
groundwater elevation in the well. The regression models were developed using
Essential Regression and Experimental Design, a free software package that integrates
with Microsoft Excel. The software can be obtained at:
http://www.geocities.coWSil icon ValleyINetworkl]0321
Two measures of how well the model fits the data are the coefficient of determination
(often referred to as r-squared), and an analysis of the model predictions of groundwater
levels to the actual data. Summaries of the nine models that were developed are
presented in Tables 4-1 through 4-9 and Figures 4-1 through 4-9. Tables 4-1 through 4-9
also present the summary data related to the variables used in the prediction of the
groundwater levels and the coefficient of determination. The coefficient of determination
(r-squared) is a measure of how well the model fits the data. For example, an r-squared
value of 0.9 means that 90 percent of the variation in the groundwater elevation can be
"explained" by the variables in the model. As can be seen, the r-squared values are high
(0.75 and higher). Tables 4-1 through 4-9 also depict a summary of the individual
matches of each data point. As can be seen, the model predictions match the actual data
well.
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Figures 4-1 through 4-9 are hydrographs of the period 1989 through 1999 (the period for
which City groundwater pumping is available). These hydrographs depict the actual data
as a solid line and the model predictions as points. Of note here is the match through the
periods of drawdown and recovery. In most cases the match is good.
As with any regression model of this type, it is imperative that the limitations of the
models be understood to minimize their potential misuse. One of the key limitations is
the fact that the models are developed for estimates of monthly groundwater levels. In
most cases, groundwater levels were not monitored on a regular monthly basis, especially
in the wells monitored by the County. Moreover, the period of data is limited and only
includes the end of one drought period.
The most severe limitation of these types of models is that the model for a particular site
is not readily transferable to another site. This means that potential new well locations
cannot be evaluated with these models. More physically based methods (finite-difference
models) would be needed to evaluate new sites. In general, models of this type should
not be used to evaluate pumping amounts that far exceed historic pumping. However, in
this case, because of the issues that arose during the early 1990s associated with City
pumping, it is unlikely that pumping would exceed historic levels to the point of
rendering the models invalid.
5.0 RESERVOIR OPERATIONS MODEL
The City currently uses a spreadsheet model of reservoir operations that was developed
several years ago and has been modified and updated periodically by City staff. The
model was developed using Quattro Pro. The work associated with updating and
enhancing the spreadsheet model included:
• Updating the model with recent data of reservoir inflow, rainfall and net
evaporation (net evaporation is calculated as pan evaporation times the
appropriate pan correction coefficient)
• Converting the model for use in Excel
• Splitting the single"worksheet"into several worksheets
• Adding the regression models of groundwater levels into the spreadsheet in order
to track the effects of pumping
• Adding a macro to automate the calculation of safe annual yield. Hitting Ctrl-y
activates the macro.
Details of the changes and enhancements (including the macro) made to the spreadsheet
are provided in Appendix A.
6.0 MANAGEMENT SCENARIOS
Management scenarios were developed in consultation with City Utilities staff. The
basic scenarios included evaluating the effects of increasing groundwater pumping on
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groundwater levels, and varying the distribution of pumping in wet, average and dry
years.
The objective of these scenarios was to provide a preliminary evaluation of the
groundwater level response to increased pumping. In addition, the scenarios provide
insight regarding the opportunities for increasing the safe annual yield of the water
supply system of the city. More detailed analyses would be necessary to specifically
evaluate subsidence potential and/or impacts of changed groundwater levels on
environmental resources.
6.1 Scenario Description
Twenty-one scenarios were completed as part of this effort: a base case where
groundwater pumping was set equal to zero in all years, ten cases where pumping was
increased using the 1-year drought index, and the same ten cases of increased pumping
using the 4-year drought index.
The base scenario assumed zero pumping to establish the safe annual yield of surface
water (i.e. Salinas Reservoir and Whale Rock Reservoir). Pumping was then increased in
scenarios, groundwater elevations were estimated using the regression models, and safe
annual yield was calculated using the spreadsheet macro.
Results of the twenty-one scenarios are summarized in Tables 6-1 and 6-2. The data that
appear in the upper parts of these tables includes annual averages of safe annual yield,
Salinas and Whale Rock operations, and average annual pumping. These values were
taken from the Summary Output sheet, column B, rows 15 to 28. The lower part of the
tables includes minimum groundwater elevations for each of the nine wells. These were
taken from column C, rows 32 to 40. For convenient reference, the historic minimum for
each well is also provided on Tables 6-1 and 6-2, as well as the spreadsheet model.
These values are included in order to compare scenario results with historic minima as a
preliminary impact assessment tool.
6.2 Safe Annual Yield
Figure 6-1 summarizes the safe annual yield for each scenario. Without pumping, safe
annual yield is 7286 AF/yr (base scenario). Assuming a constant pumping of 500 AF/yr
(Scenario 1), the safe annual yield increases to 7787 AF/yr, and increase of 501 AF/yr.
Note that the use of either the one-year or four-year drought indices results in the same
safe annual yield for Scenario 1. The assumption of pumping 500 AF/yr has been the
city's practice recently for safe annual yield calculations.
In general, the use of the four-year drought index results in slightly higher average annual
pumping and thus, slightly higher safe annual yields. This is due to several "average"
years that are preceded by dry years. Under the 1-year drought index, pumping is based
6 5-16.
on the "average year", whereas the 4-year index looks back at antecedent conditions, and
classifies such years as "dry".
As pumping increases, safe annual yield rises as summarized in Tables 6-1 and 6-2, and
graphically summarized in Figure 6-2, which depicts average annual pumping on the x-
axis, and the safe annual yield on the y-axis. In general, average annual pumping in the
1000 to 1500 AF/yr range would result in a safe annual yield of about 8000 to 9000
AF/yr. Note, however, that the increase is not linear. This is due to some variations in
the wet, average and dry pumping amounts. Most notably, Scenario 7 includes pumping
more in average years than in dry years. The dip in the dashed line (the 4-year drought
index line) at a pumping amount of 1334 is a result of this alteration in pumping by year
type. Pumping in dry years (especially towards the end of a prolonged drought) offers an
opportunity to supply water when surface supplies are limited. Since safe annual yield is
generally limited by years at the end of a drought, it follows that high pumping in dry
years will significantly increase safe annual yield.
In order to gain a better understanding of the relationship of pumping and safe annual
yield, Figure 6-3 is a plot of average annual pumping and increase in safe annual yield
from the base scenario. Note that as pumping increases above 500 AF/yr, safe annual
yield increases at more than one-to-one ratio, except in Scenario 7 where the increase in
safe annual yield is less than the average annual pumping. Again, this is due to the
pumping distribution between wet, average and dry years.
A comparison of Scenarios 3 and 10 highlight the distribution of pumping further.
Scenario 3 (4-year drought index) simulates an average annual pumping of 1072 AF/yr
distributed as follows: 1500 AF/yr in dry years, 1000 AF/yr in average years, and 500
AF/yr in wet years. Scenario 10 simulates an average annual pumping of 996 AF/yr by
assuming a constant 1000 AF/yr in all year types. (The missing 4 AF/yr in Scenario 10 is
attributed to rounding error when the annual amount is distributed over seven wells and
12 months for each well in the spreadsheet). The increase in safe annual yield in
Scenario 3 is 1393 AF/yr (for only 1072 AF/yr in pumping), whereas the increase in
Scenario 10 is only 1001 AF/yr (for 996 AF/yr in pumping). This suggests that dry year
pumping has the biggest effect on raising the safe annual yield. Therefore, pumping
distributions should be established that pump more in the dry years than in average or wet
years.
6.3 Groundwater Level Changes
Tables 6-1 and 6-2 also present the minimum groundwater elevations for each of the nine
wells tracked for each scenario. These data are summarized on Figures 6-4 through 6-12.
Historic minima and scenario minima for each scenario are plotted as bars, and the
average annual pumping for each of the two drought index approaches is plotted as lines.
Note that in most cases, groundwater elevations do not fall below historic minima.
Scenario 6, with an average annual pumping of 2499 AF/yr (1-year index) and 2566
AF/yr (4-year index) causes Auto Parkway and Medina by Border Patrol (3P2) to drop
below historic minima. Also for Scenario 6, Fire Station #4 falls to within one foot of the
7 5-17
historic minimum. Due to the potential for additional subsidence, pumping these
amounts of water is not recommended.
Average annual pumping in the range of 1000 to 1500 AF/yr would increase the safe
annual yield without causing groundwater levels to drop to the point where subsidence is
likely to occur. The potential for impact to stream flow and riparian vegetation is
unknown using the regression models, and would need to be evaluated further.
7.0 CONCLUSIONS AND RECOMMENDATIONS
The following can be concluded from this analysis:
• Groundwater pumping by the City of San Luis Obispo in the early 1990s provided
much needed water, but may have resulted in impacts. Pumping amounts as high
as 1900 AF/yr caused declines in groundwater levels in nearby wells.
• Simulations of increased groundwater pumping were completed in order to
estimate potential increases in safe annual yield. The increases in safe annual
yield from groundwater pumping are most apparent when pumping is increased in
dry years.
• Pumping in the 1000 to 1500 AF/yr range will not cause groundwater elevations
to drop below historic minima in the nine wells tracked by the model.
The results of the scenarios only yield preliminary information related to potential
impacts of increased pumping. The increase in safe annual yield must be evaluated in the
context of these potential impacts. The spreadsheet model provides a convenient and
accurate method to identify the amount of pumping that would be needed to meet
demands and increase safe annual yield. The groundwater elevation regression models
that are now part of the spreadsheet model provide good predictions of groundwater level
changes at specific points as a result of the pumping.
Issues related to subsidence, reduced stream flow, and impact to riparian vegetation
requires additional analysis. These analyses include the development, calibration and
application of a numerical model of the groundwater basin. Given the ability of the
spreadsheet model to establish specific pumping amounts to meet demand and increase
safe annual yield, a numerical model could be used to more specifically evaluate potential
impacts of pumping scenarios. Initially, pumping scenarios can be screened with the
spreadsheet model, and then detailed impact analysis could be completed with the
numerical model.
The DWR attempt to develop a groundwater model might prove helpful in this process.
Based on a review of the draft report, and a comment letter from city staff, it is clear that
the model is not calibrated, and is therefore unsuitable for use in its present form.
However, if DWR is willing to make the model input files available, some of the initial
work associated with developing a numerical model can be avoided, and some of the
obstacles to calibrating the model could be understood.
8 5-18
Key issues that could be addressed with such a model are the role of groundwater in
maintaining streamflow, and the associated role of groundwater in maintaining riparian
vegetation. A numerical model would be able to simulate the interrelationships of
streamflow, shallow groundwater and plant-water use. Once calibrated using the same
historic data that was used to develop the regression models, the model could then be
used to simulate pumping scenarios. Output from such simulations would include
changes to streamflow and changes to plant-water use.
Costs to develop and calibrate such a model would be in the $15,000 to $30,000 range
depending on the availability of the DWR model. The basic data necessary to develop
and calibrate a model of this type have been reviewed and used in this study. Time to
complete a model and have it available to run simulations and yield results that could be
used for impact analysis is about two months.
9 5-19
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