HomeMy WebLinkAbout1.21 Aquifer Sustainability Study
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TABLE OF CONTENTS
INTRODUCTION ......................................................................................................................................................... 2
BACKGROUND ........................................................................................................................................................... 2
AQUIFER CHARACTERISTICS ................................................................................................................................. 2
PRECIPITATION INFILTRATION ............................................................................................................................. 3
AQUIFER RECHARGE ............................................................................................................................................... 4
ANTICIPATED DIVERSIONS & DEPLETIONS ....................................................................................................... 5
TOTAL SPRING VALLEY AQUIFER DEMANDS ................................................................................................... 7
SUMMARY .................................................................................................................................................................. 8
FIGURE 1: SPRING VALLEY RANCH VICINITY MAP…………………………………………………………………………………………………….ATTACHED
FIGURE 2: SPRING VALLEY RANCH GEOLOGY MAP…………………………………………………………………..……………………………...ATTACHED
FIGURE 3: SPRING VALLEY RANCH 1991-2020 AVERAGE PRECIPITATION MAP…………………………….……………………………….ATTACHED
FIGURE 4: SPRING VALLEY RANCH NATIONAL LAND COVER MAP…………………………….………………………………………………….ATTACHED
TABLE 1: ESTIMATED PRECIPITATION AND INFILTRATION INTO THE SVA. *..................................................................................... 4
TABLE 2: ESTIMATED LOSSES TO FROM EVAPOTRANSPIRATION BY VEGETATION TYPE. ....................................................................... 5
TABLE 3: POTABLE WATER DEMANDS AND DEPLETIONS…………………………………………………………………………………………….ATTACHED
TABLE 4: NON-POTABLE WATER DEMANDS AND DEPLETIONS…………………………………………………………………………………….ATTACHED
TABLE 5: TOTAL POTABLE AND NON-POTABLE WATER DEMANDS AND DEPLETIONS…………….………………………………………….ATTACHED
TABLE 6: MONTHLY DEPLETION AND DELAYED RETURN FLOW FACTORS DECREED IN CASE NO. 87CW155 ........................................ 5
TABLE 7: TOTAL DIVERSIONS AND DEPLETIONS FOR SPRING VALLEY DEVELOPMENTS BASED ON DECREED PLANS FOR AUGMENTATION .... 8
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INTRODUCTION
This report has been prepared to update previous engineering evaluations related to the
sustainability of the Spring Valley Aquifer (SVA) located approximately 6 miles southeast of
Glenwood Springs, CO (Figure 1). The SVA is identified as a major alluvial aquifer resource in the
Colorado Groundwater Atlas prepared by the Colorado Geological Survey. The purpose of this
updated study is to incorporate more modern datasets into the analysis and determine if recent
changes to precipitation, temperature, and resultant hydrology have altered the results of
previous analyses, all of which found the aquifer to have sufficient water in aquifer storage and
recharge to support Spring Valley Ranch (SVR) and other planned developments.
BACKGROUND
Colorado River Engineering (CRE) has reviewed previous studies related to the SVA.1 2 3 These
studies were undertaken in the early 2000’s to quantify the water availability and compare it to
the water demands associated with the SVR development as well as other planned developments
which were all contemplated to rely on the SVA. Gamba (2000) determined the probable amount
of annual recharge to the Spring Valley Aquifer (SVA) to be 10,059 acre-feet per year on average
based on average annual precipitation (1951-1980) of 19,908 acre-feet, evapotranspiration (ET)
losses of 9,249 acre-feet, and estimated surface flow losses down Landis Creek of 600 acre-feet.
The probable annual recharge was quantified to be well above the expected total development
groundwater diversions, which are not completely consumptive to the aquifer, of 1,100 acre-
feet which included 600 acre-feet from SVR and 500 acre-feet from other developments. HRS
provided review of the Gamba analysis with a resultant recharge estimate of 4,700 acre-feet,
which estimated crop ET from free surface water evaporation. The results of these studies have
been updated by CRE utilizing more modern datasets considering recent changes in precipitation,
satellite derived vegetation types, and more detailed consumptive use estimates.
The Gamba analysis quantified the total water in storage in the aquifer to be 68,000 to 105,000
acre-feet of which 38,000 to 46,000 acre-feet is stored in the SVA and 30,000 to 60,000 acre-feet
are available in the upland volcanic materials tributary to the SVA. The HRS analysis included a
similar estimate of 82,000 acre-feet of water available in storage in the SVA. The aquifer serves
as an underground reservoir to offset dry years with lower than normal recharge and to store
water in above normal recharge years.
AQUIFER CHARACTERISTICS
The cited Gamba report provides a detailed explanation of the geologic conditions that formed
the Spring Valley Hydrologic System, which is briefly summarized herein. The SVA tributary area
1 Gamba, J. 2000. The Spring Valley Hydrologic System. Prepared for Bill Peacher.
2 HRS Water Consultants, Inc, 2000. RE: Spring Valley Ranch – Review of Jerome Gamba & Associates, Inc.
Report, The Spring Valley Hydrologic System.
3 Wright Water Engineers, Inc. 2000. Water Requirements, Water Resources, and Spring Valley Area Water
Balance. Prepared for Spring Valley Development, Inc.
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which contributes to recharge is approximately 15.4 square miles (Figure 1) and varies in
elevation from 6,870 to 9,400 feet. The aquifer was formed as soluble salts present in the
underlying Eagle Valley Evaporite formation were dissolved by groundwater resulting in collapse,
deformation, shear fracturing, and faulting of the overlying Maroon formation and volcanic rocks.
These processes resulted in the high infiltration rate and water bearing capacity of the volcanic
rocks. Millions of years of erosion have resulted in removal of softer, unconsolidated cinders and
ash from the surface and exposed the weather resistant basalt rocks. The ash and cinder lenses
that remain below the basalt provide pockets of highly porous materials that detain
groundwater, also called “hanging aquifers”. Water is channeled into these pockets via fractured
and rubblized basalt on the surface. These areas are interconnected by subsurface fractures that
slowly transmit water from higher elevations to lower elevations. The upland areas are the
primary area of recharge.
The SVA, which is a portion of the full contributing area of the aquifer, is a composite of a series
of confined aquifers within the sediments overlaying bedrock which produce artesian wells. The
confined aquifers in the lakebed sediments are comprised of sand and sandy gravel horizons
confined between layers of clay or sandy, gravely clays. The bedrock form of the lake basin is a
“half graben” with a fault on the south side of the basin. The blue gray clay layer acts as a seal
between the lake sediments and underlying volcanic rock materials.
PRECIPITATION INFILTRATION
There have been several updates to Colorado’s annual average precipitation analysis since the
1951-1980 dataset utilized by Gamba. Climate normals are updated every 10-years to reflect the
most recent 30-year period; the 1991-2020 climate normals were recently released. These data
are available at a station scale but are also available as a gridded dataset produced by the PRISM
Climate Group on an 800-meter resolution grid. This gridded data was overlain on the geologic
unit map within the SVA tributary area as defined on Figure C-1-E of the Gamba report. A geology
map is attached as Figure 2. The 1991-2020 average annual precipitation (Figure 3) was
calculated for each geologic unit which include: PPM – Pennsylvanian/Permian Maroon
Formation, Tb – Tertiary Volcanic Materials, and Ql – Quaternary Lake Sediments (aka SVA). The
average annual precipitation over the SVA tributary area was calculated to be 18,384 acre-feet
compared to the 19,908 acre-feet utilized by Gamba. This is a reduction of 1,524 acre-feet or
7.7%.
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Table 1: Estimated Precipitation and Infiltration into the SVA. *
*Note minor discrepancy in total acreage based on digitization of boundaries from Gamba, 2000.
The infiltration rates by geologic unit utilized in the Gamba analysis were also utilized in the CRE
analysis, as they were deemed to be appropriate, and are included in Table 1. The local geology
exhibits relatively quick infiltration rates. Based on the assumed infiltration rates of the geologic
units, the estimated infiltration has been quantified to be 9,318 acre-feet compared to the 10,314
acre-feet calculated by Gamba. This is an infiltration reduction of 996 acre-feet or 9.7% and is
largely attributed to the reduced precipitation inputs.
The full annual amount of ET was assumed to deplete the available precipitation inputs,
regardless of whether water was available in soil moisture and available to plants or not, which
bases the analysis on a conservative aquifer recharge amount.
AQUIFER RECHARGE
The total amount of infiltration is not realized as recharge to the aquifer due to losses from ET
and surface runoff. Using the Gamba methodology, the probable recharge was determined using
the following formula:
Recharge = Precipitation – Evapotranspiration - Landis Creek surface flows Eq 1.1
Evapotranspiration was quantified by overlaying the National Land Cover Dataset (NLCD) (Figure
4), a satellite derived depiction of land cover, on the SVA Tributary area boundary and quantifying
the area of various vegetation types. The NLCD was cross-checked with aerial photos to ensure
accurate depiction of land cover types. If discrepancies were found, the area was included with
other vegetation types supported with aerial photography. Native vegetation ET rates were
obtained from the book values from the Handbook of Applied Hydrology, however, there are also
large, irrigated pastures located within the SVA aquifer. The potential evapotranspiration (PET)
from these irrigated pastures was updated using the Lease-Fallow Tool and ASCE standardized
methodology for pasture grass using the study period 2000-2019, which was conservatively
quantified to be 30-inches per year. Other vegetation types are shown in Table 2, below.
Shrub/Scrub utilized high range values for sagebrush in western regions of 10-inches. Deciduous
forest utilized the value for aspen of 23-inches. Evergreen forest utilized the average of the values
Geologic Unit Area (ac)Infiltration
Rate
Mean
Precip
(mm)
Mean
Precip
(in)
Average
Annual
Precipitation
Volume (AF)
Estimated
Infiltration
(AF)
PPM 2132 20%612 24 4281 856
Tb 6290 60%570 22 11763 7058
Ql 1453 60%491 19 2341 1404
Total 9875 18384 9318
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for lodgepole and Engelmann spruce-fir of 17-inches. Mixed forest utilized the average value of
the deciduous and evergreen forests which is 20-inches. All of these values are greater than those
in the Gamba analysis and represent an increase in demands due to changes in climate as well as
more spatial detail to refine the vegetation types. The total annual potential ET was quantified
to be 13,842 acre-feet/year and represents a conservative value which assumes water is always
available to meet the demands of the various vegetation types.
Table 2: Estimated losses to from evapotranspiration by vegetation type.
Utilizing the Equation 1.1 results in the following estimated recharge:
Recharge = 18,384 – 13,842 - 600 = 3,942 acre-feet
These values represent average recharge conditions using conservative depletion assumptions.
This is water available for groundwater withdrawals without creating an aquifer deficit, i.e.,
“mining”, since it will be replenished on an average annual basis. The CRE estimated recharge is
6,117 acre-feet less than what was estimated by Gamba due to decreased precipitation inputs
and increased demands from evapotranspiration which are partly due to temperature and partly
due to increased spatial representativeness. The estimated annual recharge is similar to the HRS
results which quantified 4,700 acre-feet of recharge on average.
ANTICIPATED DIVERSIONS & DEPLETIONS
The development water demands for Storied Development’s amended SVR PUD plan (currently
being reviewed by Garfield County) will be less than the previously approved SVR PUD demands;
and less than the demands already decreed and covered by existing court approved
augmentation plans in Case Nos. 87CW155 and 98CW254, the latter being the operative plan for
augmentation. Basalt Water Conservancy District (BWCD) augments the structures, including
wells, surface and storage structures, which will supply water for the development. In sum, the
98CW254 augmentation plan allows for an annual water demand of 1457 acre-feet of diversions,
a total annual consumptive use of 974 acre-feet in a dry year, and an overall augmentation
requirement of 420 acre feet. The 98CW254 decree allows for modifications and reconfigurations
of the number of EQRs and amounts of irrigated acreage so long as the overall SVR PUD
consumptive use does not exceed 974 acre-feet annually. Tables 3-5 (attached) provide details
Vegetation Type Acreage (ac)ET
(in/yr)
ET Losses
(AF/yr)
Hay/Pasture/Herbaceous 756 30 1904
Shrub/Scrub 5031 10 4192
Deciduous Forest 3872 23 7421
Evergreen Forest 139 17 197
Mixed Forest 77 20 129
Total 9875 100 13842
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of the potable, non-potable, and total diversions and depletions that are augmented pursuant to
the BWCD contracts, as incorporated and approved in Case No. 98CW254.
Potable diversions, which are attributed solely to groundwater sources, total 473.1 acre-feet with
associated depletions of 177.5 acre-feet and include domestic in-house and irrigation uses
associated with 695 EQR’s and 90 acres of domestic irrigation. The non-potable diversion, of
which a portion will be satisfied by senior surface water rights, total 983.9 acre-feet with
associated depletions of 796.96 acre-feet which includes uses of non-domestic irrigation for 420
acres and 24 acres of open surface water evaporation. Overall, the total project diversion
demands are 1,457 acre-feet with associated depletions of 974 acre-feet. Again, Case No.
98CW254, paragraph 10.c. allows for modification to the number of EQRs and irrigated acreage
if the depletions do not exceed 974 acre-feet.
In contrast, Storied Development’s amended SVR PUD proposal seeks to modify certain
components of the previously approved PUD including the type and number of development
units, irrigation requirements, and to add snowmaking as a use of its non-potable water system.
CRE has calculated the water requirements for the revised PUD plan and in sum, the total water
demand for the revised PUD plan is 1,221 acre-feet/year, with total consumptive use of 688 acre-
feet/year. This is less than the contemplated and approved water demand associated with the
currently approved PUD; however, for purposes of this report and aquifer sustainability analysis,
CRE utilizes the larger acre foot demands and depletions described above and approved in the
98CW254 case.
Because most of the lands to be irrigated are located within the SVA tributary area, irrigation
return flows will accrue to the aquifer and will not be totally consumptive to the aquifer. It is also
anticipated, based on land use approvals and engineering related to the expansion of the Spring
Valley Sanitation District, that treated wastewater effluent will also be returned within the SVA
tributary area. It was estimated that approximately 25% of the treated effluent would return to
the SVA by infiltration from the discharge point(s) along the Spring Valley Drainage basin
minimizing adverse impacts to the Spring Valley environment.4 In addition, Storied Development
will have rights to use the Spring Valley Sanitation District Pipeline decreed in Case No. 00CW21
in the amount of 3.48 c.f.s., conditional, for irrigation within the Spring Valley Sanitation District
service area which encompasses the SVA tributary area. The direct use of treated effluent for
irrigation will reduce the demand for groundwater and will supplement aquifer recharge.
Lagged well depletions will be calculated pursuant to the BWCD decree (Exhibit F, Case No.
87CW155) based on the locations of the constructed wells. The well location will determine the
lagged depletion zones which are used to quantify the timing of depletions to the stream from
well pumping. Case Nos. 87CW155 and 98CW254 classified the Spring Valley wells in Groups F1
(SVA) and F2 (Upland Aquifers). The monthly depletion percentages by Well Group are shown in
4 Wright Water Engineers, Inc. March 24, 1999. RE: Spring Valley Sanitation District Service Plan Amendment
(Exhibit 4.3 to Spring Valley Sanitation District Engineering Report for Application for site approval for
modification or expansion of an existing domestic wastewater treatment plant and application and certification
procedures for lift station and interceptors dated September 1999)
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the following table which was attached as an exhibit in Case No. 87CW155. The timing of
depletions dictates the monthly augmentation replacement requirements during periods of
downstream call.
Table 6: Monthly Depletion and Delayed Return Flow Factors Decreed in Case No. 87CW155
TOTAL SPRING VALLEY AQUIFER DEMANDS
In addition to the demands associated with Spring Valley Ranch, several other subdivisions and
individual properties rely upon the Spring Valley Aquifer for all or a portion of their overall water
supplies. The following developments and associated plans for augmentation were reviewed and
are summarized in Table 7. The demands include Spring Valley Ranch, Los Amigos (Elk
Springs/Pinyon Mesa), Colorado Mountain College, Berkeley/Lake Springs Ranch, Lookout
Mountain Ranch, and individual lot owners. It is not known if Lookout Mountain Ranch relies on
the SVA for a portion of their water supplies because well construction logs and accounting are
unavailable; however, the total demand was included in the interest of being conservative. There
are approximately 30 individual properties with wells (or future wells) accessing the SVA area
with an estimated annual diversion of 30 acre-feet and an estimated depletion of 10 acre-feet,
which is based on engineering judgment. The total diversion from all developments relying on
the SVA totals approximately 1,920 acre-feet while the total depletions are approximately 1,263
acre-feet. The total diversions represent 49% of the anticipated recharge while the total
depletions represent only one third of the anticipated annual recharge to the SVA. The analysis
illustrates that the anticipated uses, based on conservative assumptions, do not result in long-
term mining of the groundwater aquifer as the average annual demands of the developments
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are met by the average annual recharge to the aquifer. In addition, these demands do not
consider the fact that a portion of the SVR irrigation demands will be met with senior, surface
water rights, which results in irrigation return flows that deep percolate and recharge the SVA.
The sustainability analysis is conservative and supports the conclusion that there is adequate
groundwater supplies for all users of the SVA. This analysis, in conjunction with a groundwater
monitoring plan, allows all SVA water users to manage the water resource in a sustainable
manner.
Table 7: Total Diversions and Depletions for Spring Valley Developments Based on Decreed Plans for
Augmentation
SUMMARY
The estimated average annual recharge of 3,942 acre-feet is more than three times the estimated
depletion of 1,263 acre-feet for all users of the SVA. Under the proposed amended PUD plan,
groundwater withdrawals for irrigation will be less than calculated due to utilization of Landis
Creek surface water rights, which have historically been used to irrigate the property, and only
using groundwater for supplemental irrigation supplies. In addition to the annual recharge, it has
been estimated by Gamba that there is 68,000 to 105,000 acre-feet of water in storage in the
SVA and upland areas which essentially serve as an underground reservoir to balance extreme
dry year and extended drought-year recharge with water demands. As was found in previous
studies, there is sufficient water in storage in the SVA and available from annual recharge to serve
all the proposed uses without injuring the groundwater resource.
The groundwater levels in the SVA will experience seasonal and year to year fluctuations due to
variability in precipitation and snowpack inputs. Each of the subdivisions that pump water from
the SVA have a long term vested interest in a comprehensive groundwater monitoring plan to
understand baseline and future groundwater conditions. A groundwater monitoring plan is
currently being developed for implementation by these water users.
Development Case No.
Annual
Diversion
(AF)
Annual
Depletion
(AF)
Spring Valley Ranch 98CW256 1457.0 974.0
Los Amigos (Elk Springs and Pinyon Mesa)98CW312 159.8 117.0
Colorado Mountain College 99CW99 132.3 53.1
Lake Springs Ranch/Berkeley W-3571 105.2 97.6
Individual Lot Owners N/A 30.0 10.0
Lookout Mountain Ranch 84CW100 36.0 11.0
Grand Total 1920.3 1262.8
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Figure:Spring Valley AquiferGeologic Units 2
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Figure:Spring Valley Aquifer1990-2020 Normal Precipitation (in)3
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Client:
Figure:Spring Valley AquiferNational Land Cover Data 4
1273: Spring Valley Ranch
In‐House Domestic
Irrigation
Total
Diversion In‐House Domestic
Irrigation
Total
Depletion
Jan 100% 23.1 0.0 23.1 1.5 0.0 1.5
Feb 100% 21.1 0.0 21.1 1.3 0.0 1.3
Mar 100% 23.1 0.0 23.1 1.5 0.0 1.5
Apr 100% 22.4 16.9 39.3 1.4 13.5 14.9
May 100% 23.1 27.0 50.1 1.5 21.6 23.1
Jun 100% 22.4 41.6 64.0 1.4 33.3 34.7
Jul 100% 23.1 47.3 70.4 1.5 37.8 39.3
Aug 100% 23.1 39.4 62.5 1.5 31.5 33.0
Sep 100% 22.4 24.8 47.2 1.4 19.8 21.3
Oct 100% 23.1 3.4 26.5 1.5 2.7 4.2
Nov 100% 22.4 0.0 22.4 1.4 0.0 1.4
Dec 100% 23.1 0.0 23.1 1.5 0.0 1.5
Total 272.7 200.4 473.1 17.2 160.3 177.5
EQR 695
Gal/EQR/day 350
Domestic Irrigation (ac)90
Lawn/Golf/Open Space Unit
Irrigation Diversion (AF/ac)2.24
Lawn/Golf/Open Space Unit
CU (AF/ac)1.79
Lawn/Golf/Open Space
Efficiency %80%
Domestic Depln (%)6.3%
Table 3: Potable Demands
Diversions (AF)Depletion (AF)
% Occupancy
Month
Diversions_Depletions
Jan 0.0 0 0.0 0 0 0
Feb 0.0 0 0.0 0 0 0
Mar 0.0 1.9 1.9 0 1.9 1.9
Apr 78.8 4.1 82.9 63.04 4.1 67.14
May 126.0 7 133.0 100.8 7 107.8
Jun 194.3 9.6 203.9 155.44 9.6 165.04
Jul 220.5 9.8 230.3 176.4 9.8 186.2
Aug 183.8 7.2 191.0 147.04 7.2 154.24
Sep 115.5 5.5 121.0 92.4 5.5 97.9
Oct 15.8 3.1 18.9 12.64 3.1 15.74
Nov 0.0 1 1.0 0 1 1
Dec 0.0 0 0.0 0 0 0
Total 934.7 49.2 983.9 747.76 49.2 796.96
Irrigated Area (ac)420
Evaporation Area (ac)24
Table 4: Non‐Potable Demands
Month Pond
Evaporation
Non‐Domestic
Irrigation Total
Diversions (AF)
Non‐
Domestic Pond Evaporation Total
Depletions (AF)
Diversions_Depletions
In‐House Irrigation Evap Total In‐House Irrigation Evap Total
Jan 23.1 0.0 0 23.1 1.5 0.0 0 1.5
Feb 21.1 0.0 0 21.1 1.3 0.0 0 1.3
Mar 23.1 0.0 1.9 25.0 1.5 0.0 1.9 3.4
Apr 22.4 95.7 4.1 122.2 1.4 76.6 4.1 82.1
May 23.1 153.0 7 183.1 1.5 122.4 7 130.9
Jun 22.4 235.9 9.6 267.9 1.4 188.7 9.6 199.7
Jul 23.1 267.8 9.8 300.7 1.5 214.2 9.8 225.5
Aug 23.1 223.2 7.2 253.5 1.5 178.6 7.2 187.2
Sep 22.4 140.3 5.5 168.2 1.4 112.2 5.5 119.2
Oct 23.1 19.2 3.1 45.4 1.5 15.4 3.1 19.9
Nov 22.4 0.0 1 23.4 1.4 0.0 1 2.4
Dec 23.1 0.0 0 23.1 1.5 0.0 0 1.5
Total 272.66 1135.10 49.20 1456.96 17.18 908.08 49.20 974.46
Table 5: Total Potable and Non‐Potable Demands
Month Total Diversion Total Depletion
Diversions_Depletions