0.0.0.0.1 November 30th, 2024
John Robin Inston
1 Background
A wealth of underground water helped to create the United States, its bountiful farmland and expansive cities, but throughout its history Americans have been squandering this incredible resource. According to the New York Times in an article from 2023 in which they monitored tens of thousands of wells and groundwater monitoring sites, nearly half of the sites they considered show evidence that water levels have declined significantly over the last 40 years as more water has been pumped out than is replenished naturally. Since the inception of the Arizona Groundwater Act of 1980 there has been an increasing urgency to take steps to halt this decline, and even hopefully reverting groundwater storage levels to their past volumes. Through the implementation of measures such as stricter water management laws, improved consumption efficiency, exploration of alternate water sources and artificial recharge, there has been some instances of successful groundwater recovery. The aim of this project is to explore one of these instances, for which we chose the Las Vegas Valley aquifer system in Nevada.
The Las Vegas Valley is a major metropolitan area located in southern Nevada. The area is home to the three largest incorporated cities in Nevada: (the fabulous) Las Vegas, Henderson, and North Las Vegas, which together have a combined population of approximately 3 million people at the end of 2024. The valley lies within both the Great Basin and Mojave desert sections of the Basin and Range physiographic province, and is nestled in between the Spring mountains to the West and the Sierra Nevada mountains to the East. [[pavelkovegasnevadagambling1999]]
Figure 1: Las Vegas Valley Map
1.1 Climate Conditions
The surrounding mountain ranges act as moisture barriers. Although more than 24 inches of precipitation fall annually in the Spring mountains, annual rainfall on the valley floor (on average) is less than 4 inches with measurable amounts (greater than 0.01 inches) seldom occurring for more than 30 days each year. Temperatures in the valley often climb above 120°F. On average the region experiences more than 125 days per year where temperatures climb above 90°F. This coupled with the low precipitation results in dry and arid climate conditions. Figure 2 below includes a plot provided by the U.S. Climate Data website displaying the monthly precipitation layered with temperature highs and lows in Las Vegas specifically averaged over the last 20 years. We see that on average no month experiences more than 1 inch of rainfall per year, and that temperatures in the summer months regularly reach over 100°F. Figure 2: Topological Map of Las Vegas Valley (Source: topographic-map.com) and Average Annual Precipitation and Temperatures (Source: U.S. Climate Data). ### Land Use
Prior to settlement, the region was known for its plentiful water resources and grassy meadows (Las Vegas translated from Spanish meaning The Meadows) which were present due to an abundance of groundwater which fueled numerous artesian wells. The presence of this oasis in the desert is what attracted settlers to the valley, with the subsequent development of the Salt Lake and Los Angeles railroad resulting in rapid population growth. Since the 1930’s Las Vegas has grown into one of the largest and most famous gaming centers and adult resort destinations in the world. This economic expansion has come a population explosion, resulting in an accelerated demand for water which was historically met with imported water from the Colorado River (sourced from the nearby Lake Mead) and with local groundwater.
Figure 3: Las Vegas Valley Population Growth. ### Aquifer System Characteristics
The water in the aforementioned artesian wells was flowing from the Las Vegas Valley aquifer system that is present under the basin. The valley is a structural trough filled with unconsolidated sediment such as sand and gravel. Scattered throughout the aquifer are pockets of consolidated and low-permeability materials such as slits and clays which increase in frequency as you approach the basin. Overlying the principal aquifers in most places is a 100-300 foot thick section of extensive clay, sand, and gravel deposits known as the near-surface reservoir. The principal aquifers and the near-surface reservoir are separated by a variably-thick, laterally discontinuous aquitard.
Figure 4: Las Vegas Valley Aquifer Hydrogeological Cross-Section.
1.2 Groundwater Conditions
The aquifer is naturally replenished by the in-flow of precipitation from the surrounding mountains. Near the margins of the valley groundwater moves freely through the course-grained sand and gravel deposits, however as the water moves further into the basin it encounters greater percentages of the lower-permeability fine-grained silts and clays which retard lateral groundwater flow. The natural recharge rate of the aquifer is around 33,000 acre-feet per year. As the valley’s population (and therefore water requirements) increased, so too did pumping rates which quickly overtook the natural recharge of the aquifer leading to a decline in water storage. Using data from the U.S. Geological Survey Online Database we constructed a plot of depth-to-water over time for a well site near the Harry Reid International Airport, shown below in Figure 2, from which we see a decline in depth-to-water (and therefore water levels) since their records began in the late 1930s.
Figure 5: Depth-to-water time series prior to intervention (Source: U.S.G.S. Online Database). ## Intervention
To arrest impacts of groundwater depletion the district opted to implement an artificial recharge program in 1988 whereby any surplus imported surface water would be injected back into the aquifer system during low water demand periods in the winter months. To assess the feasibility of conducting artificial recharge the district first conducted a pilot project in 1987 which used an existing unused production well to pump 525,000 gallons of imported water back into the aquifer. The major concern of water managers regarding artificial recharge was the resulting water chemistry. Since both the existing groundwater and the injected water from Lake Mead were high in calcium it was hypothesized to have potential for high calcite precipitation. If true, this could clog perforations in the water well casing and could potentially reduce aquifer permeability by clogging the pore space in the flow media. Fortunately, the results of this pilot program as detailed in @katzerArtificialRechargeVegas1989 found no imbalance with regard to calcium or any other chemical constituent post recharge and they concluded that artificial recharge is a feasible and viable technique for the valley to meet their increasing water demand. This was followed by a larger demonstration project whereby 1,153 acre-feet of treated Colorado River water was injected into the aquifer through existing pumps with minimal retrofitting and the results detailed in @brothersWaterBankingArtificial1990 concluded similarly that negligible calcite precipitated during recharge and recovery.
Following this successful demonstration the valley decided to expand their artificial recharge program to include 15 retrofitted existing cable-tooled drilled production wells between 1989 and 1991. From 1992 to 1993 a further 10 wells were added to the program, 7 of which were retrofitted like previous wells with the remaining 3 being newly drilled using reverse circulation drilling methods and gravel packing. However, close observation of the wells and their resulting recharge indicated that these newly constructed wells were experiencing near well bore clogging. Further evidence of this came to light when the wells were pumped during the following summer production season with flow rate declines and production capacity reductions from 30 to 60 percent. Following these findings construction of both injection-only and dual-use wells began in late 1996 using the original cable-tooled drilling methods.
In addition to the artificial recharge efforts the district began shifting away from their reliance on groundwater. As noted by the Las Vegas Valley Water District, following a historic drought in the early 2000’s the Colorado River experienced its lowest flows on record. That same year, Southern Nevada used more water than it ever had before. Recognizing the need to reduce water use the community embraced and implemented the most progressive and comprehensive water conservation program in the nation. The district adopted mandatory conservation measures including seasonal water restrictions, golf course water budgets, a grass replacement program, water waste penalties and changes to municipal codes that significantly reduce the impact of new development on water supply.
Figure 6: Las Vegas Valley Water Sources Cumulative Plot (Source: PannState InTeGrate - A Familiar History of Water and Population Growth). ## Outcomes
The Las Vegas Valley artificial recharge program is widely regarded as one of the most significant water conservation success stories involving groundwater replenishment in the United States. Examining our own data for confirmation we reproduce Figure 5, this time indicating the date that the artificial recharge program commenced as well as depth-to-water measurements since the intervention, shown below in Figure 7. We see clear evidence of not only a halting of the downwards trend but a rebounding with water levels in our given location recovering back to levels last seen in the 1960’s. Further, the current increasing trend indicating there is still further recovery on the way.
Figure 7: Depth-to-water time series in well site ID: 360349115100001 post intervention (Source: U.S. Geological Survey Online Database).
Furthermore, the districts water conservation program has been highly successful in increasing the regions water-use efficiency which has reduced their dependence on groundwater as a water source. According to the Southern Nevada Water Authority, in the 21 years between 2002 and 2023 the region experience a population growth of 52%, but in that same time period per-capita water use is down 58% and their Colorado River water consumption is down 42%. The district has also successfully transitioned away from groundwater and, per the Las Vegas Valley Water District 2024 Water Quality Report, now only 10% of the valley’s water supply comes from groundwater with the 90% majority coming from Lake Mead.
However, artificial recharge of the Las Vegas Valley reservoir ended in 2011 due the the district being unable to afford to store any of the water it received from Lake Mead in the face of a years-long drought. Despite the original plan of managing groundwater levels (with a combination of artificial recharge and groundwater pumpage reduction) due to the growth in the population groundwater pumpage levels have remained largely unchanged since the 1970’s. We produce a collection of plots using historical data provided by the Las Vegas Valley pumpage inventory on groundwater recharge, gross groundwater pumpage (before netting discharge) and Colorado River imports, shown below in Figure 8.
Figure 8: Las Vegas Valley Groundwater Recharge (Plot 1), Groundwater Pumpage (Plot 2) and Colorado River Imports (Plot 3) (Source: Las Vegas Valley Pumpage Inventory).
We see from Plot 1 in Figure 8 confirmation that groundwater artificial recharge began in 1988 and peaked in the early 2000s - where a little over 30,000 acre-feet of water was injected into the aquifer - and ended in 2011. However, we see from Plot 2 in Figure 8 that groundwater pumpage levels have remained roughly constant around 70,000 acre-feet per year since the 1970’s and there is no real evidence that the total groundwater pumpage is decreasing. This has sparked some concern among hydrologists of an impending relapse of the problem since the 30,000 acre-feet per annum natural recharge is not sufficient to balance pumpage. Further, we see from Plot 3 of Figure 8 that although there has been some decrease in Colorado River consumption, the district still imports over 400,000 acre-feet per year and as Lake Mead water levels decrease Las Vegas will be entitled to smaller quantities. It is important to note however that the valley’s consumptive water use is down 30% in the last 20 years and that approximately 99% of all indoor water use is recycled and returned to Lake Mead. This means that although their pumpage is high, their consumptive water use is hovering slightly below 200,000 acre-feet, well below their current Lake Mead quota of 300,000 acre-feet. In short, the district is still importing large quantities of water from lake mead but is consuming a significantly smaller portion.
2 Summary
The Las Vegas Valley region of Southern Nevada implemented both an artificial recharge program as well as an aggressive water conservation plan in an attempt to combat the decreasing groundwater levels observed in the Las Vegas Valley aquifer. The program was a clear success with data suggesting a significant recovery of groundwater levels which as of 2024 have reverted back to levels not seen historically since the 1960’s. Although the problem of a growing population and shrinking water sources persists for the region, the groundwater problem is widely considered to be solved although some express concerns over the continued pumpage levels. Despite the challenges presented by climate change, Southern Nevada remains one of the most water-secure communities reliant on the Colorado river.
2.0.0.0.1 References
- Brothers, K., & Katzer, T. (1990). Water banking through artificial recharge, Las Vegas Valley, Clark County, Nevada. Journal of Hydrology, 115(1–4), 77–103. https://doi.org/10.1016/0022-1694(90)90199-8
- Johnson, M., Cole, E., & Brothers, K. (n.d.). Artificial recharge in the Las Vegas Valley: An Operational history.
- Katzer, T., & Brothers, K. (1989). Artificial Recharge in Las Vegas Valley, Clark County, Nevada. Groundwater, 27(1), 50–56. https://doi.org/10.1111/j.1745-6584.1989.tb00007.x
- Pavelko, M., Wood, D., & Laczniak, R. (1999). Las Vegas, Nevada—Gambling with water in the desert. US Geological Survey, 1182, 49–64.
- Ranatunga, T., Tong, S. T. Y., Sun, Y., & Yang, Y. J. (2014). A total water management analysis of the Las Vegas Wash watershed, Nevada. Physical Geography, 35(3), 220–244. https://doi.org/10.1080/02723646.2014.908763
- Sullivan, A. (n.d.). LAS VEGAS VALLEY HYDROGRAPHIC BASIN 13-212.
- Wilson, J. W. (2021). Discharge Data Collection and Analysis and Implications for Surface-Water/Groundwater Interactions in the lower Las Vegas Wash, Clark County, Nevada, 2016–18 (Scientific Investigations Report No. 5034; Scientific Investigations Report).
- Donovan, David & Katzer, Terry & Brothers, Kay & Cole, Erin & Johnson, Michael. (2002). Cost-Benefit Analysis of Artificial Recharge in Las Vegas Valley, Nevada. Journal of Water Resources Planning and Management-asce - J WATER RESOUR PLAN MAN-ASCE. 128. 10.1061/(ASCE)0733-9496(2002)128:5(356).
- Draft Hydrogeological conceptual site model report, Avis and National Facilities McCarran International Airport Las Vegas, Nevada, Geo Blue Consulting Inc. (2018).
- Plume, R. W. (1989). Ground-water conditions in Las Vegas Valley, Clark County, Nevada—Part 1 Hydrogeologic Framework. In Water Supply Paper (Nos. 2320-A). Dept. of the Interior, U.S. Geological Survey ;Books and Open-File Reports Section [distributor]. https://doi.org/10.3133/wsp2320A
- Plume, R. W. (1996). Ground-water Conditions in Las Vegas Valley, Clark County, Nevada—Part 2 Hydrology and simulation of ground-water flow. U.S. Department of the Interior, U.S. Geological Survey.