Response 2 SNWA; NO Water Theft 4 Developers! (Part 1)

Overheard at the public comment meeting;

"The SNWA pipeline is a stimulus package for developers."




Statement from Confederated Tribes of the Goshute;

"We, the Confederated Tribes of the Goshute, reside in an isolated oasis in the foothills of the majestic Deep Creek Mountains on what is now the Utah/ Nevada state line. Our reservation lies in one of the most sparsely populated regions of the United States, and it has always been our home. Resulting from this isolation, we have benefited by retaining strong cultural ties to Goshute land, our traditions, and a resolute determination to protect our ways. Ironically, water, the most elemental resource in our basin, is the very thing developers now seek to extract and send 250 miles away for Las Vegas suburbs. The Southern Nevada Water Authorities’ pipeline proposal would draw 125,000-212,000 acre feet per year from the Great Salt Lake Watershed Basin lowering the water table, drying up our springs, and fundamentally changing access to water over this vast region for plants, wildlife, and people.

Even a slight reduction in the water table will result in a cascade of wildlife impacts directly harming our ability to engage in traditional practices of hunting, gathering, and fishing on ancestral lands. As our Chairman Rupert Steele has pointed out; “if we lose our language or our lands, we will cease to be Goshute people.” This issue is the biggest threat to the Goshute way of life since European settlers first arrived on Goshute lands more than 150 years ago.

Even though our people have been challenging this project for many years, we have largely been ignored, and told not to be concerned as “the aquifer surely ends at your reservation boundary and this pipeline will not affect you.” Thus far, studies conducted by the project’s proponents have failed to accurately describe Goshute interests, our history, land rights, or our concerns. The BLM is creating a Draft Environmental Impact Statement, due out in Spring, 2011, and the Nevada State Engineer will be reviewing water rights request beginning in 2011. The Goshute people request help in getting our rights acknowledged and concerns addressed as this water grab is being considered. Please help by assisting the Tribe or our partners in defending our Goshute way of life, in the place we have called home for thousands of years."

http://www.goshutewater.org/





Here's the report about the geology of the aquifer and how it is vulnerable to permanant collapse if the proposed SNWA pipeline makes their extractions. This would appeal to geology students, some sections discuss the political and finacial goals of developers as related to the pipeline. This would be more interest to the eco-activists, who can just skim through the geology details. Aquifers everywhere need protection!

The details are needed to show the NV State Engineers (who held the public comment meeting) that the SNWA is overlooking the geological complexity of the aquifer they plan to extract billions of gallons/year from.





Maintaining and Protecting Nevada’s Aquifers from SNWA Pipeline Extraction

Summary and Introduction – pg. 2

Pattern of Aquifer Overdrafts and Land Subsidence in Las Vegas Valley – pg. 3 - 6

Hydrogeology of Basin and Range Carbonate Karst Aquifer System – pg. 6 - 8

Pahrump Valley Subsidence from Extractions of a Partly Drained Closed Basin Aquifer – pg. 8 -13

Carbonate Aquifers Collapse Potential Visible in Lehman Caves Talus Room – pg. 13 - 14

Pole Canyon Limestone Members Form Solution Caves of Varying Grades of Strength – pg. 14 – 16

Metamorphic Core Complex of Northern Snake Range Decollement (NSRD) Show Strata Instability, Excessive Faulting and Multiple Variations in Marble Grade

Part I – Boudinage and Mylonitic Marble in the Spring Mountain Quadrangle (SMQ) – pg. 16 -19

Part II – Ductile and Brittle Faulting Found at the Pole Canyon Limestone Upper and Lower Plate Boundary in Miller Basin – pg. 19 -21

Part III – NSRD Metamorphic Core Complex Upper and Lower Plate Relations as Rolling Hinge Model of Fault Movement – pg. 21 – 29

Learning from Overdraft Caused Limestone Aquifer Cavern Collapse in Florida – pg. 29 - 31

Prior Species Extinctions from Loss of Seeps and Springs Following Overdraft – pg. 31 - 32

Protecting Aquifer Dependent Seep and Spring Habitats for Endemic Spring Snails – pg. 32 – 33

Additional Probable Financial Beneficiaries of the SNWA Pipeline Revealed by Location Choices and Contribution History – pg. 33 -37

Exposing Myths of SNWA Public Relations Propaganda – pg. 37 -38

Restoring Hydrological Sanity by Maintaining and Protecting Aquifers from Overdraft – pg. 38 -43

More Reasonable Water Acquisition Methods for Las Vegas Valley than the Snake/Spring Valley Pipeline – pg. 43 - 46

Conclusion – pg. 46 - 49

References – pg. 49 - 52


Summary

This report is in response to the Southern Nevada Water Authority’s (SNWA) proposed 287 mile pipeline moving 41 billion gallons of extracted groundwater every year from the Snake and Spring Valley aquifer system in White Pine County towards the Las Vegas Valley region. Evidence will show that the ecological repercussions of excessive groundwater extractions as planned by the SNWA pipeline will likely be land subsidence, fracturing, fissuring, aquifer cavern collapse, seeps and springs drying out and causing extinctions of endemic species such as spring snails. The SNWA pipeline is another example of a continuing pattern of groundwater overdraft of aquifers in southern Nevada resulting from perpetual sprawling suburban development in the greater Las Vegas Valley. The purpose of this report is to demonstrate the overall negative ecological and hydrological effects from the SNWA’s over dependency on aquifer water and introduces probable financial motives by SNWA officials and certain developers in pushing the people of Nevada to believe that the pipeline from the distant Snake and Spring Valley aquifer system is even needed.

Introduction

Recent protests throughout Nevada and neighboring Utah pertain to the proposal by SNWA to remove 41 billions of gallons of groundwater yearly from the Spring and Snake Valley aquifer system by constructing an approximately 287 mile long pipeline. The Spring and Snake Valley aquifer system is found throughout a network of solution carbonate karst caverns found along a strata layer of slightly metamorphosed limestone of various degrees of strength. The Snake and Spring Valley aquifer is one continuous underground body of water that slowly flows through carbonate or karst caverns located below unconsolidated sedimentary basin fill stretching across the Snake Valley, Snake Range and Spring Valley. The recharge points for these aquifers are the Schell Creek Range and the northern and southern Snake Ranges that receive above average yearly precipitation due to their high elevation above the surrounding valleys. The rainwater then percolates down through sediment strata layers into the carbonate limestone layer where aquifer caverns enlarged by dissolution store rainwater as groundwater. This groundwater eventually discharges as surface springs, seeps and local wells that provide life to the aquifer dependent ecosystems and humans of the Snake and Spring Valleys. However, the aquifer cavern roofs far below ground that store the aquifer water constantly depend on the buoyant upwards directional support of the water itself to prevent the heavy layers of basin sediment fill from eventually collapsing the cave’s roof inwards. This process of aquifer cavern collapse is related to land subsidence and has been documented to occur regularly in similar limestone aquifers found elsewhere as a result of aquifer overdraft. In addition, aquifer overdraft almost immediately drops the groundwater table to levels below their prior discharge points at seeps and springs. Species such as spring snails are endemic to these seeps and springs, meaning they are only found in these specific habitats and no place else on Earth. If the springs dry out from a drop in groundwater level, the endemic spring snails will become extinct. This is not new, as the Las Vegas region developed more desert land with lawns and golf courses, groundwater withdrawals from nearby aquifers became the leading factor responsible for the extinction of the Vegas Valley Leopard Frog and the Vegas dace that were both endemic to those specific springs that dried up following overdraft.

Pattern of Aquifer Overdrafts and Land Subsidence in Las Vegas Valley

This latest aquifer draining quest by SNWA officials will result in the same eventual depletion of the Snake and Spring Valley’s groundwater, followed by land subsidence, dried out springs, endemic species extinctions and dust storms as with many other aquifers tapped dry by the developers of the Las Vegas Valley in the prior decades. The regional aquifers around Las Vegas Valley were overdrafted decades ago, causing land around the Las Vegas Valley to subside by several feet in many regions, more in some places than others. This overdraft mostly occurred during the recent decades of Las Vegas Valley’s rapid development when many newcomers were somewhat unaware of the sensitive dynamics of the region’s underground water storage. Over-abstraction, overdraft or excessive lowering of groundwater is one of the leading causes of subsidence and is documented to regularly occur in all nations around the globe. Parts of the Las Vegas valley have subsided over 1.5 meters as a result of over-abstraction with rainfall averages of only 100 mm/year (worldwaterday.org).

Indigenous people of the Las Vegas Valley were able to live for at least several hundreds of years in this extreme desert environment without overdrafting the region’s aquifers and drying out surface springs. Unfortunately modern day developers in the Las Vegas Valley have managed to overdraft aquifers and dry up the region’s springs in just a few decades due to carelessness and ignorance. The chronology of aquifer withdrawal and subsidence in Las Vegas Valley shows that an ancient aquifer such as the Snake and Spring Valley karst system that required centuries of percolating rainwater to form caverns full of filtered groundwater can be drained dry in mere decades. In the Snake and Spring Valley aquifer system the rate of recharge from precipitation is measured in centuries while the rate of discharge from extractions and spring outflow is measured in mere months. The scales are balanced much more in favor of rapid emptying of the aquifer than the aquifer becoming refilled in the event of overdraft. Though the material substrate in the Las Vegas aquifer is sedimentary while the Snake and Spring Valley aquifer is slightly metamorphosed limestone of varied grades of strength, general rules of physics apply to conditions such as overdraft and subsidence in all aquifers.

The influence of gravity on dewatered aquifer caverns or lowered groundwater levels is nearly identical, the absence of water in either the pore spaces of sedimentary groundwater or the aquifer cavern in carbonate karst systems results in a loss of upward directional buoyant pressure and the eventual lowering of the ground to fill in the spaces emptied of water.

Since 1925 fissures were documented as appearing in the Las Vegas Valley, many forming as small tension cracks in sediment above the water table and are believed to enlarge from mechanical piping. The fissure continues to grow until it is visible after breaking through the sedimentary cover bridging the pipe, sometimes causing lines of small potholes to form on the surface (Bell).

In Las Vegas Valley land began subsiding around 1935, documented during studies related to Hoover Dam’s construction. Regional monitoring programs since 1935 have documented a widespread shallow sinking of land along Boulder Canyon around 19 km north of Hoover Dam, showing a southeastward tilt of 10-12 cm resulting from loading of stored water (Bell).

The Las Vegas Valley’s hydrologic basin is mostly hundreds of meters of sedimentary alluvial deposits, with coarser grains around the margins and fine grained in the middle. The aquifers are confined and semi-confined at 200-300 meter depths. Decreases in sediment volume from groundwater removal result in subsiding of land. The increase in effective stress on silts and clays force permanent rearrangement of fine grained particles. Fine grained sediments show greater tendency to compact than coarse grained deposits (Bell).

Many residents from the Windsor Park section of North Las Vegas were forced to relocate following years of repairing twisted homes, schools, roads and other infrastructure deformations. The homes in Windsor Park are being destroyed by subsidence related slow movement and rupture of the ground below them. One effect of long term subsidence is a change in locations of frequently flooded areas (Helm).

In 1946 annual groundwater withdrawals from the Vegas Valley were greater than annual recharge, and have consistently exceeded recharge by two to three times the amount. The long term effect of constant overdraft is witnessed in some locations where water levels have declined by more than 90 meters (Bell).

From elevation benchmarks at various locations around the Valley a map of cumulative subsidence was formed in 1963. The ’63-’87 subsidence map shows depths of greater than five feet occurred near Craig Rd. and Ranch Dr. in the northwest Valley . From 1963-80 the cumulative subsidence map was published in Nevada Geology’s summer ’89 issue (no.3), though since recent development has destroyed elevation benchmarks future maps cannot use the original base data. Two geodetic surveys (GPS) were run in 1990-91 to try to establish new base data (Helm).

Nevada Bureau of Mines and Geology (NBMG) scientists John Bell and Jon Price have collected earlier data until 1980 and updated this with new data that ended in 1989. The annual rate of subsidence since 1980 has remained close to what it was prior to 1980. This leveling off could reflect common sense decisions to manage the groundwater differently, instead of overdrafting the aquifers year round they were finally reinjected with runoff during the wetter winter season (Helm).

According to Nevada Geology (No. 3, Summer 1989) the cause of Las Vegas Valley’s subsidence is caused by groundwater withdrawal as pumping from aquifers has exceeded the natural recharge rates by 25-35 k acre-feet per year since the mid-1940s. In 1968 the region’s groundwater withdrawal had reached a maximum yearly rate of 88k acre feet, though in the years after decreased to 68k acre-feet per year after imports from Lake Mead began. Then in 1987 the Las Vegas Valley Water District began an artificial recharge program during times of lower usage, from 10-20k ac.-ft./yr. (Helm).

The effects of groundwater overdraft are continuous declines of water levels and the reduced artesian spring pressure throughout the greater Las Vegas Valley basin. Reduction of upwards pressure from the groundwater below allows the effective stress to increase at depths. This occurs because the percentage of the overburden’s weight supported by contact between the grains increases while the percentage supported by interstitial water inside of the pore spaces decreases. An increase in effective stress throughout the sediment source material of withdrawal causes pore structures to yield to gravitational pressure and compress. The cumulative effects of individual pore space compression results in land subsidence as the gradually increasing density of grains at depths moves upwards (Helm).

In 1963 scientists documented the center of the valley had subsided by 1 meter and by 1980 by 1.5 meter. One broad subsiding bowl covers the central portion of the valley, while three smaller subsiding bowls cover downtown, the southern strip and the northwestern region (Bell).
Near fault lines subsidence causes uneven tilting as rates differ on opposite sides across the fault. There is documented evidence of steep gradient of differential subsidence near the Eglington fault with a 2 ft. contour line in close proximity to a five ft. contour line (Helm).
Several linear and curvilinear north to northeast faults cross the valley, raising scarps by 50 meters in some places. Studies have shown that the faults are preferred sites for local, subsidence created vertical movements (Bell).

Between 1978-91 data for four contour lines across three separate fault zones show constant rates of movement. Along the northeast trending Eglington scarp between 1978-85, the data on elevation shows subsiding on the upthrown northwest was subsiding at 5 cm/yr. Along all four contour lines the most extreme differences in elevation occur either along the central portion of the scarp or begin their differential readings at the scarp. All four lines show and antithetic movement opposing the original geologic displacement (Bell).

“Areas within the valley that have been heavily pumped and show large water-level declines have also been the sites of major elevation change, surface deformation, and damage” (Bell).

Subsidence fissures are a result of long narrow cracks from depths migrating upwards to meet the surface. When erosion transports sediments into the fissure it forms a line of potholes and gullies at the surface. Subsidence fissures from horizontal aquifer movements at depths are different from desiccation cracks from drying of near surfaces clays. Most fissures are near already existing faults throughout Las Vegas Valley, 45% percent of fissure lengths are within 500 ft. of the nearest fault and 82 % are within 1,200 feet of the nearest fault (Helm). Throughout the eight zones of fissuring show close relations with geological faults, a result of the fault’s tensile strains being the ideal sites for fissuring (Bell).

The reason for cracks at depths along already existing faults is from downwards aquifer movement being lowered by groundwater withdrawals. Geological heterogeneities and aquifer hydraulics are closely interdependent regarding subsurface fissuring (Helm).

If groundwater discharge center is located on the upthrown side of a fault, the upthrown side moves downwards at greater distance when compared to the downthrown side, opposing the prior geological direction of motion. Geological structure seems to determine the locations of fissures while groundwater hydraulics determines the direction, magnitude and timing of subsidence related fissuring (Helm).

Based upon their dedicated research, the scientists at NBMG reached certain conclusions to mitigate subsidence hazards in Las Vegas Valley;

1) Reduce net annual groundwater withdrawal to level of net annual recharge by reducing dependency on groundwater withdrawals and increasing aquifer injection recharge.

2) Continue to recognize hazards zones based upon location of faults or existing subsidence related fissures.

3) In regions prone to fissuring encourage drought tolerant native landscaping to prevent needless withdrawals from sensitive underlying groundwater.

4) Establish a Las Vegas Valley Subsidence District to be responsible for water policy related to subsidence.

5) Monitor and track locations with subsidence and fissuring. Even if future reductions in withdrawals equal levels of annual recharge subsidence would continue for 5-10 years and fissures would increase from erosion. This would help noticing changes in runoff and erosion patterns.

6) Continue research in geological causes of horizontal movements, and subsidence related cracks at depths and fissuring. Network and compare research with other places experiencing fissuring (Helm).

Despite improvements in future rates of subsidence, the winter aquifer injections were not able to recover the subsidence prior to 1986-87 when the recharge program began. Near Las Vegas downtown Post Office six inches of subsidence occurred prior to 1950. These regions then were measured at a steady rate of subsidence until 1987 (Helm).

The lessons learned from the excessive groundwater withdrawal and resulting subsidence in the Las Vegas Valley seem to be ignored by the current establishment of the SNWA. The recommendations from the NBMG include lowering dependency on groundwater, to recharge aquifers at greater rates than those of discharge and withdrawals. This advisory from the NBMG is not because the scientists “don’t want to see Las Vegas grow” as the SNWA public relations machine would have people believe. The NBMG scientists’ sage advice is solely for the reason of protecting and maintaining Nevada’s aquifers for the future generations of people, plants, animals and entire ecosystems that depend upon the aquifers remaining full for their survival. The greater rates of aquifer recharge recommendations from the NBMG scientists also apply to the limestone based karst carbonate aquifers found far to the north of the Las Vegas Valley.


Hydrogeology of Basin and Range Carbonate Karst Aquifer System

The Snake and Spring Valley carbonate karst aquifer system is part of the greater Basin and Range province geology with rising mountain ranges running parallel at length with falling valleys. The source of current recharge is the surrounding Schell Creek Range and northern and southern Snake Range. The aquifer stretches between the two valleys and beneath the Snake Range. Aquifers in the Basin and Range Province that formed and filled in much wetter climates than that of our modern deserts experience far greater potential yearly losses from discharge and extraction than they gain from precipitation. During the wet prehistoric climate the aquifer gained water, while in the modern desert climate the aquifer suffers a net loss. This climactic inconsistency results in the Basin and Range aquifers at spring and seep level being very sensitive to overdraft of the groundwater. At the extraction rates proposed by SNWA to the tune of billions of gallons per year, expected results from the proposed pipeline would be excessive losses from the aquifer with minimal ability to recharge under modern climactic conditions. As the groundwater drops below levels where inadequate recharge raises the levels, the seeps and springs can go permanently dry. Having this understanding of climactic influences on geology is causing those scientists with ethics to experience grave concerns about the outcome of the proposed SNWA pipeline.

Most desert basins such as Snake and Spring Valley have land sloping from rising mountain blocks towards a central depression with a central drainage that is usually dry. Some valleys have playas that are remnants of seasonally sporadic lakes, many containing alkaline water with large amounts of dissolved salts and minerals (USGS Atlas).

Three main types of aquifers in Basin and Range Province are volcanic rock, carbonate rock and basin fill aquifers. The carbonate rocks under Snake and Spring Valley are dolomites and metamorphosed limestone mostly from the Mesozoic and Paleozoic ages. Drill test results show intervals of carbonate caverns exist from 5,000 to 15,000 feet in depths (USGS Atlas).

Many of Nevada’s eastern carbonate rocks have minerals such as quartzite, shale, siltstone and limestone deposited from earlier eras lying beneath them, with their minimal permeability forming a lower limit to the carbonate rocks. Carbonate rock aquifers exhibit two parts; the upper plate rocks from Late Triassic to the Early Mississippian age that are mainly limestone with smaller amounts of dolomite and interbedded with shale and sandstone and lower plate rocks of limestone and dolomite from Middle Devonian to Middle Cambrian age with hardly any amounts of clastic material (USGS, Atlas).

The primary strata that is a source of aquifer caverns in the Snake and Spring Valley is Cambrian Pole Canyon Limestone, usually found above a layer of Pioche Shale and below the Lincoln Peak Limestone and Dunderberg Shale strata layers. Shale is notorious for being fragile, flaky and crumbly. By sandwiching two different grades of metamorphosed limestone between two layers of shale, the aquifer bearing strata is seated on some rather crumbly bread slices.

There are great variations in thickness of carbonates from deep erosion and structural deformation, saturated up until 10,000 feet and with total depth in some locations as great as 15,000 feet. The thickness and position of the carbonates generally allows the saturated zone of an individual aquifer to begin at several thousand feet throughout most of the areal extent of the aquifer system. This type of aquifer is completely unsaturated only in the vicinity of the outcrop and is found everywhere except atop buried structural highs (USGS, Atlas).

These carbonates are very fractured and locally brecciated, with rocks consisting of angular clastic fragments. Clastic fragments found in monomictic breccias have the same composition, while polymictic breccias contain clastic fragments of varied compositions. Gravel size in sedimentary breccias are more than 30% of fragments at (>2mm) and angular clasts are produced by either physical weathering or brittle deformation of surrounding rocks. The angular shape of the clastic fragments indicates near distance transport. Sedimentary breccias form at the foot of slopes with talus or nearby active faults. Karst breccias result from erosion, dissolution and collapse of limestone. Pressure solution from high local stresses at points of contact between angular limestone fragments, marble, or chert can result in interpenetration of clastic fragments (enotes.com).

Specific outcrops found within the aquifer system can have three or more sets of joints, one or more high angle faults and one or more brecciated zones. Near the Nevada Test Site north of Las Vegas the joints and most of the faults in carbonate rocks are fractures with steep angles.

Brecciation usually happens along faults showing only a few feet of displacement and does not automatically indicate displacement from larger magnitude quakes. Joint density correlates with type of rock, with fine grained carbonates having the greatest joint density. The fine grained joints usually divide rocks into blocks from one inch to a few inches on each side. The medium grained carbonates are separated into blocks from a few inches to one foot per side. Coarse grained carbonate rock blocks range from 6 inches to two feet on each side (USGS, Atlas).

The outcrops usually contain secondary openings locally along the bedding planes, some from subaerial weathering and some from dissolution of the rock. Dissolution results in smooth tabular openings, while weathering alone would have tightly closed bedding and joint planes (USGS, Atlas).

Most Basin and Range groundwater flow systems are either in individual basins or two or more hydraulically connected basins where groundwater flows into a final discharge point or collects in a sink. Except for Colorado River drainage, the water does not leave the Great Basin and is used by desert plants. Basin and Range aquifer boundaries are the impermeable rocks of mountain ranges, with most water traveling through the basin fill deposits towards the carbonate caverns beneath. Basin fill groundwater is replenished from snowmelt entering fractures in bedrock channels and working down through alluvial fans. Water from summer thunderstorms usually is soaked up by dry soil and does not percolate far down enough to resupply groundwater. The mountains with carbonate rocks usually lack runoff as most water enters the permeable carbonate aquifer system (USGS Atlas).

There are four types of aquifer classificatons; undrained closed basins, partly drained closed basins, drained closed basins and the terminal sink basin. The undrained closed basins are simple single valleys surrounded by underlying impermeable bedrock with no interbasin flow and a central discharge point into a playa or sink. The partly drained closed basins are surrounded by moderately permeable bedrock with some flow out of the basin at the downgradient site and some playa evaporation on the upgradient site. The drained closed basins have deep aquifers that discourage evapotranspiration though are surrounded by highly permeable bedrock that enables all recharge to leave the basin. The terminal sink basin is surrounded by highly permeable bedrock to enable flow from several connected basins to enter the basin and collect in a playa (USGS Atlas).

Partly drained closed basins have permeable bedrock below them and are often hydraulically connected systems spanning across several valleys. In some cases, stream courses can connect several basins that are not closed. However, the Snake and Spring Valley aquifer is not connected by surface streams, though is connected below ground by interbasin flow between the two valleys beneath the Snake Range. This would classify the Snake and Spring Valley aquifer as a partly drained closed basin, though with deeper groundwater storage capacity that discourages upgradient evapotranspiration. With the exception that the aquifer is deep below ground, the similarities are close enough to consider the aquifer as a partly drained closed basin (USGS Atlas).

Pahrump Valley Subsidence from Extractions of a Partly Drained Closed Basin Aquifer

Many aquifers throughout Nevada exhibit similar characteristics of a partly drained closed basin. One example is the Pahrump Valley, covering around 1,050 square miles between Inyo and San Bernardino Counties in CA and Nye and Clark Counties in NV. The water source for this aquifer complex is the Spring Mountains (not the same Spring Mountains neighboring the Snake Range) on the northeastern border of the basin. On the southwest slope of the mountains, large alluvial fans drain the canyons descending from Charleston Peak, called the Pahrump and Manse Fans (USGS Atlas).

The intervalley groundwater flow of Pahrump Valley heads southwest to low elevations bordering the Amargosa River. The downgradient discharge point is between the towns of Tecopa and Shoshone, between 10-15 miles southwest of the Pahrump Valley’s topographic boundary. Thrust faults exposed in these mountains also displaced low permeability clastic rocks nearby the water storing permeable carbonate rocks, thus restricting aquifer flow. This is evidenced above ground by stands of mesquite and springs along the northwestern side of the fault, showing the barrier causes groundwater to move parallel along the barriers until it emerges at the surface. Throughout the fault, there are sections of broken rock and fractures where some permeability allows escape of water from the fault zone (USGS Atlas).

Similar to the Snake and Spring Valley aquifer, the Pahrump aquifer consists of two distinct segments; the carbonate rock karst caverns found beneath the valley, and the top layer of sedimentary basin fill, consisting of accumulated unconsolidated deposits of debris that fill the structural elongated bowl of the valley floor. Other similarities include the lifting mountain ranges that capture recharge and the falling valley basins that collect the groundwater in karst caverns under sediment fill, discharging in seeps and springs. In the Pahrump Valley, the groundwater is transported to the nearby Chicago and Amargosa River Valley southwest of the source. Due to the depth of the fill, the wells are drilled to extract water from the basin-fill aquifer (USGS Atlas).

In the Pahrump Valley, the carbonate aquifer is at such a great depth that drilling it would be nearly impossible, and as a result all water is withdrawn from the basin fill layer of sediments (USGS Atlas).

Since there are no wells in the Pahrump Valley’s carbonate karst aquifer, this makes it an excellent example of how an aquifer is supposed to replenish from precipitation and discharge into springs and streams at a reliable rate. The carbonate rocks of the Pahrump Valley aquifer system are of Triassic to Cambrian age and appear as outcrops in the Spring Mountains and also underneath the basin fill of the valley. From the Spring Mountains the aquifer extends southwestwards through the Nopah and Resting Springs Range into the California and Chicago Valleys. Localized solution openings and interconnected fractures allow the groundwater to move through the rocks.

The estimated transmissivity measured at 10 wells outside the valley was from 130 to 120,000 feet squared per day. The greater the transmissivity, the more water the aquifer will yield. The wide range in transmissivity numbers for the Pahrump Valley may be a result of variations resulting from faulting and the number and size of solution openings (USGS, Atlas).
Transmissivity (T) is equal to the volume of water that flows through a randomly selected cross section of an aquifer measuring 1 ft. x aquifer thickness (b) under a hydraulic gradient of 1 ft./1 ft. over a selected time frame such as 24 hours. Transmissivity is written as ft2/day because if T = Kb, then T = (ft./day)(ft./1). The measure of (T) is also equal to hydraulic conductivity (K) times aquifer thickness (b), or expressed as T = Kb (NC water). This indicates that subsidence would bear a direct effect on the lowering of transmissivity values by decreasing the thickness of the aquifer. The transmissivity values reflect on the amount of water stored in the aquifer (NCwater).

Similar to the Snake and Spring Valley, the Pahrump Valley basin fill aquifer is composed of unconsolidated alluvial and lacrustine deposits that partly fill the valley. Coarser grained material is found on the sides, while fine grained material occurs in the valley center. The 650 square mile areal extent of the aquifer is two thirds of the entire valley floor. To the northeast, northwest and southwest, the aquifer boundaries are the consolidated rocks of the Spring, Resting Springs, Nopah and Kingston Mountains. The southeastern boundary is a topographic high that separates the Pahrump and Mesquite Valleys (USGS, Atlas). The Spring Mountains mentioned in this section refers to a different mountain found in Clark County and is not connected to the Spring Valley aquifer other than the same first name.

The wells drilled into the Pahrump’s basin fill aquifer are from 50 feet to over 1,000 feet deep, none penetrating the basin fill except a few at the margins. The estimation of basin fill width was from geophysical measurements with maximum thickness of 4,800 feet in the valley center, with the thickest accumulations in the axis parallel to the valley length with some variations in the south end attributable to faulting (USGS, Atlas).

There are often extreme differences in transmissivities between different aquifers. In North Carolina’s coastal plain karst aquifers, some Cretaceous age aquifers have transmissivities from 100 to 1,000 ft2/day while Eocene age Castle Hayne Limestone can be as high as 50,000 ft2/day (NCwater.org).

Virtually all the Pahrump Valley’s aquifer water comes from precipitation, with ground water recharge percolating from the mountains down through bedrock fractures to zones of saturation, and the remainder on upper slopes of alluvial fans percolating into basin fill until reaching saturation. The flow is from recharge areas near the Spring Mountains southwestwards across the valley towards the Nopah Range. Evapotranspiration occurred in areas of shallow groundwater and additional water loss from subsurface outflow beneath the Nopah Range. The 2,600 foot contour lines on maps indicate the hydraulic gradient if the northwestern section of the valley is towards the Ash Meadows discharge site in the Amargosa desert north and west of Pahrump Valley. Instead, the majority of the groundwater flow is discharged along an area of the Amargosa River between the towns of Shoshone and Tecopa to the southwest of the Pahrump Valley (USGS, Atlas).

The Pahrump Valley’s agriculture depended upon two large springs for many years. In the late 1800’s, Bennetts Spring discharged nearly 7.5 cubic ft. per sec. (5,430 ac. ft./yr.) and Manse Springs nearly 6 cubic ft. /sec. (4,340 ac. ft./yr.) until 1913 when groundwater withdrawals began. Soon thereafter the springflow decreased drastically until Bennetts Spring ceased flow in 1959 and not one drop coming out of Manse Spring during the 1975 irrigation season. By this time, 9,800 ac.ft./yr. of spring discharge was diverted as groundwater withdrawals (USGS, Atlas).
The Pahrump Valley’s first well was drilled in 1910 and by 1916 there were 28 operational wells, with 15 of them flowing. Wells and withdrawals increased thereafter until the mid ‘40s when the first large capacity wells were drilled. These newly installed wells increased aquifer water discharge from wells from 4,000 to 28,000 ac. ft./yr. (USGS, Atlas).

Additional residential growth increased aquifer withdrawals throughout the ‘60s and ‘70s, resulting in noticeable effects as springs ceased to discharge and ground water levels began to decline. Differences between annual rates of decline depend upon the distribution of the withdrawal and the hydraulic properties of basin fill and well depth. The regions nearest the greatest concentration of wells showed groundwater level drops of 100 feet, while those with less concentrated wells showed less decline (USGS, Atlas). Other factors are variations within the aquifer itself, both rate of extraction and aquifer topography can effect the rate of the aquifer’s transmissivity.

Tranmissivity estimates only represent the top 1,000 feet of the aquifer, and some variations occur from deposition of coarser material and the water table’s positioning. Transmissivity values are shown to increase from the mountain’s edge where saturated materials are thin towards the center where the water table rises to meet the flattening land surface. The increasing thickness of the coarse materials saturated by groundwater give the greatest transmissivity values of 4,000 feet squared per day in the Pahrump and Manse Fans. Transmissivity values then decrease in almost parallel bands across the valley to less than 1,000 feet squared per day as the sediment layer’s saturated thickness decreases towards the mountain’s edge (USGS, Atlas).

Though citing the Pahrump Valley’s groundwater level dropping and causing subsidence in a sedimentary basin fill aquifer could be invalidated with contrasting the differences of the Snake and Spring Valley aquifer’s limestone carbonate karst aquifer system, the common theme in terms of physics of groundwater removal resulting in a lack of countergravitational buoyancy applies to both. The differences are primarily of scale; the sedimentary basin fill aquifer’s groundwater is stored in billions of almost microscopic and nearly equally sized and spaced pore spaces found between near equally sized and spaced sediment grains and the limestone carbonate aquifer water is stored in much smaller numbered yet much larger and also unequally sized and spaced karst caverns. In the Snake and Spring Valley aquifer system, the layer most responsible for forming aquifer caverns is Cambrian Pole Canyon Limestone (PCL). Surrounding the PCL layer are strata layers of Pioche Shale, Dunderberg Shale, Lincoln Peak Limestone and other limestone strata of varying strength. The limestone caverns could be thought of as a vertically thin and widespread extended layer of a water storing “pore space” sandwiched between much thicker strata layers of “sediment grains” incapable of storing water. Generally speaking the “sediment grain” below the limestone layer is Pioche Shale and the other “sediment grain” layer directly above the limestone is Lincoln Peak Limestone and Dunderberg Shale. Removing water from the pore spaces of sedimentary fill aquifers causes gravity to force the sediment grain formerly found above each now dewatered pore space downwards to fill and replace the empty pore space until it touches the sediment grain formerly found below the pore space.
Another difference would be in the rates of timing of land subsidence, overdraft of a sedimentary basin fill aquifer will result in an immediate yet gradual subsidence as small pores spaces empty of water and sediment grains slowly move downwards to fill the pore space emptied of water.

When a carbonate aquifer cavern roof loses support from countergravitational buoyant groundwater, the collapse of the large empty cavern can take a much longer time yet be a sudden dramatic incident once the fracture occurs and weakens the cavern’s structural integrity. The reason is the extreme size variations of the two aquifer’s water containing cavities, the billions of micro-sized pore spaces in sediment basin fill aquifers will begin emptying gradually immediately after groundwater level drops, moving sediment grains downwards along with the groundwater level movement. The limestone karst aquifer caverns are house sized and relatively solid, so an initial groundwater drop can partially empty a karst cavern over months or even years before the cavern’s roof stability becomes compromised and suddenly collapses. Immediately following groundwater overdraft, the falling groundwater levels will not show any subsidence on the surface for as long as the karst aquifer cavern remains stable. Metamorphosed limestone aquifer caverns as those found in the Snake and Spring Valley aquifer system are able to maintain their structural integrity for years and suddenly collapse several meters in seconds without warning. After enough time of being dewatered the weight of the overburden will fracture the roof along stress points of the karst aquifer cavern roof and result in a sudden collapse and subsidence after the overburden’s pressure from gravity overwhelms the karst cavern’s unsupported roof.

A similarity between the Pahrump sedimentary aquifer and the Snake and Spring Valley carbonate aquifer is in overall reduction of transmissivity following overdraft. Transmissivity is effected on both sides of the equation (T = Kb), with K = (ft./day) and b = (ft./1). The hydraulic conductivity (K) is measured as the amount of water flowing through a 1 foot square of a cross section of the aquifer over one 24 hour day. The height of the aquifer (b) is measured as one foot width times however many feet in vertical height the aquifer reaches. The effect of overdraft is reducing the amount of water and the height of the aquifer by cavern collapse and subsidence.

Carbonate Aquifers Collapse Potential Visible in Lehman Caves Talus Room

Although the Snake and Spring Valley aquifer system’s variations of different degrees of strength of metamorphosed limestone carbonate aquifer may endure greater overall overburden stress for a longer duration than the Pahrump and Las Vegas Valley’s sedimentary basin and fill aquifer, the same concept of overburden stress applies to karst cavern collapse and land subsidence. Removal of groundwater ends the upwards directional countergravitational buoyant pressure on the cavern’s roof, causing gravity to move the roof downwards to lower elevation positions in the cavern now emptied of the water’s prior support. Once an aquifer cavern becomes strained from gravity by unsupported overburden and collapses, the cavern can never be restored to the original capacity. The aquifer cavern’s roof lowers and moves closer to the floor, resulting in less empty space to store groundwater. One would hope that SNWA officials would reach the conclusion that groundwater in an undisturbed aquifer cavern that has not yet subsided from climactic dewatering is more desirable than an overdrafted aquifer cavern that has collapsed and is no longer capable of maintaining artesian springs or the aquifer cavern’s original storage capacity.

Limestone based karst aquifers like the caverns found beneath Snake and Spring Valley are eventually vulnerable to the same forces of gravitational collapse that caused the subsidence in Pahrump and Las Vegas Valley. Limestone cavern subsidence is not a result of pore spaces between grains growing smaller, it occurs as large hollowed out caverns deep beneath the Earth can no longer support the overburden following drops in groundwater level. One example of an earlier limestone karst cavern roof lowering collapse can be witnessed firsthand by a visit to Lehman Caves at Great Basin National Park. In this situation Lehman Cave’s Talus Room shows the results of a prehistoric karst cavern collapse caused by a climatic influenced lowering of groundwater tables. As the groundwater level dropped inside of the Talus Room’s caverns, the overburden’s gravitational pressure without any upwards buoyant support pressure from groundwater caused stress fractures in the metamorphosed limestone and caused the karst cavern’s roof to move downwards as a sudden collapse.

The great number and large sizes of intact Lehman Cave mineral formations indicates that very little shifting occurred except in the Talus Room. The numerous rockfalls in the Talus Room were likely a result of water draining out of the cave following climate change to drier conditions and removing buoyant support from the ceiling’s weight. The original strength of the bedrock itself was not weakened by the lowering water table. However, the original strength of the bedrock alone may never have been able to support the cavern’s roof without the additional countergravitational support of the groundwater. The process of the Talus Rooms’s ceiling collapse began when the water drained out of the cave and has not yet reached a stable point due to the room’s large size and is possibly enhanced by nearby faults. It is probable that several old cave passages are buried beneath the Talus Room’s layers of rubble (NPS).
The Snake and Spring Valley aquifer system did not fill up over just a few years of rainfall. These aquifer caverns consist of metamorphosed limestone that filled after corrosion of the limestone by slightly acidic carbonic acid from downward percolating rainwater. The duration of time that was needed for this aquifer to fill completely to the point of having artesian springs was several thousands of years. The climate during the time of formation was also much wetter with greater precipitation rates than current times.

Above the aquifer caverns lies the overburden of eroded sediment from the surrounding mountains. The Basin and Range geology of the Snake and Spring Valleys is lifting the mountains and lowering the valleys. The aquifer caverns are buried underneath the overburden in the valley. The water in the aquifer cavern is responsible for supporting the tremendous weight of the overburden. Removal of the aquifer water would result in the weight of the overburden resting solely on the limestone cavern’s roof itself. Both the caverns of Lehman Caves Talus Room and the modern day aquifers under the Snake and Spring Valley are considered solution caves, formed by carbon dioxide dissolved in rainwater and acting as an acid on the karst bedrock.

Pole Canyon Limestone Members Form Solution Caves of Varying Grades of Strength

The geological materials that form karst solution caves such as Lehman Cave’s Talus Room are limestone, dolomite, gypsum, salt and marble, and all are dissolved over time by contact with slightly acidic rainwater. The material of Lehman Caves and the Snake and Spring Valley aquifers is a type of metamorphosed limestone that became low grade marble, named Pole Canyon limestone for Pole Canyon on the west side of the southern Snake Range’s Mount Washington. Pole Canyon limestone conformably overlies the Pioche Shale and unconformably (thrust faulted) underlies the new Lincoln Peak Limestone. The age is Middle Cambrian, determined by the presence of trilobite fossils (ngmdb.usgs.gov).

Pole Canyon limestone is composed of massive gray and white limestone and is divided into five members (ascending);

1) Member A; 415 ft thick; composed of massive dark gray crystalline limestone; locally white calcite blebs, veinlets, and Girvanella type forms produce mottled texture; at 30 and 85 ft above base, two yellowish gray quartzite units 3-5 ft thick are present

2) Member B; 630 ft thick; massive, moderately coarse-grained, light gray limestone

3) Member C; 160-320 ft thick; similar to Member A

4) Member D; 220-380 ft thick; massive light gray limestone similar to Member B

5) Member E; about 340 ft thick; light to medium gray limestone with pale red to yellow-brown argillaceous partings, and much varicolored shale near the top. Overall thickness is about 2000 ft.

According to the above categories, there is considerable variation in the types of limestone and the height of each strata layer. One question relevant to the proposed SNWA pipeline is;

“Are certain members of Pole Canyon Limestone more vulnerable to fracture than others?”

To find an answer to the above question an understanding of the chemistry of limestone formation and dissolution is needed. The karst material of Pole Canyon limestone formed the many interconnected solution caverns of Lehman Caves. The Pole Canyon Limestone formed as an ancient shallow sea deposited calcium carbonate (CaCO3) or calcite on the sea floor in thick layers over 500 mya (NPS). Over time the limestone went through various stages of metamorphism, though as is clearly shown in the different strata layers each member is unique.

Solution karst caves form by chemical dissolution of bedrock by carbonic acid in percolating rainwater. Calcite in limestone can dissolve in weakly acidic rainwater. Carbonic acid is the most common form of acid rain as near atmospheric CO2 reacts with rainwater according to the following reaction;

H2O + CO2 --> H2CO3 (carbonic acid)

This same reaction occurs in soil air at higher rates as CO2 levels in soil can be 300 times higher than the 10% levels of CO2 in the air. When carbonic acid contacts limestone, the reaction dissolves calcite into liquid solution as follows;

CaCO3 + H2CO3 --> Ca+2 + 2(HCO3-) (calcium bicarbonate solution)

Limestone dissolution occurs in the vadose zone where initial contact with acid occurs, located in the aerated zone beneath the soil zone. Cave formation does not occur here, the rock dissolves from the top down (NPS).

Limestone dissolution and cavern formation occurs mostly at or slightly below the surface of the water table for three main reasons;

1) The water in the saturated (phreatic) zone moves slower than water percolating down and is in contact with the rock for a longer dissolving time.

2) The top of the saturated zone (groundwater level) receives acidic water from above, being closer to the source causing more dissolution to occur near the top than deeper.

3) Mixing two different water chemistries causes dissolution even if both were saturated with calcite, this occurs when surface water chemically mixes with CO2 rich groundwater.

These three factors combined create strong tendencies for cavern development at or slightly below the existing water table at time of formation. This can be observed as Lehman Cave’s plane is level, despite being inclined from later uplift and tilting (NPS).

Several varieties of mineral formations show characteristics of the cavern’s material. Lehman Caves have over 300 shield formations, an unusually large concentration of these two oval parallel plates that contain a thin crack between them. The theory for shield formation is when water under pressure moves through thin fractures in limestone it deposits calcite on either side of the crack, building calcite plates with thin water filled cracks in between the plates. Shield formations often form in caves with highly fractured limestone like Lehman Caves (NPS).

The great number and large sizes of intact Lehman Cave mineral formations indicates that very little shifting occurred except in the Talus Room. The numerous rockfalls in the Talus Room were likely a result of water draining out of the cave following climate change to drier conditions and removing buoyant support from the ceiling’s weight. The original strength of the bedrock itself was not weakened by the lowering water table. However, the original strength of the bedrock alone may never have been able to support the cavern’s roof without the additional countergravitational support of the groundwater. The process of the Talus Rooms’s ceiling collapse began when the water drained out of the cave and has not yet reached a stable point due to the room’s large size and is possibly enhanced by nearby faults. It is probable that several old cave passages are buried beneath the Talus Room’s layers of rubble (NPS).
In the case of the Talus Room cavern collapse the lowering water table was a result of a changing prehistoric climate. If this is compared to the proposed SNWA pipeline it becomes clear that lowering groundwater levels for any reason would be directly correlated to collapse of an aquifer cavern’s roof. However, the future collapse of Snake and Spring Valley aquifer system would be a result of human induced lowering of the water table, not a result of a changing climate.

Temperature and airflow in caves are less variable than on the surface, creating near constant conditions. Mineral formations develop at timescales much longer than human observation can observe in the present day. This results in changes caused by humans exceeding that which may be anticipated by extrapolating data based upon surface conditions and expectations, possibly causing impacts that last for centuries (NPS). There are also examples of modern day karst aquifer contamination resulting from overdraft of groundwater.

Other regions with similar karst aquifers have witnessed the effects of groundwater overdraft. The Great Swamp of NY between Putnam and Dutchess Counties occupies a valley worn into exposed carbonate rocks such as dolomitic marble, semi-pure marble and carbonate sandstones covered by glacial fill. Wells pumping out of this carbonate aquifer reversed normal groundwater discharge and caused contaminants to be drawn down into the aquifer system (Mead).

Similar to Lehman Caves and Snake/Spring Valley aquifer caves, the Great Swamp carbonate aquifer formation are fractured caves of various grades of marble below sedimentary glacial fill. The carbonate bedrock lies directly beneath the glacial sediments, creating a permeable path for downwards groundwater infiltration (Mead). In the Snake and Spring Valley aquifer system the sedimentary basin fill derived from erosion of mountains reacts the same way to infiltration.

In another study done in Western Ukrainian’s gypsum karst caves, it was demonstrated that loss of buoyant support of the cavern’s roof due to lowered groundwater was one of the most important factors in triggering a breakdown collapse. This loss of buoyancy occurred as the Miocene aquifer lost its’ prior state of confinement due to geological factors. The loss of buoyant support can disrupt the previously metastable state of the cavern’s roof at points where other geological and speleogenetic factors have already brought the roof’s resistance to failure, or bridging capacity, close to critical levels. This process is shown to happen repeatedly in quarries where massive groundwater withdrawal from the Miocene aquifer resulted in sudden drops of the potentiometric surfaces and intensifications of collapse and subsidence formations near the quarries (Klimchouk). Though gypsum is a much more fragile type of karst cavern than is low grade marble or even limestone, it may just be another difference in time until the eventual collapse of the Pole Canyon limestone cavern’s roof occurs.