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DOE OSTI2015

Evapotranspiration Within the Groundwater Model Domain of the Tuba City, Arizona, Disposal Site Interim Report

The revised groundwater model includes estimates of evapotranspiration (ET). The types of vegetation and the influences of ET on groundwater hydrology vary within the model domain. Some plant species within the model domain, classified as phreatophytes, survive by extracting groundwater. ET within these plant communities can result in a net discharge of groundwater if ET exceeds precipitation. Other upland desert plants within the model domain survive on meteoric water, potentially limiting groundwater recharge if ET is equivalent to precipitation. For all plant communities within the model domain, excessive livestock grazing or other disturbances can tip the balance to a net groundwater recharge. This task characterized and mapped vegetation within the groundwater model domain at the Tuba City, Arizona, Site, and then applied a remote sensing algorithm to estimate ET for each vegetation type. The task was designed to address five objectives: 1. Characterize and delineate different vegetation or ET zones within the groundwater model domain, focusing on the separation of plant communities with phreatophytes that survive by tapping groundwater and upland plant communities that are dependent on precipitation. 2. Refine a remote sensing method, developed to estimate ET at the Monument Valley site, for application at the Tuba City site. 3. Estimate recent seasonal and annual ET for all vegetation zones, separating phreatophytic and upland plant communities within the Tuba City groundwater model domain. 4. For selected vegetation zones, estimate ET that might be achieved given a scenario of limited livestock grazing. 5. Analyze uncertainty of ET estimates for each vegetation zone and for the entire groundwater model domain.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2016

Groundwater Remediation in a Flood plain Aquifer at Shiprock, New Mexico - 16097

The Shiprock, New Mexico, Disposal Site is the location of a former uranium and vanadium ore-processing mill that operated from 1954 to 1968. During milling years, tailings leachate and other mill-related liquids migrated into underlying sediments, resulting in the contamination of groundwater beneath the river terrace containing the mill and a nearby flood plain alluvial aquifer adjacent to the San Juan River. The US DOE Office of Legacy Management manages the site and is implementing the current groundwater remedy for the flood plain area, consisting of natural flushing supplemented by extraction of alluvial groundwater to accelerate reduction in contaminant concentrations. The alluvial aquifer contains elevated concentrations of sulfate, uranium, and nitrate derived largely from the historical eastward migration of groundwater through terrace alluvium and underlying bedrock to the flood plain. The bedrock formation beneath the entire Shiprock site is Mancos shale, which forms an escarpment that separates the terrace from the flood plain. Contamination in the flood plain alluvial aquifer is influenced by a dynamic flow system that changes seasonally and from year to year. Simulations with a flood plain flow model based on monitored groundwater levels and coincident river-surface elevations show that background groundwater flow, unaffected by remediation pumping, is characterized by distinct hydraulic conditions for three separate periods each year. During months of low river flow in winter and early spring, river losses to the aquifer create a large hypo-rheic zone in the south half of the flood plain. Simultaneously, recharge of continually flowing surface water emptying onto the north half of the flood plain from Bob Lee Wash creates divergent, radial flow from the wash outlet and diverts northwestward-migrating contaminant plumes to the northeast where they discharge to the river. Subsequent bank storage processes in May and June from high snow melt runoff change background flow directions, increasing the spread of contamination in the alluvium. From mid-summer through fall, river flows are typically similar in value to those observed prior to the onset of snow melt, but groundwater elevations in the aquifer are noticeably lower than water levels observed in winter and early spring and may induce river losses to the aquifer. Several different contaminant sources have impacted the alluvial aquifer. Much of the contamination originated as tailings leachate and raffinate wastewater that infiltrated the terrace subsurface and eventually reached the flood plain at the escarpment. Additional sources of contamination in the aquifer included discharge of mill effluent to Bob Lee Wash and a pond at the base of the escarpment, and aqueous mobilization of solid-phase contamination in the form of windblown deposits and contaminants adsorbed to alluvial sediments. The degree to which remnant, secondary-source processes, such as leaching of solid-phase deposits and discharge of terrace groundwater across the escarpment, contribute to existing contaminant plumes is uncertain. The flood plain remediation system consists of two groundwater extraction wells, pumping from two horizontal wells placed in trenches near the base of the escarpment, a sump collecting discharges from seeps in the escarpment wall, and discharge of collected water to an evaporation pond on the terrace. Flow-model simulations indicate that most contaminated areas are captured by remediation pumping, resulting in several zones of freshwater between the river and the escarpment. Despite the highly transient nature of groundwater flow beneath the flood plain and the varied contaminant sources, initiation of remediation pumping in 2003 and coordinated operation of the system's various components have successfully removed large amounts of contaminant mass and helped prevent discharge of contaminated groundwater to the river. The percentage reductions of dissolved sulfate, uranium, and nitrate mass in the aquifer estimated for the first 9 years of remediation range from 40% to 72%. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2018

Hydrogeologic Setting and Groundwater Contaminant Conceptual Model, Los Alamos National Laboratory - 18681

Our understanding of the complex hydrogeologic setting at Los Alamos National Laboratory (LANL) has evolved significantly over the previous decade. The semi-arid, high-desert setting consists of generally dry drainages in canyons that head either in uplands to the west of the Laboratory or on the high mountain-front plateau where the Laboratory is spread across approximately 40 square miles. Some canyon drainages have spatially limited reaches with perennial surface water from natural runoff and spring sources and/or from permitted effluent sources. The Rio Grande rift basin-fill stratigraphy includes primary volcanic deposits from pyroclastic eruptions and intermediate composition lava flows, and fluvial and alluvial fan deposits. Highly complex and spatially variable stratigraphy governs the occurrences of three groundwater zones: shallow 'alluvial groundwater', deep perched groundwater, and a thick 'regional' aquifer that is the water supply for the community of Los Alamos and the Laboratory. The current understanding of the hydrogeologic framework was greatly expanded as part of a site hydrogeologic characterization program completed in 2005. Higher resolution information exists in area where more detailed groundwater characterization has been conducted to support specific project objectives. A conceptual model for occurrences of contaminants in each of the three groundwater zones is governed by four factors: contaminant solubility/mobility, the mass of historical release, presence of a continuous hydraulic driver, and 'breakthrough' geology, defined as geology conducive to infiltration of shallow groundwater to deeper groundwater zones. The Laboratory's operations were historically somewhat spatially partitioned. Most of the radionuclide work (and historical releases) was conducted in the northern portion of the Laboratory, high explosives work has taken place exclusively in the southwestern portion, and treated sanitary effluent was released at a limited number of outfalls. Historical material disposal areas (MDAs) are dispersed throughout the Laboratory, and active MDAs are all located on a single dry mesa at the eastern part of the Laboratory. Evidence exists of transport of anthropogenic constituents to the regional aquifer. However, years of groundwater characterization have identified only two cases for which all the key factors were present and a complete contaminant pathway to the regional aquifer has resulted in contamination at levels above an applicable standard. Hexavalent chromium contamination associated with historical use as a corrosion inhibitor in a power plant is present beneath Mortandad Canyon, and RDX contamination associated with high-explosives machining is present beneath Canon de Valle. Each of those project areas have refined conceptual and numerical models that address contaminant migration pathways, spatial extent of perched-intermediate groundwater, mass balance, and fate and transport within the regional aquifer. No known contamination is present in the regional aquifer associated with MDAs, and interestingly, much of the Laboratory's groundwater background dataset is from wells located beneath active MDAs. A comprehensive groundwater monitoring program is in place to track the evolution of chromium and RDX contamination, and to identify any other constituents that may be attributable to historical LANL operations. (authors)

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DOE OSTI2017

Hydrochemical and isotopic characteristics of groundwater in the northeastern Tennger Desert, northern China

Groundwater is typically the only water source in arid regions, and its circulation processes should be better understood for rational resource exploitation. Stable isotopes and major ions were investigated in the northeastern Tengger Desert, northern China, to gain insights into groundwater recharge and evolution. In the northern mountains, Quaternary unconsolidated sediments, exposed only in valleys between hills, form the main aquifer, which is mainly made of aeolian sand and gravel. Most of the mountain groundwater samples plot along the local meteoric water line (LMWL), with a more depleted signature compared to summer precipitation, suggesting that mountain groundwater was recharged by local precipitation during winter. Most of the groundwater was fresh, with total dissolved solids less than 1 g/L; dominant ions are Na{sup +}, SO{sub 4}{sup 2−} and Cl{sup −}, and all mineral saturation indices are less than zero. Evaporation, dissolution and cation exchange are the major hydrogeochemical processes. In the southern plains, however, the main aquifers are sandstone. The linear regression line of δD and δ{sup 18}O of groundwater parallels the LMWL but the intercept is lower, indicating that groundwater in the plains has been recharged by ancient precipitation rather than modern. Both calcite and dolomite phases in the plains groundwater are close to saturation, while gypsum and halite can still be dissolved into the groundwater. Different recharge mechanisms occur in the northern mountains and the southern plains, and the hydraulic connection between them is weak. Because of the limited recharge, groundwater exploitation should be limited as much as possible.

58 GEOSCIENCES↗

DOE OSTI2017

A groundwater salinity hotspot and its connection to an intermittent stream identified by environmental tracers (Mt Lofty Ranges, South Australia)

High and variable levels of salinity were investigated in an intermittent stream in a high-rainfall area (∼800 mm/year) of the Mt. Lofty Ranges of South Australia. The groundwater system was found to have a local, upslope saline lens, referred to here as a groundwater salinity ‘hotspot’. Environmental tracer analyses (δ{sup 18}O, δ{sup 2}H, {sup 87/86}Sr, and major elements) of water from the intermittent stream, a nearby permanent stream, shallow and deep groundwater, and soil-water/runoff demonstrate seasonal groundwater input of very saline composition into the intermittent stream. This input results in large salinity increases of the stream water because the winter wet-season stream flow decreases during spring in this Mediterranean climate. Furthermore, strontium and water isotope analyses demonstrate: (1) the upslope-saline-groundwater zone (hotspot) mixes with the dominant groundwater system, (2) the intermittent-stream water is a mixture of soil-water/runoff and the upslope saline groundwater, and (3) the upslope-saline-groundwater zone results from the flushing of unsaturated-zone salts from the thick clayey regolith and soil which overlie the metamorphosed shale bedrock. The preferred theory on the origin of the upslope-saline-groundwater hotspot is land clearing of native deep-rooted woodland, followed by flushing of accumulated salts from the unsaturated zone due to increased recharge. This cause of elevated groundwater and surface-water salinity, if correct, could be widespread in Mt. Lofty Ranges areas, as well as other climatically and geologically similar areas with comparable hydrogeologic conditions.

58 GEOSCIENCES↗

DOE OSTI2018

Groundwater mixing and mineralization processes in a mountain–oasis–desert basin, northwest China: hydrogeochemistry and environmental tracer indicators

Hydrogeochemistry and environmental tracers ({sup 2}H, {sup 18}O, {sup 87}Sr/{sup 86}Sr) in precipitation, river and reservoir water, and groundwater have been used to determine groundwater recharge sources, and to identify mixing characteristics and mineralization processes in the Manas River Basin (MRB), which is a typical mountain–oasis–desert ecosystem in arid northwest China. The oasis component is artificial (irrigation). Groundwater with enriched stable isotope content originates from local precipitation and surface-water leakage in the piedmont alluvial–oasis plain. Groundwater with more depleted isotopes in the north oasis plain and desert is recharged by lateral flow from the adjacent mountains, for which recharge is associated with high altitude and/or paleo-water infiltrating during a period of much colder climate. Little evaporation and isotope exchange between groundwater and rock and soil minerals occurred in the mountain, piedmont and oasis plain. Groundwater δ{sup 2}H and δ{sup 18}O values show more homogeneous values along the groundwater flow direction and with well depths, indicating inter-aquifer mixing processes. A regional contrast of groundwater allows the {sup 87}Sr/{sup 86}Sr ratios and δ{sup 18}O values to be useful in a combination with Cl, Na, Mg, Ca and Sr concentrations to distinguish the groundwater mixing characteristics. Two main processes are identified: groundwater lateral-flow mixing and river leakage in the piedmont alluvial–oasis plain, and vertical mixing in the north oasis plain and the desert. The {sup 87}Sr/{sup 86}Sr ratios and selected ion ratios reveal that carbonate dissolution and mixing with silicate from the southern mountain area are primarily controlling the strontium isotope hydrogeochemistry.

58 GEOSCIENCES↗

DOE OSTI2018

Groundwater sustainability and groundwater/surface-water interaction in arid Dunhuang Basin, northwest China

The Dunhuang Basin, a typical inland basin in northwestern China, suffers a net loss of groundwater and the occasional disappearance of the Crescent Lake. Within this region, the groundwater/surface-water interactions are important for the sustainability of the groundwater resources. A three-dimensional transient groundwater flow model was established and calibrated using MODFLOW 2000, which was used to predict changes to these interactions once a water diversion project is completed. The simulated results indicate that introducing water from outside of the basin into the Shule and Danghe rivers could reverse the negative groundwater balance in the Basin. River-water/groundwater interactions control the groundwater hydrology, where river leakage to the groundwater in the Basin will increase from 3,114 × 10{sup 4} m{sup 3}/year in 2017 to 11,875 × 10{sup 4} m{sup 3}/year in 2021, and to 17,039 × 10{sup 4} m{sup 3}/year in 2036. In comparison, groundwater discharge to the rivers will decrease from 3277 × 10{sup 4} m{sup 3}/year in 2017 to 1857 × 10{sup 4} m{sup 3}/year in 2021, and to 510 × 10{sup 4} m{sup 3}/year by 2036; thus, the hydrology will switch from groundwater discharge to groundwater recharge after implementing the water diversion project. The simulation indicates that the increased net river infiltration due to the water diversion project will raise the water table and then effectively increasing the water level of the Crescent Lake, as the lake level is contiguous with the water table. However, the regional phreatic evaporation will be enhanced, which may intensify soil salinization in the Dunhuang Basin. These results can guide the water allocation scheme for the water diversion project to alleviate groundwater depletion and mitigate geo-environmental problem.

58 GEOSCIENCES↗

DOE OSTI2018

Distribution and migration mechanism of fluoride in groundwater in the Manas River Basin, Northwest China

Elevated fluoride (F) concentration in groundwater is posing a public health risk in the Manas River Basin (MRB), Northwest China. Based on the characterization of regional groundwater flow, 90 groundwater samples from aquifers were analyzed, along with top-soil leachate and pore-water samples from aquitards. Stable oxygen (δ{sup 18}O) and hydrogen isotopes, radiocarbon and hydrochemical analyses of the groundwater and pore-water samples were conducted to trace groundwater hydrological and hydrochemical processes and thereby understand the distribution and migration mechanism of F. The groundwater is recharged by meteoric precipitation through vapor condensation processes in the Tianshan Mountains. The F concentration in groundwater samples from this basin ranged from 0.11 to 48.15 mg/L (mean 2.56 mg/L). In 37 of the 90 groundwater samples, the F concentrations were above the safe level for drinking water. The F concentrations progressively increased with the residence time and well depths in the northwest of the alluvial-fluvial plain, where groundwater is overexploited for agricultural and domestic use. Positive correlations between F and sodium (Na)/calcium (Ca) indicate that the enrichment and migration of F are influenced by cation exchange processes under high-Na and alkaline pH conditions. The relationships between δ{sup 18}O and F and chloride (Cl) concentrations were nonlinear due to leaching and mixing processes. This shows that vertical leaching by irrigation return flow and mixing with pore water are the dominant processes driving the migration of F in the groundwater flow system of MRB, in addition to geochemical processes.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2016

Rare earth element behavior during groundwater–seawater mixing along the Kona Coast of Hawaii

Groundwater and seawater samples were collected from nearshore wells and offshore along the Kona Coast of the Big Island of Hawaii to investigate rare earth element (REE) behavior in local subterranean estuaries. Previously we saw that submarine groundwater discharge (SGD) is the predominant flux of terrestrial waters to the coastal ocean along the arid Kona Coast of Hawaii. Groundwater and seawater samples were filtered through 0.45 μm and 0.02 μm pore-size filters to evaluate the importance of colloidal and soluble (i.e., truly dissolved ionic species and/or low molecular weight [LMW] colloids) fractions of the REEs in the local subterranean estuaries. Mixing experiments using groundwater collected immediately down gradient from a wastewater treatment facility (WWTF) proximal to the Kaloko-Hanokohau National Historic Park, and more “pristine” groundwater from a well constructed in a lava tube at Kiholo Bay, were conducted with local seawater to study the effect of solution composition (i.e., pH, salinity) on the concentrations and fractionation behavior of the REEs as groundwater mixes with seawater in Kona Coast subterranean estuaries. The mixed waters were also filtered through 0.45 or 0.02 μm filters to ascertain the behavior of colloidal and soluble fractions of the REEs across the salinity gradient in each mixing experiment. Concentrations of the REEs were statistically identical (two-tailed Student t-test, 95% confidence) between the sequentially filtered sample aliquots, indicating that the REEs occur as dissolved ionic species and/or LMW colloids in Kona Coast groundwaters. The mixing experiments revealed that the REEs are released to solution from suspended particles or colloids when Kona Coast groundwater waters mix with local seawater. The order of release that accompanies increasing pH and salinity follows light REE (LREE) > middle REE (MREE) > heavy REE (HREE). Release of REEs in the mixing experiments is driven by decreases in the free metal ion activity in solution and the concomitant increase in the amount of each REE that occurs in solution as dicarbonato complexes [i.e., Ln(CO 3 ) 2 - ] as pH increases across the salinity gradient. Input-normalized REE patterns of Kona Coast groundwater and coastal seawater are nearly identical and relatively flat compared to North Pacific seawater, indicating that SGD is the chief source of these trace elements to the ocean along the Kona Coast. Additionally, REE concentrations of the coastal seawater are between 10 and 50 times higher than previously reported open-ocean seawater values from the North Pacific, further demonstrating the importance of SGD fluxes of REEs to these coastal waters. Altogether, these observations indicate that large-scale removal of REEs, which characterizes the behavior of REEs in the low salinity reaches of many surface estuaries, is not a feature of the subterranean estuary along the Kona Coast. A large positive gadolinium (Gd) anomaly characterizes groundwater from the vicinity of the WWTF. The positive Gd anomaly can be traced to the coastal ocean, providing further evidence of the impact of SGD on the coastal waters. Estimates of the SGD fluxes of the REEs to the coastal ocean along the Kona Coast (i.e., 1.3–2.6 mmol Nd day -1 ) are similar to recent estimates of SGD fluxes of REEs along Florida’s east coast and to Rhode Island Sound, all of which points to the importance of SGD as significant flux of REEs to the coastal ocean.

58 GEOSCIENCES↗

DOE OSTI2016

An ecology-oriented exploitation mode of groundwater resources in the northern foot of Tianshan Mountain, China

In recent years, ecological degradation caused by irrational groundwater exploitation has been of growing concern in arid and semiarid regions. To address the groundwater-ecological issues, this paper proposes a groundwater-resource exploitation mode to evaluate the tradeoff between groundwater development and ecological environment in the northern Tianshan Mountains, northwest China’s Xinjiang Uygur Autonomous Region. Field surveys and remote sensing studies were conducted to analyze the relation between the distribution of hydrological conditions and the occurrence of ecological types. The results show that there is a good correlation between groundwater depth and the supergene ecological type. Numerical simulations and ecological assessment models were applied to develop an ecology-oriented exploitation mode of groundwater resources. The mode allows the groundwater levels in different zones to be regulated by optimizing groundwater exploitation modes. The prediction results show that the supergene ecological quality will be better in 2020 and even more groundwater can be exploited in this mode. This study provides guidance for regional groundwater management, especially in regions with an obvious water scarcity.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2014

Modeling the Effects of Groundwater-fed Irrigation on Terrestrial Hydrology over the Conterminous United States

Human alteration of the land surface hydrologic cycle is substantial. Recent studies suggest that local water management practices including groundwater pumping and irrigation could significantly alter the quantity and distribution of water in the terrestrial system, with potential impacts on weather and climate through land-atmosphere feedbacks. In this study, we incorporated a groundwater withdrawal scheme into the Community Land Model version 4 (CLM4). To simulate the impact of irrigation realistically, we calibrated the CLM4 simulated irrigation amount against observations from agriculture census at the county scale over the conterminous United States (CONUS). The water used for irrigation was then removed from the surface runoff and groundwater aquifer according to a ratio determined from the county-level agricultural census data. Based on the simulations, the impact of groundwater withdrawals for irrigation on land surface and subsurface fluxes were investigated. Our results suggest that the impacts of irrigation on latent heat flux and potential recharge when water is withdrawn from surface water alone or from both surface and groundwater are comparable and local to the irrigation areas. However, when water is withdrawn from groundwater for irrigation, greater effects on the subsurface water balance were found, leading to significant depletion of groundwater storage in regions with low recharge rate and high groundwater exploitation rate. Our results underscore the importance of local hydrologic feedbacks in governing hydrologic response to anthropogenic change in CLM4 and the need to more realistically simulate the two-way interactions among surface water, groundwater, and atmosphere to better understand the impacts of groundwater pumping on irrigation efficiency and climate.

conterminous↗

DOE OSTI2025

Long-term hydro-economic analysis tool for evaluating global groundwater cost and supply: Superwell v1.1

Abstract. Groundwater plays a key role in meeting water demands, supplying over 40 % of irrigation water globally, with this role likely to grow as water demands and surface water variability increase. A better understanding of the future role of groundwater in meeting sectoral demands requires an integrated hydro-economic evaluation of its cost and availability. Yet substantial gaps remain in our knowledge and modeling capabilities related to groundwater availability, recharge, feasible locations for extraction, extractable volumes, and associated extraction costs, which are essential for large-scale analyses of integrated human–water system scenarios, particularly at the global scale. To address these needs, we developed Superwell, a physics-based groundwater extraction and cost accounting model that operates at sub-annual temporal and at the coarsest 0.5° (≈50 km × 50 km) gridded spatial resolution with global coverage. The model produces location-specific groundwater supply–cost curves that provide the levelized cost to access different quantities of available groundwater. The inputs to Superwell include recent high-resolution hydrogeologic datasets of permeability, porosity, aquifer thickness, depth to water table, recharge, and hydrogeological complexity zones. It also accounts for well capital and maintenance costs, as well as the energy costs required to lift water to the surface. The model employs a Theis-based scheme coupled with an amortization-based cost accounting formulation to simulate groundwater extraction and quantify the cost of groundwater pumping. The result is a spatiotemporally flexible, physically realistic, economics-based model that produces groundwater supply–cost curves. We show examples of these supply–cost curves and the insights that can be derived from them across a set of scenarios designed to explore model outcomes. The supply–cost curves produced by the model show that most (90 %) nonrenewable groundwater in storage globally is extractable at costs lower than USD 0.57 m−3, while half of the volume remains extractable at under USD 0.108 m−3. The global unit cost is estimated to range from a minimum of USD 0.004 m−3 to a maximum of USD 3.971 m−3. We also demonstrate and discuss examples of how these cost curves could be used by linking Superwell's outputs with other models to explore coupled human–environmental system challenges, such as water resources planning and management, or broader analyses of multisectoral feedbacks.

Global Change Analysis Model (GCAM)↗

NASA NTRS2019

Application of GRACE to the Estimation of Groundwater Storage Change in a Data-poor Region: A Case Study of Ngadda Catchment in the Lake Chad Basin

The present study is to explore the feasibility of GRACE-based estimation of a groundwater storage change in a data-poor region using a case study of the Ngadda catchment in the Lake Chad Basin. Although the Ngadda catchment has only one set of in situ time series data of groundwater from 2006 to 2009 and a limited number of groundwater measurements in 2005 and 2009, GRACE-based groundwater storage change can be evaluated against thein situ groundwater measurements combined with specific yield data. The cross-correlation analysis in the Ngadda catchment shows that maximum rainfall reached in July and August, whereas both the maximum total water storage anomaly and the maximum groundwater storage anomaly occurred 2months later. Whereas the mean annual amplitude of total water storage anomaly is about 17cm from both the average total water storage anomaly from three mascon products and the one from three spherical harmonic products, the mean annual amplitude of soil moisture storage anomaly is substantially varied from5.58cm for CLM to about 14cm for NOAH and Mosaic. The goodness-of-fit tests show that CLM soil moisture produces the closest estimation of groundwater storage anomaly to thein situ groundwater measurements. The present study shows that GRACE-based estimation for groundwater storage anomaly can be a cost-effective and alternative tool to observe how groundwater changes in a basin scale under the limitation of modelling and in situ data availability.

GRACE↗

DOE OSTI2019

A Transdisciplinary Approach to Characterize Hydrological Controls on Groundwater-Dependent Ecosystem Health

Sustainable groundwater management provides an opportunity for environmental water needs to be considered and secured by establishing appropriate groundwater thresholds. Ecosystems that require access to groundwater for some or all their water requirements are referred to as groundwater dependent ecosystems (GDEs). However, large data gaps often exist around the cause-and-effect relationships between groundwater conditions and the impacts they have on GDEs. These data gaps are largely attributed to a lack of shallow monitoring wells near GDEs, and a lack of practical biological metrics to characterize ecosystem health. This transdisciplinary study explores the use of geophysics (electrical resistivity tomography) to fill in our understanding of shallow subsurface soil-hydrological conditions within GDEs. In addition, we develop an approach to characterize ecosystem health within GDEs, using groundwater-dependent vegetation (phreatophytes) as indicators. Ten vegetation variables were used to characterize six biological indicators—growth, diversity, recruitment, structure, native plant dominance, and survivorship—which were used to infer ecosystem health conditions. Health indicators for groundwater-dependent vegetation were found to directly correlate with subsurface conditions, where greater groundwater availability (higher soil moisture content and shallower groundwater levels) was associated with “healthier” vegetation. This study provides a new approach to integrate hydrological, geophysical, and biological data to strengthen monitoring programs and inform water resource management decisions.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2016

Estimating groundwater dynamics at a Colorado River floodplain site using historical hydrological data and climate information

Long-term prediction of groundwater dynamics is important for assessing water resources and their impacts on biogeochemical cycling. However, estimating future groundwater dynamics is challenging due to the wide range of spatiotemporal scales in hydrological processes and uncertainty in future climate conditions. In this study, we develop a Bayesian model to combine small-scale historical hydrological data with large-scale climate information to estimate groundwater dynamics at a floodplain site in Rifle, Colorado. Although we have only a few years of groundwater elevation measurements, we have 47 years of streamflow data from a gaging station approximately 43 km upstream and long-term climate prediction on the Upper Colorado River Basin. To estimate future daily groundwater dynamics, we first develop a time series model to downscale the monthly streamflow derived from climate information to daily streamflow, and then transform the daily streamflow to groundwater dynamics at the downstream floodplain site. We use Monte Carlo methods to estimate future groundwater dynamics at the site through sampling from the joint posterior probability distribution. The results suggest that although future groundwater levels are expected to be similar to the current levels, the timing of the high groundwater levels is predicted to occur about 1 month earlier. The developed framework is extendable to other sites to estimate future groundwater dynamics given disparate data sets and climate projections. Additionally, the obtained estimates are being used as input to a site-specific watershed reactive transport models to predict how climate-induced changes will influence future biogeochemical cycling relevant to a variety of ecosystem services.

58 GEOSCIENCES↗

DOE OSTI2018

Importance of river water recharge to the San Joaquin Valley groundwater system

Groundwater is not a sustainable resource, unless abstraction is balanced by recharge. Identifying the sources of recharge in a groundwater basin is critical for sustainable groundwater management. In this paper, we studied the importance of river water recharge to groundwater in the south-eastern San Joaquin Valley (24,000 km 2 , population 4 million). We combined dissolved noble gas concentrations, stable isotopes, tritium, and carbon-14 analyses to analyse the sources, mechanisms, and timescales of groundwater recharge. Area-representative groundwater sampling and numerical model input data enabled a stable isotope mass balance and quantitative estimates of river and local recharge. River recharge, identified by a lighter stable isotope signature, represents 47 ± 4% of modern groundwater in the San Joaquin Valley (recharged after 1950) but only 26 ± 4% of premodern groundwater (recharged before 1950). This implies that the importance of river water recharge in the San Joaquin valley has nearly doubled and is likely the result of a 40% increase in total recharge, caused by river water irrigation return flows and increased stream depletion and river recharge due to groundwater pumping. Compared with the large and long-duration capacity for water storage in the subsurface, storage of water in rivers is limited in time and volume, as evidenced by cold river recharge temperatures resulting from fast infiltration and recharge. Finally, groundwater banking of seasonal surface water flows and expansion of managed aquifer recharge practices therefore appear to be a natural and promising method for increasing the resilience of the San Joaquin Valley water supply system.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2018

Assessment and uncertainty analysis of groundwater risk

Highlights: • Two different sets of index evaluation systems for shallow and deep groundwater were built respectively. • Ordinary kriging was adopted to execute the interpolation estimation. • The synthetic weights were adopted merging the AHP method and PCA method. • The uncertainty of groundwater risk was analyzed by indicator kriging. Groundwater with relatively stable quantity and quality is commonly used by human being. However, as the over-mining of groundwater, problems such as groundwater funnel, land subsidence and salt water intrusion have emerged. In order to avoid further deterioration of hydrogeological problems in over-mining regions, it is necessary to conduct the assessment of groundwater risk. In this paper, risks of shallow and deep groundwater in the water intake area of the South-to-North Water Transfer Project in Tianjin, China, were evaluated. Firstly, two sets of four-level evaluation index system were constructed based on the different characteristics of shallow and deep groundwater. Secondly, based on the normalized factor values and the synthetic weights, the risk values of shallow and deep groundwater were calculated. Lastly, the uncertainty of groundwater risk assessment was analyzed by indicator kriging method. The results meet the decision maker's demand for risk information, and overcome previous risk assessment results expressed in the form of deterministic point estimations, which ignore the uncertainty of risk assessment.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2015

Determination of the origin of dissolved inorganic carbon in groundwater around a reclaimed landfill in Otwock using stable carbon isotopes

Highlights: • Research showed the origin of DIC in the groundwater around a reclaimed landfill. • Carbon isotope was used to evaluate the contributions of carbon from different sources. • The leachate-contaminated water was isotopically distinct from the natural groundwater. • DIC in the natural groundwater comes from organic matter and dissolution of carbonates. • In the contaminated water, DIC comes from organic matter in the aquifer and landfill. - Abstract: Chemical and isotopic analyses of groundwater from piezometers located around a reclaimed landfill in Otwock (Poland) were performed in order to trace the origin of dissolved inorganic carbon (DIC) in the groundwater. Due to differences in the isotopic composition of carbon from different sources, an analysis of stable carbon isotopes in the groundwater, together with the Keeling plot approach and a two-component mixing model allow us to evaluate the relative contributions of carbon from these sources in the groundwater. In the natural (background) groundwater, DIC concentrations and the isotopic composition of DIC (δ{sup 13}C{sub DIC}) comes from two sources: decomposition of organic matter and carbonate dissolution within the aquifer sediments, whereas in the leachate-contaminated groundwater, DIC concentrations and δ{sup 13}C{sub DIC} values depend on the degradation of organic matter within the aquifer sediments and biodegradation of organic matter stored in the landfill. From the mixing model, about 4–54% of the DIC pool is derived from organic matter degradation and 96–46% from carbonate dissolution in natural conditions. In the leachate-contaminated groundwater, about 20–53% of the DIC is derived from organic matter degradation of natural origin and 80–47% from biodegradation of organic matter stored in the landfill. Partial pressure of CO{sub 2} (P CO{sub 2}) was generally above the atmospheric, hence atmospheric CO{sub 2} as a source of carbon in DIC pool was negligible in the aquifer. P CO{sub 2} values in the aquifer in Otwock were always one to two orders of magnitude above the atmospheric P CO{sub 2}, and thus CO{sub 2} escaped directly into the vadose zone.

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