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Groundwater transit time distributions (TTDs) describe the spectrum of flow‐weighted apparent ages of groundwater from aquifer recharge to discharge. Regional‐scale TTDs in stream baseflow are often estimated from numerical models with limited calibration from groundwater sampling and suggest much younger groundwater discharge than has been observed by discrete age‐dating techniques. We investigate both local and regional‐scale groundwater TTDs in the Upper Middle Loup watershed (5,440 km2) overlying the High Plains Aquifer in the Nebraska Sand Hills, USA. We determined flow‐weighted apparent ages of groundwater discharging through the streambed at 88 discrete points along a 99 km groundwater‐dominated stream segment using 3H, noble gases, 14C, and groundwater flux measurements at the point‐scale (<7.6 cm diameter). Points were organized in transects across the stream width (3–10 points per transect) and transects were clustered in five sampling areas (10–610 m in stream length) located at increasing distances along the stream. Groundwater apparent ages ranged from 0 to 8,200 years and the mean groundwater transit time along the 99 km stream is >3,000 years. TTDs from upstream sampling areas were best fit by distributions with a narrow range of apparent ages, but when older groundwater from downstream sampling areas is included, the regional TTD is scale dependent and the distribution is better described by a gamma model (α ≈ 0.4) which accommodates large fractions of millennial‐aged groundwater. Observations indicate: (a) TTDs can exhibit spatial variability within a watershed and (b) watersheds can discharge larger fractions of old groundwater (>1,000 years) than commonly assumed. Plain Language Summary: The transit time distribution (TTD) of an aquifer describes the complete spectrum of groundwater transit times from the water table to discharge at an outlet such as a stream. TTDs indicate how susceptible aquifers may be to contamination, drought, or pumping. Despite their importance, TTDs are difficult to quantify and many approaches either overestimate the relative fraction of young groundwater discharge or have been conducted in small systems (areas <1,000 km2 or stream lengths <10 km). We measured the flow‐weighted transit time of groundwater through the streambed at 88 points along a 99 km stream in the Upper Middle Loup watershed (5,440 km2) in Nebraska, USA to estimate TTDs of the underlying High Plains Aquifer at various spatial scales. Transit times ranged from 0 to 8,200 years and the mean transit time is >3,000 years along the entire study stream with substantially older groundwater discharge downstream. Upstream TTDs were well‐fit by distributions with larger fractions of young groundwater (<15 years), but TTDs for the entire study stream were better fit by distributions that accommodate large fractions of millennial‐aged groundwater. Our findings suggest that TTDs can vary within a watershed and ancient groundwater may be a significant component of modern streamflow. Key Points: Groundwater age and flux along a 99 km stream overlying the High Plains Aquifer, USA yield transit time distributions (TTD) and meanGroundwater ages ranged from 0 to 8,200 years with mean age >3,000 years and increasing apparent age of baseflow downstreamTTD evolution with stream distance suggests streams overlying regional systems may intersect large fractions of millennial‐aged groundwater [ABSTRACT FROM AUTHOR]

Copyright of Hydrogeology Journal is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Summary: A complementary relationship (CR) between potential (ETp ) and actual (ETa ) evapotranspiration is obtained using two independent approaches, and the role of soils in formulating the CR is also investigated. The first approach employs a process-based vadose zone model that links ETp and ETa by explicitly considering the role of soils. To drive the model, hydrometeorological, soil, and remotely sensed physiological data are utilized. The second approach utilizes the products of the MODIS_ET (MOD16) data set that is derived from remotely sensed data by mainly considering above-ground land surface processes. Both methods are applied for 42 sites across Nebraska, USA. The results from both approaches demonstrate the CR; however, according to previously reported ETa and ETp values, MODIS_ET tends to underestimate ETa in wet years and systematically overestimate ETp across Nebraska, which leads the wet surface evapotranspiration (ETw ) to decrease with increasing annual humidity index Φ (i.e., the ratio of annual precipitation over annual ETp ). In comparison, ETw estimated from the vadose zone model is similar to ETw calculated from the Priestley–Taylor equation, and remains relatively constant over a wide range of annual Φ with a mean of 105.1cm/year. Both approaches show that ETw is not sensitive to soil properties, which provides a rationale for combining historical data from different locations to study the impact of climate variability on evapotranspiration processes at annual time scales. [ABSTRACT FROM AUTHOR]

Groundwater beneath the former Nebraska Ordnance Plant (NOP) is contaminated with the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and trichloroethene (TCE). Previous treatability experiments confirmed that permanganate could mineralize RDX in NOP aquifer material. The objective of this study was to determine the efficacy of permanganate to transform RDX in the field by monitoring a pilot-scale in situ chemical oxidation (ISCO) demonstration. In this demonstration, electrical resistivity imaging (ERI) was used to create two-dimensional (2-D) images of the test site prior to, during, and after injecting sodium permanganate. The ISCO was performed by using an extraction-injection well configuration to create a curtain of permanganate. Monitoring wells were positioned downgradient of the injection zone with the intent of capturing the permanganate-RDX plume. Differencing between ERI taken preinjection and postinjection determined the initial distribution of the injected permanganate. ERI also quantitatively corroborated the hydraulic conductivity distribution across the site. Groundwater samples from 12 downgradient wells and 8 direct-push profiles did not provide enough data to quantify the distribution and flow of the injected permanganate. ERI, however, showed that the permanganate injection flowed against the regional groundwater gradient and migrated below monitoring well screens. ERI combined with monitoring well samples helped explain the permanganate dynamics in downgradient wells and support the use of ERI as a means of monitoring ISCO injections. [ABSTRACT FROM AUTHOR]

Mean annual recharge in the Sand Hills of Nebraska (USA) over the 2000-2009 period was estimated at a 1-km spatial resolution as the difference of mean annual precipitation ( P) and evapotranspiration (ET). Monthly P values came from the PRISM dataset, while monthly ET values were derived from linear transformations of the MODIS daytime land-surface temperature values into pixel ET rates with the help of ancillary atmospheric data (air temperature, humidity, and global radiation). The study area receives about 73 mm of recharge (with an error bound of ±73 mm) annually, which is about 14 ± 14% of the regional mean annual P value of 533 mm. The largest recharge rates (about 200 ± 85 mm or 30 ± 12% of P) occur in the south-eastern part of the Sand Hills due to smoother terrain and more abundant precipitation (around 700 mm), while recharge is the smallest (about 40 ± 59 mm or 10 ± 14% of P) in the western part, where annual precipitation is only about 420 mm. Typically, lakes, wetlands, wet inter-dunal valleys, rivers, irrigated crops (except in the south-eastern region) and certain parts of afforested areas in the south-central portion of the study area act as discharge areas for groundwater. [ABSTRACT FROM AUTHOR]

Potentiomanometers (PMs) are commonly used to determine flux directions across interfaces between surface waters and aquifers. We describe a complementary function: estimating small-scale hydraulic conductivity ( K) in a lakebed, using the constant-head injection test (CHIT) by Cardenas and Zlotnik (2003) with the PM designed by Winter et al. (1988). A piezometer with a small screen is inserted into the lakebed. Local head potential is obtained by measuring the head difference between the test point and the aquifer interface. The piezometer is then used for water injection. This technique is illustrated by measurements taken from Alkali Lake in the Sand Hills, Nebraska, United States. Lakebed K and seepage fluxes ranged from 0.037 to 0.090 m/d and Darcy velocities ranged from 0.004 to 0.027 m/d. Results were consistent with the supplementary data gathered using a modified CHIT and a cone penetrometer. The compact size of the device and the small volumes used for injection enable this method to estimate lakebed K values as low as 0.01 to 0.1 m/d, a range seldom explored in lake-aquifer interface systems. [ABSTRACT FROM AUTHOR]

Copyright of Hydrogeology Journal is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Groundwater beneath the former Nebraska Ordnance Plant (NOP) is contaminated with the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The current pump and treat facility is preventing offsite migration but does not offer a short-term solution. Our objective was to quantify the effectiveness of permanganate to degrade RDX in situ. This was accomplished by performing laboratory treatability experiments, aquifer characterization, and a pilot-scale in situ chemical oxidation (ISCO) demonstration. Treatability experiments confirmed that permanganate could mineralize RDX in the presence of NOP aquifer solids. The pilot-scale ISCO demonstration was performed using an extraction-injection well configuration to create a curtain of permanganate between two injection wells. RDX destruction was then quantified as the RDX-permanganate plume migrated downgradient through a monitoring well field. Electrical resistivity imaging (ERI) was used to identify the subsurface distribution of permanganate after injection. Results showed that RDX concentrations temporally decreased in wells closest to the injection wells by 70% to 80%. Observed degradation rates (0.12 and 0.087/d) were lower than those observed under laboratory batch conditions at 11.5 °C (0.20/d) and resulted from lower than projected permanganate concentrations. Both ERI and spatial electrical conductivity measurements verified that permanganate distribution was not uniform throughout the 6.1-m (20 feet) well screens and that groundwater sampling captured both treated and nontreated groundwater during pumping. Although heterogeneous flow paths precluded a uniform permanganate distribution, pilot-scale results provided proof-of-concept that permanganate can degrade RDX in situ and support permanganate as a possible remedial treatment for RDX-contaminated groundwater. [ABSTRACT FROM AUTHOR]

Summary: We investigate the conditions and criteria for identifying flow regimes in shallow groundwater-dominated lakes in arid environments. Relying on regional and local water-table configurations, we develop a steady-state model of lake-aquifer systems. Using dimensional analysis and numerical simulations, we investigate relationships between groundwater inflow and outflow. A new framework uses the concepts of gradient ratio G, defined as the ratio of regional-to-local head gradients, and the topohydrologic offset (THO), defined as the difference between actual lake level and regional groundwater level. It is shown that a critical value of gradient ratio, Gc , exists for each aquifer thickness and anisotropy in hydraulic conductivity. In cases where the regional gradient is larger than the local gradient and G > Gc , the lake is “flushed” in a flow-through regime. When the local gradient is larger than the regional head gradient and G < Gc , the lake gains and large evaporative losses occur. Gc becomes progressively larger with increasing aquifer thickness and anisotropy in hydraulic conductivity. Variation in the ratio of groundwater inflow-to-outflow as G approaches Gc is consistent with observed variability in lake salinity. Transition from flow-through conditions to other regimes in groundwater-dominated lakes in arid environments may occur with subtle changes in the gradient ratio, regional water-table slope, lake size, and THO. Finally, we discuss the feasibility of using a geographic information system to delineate the potential for lake to retain solutes in areas with numerous lakes, similar to Nebraska, USA. [Copyright &y& Elsevier]

Conductivity and major ion chemistry data were analyzed for a suite of Nebraska (USA) natural lakes, reservoirs, sand pits, and barrow pits to evaluate the magnitude of climatic versus non-climatic influence on ionic concentration and composition. In both natural lakes and sand and barrow pits, conductivity is positively related to longitude and reflects decreasing effective moisture from east to west. Reservoirs showed no relationship between lake conductivity and location, probably because the reservoirs are very strongly influenced by groundwater and surface water inflow and have shorter residence times relative to the other lake types. At smaller spatial scales, conductivity among natural lakes is variable. Lakes that are at low elevation within a groundwater flow system were fresh, because of substantial input of fresh groundwater. In contrast, lakes at high elevation exhibited a wide range of conductivity, probably because of differences in the degree of connection to groundwater and surface to volume ratio impacts on evaporation rates. Differences also were evident among natural lakes in terms of their response to seasonal changes in precipitation. Sub-saline and saline lakes showed more seasonal variation in conductivity than freshwater lakes, and lakes in the more arid part of the state showed larger responses to precipitation change than those in areas to the east that receive higher precipitation. [ABSTRACT FROM AUTHOR]

A permeameter was designed and constructed, and its theory of operation was developed; it has a subsurface probe for internal permeability measurements in poorly consolidated sediments. It was used to measure the permeability of sand at five depths in the upper 1.3 m (4.3 ft) of structural features on a large vegetated composite crescentic dune in the Nebraska Sand Hills. Permeability measurements were taken at 34 locations on the dune's steep leeward face, its long windward slope, fans below the leeward face, blowouts, peaks, crests, transverse ridges, benches, and the heads of gullies. Correlation of the permeabilities with the morphology of the dune is illustrated by a plot of the depth-averaged permeabilities and dune profile along a transect perpendicular to the lee face. Some of the permeabilities do not match the values anticipated on the basis of the theory of sedimentation (and experimental results) for active crescentic dunes. Although the large-scale features of the dune studied were evidently because of the original eolian sedimentation, the permeability was found to be strongly influenced by dune degradation processes involving running water and animal activity. [ABSTRACT FROM AUTHOR]

• MTT's ranged from 1 to 397 and 820 to 7968 years for upstream and downstream sites. • TTDs at upstream sites were dominated by young groundwater from shallow flow paths. • TTDs at upstream sites sensitive to riverbed hydraulic conductivity. • Recharge parameterization had greatest influence on the MTTs at downstream sites. Groundwater transit time distributions (TTDs), the travel times through the aquifer from recharge at the water table to discharge at the surface water body, provide critical information on the timescales of hydrologic response of subsurface flow systems. We investigated the effects of spatial patterns of recharge, aquifer heterogeneity, and systematic variation in riverbed hydraulic conductivity on the mean transit times (MTTs) and TTDs of groundwater discharge to the Upper Middle Loup River (UMLR) and headwaters using a 3D-steady-state MODFLOW USG-MODPATHD3DU model. This 5436 km2 watershed overlies the High Plains Aquifer in the Sand Hills of Nebraska, USA. Modeled MTTs differed by up to three orders of magnitude from upstream to downstream in a 158 km section of the UMLR under varying recharge, aquifer, and riverbed heterogeneity scenarios. The simulated MTTs ranged from 1 to 397 years for upstream sites and 820 to 7968 years for the downstream sites. The TTDs at upstream sites were dominated by young groundwater from shallow flow paths and were sensitive to changes in riverbed hydraulic conductivity. Recharge parameterization had greater influence on the shape of the TTDs and magnitude of MTTs at the downstream sites, where much older groundwater discharged to the UMLR. Overall, spatial trends in transit times under varying model scenarios provided important information for refined conceptualization and calibration of future numerical models. [ABSTRACT FROM AUTHOR]