Aaron R. Mittelstet

• Doctor of Philosophy
• Professor (Associate) at University of Nebraska–Lincoln

Nitrate contamination of groundwater is a growing concern in east-central Nebraska. This study investigates nitrogen transport through the vadose zone and its impact on groundwater quality in Nebraska' Lower Loup Natural Resources District (LLNRD). The project analyzed data collected from 20 soil cores (16 shallow and 4 deep cores) ranging from 4m...

Splays—fan‐shaped depositional landforms produced by overbank deposition by unconfined flows—can damage structures, degrade arable land and incur substantial mitigation costs. Splay‐related hazards along many rivers are likely to worsen with the increasing magnitude and frequency of major floods. The highly incomplete understanding of splays on bra...

Urban and storm water retention ponds eventually become eutrophic after years of receiving runoff water. In 2020, a novel biological and chemical treatment was initiated to remove accumulated nutrients from an urban retention pond that had severe algae and weed growth. Our approach installed two 6.1 m × 6.1 m floating treatment wetlands (FTWs) and...

Characterizing the 3D distribution of hydraulic properties in glacial sediments is challenging due to fine‐scale heterogeneity and complexity. Borehole lithological data provide high vertical resolution but low horizontal resolution. Geophysical methods can fill gaps between boreholes, providing improved horizontal resolution but low vertical resol...

Imagery from fixed, ground-based cameras is rich in qualitative and quantitative information that can improve stream discharge monitoring. For instance, time-lapse imagery may be valuable for filling data gaps when sensors fail and/or during lapses in funding for monitoring programs. In this study, we used a large image archive (>40,000 images from...

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 observe...

Highlights Treatment improved water quality at Densmore Pond but not Wilderness Ridge Pond. Macroinvertebrate populations showed context-dependent patterns from FTW treatment. FTWs acted as refugia for ponds with eutrophic conditions. Macroinvertebrate predator species diversification increased at Densmore Pond. Abstract. In urban landscapes, stor...

A new approach for measuring fluxes across surface water—groundwater interfaces was recently proposed. The Automatic Seepage Meter (ASM) is equipped with a precise water level sensor and digital memory that analyzes water level time series in a vertical tube inserted into a streambed (Solomon et al., 2020, https://doi.org/10.1029/2019WR026983 ). Th...

Highlights High rates of streambank erosion are detrimental to the stability and function of streams. This collection brings together six studies that represent key advances in streambank erosion research. Current research directions on streambank erosion, erodibility characterization, and sediment loading are presented. Future research directions...

The Nebraska Sand Hills (NSH) is considered a major recharge zone for the High Plains Aquifer in the central United States. The uncontrolled expansion of the eastern redcedar ( Juniperus Virginiana ) under climate warming is posing threats to surface water and groundwater resources. The combined impact of land use and climate change on the water ba...

Worldwide, tree or shrub dominated woodlands have encroached into herbaceous dominated grasslands. While very few studies have evaluated the impact of Eastern Redcedar (redcedar) encroachment on the water budget, none have analyzed the impact on water quality. In this study, we evaluated the impact of redcedar encroachment on the water budget in th...

Efforts to improve water quality of eutrophic ponds often involve implementing changes to watershed management practices to reduce external nutrient loads. While this is required for long-term recovery and prevention, eutrophic conditions are often sustained through the recycling of internal nutrients already present within the waterbody. In partic...

China established the Giant Panda National Park for ecological and species protection. New management requirements will impact the socioeconomic sustainability of the Baoxing River watershed, the core area of the national park. In this study, the Soil and Water Assessment Tool model was used to explore future temporal distribution and magnitude of...

Image‐based water level measurements offer data quality assurance through visual verification that no other method can provide. GaugeCam Remote Image Manager‐Educational 2 (GRIME2) is a mature, open‐source commercial friendly software application that automatically detects and measures water level in laboratory and field settings. The software reli...

We utilized 251 measurements from a recently developed automated seepage meter (ASM) in streambeds in the Nebraska Sand Hills, USA to investigate the small‐scale spatial variability of groundwater seepage flux (q) and the ability of the ASM to estimate mean q at larger scales. Small‐scale spatial variability of q was analyzed in five dense arrays,...

Stream reach mass-balance can be used to estimate groundwater age at scales larger than individual wells. Atmospherically derived SF6 was used as an age-dating tracer in reach mass-balance analysis of six stream reaches in Nebraska, USA. The goal was to estimate the flow-weighted mean SF6 concentration in the groundwater discharge to each reach (Cg...

Stabilization projects are increasingly used to mitigate the effects of anthropogenic streambank erosion, yet the effectiveness of stabilization has been insufficiently measured. Sound monitoring practices inform adjustments in implementation and maintenance, which improve engineered effectiveness. Thus, the objectives of this study were to: 1) mea...

Study region The Nebraska Sand Hills (NSH) lies in the western part of Nebraska, United States. We chose the north-eastern, central, and western parts of NSH with distinct climate, topography, and hydrology. Study focus The study assesses the response of hundreds of shallow groundwater-fed lakes to drought. Total lake area (TLA), determined by cla...

Data‐driven models used for predicting soil temperature usually have increasing errors with increasing depth. By exploring the integration of knowledge‐based and machine learning approaches, this study used a novel transformation of meteorological variables to increase prediction accuracy of soil temperature with increasing soil depth. Using datase...

The spatial variability of groundwater levels is often inferred from sparsely located hydraulic head observations in wells. The spatial correlation structure derived from sparse observations is associated with uncertainties that spread to estimates at unsampled locations. In areas where surface water represents the nearby groundwater level, remote...

There is a need to evaluate high surface nitrate concentrations across agricultural watersheds, both spatially and temporally, to increase understanding of source and timing of nitrogen loads in streams and rivers. Bazile Creek is a high-nitrate stream originating in the agriculturally intensive Bazile Groundwater Management Area of Eastern Nebrask...

In this study, we explored the use of statistical machine learning and long-term groundwater nitrate monitoring data to estimate vadose zone and saturated zone lag times in an irrigated alluvial agricultural setting. Unlike most previous statistical machine learning studies that sought to predict groundwater nitrate concentrations within aquifers,...

Excess nutrients and herbicides remain two major causes of waterbody impairment globally. In an attempt to better understand pollutant sources in the Big Sandy Creek Watershed (BSCW) and the prospects for successful remediation, a program was initiated to assist agricultural producers with the implementation of best management practices (BMPs). The...

Highlights A sediment monitoring system was developed to measure erosion and deposition. The system uses an ultrasonic sensor to take high-frequency measurements. Tests in controlled lab settings showed high accuracy. The system measured 43.5 cm of deposition from two storm events. Abstract . Monitoring erosion at high temporal resolution can prov...

Highlights Stabilization structures are only effective at stabilized segments. Erosion increased in two of the six segments in the post-stabilization period. Deposition decreased in all six segments in the post-stabilization period. Jetties are effective at reducing erosion but are also prone to fail. Abstract . The effectiveness of streambank sta...

Time-lapse imagery of streams and rivers provide new qualitative insights into hydrologic conditions at stream gauges, especially when site visits are biased toward baseflow or fair-weather conditions. Imagery from fixed, ground-based cameras is also rich in quantitative information that can improve streamflow monitoring. For instance, time-lapse i...

Neonicotinoids have been the most commonly used insecticides since the early 1990s. Despite their efficacy in improving crop protection and management, these agrochemicals have gained recent attention for their negative impacts on non-target species such as honeybees and aquatic invertebrates. In recent years, neonicotinoids have been detected in r...

Currently little is known of newer pesticide classes and their occurrence and persistence in recreational lakes. Therefore, the objectives of this study were to (1) assess average pesticide concentrations and loadings entering recreational lakes in three distinct watersheds throughout the growing season, (2) evaluate pesticide persistence longitudi...

The robustness of groundwater vulnerability assessment using the conventional DRASTIC is highly dependent on the quality of the input data. The current study investigates the vulnerability of the western part of the coastal aquifer of the Gaza Strip, part of the Palestinian Territories, towards the southern-western part of Gaza coastal aquifer. The...

This study evaluated the spatial variability of streambed vertical hydraulic conductivity (K v ) in different stream morphologies in the Frenchman Creek watershed, western Nebraska, using different variogram models. Streambed K v values were determined in‐situ using permeameter tests at ten sites in Frenchman, Stinking Water and Spring Creeks durin...

Abstract. In this study, we explored the use of statistical machine learning and long-term groundwater nitrate monitoring data to estimate vadose-zone and saturated-zone lag times in an irrigated alluvial agricultural setting. Unlike most previous statistical machine learning studies that sought to predict groundwater nitrate concentrations within...

A wide variety of antibiotics and other pharmaceuticals are used in livestock production systems and residues passed to the environment, often unmetabolized, after use and excretion. Antibiotic residues may be transported from manure-treated soils via runoff and are also capable of reaching surface and groundwater systems through a variety of pathw...

Accurate prediction of Escherichia coli contamination in surface waters is challenging due to considerable uncertainty in the physical, chemical and biological variables that control E. coli occurrence and sources in surface waters. This study proposes a novel approach by integrating hydro-climatic variables as well as animal density and grazing pa...

Streambeds are critical hydrological interfaces: their physical properties regulate the rate, timing, and location of fluxes between aquifers and streams. Streambed vertical hydraulic conductivity (Kv) is a key parameter in watershed models, so understanding its spatial variability and uncertainty is essential to accurately predicting how stresses...

The 2010 dam breach and consequent anomalous flood event on the Cedar River in Nebraska, USA provided an opportunity to study the following objectives: (1) evaluate the impact of an extreme flood event on streambank retreat along a 45 km stretch relative to the average annual retreat; (2) quantify the changes in streambank retreat for each km segme...

The sustainability of groundwater resources for agricultural and domestic use is dependent on both the groundwater recharge rate and the groundwater quality. The main purpose of this study was to improve the understanding of the timing, or seasonality, of groundwater recharge through the use of stable isotopes. Based on 768 groundwater samples coll...

Groundwater discharge though streambeds is often focused toward discrete zones, indicating that preliminary reconnaissance may be useful for capturing the full spectrum of groundwater discharge rates using point-scale quantitative methods. However, many direct-contact reconnaissance techniques can be time-consuming, and remote sensing (e.g., therma...

Recent pathogenic Escherichia coli contamination of fresh vegetables that originated from irrigation water has increased awareness and importance of identifying sources of E. coli entering agroecosystems. However, inadequate methods for accurately predicting E. coli occurrence and sources in waterways continue to limit the identification of appropr...

Atrazine, one of the most widely used herbicides in the world, threatens human health along with terrestrial and aquatic biota. Recent reports have found atrazine in drinking water to be associated with increased birth defects and incidences of Non-Hodgkin's Lymphoma, with higher levels of significance from exposure to both atrazine and nitrate-N....

Nebraskan streams contribute excess nitrogen to the Mississippi/Atchafalaya River Basin and Gulf of Mexico, which results in major water-quality impairments. Reducing the amount of nitrogen (N) exported in these streams requires the use of best management practices (BMPs) within the landscape. However, proper BMP utilization has rarely been statist...

Highlights A floating treatment wetland design was evaluated for water quality improvements. Nitrate-N removal rates were quantified using spent coffee grounds as a carbon source. Nitrate-N removal rates increased throughout the growing season Abstract. The Midwestern U.S. is vulnerable to eutrophic conditions from high nutrient concentrations. Flo...

Groundwater is vital worldwide for water supply, agriculture and industry. Nearly 60% of all water use in Nebraska is from groundwater. Over 90% of groundwater is used for irrigation in Nebraska, which has the largest area of irrigated land in the United States. Many Nebraskans depend on groundwater for drinking water, both from private wells and m...

In the Great Plains of the central United States, water resources for human and aquatic life rely primarily on surface runoff and local recharge from rangelands that are under rapid transformation to woodland by the encroachment of Eastern redcedar (redcedar; Juniperus virginiana) trees. In this synthesis, the current understanding and impact of re...

Shallow aquifers are prone to nitrate contamination worldwide. In western Nebraska, high groundwater nitrate concentrations ([NO3⁻]) have resulted in the exploration of new groundwater and nitrogen management regulations in the North Platte Natural Resources District (NPNRD). A small region of NPNRD (“Dutch Flats”) was the focus of intensive ground...

This study compared three approaches, regional averaging, nearest neighbor, and donor techniques, to regionalize parameters in the Soil and Water Assessment Tool (SWAT) on eleven watersheds located in the Dissected Plains, Plains, and Rolling Hills Landforms in the eastern portion of the State of Nebraska, USA. Within the Rolling Hills Landform, th...

Core Ideas Numerical models were calibrated with in situ infiltration and leaching data. We evaluated how increasing model complexity improves predictions. Critical parameters included field‐measured hydraulic conductivity and dispersivity. A one‐dimensional dual‐porosity model was adequate for long‐term simulations. Phosphorus and N are critical...

Excessive sediment is a major pollutant to surface waters worldwide. In some watersheds, streambanks are a significant source of this sediment, leading to the expenditure of billions of dollars in stabilization projects. Although costly streambank stabilization projects have been implemented worldwide, long-term monitoring to quantify their success...

A wide range of mathematical models are available for predicting phosphorus (P) losses from agricultural fields, ranging from simple, empirically based annual time‐step models to more complex, process‐based daily time‐step models. In this study, we compare field‐scale P‐loss predictions between the Annual P Loss Estimator (APLE), an empirically bas...

Due to a shortage of available phosphorus (P)‐loss datasets, simulated data from an accurate quantitative P transport model could be used to evaluate a P Index. The objective of this study was to compare predictions from the Texas Best Management Practice Evaluation Tool (TBET) against measured P‐loss data to determine whether the model could be us...

The hydrologic function of riverbeds is greatly dependent upon the spatiotemporal distribution of hydraulic conductivity and grain size. Vertical hydraulic conductivity (Kv) is highly variable in space and time, and controls the rate of stream–aquifer interaction. Links between sedimentary processes, deposits, and Kv heterogeneity have not been wel...

Study region: Rwanda is a landlocked country in Africa with precipitation ranging from 800 mm yr−1 in the east to 1500 mm yr−1 in high-altitude regions in the north and west. Study focus: Streamflow estimation is an important task that is required in water resource assessments due to its importance in planning, decision-making and economic developm...

Truncated distributions of pelagophilic fishes have been observed across the Great Plains of North America, with water use and landscape fragmentation implicated as contributing factors. Developing conservation strategies for these species is hindered by the existence of multiple competing flow regime hypotheses related to species persistence. Our...

Streambanks can be a significant source of sediment and phosphorus to aquatic ecosystems. Although the streambank-erosion routine in the Soil and Water Assessment Tool (SWAT) has improved in recent versions, the recently developed routine in SWAT 2012 has undergone limited testing, and the lack of site or watershed specific streambank data increase...

In some watersheds, streambanks are a source of two major pollutants, phosphorus (P) and sediment. P originating from both uplands and streambanks can be transported and stored indefinitely on floodplains, streambanks, and in closed depressions near the stream. The objectives of this study were to (1) test the modified streambank erosion and instre...

Wetlands and coastal systems are unique, highly productive, and often threatened landscapes that provide a host of services to both humans and the environment. This article introduces a five-article Wetlands and Coastal Systems Spe-cial Collection that evolved from a featured session at the 2015 ASABE Annual International Meeting in New Orleans, Lo...

Classic agricultural-conservation practices may not address decades of phosphorus (P) accumulation, known as legacy P. Identifying and quantifying legacy P sources are necessary to identify the most cost-efficient conservation practices. A method was developed to identify and quantify legacy P at the watershed scale using a mass-balance approach an...

This paper compares the water rights of several states in the southern United States. Each state differs in the quantity of precipitation they receive and the method they appropriate their surface and ground water.

The number of states that have adopted numeric nutrient water-quality standards has increased to 23, up from ten in 1998. One state with both stream and reservoir phosphorus (P) numeric water- quality standards is Oklahoma. There were two primary objectives of this research: (1) determine if Oklahoma was meeting the stream and reservoir numeric wat...

Surface water bodies can be impaired by turbidity and excessive sediment loading due to urban development, construction activities, and agricultural practices. Turbidity has been considered as a proxy for evaluating water quality, aquatic habitat, and aesthetic impairments in surface waters. The US Environment Protection Agency (USEPA) has listed t...

Crop yields and salinity levels in the North Fork of the Red River (North Fork River) basin, located in southwestern Oklahoma and the Texas Panhandle, were analyzed based on the diverse climate in the region. Saline irrigation water is a major problem in the basin. The Elm Fork Creek flows through salt mines, making the creek and its receiving stre...

Crop yields and salinity levels in the North Fork of the Red River (North Fork River) basin, located in southwestern Oklahoma and the Texas Panhandle, were analyzed based on the diverse climate in the region. Saline irrigation water is a major problem in the basin. The Elm Fork Creek flows through salt deposits, making the creek and its receiving s...

The floodplains of many gravel-bed streams have a general stratigraphy that consists of a layer of topsoil covering gravel-dominated subsoil. Previous research has demonstrated that this stratigraphy can facilitate preferential groundwater flow through focused linear features, such as paleochannels, or gravelly regions within the vadose zone. These...

Assuming homogeneity in alluvial aquifers is convenient, but limits our ability to accurately predict stream-aquifer interactions. Research is needed on (i) identifying the presence of focused, as opposed to diffuse, groundwater discharge/recharge to streams and (ii) the magnitude and role of large-scale bank and transient storage in alluvial flood...

Streams throughout the North Canadian River watershed in northwest Oklahoma, USA have elevated levels of nutrients and sediment. Soil and Water Assessment Tool (SWAT) was used to identify areas that likely contributed disproportionate amounts of Phosphorus (P) and sediment to Lake Overholser, the receiving reservoir at the watershed outlet. These s...

High streambank erosion and failure rates on streams in the Ozark ecoregion of Oklahoma may be attributed to land use change and degradation of riparian areas. Numerous benefits may be achieved from streambank stabilization, but methods are needed to determine the most critical reaches for investing limited funds. Rapid geomorphic assessments (RGAs...

The Soil and Water Assessment Tool is widely used to predict the fate and transport of phosphorus (P) from the landscape through streams and rivers. The current in-stream P submodel may not be suitable for many stream systems, particularly those dominated by attached algae and those affected by point sources. In this research, we developed an alter...

Estimating future flow and salinity levels within stream systems and crop yields based on future climate conditions is an increasingly important issue. Saline irrigation water is a major concern in the North Fork of the Red River watershed in southwestern Oklahoma. The Elm Fork Creek flows through salt deposits, making the creek and its receiving s...

Models for contaminant transport in streams commonly idealize transient storage as a well mixed but immobile system. These transient storage models capture rapid (near-stream) hyporheic storage and transport, but do not account for large-scale, stage-dependent interaction with the alluvial aquifer. The objective of this research was to document tra...

Highlights ► Streambank seep originated from preferential flow pathway in riparian floodplain. ► Seep flow pathway was a localized zone of relatively clean gravel. ► Pathways may impact water and solute distribution without surfacing at the bank. ► Streambank seepage commenced at 40 cm of hydraulic head and peak at 3.5 L/min. ► Trench injection sys...

Phosphorus (P) loading to streams can occur by both surface runoff and subsurface transport, with subsurface P transport often assumed negligible. Groundwater P concentrations in alluvial aquifers can be significant, especially in preferential flow paths (PFPs). The objectives of this research were to quantify subsurface P transport rates at two si...

Mittelstet, Aaron R., Michael D. Smolen, Garey A. Fox, and Damian C. Adams, 2011. Comparison of Aquifer Sustainability Under Groundwater Administrations in Oklahoma and Texas. Journal of the American Water Resources Association (JAWRA) 1-8. DOI: 10.1111/j.1752-1688.2011.00524.x Abstract: We compared two approaches to administration of groundwater l...

Sediment in stormwater from construction site runoff consists of particles having a variety of sizes. Fine particles such as fine silt, clay, and colloidal particles remain in suspension and in many cases cannot be effectively removed by conventional sediment control practices. The US EPA is targeting fine silt, clay, and colloids in stormwater as...

In the Ozark ecoregion of Oklahoma, Arkansas and Missouri, the erosion of carbonate bedrock (primarily limestone) by slightly acidic water has left a residuum of chert gravel, producing gravel-bed streams and floodplains generally consisting of coarse chert gravel overlain by a mantle (1 to 300 cm) of gravelly loam or silt loam. Previous research h...

While surface runoff is considered to be the primary transport mechanism for phosphorus (P), subsurface transport through coarse subsoil to gravel bed streams may be significant and represent a source of P not alleviated by current conservation practices (e.g., riparian buffers). Previous research has documented P transport in a preferential flow p...

The Ozark region of eastern Oklahoma is characterized by limestone and sandstone plateaus partitioned by steep-sided stream valleys and occupied by clear, gravel-bed streams. Alluvial floodplain soils are generally cherty with high hydraulic conductivities. Many areas have alluvial floodplains consisting of a mantle of alluvial soils overlying thic...

A better understanding of stream-aquifer interactions is needed both for water policy and for quantifying its impact on stream chemistry and water quality. Assuming homogeneity in alluvial aquifers is convenient but limits our understanding of stream-aquifer interactions. Previous research in the Ozark ecoregion, which is characterized by cherty so...

The ground water policies of Oklahoma and the Texas Panhandle Groundwater Conservation District (GCD) were compared to determine their effects on sustainability of an alluvial aquifer. The Oklahoma policy permits each well to withdraw water at a rate projected to allow not more than 50% of wells to go dry in 20 years. The GCD policy permits pumping...