Timothy K. Gates

Assessing selenium contamination in the irrigated stream-aquifer system of the Arkansas River, Colorado.

Prudent interventions for reducing selenium (Se) in groundwater and streams within an irrigated river valley must be guided by a sound understanding of current field conditions. An emerging picture of the nature of Se contamination within the Lower Arkansas River Valley in Colorado is provided by data from a large number of groundwater and surface water sampling locations within two study regions along the river. Measurements show that dissolved Se concentrations in the river are about double the current Colorado Department of Public Health and Environment (CDPHE) chronic standard of 4.6 microg L(-1) for aquatic habitat in the upstream region and exceed the standard by a factor of 2 to 4 in the downstream region. Groundwater concentrations average about 57.7 microg L(-1) upstream and 33.0 microg L(-1) downstream, indicating a large subsurface source for irrigation-induced dissolution and mobilization of Se loads to the river and its tributaries. Inverse correlation was found between Se concentration and the distance to the closest identified shale in the direction upstream along the principal groundwater flow gradient. The data also exhibited, among other relationships, a moderate to strong correlation between dissolved Se and total dissolved solids in groundwater and surface water, a strong correlation with uranium in groundwater, and power relationships with nitrate in groundwater. The relationship to nitrate, derived primarily from N fertilizers, reveals the degree to which dissolved Se depends on oxidation and inhibited reduction due to denitrification and suggests that there are prospects for reducing dissolved Se through nitrate control. Current and future results from these ongoing studies will help provide a foundation for modeling and for the discovery of best management practices (BMPs) in irrigated agriculture that can diminish Se contamination.

Modeling variably saturated multispecies reactive groundwater solute transport with MODFLOW-UZF and RT3D

A numerical model was developed that is capable of simulating multispecies reactive solute transport in variably saturated porous media. This model consists of a modified version of the reactive transport model RT3D (Reactive Transport in 3 Dimensions) that is linked to the Unsaturated-Zone Flow (UZF1) package and MODFLOW. Referred to as UZF-RT3D, the model is tested against published analytical benchmarks as well as other published contaminant transport models, including HYDRUS-1D, VS2DT, and SUTRA, and the coupled flow and transport modeling system of CATHY and TRAN3D. Comparisons in one-dimensional, two-dimensional, and three-dimensional variably saturated systems are explored. While several test cases are included to verify the correct implementation of variably saturated transport in UZF-RT3D, other cases are included to demonstrate the usefulness of the code in terms of model run-time and handling the reaction kinetics of multiple interacting species in variably saturated subsurface systems. As UZF1 relies on a kinematic-wave approximation for unsaturated flow that neglects the diffusive terms in Richards equation, UZF-RT3D can be used for large-scale aquifer systems for which the UZF1 formulation is reasonable, that is, capillary-pressure gradients can be neglected and soil parameters can be treated as homogeneous. Decreased model run-time and the ability to include site-specific chemical species and chemical reactions make UZF-RT3D an attractive model for efficient simulation of multispecies reactive transport in variably saturated large-scale subsurface systems.

The influence of nitrate on selenium in irrigated agricultural groundwater systems.

Selenium (Se) contamination of groundwater is an environmental concern especially in areas where aquifer systems are underlain by Se-bearing geologic formations such as marine shale. This study examined the influence of nitrate (NO₃) on Se species in irrigated soil and groundwater systems and presents results from field and laboratory studies that further clarify this influence. Inhibition of selenate (SeO₄) reduction in the presence of NO₃ and the oxidation of reduced Se from shale by autotrophic denitrification were investigated. Groundwater sampling from piezometers near an alluvium-shale interface suggests that SeO₄ present in the groundwater was due in part to autotrophic denitrification. Laboratory shale oxidation batch studies indicate that autotrophic denitrification is a major driver in the release of SeO₄ and sulfate. Similar findings occurred for a shale oxidation flow-through column study, with 70 and 31% more reduced Se and S mass, respectively, removed from the shale material in the presence of NO₃ than in its absence. A final laboratory flow-through column test was performed with shallow soil samples to assess the inhibition of SeO₄ reduction in the presence of NO₃, with results suggesting that a concentration of NO₃ of approximately 5 mg L or greater will diminish the reduction of SeO₄. The inclusion of the fate and transport of NO₃ and dissolved oxygen is imperative when studying or simulating the fate and transport of Se species in soil and groundwater systems.

Simulating variably-saturated reactive transport of selenium and nitrogen in agricultural groundwater systems

Selenium (Se) contamination in environmental systems has become a major issue in many regions world-wide during the previous decades, with both elevated and deficient Se concentrations in groundwater, surface water, soils and associated cultivated crops reported. To provide a tool that can assess baseline conditions and explore remediation strategies, this paper presents a numerical model capable of simulating the reactive transport of Se species in large-scale variably-saturated groundwater systems influenced by agricultural practices. Developed by incorporating a Se reaction module into the multi-species, variably-saturated reactive transport model UZF-RT3D, model features include near-surface Se cycling due to agricultural practices, oxidation-reduction reactions, and the inclusion of a nitrogen (N) cycle and reaction module due to the dependence of Se transformation and speciation on the presence of nitrate (NO₃). Although the primary motivation is applying the model to large-scale systems, this paper presents applications to agricultural soil profile systems to corroborate the near-surface module processes that are vital in estimating mass loadings to the saturated zone in large-scale fate and transport studies. The first application jointly tests the Se and N modules for corn test plots receiving varying loadings of fertilizer, whereas the second application tests the N module for fertilized and unfertilized test plots. Results indicate that the model is successful in reproducing observed measurements of Se and NO₃ concentrations, particularly in lower soil layers and hence in regards to leaching. For the first application, the Ensemble Kalman Filter (EnKF) is used to condition model parameters, demonstrating the usefulness of the EnKF in real-world reactive transport systems.

Assessing the effectiveness of land and water management practices on nonpoint source nitrate levels in an alluvial stream–aquifer system

The search for ways to allay subsurface nitrate pollution and loading to streams over broad regional landscapes is taken up using a calibrated groundwater model supported by extensive field data. Major processes of transport and chemical reaction are considered in the irrigated vadose zone and the underlying alluvial aquifer in interaction with Colorados Lower Arkansas River and its tributaries. Simulation of a variety of best management practices reveals that there is potential to lower regional nitrate concentrations in groundwater by up to about 40% and mass loading to the river network by up to 70% over a four-decade span. Over the 27BMP scenarios considered in this study, the most effective singular measures are reduction of fertilizer application and sealing of irrigation canals, while combinations of reduced fertilizer application, reduced irrigation application, canal sealing, and enhanced riparian buffer zones are predicted to have the greatest overall impact. Intermittent fallowing of 25% of the land to lease irrigation water also is found to be promising, resulting in a forecasted decrease of about 15% in nitrate groundwater loading to streams. Due to the strong similarity between the study region and other irrigated, fertilized alluvial river valley stream-aquifer systems worldwide, results of this study are expected to be broadly applicable.

Assessing Irrigation-Induced Selenium and Iron in the Lower Arkansas River Valley in Colorado

An intense field investigation was performed in an irrigated alluvial valley along a 61.6 km portion of the Arkansas River in Colorado. 54 monitoring wells and 21 surface water locations (including six locations in the Arkansas River) were sampled twelve times over the past twenty months. Results indicated selenium concentrations ( C Se ) in the ground water ranging from less than 0.4 to approximately 3760 μg/L with a median concentration of about 17 μg/L. Ground water C Se samples taken from wells located in alluvial material ranged from less than 0.4 to 166 μg/L with a median concentration of 12.2 μg/L while samples taken from monitoring wells located in slopewash and shale derived material ranged from less than 0.4 to 3760 μg/L with a median concentration of 30.8 μg/L. The surface water C Se samples ranged from approximately 1.6 to 43.2 μg/L with a median concentration of about 11 μg/L. Iron concentration ( C Fe ) ranged from less than 5 to about 1560 μg/L; however, only about 25 percent of the samples indicated dissolved Fe above the analytical detection limit. Total recoverable Fe concentration ( C Fe-tree ) in samples in the surface water ranged from about 25.4 to 118,000 μg/L, with a median concentration of about 361 μg/L. A non-linear single-variant relationship was developed between C Se and EC in the surface water (r 2 = 0.34) and in the ground water (r 2 = 0.39). In the surface water samples, a linear multi-variant relationship was developed to predict C Se from C SO4 and C NO3 (r 2 = 0.55). Also, a non-linear relationship was developed between C Se from C SO4 in the ground water (r 2 = 0.55). About 40 independently monitored surface water locations and 55 monitoring wells were sampled to test the reliability of these models. A preliminary Se mass balance for the Study Area was completed over a one-year period from approximately April 1, 2003 to March 31, 2004. The total Se load to the Arkansas River was estimated at approximately 15.6 kg-Se per km per year. The total Se load from irrigation canals in the Study Area was estimated at approximately 1086 kg-Se per year while about 959 kg-Se per year was returned to the river.

Sediment and microbial fouling of experimental groundwater recharge trenches

Abstract A common method of recharging groundwater is by the use of injection wells and/or recharge trenches. With time the recharge capacities of the wells/trenches progressively decline. Deposition of suspended fines in the recharge water and growth of microorganisms in the aquifer are common causes of this decline. This paper presents an investigation of the relative significance of these two factors under controlled laboratory conditions. Large-scale physical models of recharge trenches were conducted in the laboratory to monitor the decline with time of the recharge capacity under controlled conditions. The physical models consisted of four hydraulically separate cells in which six different experiments were conducted. In three of the experiments microorganism were added as an inoculant. A nutrient and carbon fine solution was constantly injected into the influent stream entering through the inflow pipe. Both carbon fines and microorganisms caused plugging of the model recharge trenches in the laboratory. However, initialy the microbes appeared to have a beneficial effect by hindering the transport of the carbon fines from the gravel pack in the trench. Later the microbes contributed to the plugging of the gravel pack. A significant correlation was determined between the extent of carbon fine deposition and microbial growth. In the experiment using a biodegradable slurry, microbial growth did not affect the recharge capacity of the trench. One laboratory experiment involved the introduction of silt as a source of sediment fines to the model recharge trench. This experiment simulated conditions often found in the field when no carbon fine adsoprtion system is used and natural surface water is recharged into aquifer. This research will be useful in understanding the relative importance of factors contributing to the decline of recharge capacity observed in the field.

River GeoDSS for Agroenvironmental Enhancement of Colorado’s Lower Arkansas River Basin. I: Model Development and Calibration

The Lower Arkansas River (LAR) Basin in Colorado, like many intensively irrigated river basins in the Western United States, faces a variety of problems associated with inefficient irrigation, seepage from earthen canals, and inadequate drainage facilities. Upward flows from high water tables have salinized and waterlogged agricultural soils of the Valley, contributing to reduced crop yields and nonbeneficial water consumption on adjacent uncultivated lands. River water quality has also suffered since intensive irrigation of alluvial soils results in evaporative concentration and the accelerated dissolution of inherent salts and other mineral pollutants into the underlying aquifer, appearing as return flows that threaten the ecological health of the river. A geographic information system-based river basin decision-support system (River GeoDSS) has been developed and applied to the LAR Basin for assessing basinwide strategies for improving agricultural productivity, salvaging water from nonbeneficial consu...

River GeoDSS for Agroenvironmental Enhancement of Colorado’s Lower Arkansas River Basin. II: Evaluation of Strategies

Research conducted at the field and regional scales in the Lower Arkansas River (LAR) Valley of Colorado has identified water management alternatives with potential for enhancing agroenvironmental conditions in the basin by reducing waterlogging and soil salinity, salt loadings to the river, and nonbeneficial evapotranspiration in the irrigated stream-aquifer system. The LAR geospatial decision support system (GeoDSS), presented in a companion paper as a customized version of the generalized River GeoDSS, is applied to the evaluation of the feasibility and performance of water management strategies at the basin scale. The LAR GeoDSS allows comparative evaluation of management options for improving irrigation efficiency, minimizing water shortages, and improving water quality at selected control points by augmenting groundwater return flows through dynamic regulation of reservoir releases to abide by legal and administrative constraints on river operations. Results show that conditions favorable to increas...

Designing Furrow-Irrigation Systems for Improved Seasonal Performance

ABSTRACT Atraditional approach to furrow irrigation system design was evaluated to determine its effect on the seasonal performance of a typical farmer-managed system. Results indicate the need for an improved design approach that better accounts for seasonal changes in field conditions. Recently, attention has been focused on the need for more wise and efficient use of water on the farm. Irrigated agriculture, in particular, is faced with the dilemma of using a diminishing supply of water to meet an increasing demand for food and farm income. Much of the responsibility to meet this challenge falls upon the agricultural engineering profession. Traditional methods of system design and operation need to be evaluated and new methods developed that will help to strengthen irrigated agriculture as an economically justifiable, productive, and socially responsible endeavor. Surface irrigation (used here to refer to surface flooding by gravity flow) is the method used on most of the worlds irrigated land (Goldberg, 1974). Improving irrigated agriculture on a worldwide scale must address the problems related to surface irrigation. Many surface irrigation systems ideally can supply crop water needs at field application efficiencies of 70 to 85 percent (Willardson, 1972; and Merriam and Keller, 1978). Studies conducted around the world indicate typical efficiencies to be only 40 to 50 percent (Bos and Nugteren, 1974; Clyma and Ali, 1977; Kruse and Heermann, 1977; Clyma et al., 1975). Solving the problems of inefficient surface irrigation practices requires the use of good system design. A good irrigation system design is the product of a reliable design procedure and the appropriate application of that procedure. The design procedure should be based on a model that adequately predicts system hydraulics and performance. It is used to select the values of system and operational (or management) variables that constitute the right conditions for achieving a specified level of performance. If the system is built and given to the operator without instructions as to how the system should be operated, the system cannot be expected to function to produce the desired results. Traditionally, design procedures for surface irrigation have specified operational variables based on field conditions during the period of peak consumptive use of the crop (USDA, 1979; Corey and Hart, 1974; Withers and Vipond, 1974). Thus, the design does not account for the change in field conditions during a cropping season. In the case of the SCS method and various other empirical methods of furrow irrigation system design, operational variables are explicitly selected in the design process. However, when system and operational variables are specified and implemented based upon conditions existing in the field at one point in time, the resulting system performance may be poor. How do furrow intake rates and required water application depths vary on a typical farmer managed furrow irrigation system? What system performance would result if each irrigation, as scheduled by the farmer, were operated with the flow rate and inflow time designed for the peak capacity period? If seasonal variations in field conditions are considered in design, what changes in the system or its operation might be needed to achieve acceptable performance through the season? This study was conducted to address these questions.