Stream-aquifer and in-stream processes affecting nitrogen along a major river and contributing tributary.

This study assesses the spatio-temporal patterns of water and nutrient mass exchange in a stream-riparian system of a major river and a contributing tributary in an irrigated semi-arid region. Field monitoring is performed along reaches of the Arkansas River (4.7km) and Timpas Creek (2.

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 Colorado's 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.

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).

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.

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.

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.