THE TRANSPORT OF ANTHROPOGENIC AND NATURAL CONTAMINANTS (TANC) IN THE HIGH PLAINS AQUIFER, NEBRASKA

 

Michael J. Turco, Hydrologist, U.S. Geological Survey, Houston, Texas, and Matthew K. Landon, Hydrologist, U.S. Geological Survey, Lincoln, Nebraska

 

Proceedings of the Conference on High Plains Groundwater Resources; Challenges and Opportunities, December 7-9, 2004, Lubbock, Texas.

 

ABSTRACT

 

            The High Plains aquifer is a major source of water to most of the municipal, industrial and agricultural users in Nebraska. As part of the U.S. Geological Survey (USGS) National Water Quality Assessment (NAWQA) Program, an investigation of the Transport of Natural and Anthropogenic Contaminants (TANC) is underway near York, Nebraska. The goal of the NAWQA TANC topical study is to answer the question: What are the primary anthropogenic and natural contaminant sources, aquifer processes, and well characteristics that control the transport and transformation of contaminants along flow paths to supply wells in representative water-supply aquifers? The TANC project in the High Plains aquifer is conducted on both large and small spatial scales.

            The TANC large-area ground-water study encompasses a 150 square-mile area around York in east-central Nebraska. It is located in Quaternary alluvial deposits that comprise the High Plains aquifer in eastern and central Nebraska, and Kansas, that are hydraulically connected to and derived from the Ogallala Formation, which is the largest hydrogeologic unit of the High Plains aquifer. The study area has relatively low relief and is oriented northwest to southeast along a regional ground-water-flow path that is consistently indicated by historical water-table maps and regional ground-water-modeling studies. The TANC large-area study includes compilation and analysis of retrospective water-quality data and ground-water-flow modeling to identify zones of contribution to public supply wells. These data were used to facilitate statistical analysis of hydrogeochemical and land use variables that influence the occurrence of contaminants in public supply wells. A conceptual hydrogeochemical model based on retrospective data showing hydrogeology, oxidation-reduction chemistry, and a water budget was developed and used along with additional ancillary data to construct a MODFLOW ground-water-flow model.

            The ground-water-flow model was developed using MODFLOW-2000, which incorporates parameter estimation. The model consists of six layers that vertically account for the main water-yielding zones and confining units in the High Plains aquifer. The model was calibrated using historical water-level and surface-water-discharge data.  Results from the large-area model showed that withdrawals from the confined zones (layers in the model) cause downward head gradients indicating ground-water movement from the unconfined to the confined water-yielding zones. The calibrated model was used to simulate the areas contributing recharge to the City of York municipal supply wells so that a monitoring-well network could be installed for a specific flow path for the small-scalearea investigation.

            The small-area investigation includes the installation of monitoring wells along a ground-water-flow path to a supply well, and simulations of ground-water flow and solute transport to understand the processes controlling contaminant movement and to refine hypotheses based on the large-area work. Thirty-six monitoring wells were installed during spring 2003 and 2004 along or adjacent to the zone of contribution to a selected York public supply well. The installed monitoring wells predominantly are located in irrigated agriculture and urban areas. Seven well clusters include wells screened in each of the three major water-yielding zones and 1515 additional wells are screened near the water table. Samples for ground-water chemistry, age, and isotopic tracers were collected during fall 2003. Sampling of selected wells and constituents will continue through spring 2005.

            The USGS used a depth-dependent sampler and dye injector in June 2004 to determine the velocity profile of the selected supply well to identify the primary screen intervals contributing flow. Water samples subsequently were collected from several depths in the supply well using a bladder pump that fits into a 1.25-inch-diameter access tube in well to better understand where contaminants are entering the well screen and where well-bore mixing might be occurring. Results of the sampling have refined the conceptual model of the flow system and suggest that there are localized areas in the confined zones where downward moving water from the unconfined zone has mixed with confined waters. .

            Nitrogen and oxygen isotopic values of nitrate indicate de-nitrification is occurring as water moves deeper into the system. Cores of the upper confining unit were collected in April 2004 to be analyzed for pore-water chemistry to better understand the effect of denitrification in the confining unit on nitrate transport. Preliminary results show rapid decreases in nitrate concentrations across the confining unit. Isotopic data and modeling will be used to estimate the contribution of denitrification and historical changes in nitrate loading on nitrate transport through the confining layer.

            Project results will be compared with those of other TANC studies across the country to better understand variables controlling contaminant movement to supply wells across a range of hydrogeologic settings.